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Offline Ed_Chambley

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Series caps differential voltage
« on: March 01, 2017, 03:33:58 pm »
Curious if anyone knows a formula to determine what is the maximum voltage of dissimilar voltage and farad caps.  For instance, you have a 1uf/400 cap in series with a 22uf/50v cap.  If placed in series what would be the total voltage on each cap is 50vdc were applied?

Then if you had 3 in series, say 10uf/350 and 50uf/500 and finally 16uf/160 and applies 160 vdc, what would each cap voltage measure?


Offline silverfox

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Re: Series caps differential voltage
« Reply #1 on: March 01, 2017, 03:46:31 pm »
I will go with: Under DC conditions the voltage drop on each capacitor will be proportional to the equivalent DC circuit resistance. Very likely and for all practical purposes each resistance will be equal. So find the leakage current and do the series resistance math. I believe, if you are going to use each capacitor as a filter for a dropped voltage however, there will be a complicated relationship based on the charge discharge cycles resulting from the changing load. Then you're dealing with a sort of quasi-AC theory.

As always, refutations gratefully accepted.

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Offline sluckey

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Re: Series caps differential voltage
« Reply #2 on: March 01, 2017, 03:52:33 pm »
I would be more curious to know if you applied 300V across a 1µF cap connected in series with a .01µF what would be the voltage measured across each cap?
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Offline PRR

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Re: Series caps differential voltage
« Reply #3 on: March 01, 2017, 04:44:52 pm »
In the first nano-instant, they divide as (inverse of) capacity.

Long-term they divide, as (inverse of) leakage, as silverfox says.

> 1uf/400 cap in series with a 22uf/50v

As the 1uFd is likely to be Film (plastic), and the 22u is likely to be electrolytic (soggy rust) with 1000X the leakage, I would wild-pitch 49++V on the 1u and <1V on the 22u after a few seconds.

Offline silverfox

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Re: Series caps differential voltage
« Reply #4 on: March 02, 2017, 01:48:01 am »
After getting on the road today another thought occurred to me regarding the use of stacked caps as a series of filter capacitors. Capacitors  of different values in series will have a proportionate capacitance to the total capacitance which is calculated according to the formulas on this page: http://www.electronics-tutorials.ws/capacitor/cap_7.html

Personally I don't know enough to be certain the source is correct. But it seems logical. The source page goes into a good explanation.

Capacitors in Series Summary Then to summarise, the total or equivalent capacitance, CT of a circuit containing Capacitors in Series is the reciprocal of the sum of the reciprocals of all of the individual capacitance’s added together.
Also for capacitors connected in series, all the series connected capacitors will have the same charging current flowing through them as iT = i1 = i2 = i3 etc. Two or more capacitors in series will always have equal amounts of coulomb charge across their plates.
As the charge, ( Q ) is equal and constant, the voltage drop across the capacitor is determined by the value of the capacitor only as V = Q ÷ C. A small capacitance value will result in a larger voltage while a large value of capacitance will result in a smaller voltage drop.
silverfox.

Offline HotBluePlates

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Re: Series caps differential voltage
« Reply #5 on: March 02, 2017, 02:19:16 pm »
I would be more curious to know if you applied 300V across a 1µF cap connected in series with a .01µF what would be the voltage measured across each cap?

... A small capacitance value will result in a larger voltage while a large value of capacitance will result in a smaller voltage drop. ...

Right; though the long-way of explaining had the feel of a math proof.

Ever calculated "capacitive reactance"?  We had to do it a bunch when first covering caps in an electronics class.

Xc = 1/(2*π*f*C)   (expressed in Ohms)
 - Bottom-line: Smaller µF = More Ω

So you can make an a.c. voltage divider consisting only of caps.  The voltage will divide as the inverse-ratio of cap values.  For Sluckey's example of 1µF and 0.01µF, 100-parts of the applied voltage will drop across the 0.01µF cap and 1-part will drop across the 1µF cap.  If you assume an a.c. frequency, you can prove this by calculating Xc for each cap, then use whatever method you prefer for voltage drops in a divider.

Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #6 on: March 02, 2017, 02:49:29 pm »
I would be more curious to know if you applied 300V across a 1µF cap connected in series with a .01µF what would be the voltage measured across each cap?

... A small capacitance value will result in a larger voltage while a large value of capacitance will result in a smaller voltage drop. ...

Right; though the long-way of explaining had the feel of a math proof.

Ever calculated "capacitive reactance"?  We had to do it a bunch when first covering caps in an electronics class.

Xc = 1/(2*π*f*C)   (expressed in Ohms)
 - Bottom-line: Smaller µF = More Ω

So you can make an a.c. voltage divider consisting only of caps.  The voltage will divide as the inverse-ratio of cap values.  For Sluckey's example of 1µF and 0.01µF, 100-parts of the applied voltage will drop across the 0.01µF cap and 1-part will drop across the 1µF cap.  If you assume an a.c. frequency, you can prove this by calculating Xc for each cap, then use whatever method you prefer for voltage drops in a divider.
HBP, Glad you brought Sluckey's example in.  This is a discussion I was having with him.  The purpose for the question is if I am using a cap sub box that is 200v, is it possible to make a probe to use this same box to test values in a 300 to 500vdc coupler.  Since the cap box will series switch this will put too much voltage the caps internally.

In this instance it is an older Cap Ranger by Ohmite and is very nice.  I just wanted to insure I did not fry it or shock myself or even overvolt the switches and construction.

He suggested me testing first bu placing serise caps in an amp and measuring to see what I come up with.  I plan to do this soon.

Seems the voltage is not a rather simple thing to determine.  So I am coming clean here with my reason for the question.   It is not so much filtration and saving a piece of test gear that if I were to make would cost a fortune.

Do you know a viable solution other than using higher voltage caps?

Offline HotBluePlates

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Re: Series caps differential voltage
« Reply #7 on: March 02, 2017, 03:29:44 pm »
... if I am using a cap sub box that is 200v, is it possible to make a probe to use this same box to test values in a 300 to 500vdc coupler.  Since the cap box will series switch this will put too much voltage the caps internally.
...
Do you know a viable solution other than using higher voltage caps?

None that I would do.  My post was about A.C. voltage division with caps.

At D.C. a cap looks like an open-circuit; therefore, voltage division for d.c. is all about leakage current as mentioned earlier.

And why even with two same-value caps in series, voltage-sharing resistors across the caps.  Yeah, people call the 220kΩ resistors "bleeders" and they do that.  But Leo coulda only paid for 1 resistor if he didn't have to ensure equal-voltage across the series caps.

Offline PRR

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Re: Series caps differential voltage
« Reply #8 on: March 02, 2017, 03:35:13 pm »
> a 300 to 500vdc coupler.

What is that?

Offline HotBluePlates

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Re: Series caps differential voltage
« Reply #9 on: March 02, 2017, 03:51:13 pm »
> a 300 to 500vdc coupler.

What is that?

I think Ed is trying to have a coupling cap substitution box rated over 300vdc.

Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #10 on: March 02, 2017, 03:51:53 pm »
> a 300 to 500vdc coupler.

What is that?
Sorry, Coupling caps.  Since the cap ranger is made for series testing I had concerns about putting it in a circuit where the voltage exceeds 200v that the Ohmite Cap Ranger is designed for.


So said another way, is there a way I can use this device in higher voltage areas by putting a 10X cap say an Orange Drop 630 V in series and not damage the unit?

Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #11 on: March 02, 2017, 03:54:29 pm »
> a 300 to 500vdc coupler.

What is that?

I think Ed is trying to have a coupling cap substitution box rated over 300vdc.
yes, thanks for clarifying my thoughts.

Offline HotBluePlates

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Re: Series caps differential voltage
« Reply #12 on: March 02, 2017, 04:05:42 pm »
... if I am using a cap sub box that is 200v, is it possible to make a probe to use this same box to test values in a 300 to 500vdc coupler.  Since the cap box will series switch this will put too much voltage the caps internally.
...
Do you know a viable solution other than using higher voltage caps?

None that I would do.  ...

I think I see now where Sluckey was going.

Dunno if your cap box has two different-colored binding posts.  You could connect a 1µF (or larger) cap of 400-630v in series between a binding post and the rest of the internals of the sub-box.  If you always connect that binding post to the highest-voltage point in your test setup, it will block the d.c. volts and keep the stock caps protected.

The only risk I can think of is the initial surge when connecting this in-circuit.

Offline sluckey

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Re: Series caps differential voltage
« Reply #13 on: March 02, 2017, 04:19:55 pm »
Ed, you have the perfect test bed for this ... Your AC-15 has two series connected caps for a coupling cap. Look at page 2 of this pdf...

     http://sluckeyamps.com/VAC15/Vox_AC15.pdf

Now look at the EF86 preamp tube. It has 98vdc on the plate that is connected to C12. The other side of C12 connects to switch selectable tone caps. Just measure the voltage on the right side of C12 to see how much (if any) dc volts is felt on the  selected 'tone' cap.

This is basically what you want to do with the cap ranger. A good rule of thumb is to use an external cap that is 10X the value of the cap you want to select inside the cap ranger.

I may just go to the shop and make this check on my AC-15 right now.
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Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #14 on: March 02, 2017, 04:22:28 pm »
HBP,
Attached is a photo of the cap ranger.  It looks as if it does indeed have as you asked.  Could you please let me know if this is what you meant and remember you are explaining to a 6th grader. :icon_biggrin:


Just kidding, but I believe this will work.  Thoughts appreciated.

Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #15 on: March 02, 2017, 04:26:33 pm »
Ed, you have the perfect test bed for this ... Your AC-15 has two series connected caps for a coupling cap. Look at page 2 of this pdf...

     http://sluckeyamps.com/VAC15/Vox_AC15.pdf

Now look at the EF86 preamp tube. It has 98vdc on the plate that is connected to C12. The other side of C12 connects to switch selectable tone caps. Just measure the voltage on the right side of C12 to see how much (if any) dc volts is felt on the  selected 'tone' cap.

This is basically what you want to do with the cap ranger. A good rule of thumb is to use an external cap that is 10X the value of the cap you want to select inside the cap ranger.

I may just go to the shop and make this check on my AC-15 right now.
Yep, but my AC15 is in Asheville right not and I am in Atlanta, but I do have another amp, quite a few actually, that have cap switching.  They are not in head boxes.


After I made the AC15 with 12, I sort of got carried away with keeping gain. :icon_biggrin:   I actually put on on my Silverface Super Reverb.

Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #16 on: March 02, 2017, 04:34:33 pm »
Ed, you have the perfect test bed for this ... Your AC-15 has two series connected caps for a coupling cap. Look at page 2 of this pdf...

     http://sluckeyamps.com/VAC15/Vox_AC15.pdf

Now look at the EF86 preamp tube. It has 98vdc on the plate that is connected to C12. The other side of C12 connects to switch selectable tone caps. Just measure the voltage on the right side of C12 to see how much (if any) dc volts is felt on the  selected 'tone' cap.

This is basically what you want to do with the cap ranger. A good rule of thumb is to use an external cap that is 10X the value of the cap you want to select inside the cap ranger.

I may just go to the shop and make this check on my AC-15 right now.
Well you gotta get a beer anyway. :wink:   Midas well.

Offline jjasilli

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Offline HotBluePlates

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Re: Series caps differential voltage
« Reply #19 on: March 02, 2017, 04:53:47 pm »
HBP,
Attached is a photo of the cap ranger.  It looks as if it does indeed have as you asked.  Could you please let me know if this is what you meant ...

I had hoped for only 2 binding posts, or a plain-metal 3rd post.

I suspect the Black post is attached to the case.

One of the Red posts is probably common to one side of all caps.  The other Red post is probably common to all switches (on the other side of the caps).

If it were me, I'd add my blocking cap to the first of those Red posts (common to all caps) and conspicuously mark that post to always be attached to the highest voltage.

However, the Cap Ranger looks like a small case.  Will a 1µF 630v film cap fit inside?

Offline sluckey

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Re: Series caps differential voltage
« Reply #20 on: March 02, 2017, 04:58:38 pm »
I'm back. I have 100vdc on the left side of C12. I have 0vdc (well maybe a few mV) on the right side of C12. So, I'd say that C12 blocked dc from reaching the smaller caps on the selector switch.

This still does not tell if there is a big transient when you first connect the caps to the circuit, nor how that transient will divide across each of the two series caps.

If I had a cap ranger I'd give it a try. The worst that will happen is you pop a cap. In that case, just replace the popped cap with a higher rated cap.

I'd still like to hear someone give a definitive answer to the question, keeping in mind we ain't talking about AC voltage. I suppose a transient spike could be considered AC...
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Offline HotBluePlates

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Re: Series caps differential voltage
« Reply #21 on: March 02, 2017, 05:06:00 pm »
This still does not tell if there is a big transient when you first connect the caps to the circuit, nor how that transient will divide across each of the two series caps.
...
I'd still like to hear someone give a definitive answer to the question, keeping in mind we ain't talking about AC voltage. I suppose a transient spike could be considered AC...

I don't really know.  My gut says the new blocking cap will try to charge to the B+, which implies it will pass a brief big current through the old cap.  I think "voltage division" doesn't really matter in this case.  You/Ed are hoping the new cap will block all d.c. off the old caps.

Offline sluckey

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Re: Series caps differential voltage
« Reply #22 on: March 02, 2017, 05:11:31 pm »
Quote
You/Ed are hoping the new cap will block all d.c. off the old caps.
Or just that it would divide the voltage such that the ranger could be used to experiment with different value plate coupling caps.

Here's a little better pic of the ranger...
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Offline HotBluePlates

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Re: Series caps differential voltage
« Reply #23 on: March 02, 2017, 05:27:15 pm »
As the 1uFd is likely to be Film (plastic), and the 22u is likely to be electrolytic (soggy rust) with 1000X the leakage, I would wild-pitch 49++V on the 1u and <1V on the 22u after a few seconds.

For two non-leaky caps, you've already measured ~0v after the first cap.  Good enough for me.

Offline silverfox

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Re: Series caps differential voltage
« Reply #24 on: March 02, 2017, 05:32:15 pm »
Maybe I'm getting something lost in the translation here but: We have a cap substitution box with known values. And now we want to add various other caps in series with the known values? You'll then have to do the math and if this is in an AC circuit,...

silverfox.

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Re: Series caps differential voltage
« Reply #25 on: March 02, 2017, 06:01:31 pm »
Maybe I'm getting something lost in the translation here but: We have a cap substitution box ...

Rated for 200vdc.  Ed wants to use it in real amp circuits with >200vdc.

Offline sluckey

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Re: Series caps differential voltage
« Reply #26 on: March 02, 2017, 06:22:25 pm »
Ed, just a suggestion... Open up the ranger box and disconnect anything connected to that black banana binding post. I know you want to see what's inside anyway.  :icon_biggrin:

Put it all back together. Now mount your external blocking cap between the black binding post and the middle red binding post. Your test leads will connect to the black post and the red post on the outside.

Remember you want to use an ext. blocking cap that's 10X (or more) the value of the anticipated cap value you will switch in.
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Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #27 on: March 02, 2017, 07:12:35 pm »
Ok i have some feedback. When connected series i actually simply put a cap 630v connected to the right and single probe to right red. First to 193vdc and no measueable voltage INSIDE at the base of the post. Nothing! This one was a 1uf. Switch on with 2 meters, analog and digital. No change no voltage. Next I tried a audiophiler poly 10uf. Same results.


Moved up to PI node and 327v. Same results. Bravely up to power tube plates at 467vdc and same.


So now I am happy but confused. How is the large blocking when in series?


Yes, Sluckey I actually took it apart first. You do not really want o replace caps in this thing. Doable, yes. Want to do,no.


The black binding post is soldered to a standoff which bolts to the case. I have no idea why? Anyone? To disconnect is simple and is a good idea to pin the large cap and then you could simply use the reds for under 200v and place whatever 10x cap. Nice idea provide the chassis ground serves no purpose. I assume the black is to share a ground with whatever you may be testing to reduce noise. :dontknow:


I still not cannot wrap my mind a series cap blocking when when used in filter positions we depend upon increased voltage. I guess I need to email Michael Faraday.
« Last Edit: March 02, 2017, 07:14:37 pm by Ed_Chambley »

Offline jjasilli

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Re: Series caps differential voltage
« Reply #28 on: March 02, 2017, 07:45:01 pm »
Yes, what I think Hotblue, and the sites I posted, are revealing is that the result is dependent upon voltage, frequency, and the position of the cap in a series string of caps; and that this relationship is complex.  Also, for B+ the DC is not pure; the ripple component looks like AC to the cap.  So unless you're willing to do the grueling math, you're in the dark.

Bottom line, it does not appear that the Cap Ranger is useful for B+ Filtering.  It seems more useful to test for: (i) smaller value cathode bypass cap; (ii) caps in the signal path.  For (ii) if high DC operating voltage is an issue, use a higher voltage blocking cap in series with the Cap Ranger.  The value of the blocking cap should follow the Rule of Tens so as not to skew the value of the cap under test.  Hope this helps.

In the past PRR has pointed out that true decade boxes with lots of incremental values are great for testing laboratories, but may not really suit the needs of the amp builder.  A homemade box with a number of likely values, of suitable voltage or current handling is another choice.  E.g., a pF box with values of 10, 47, 100, 220, 330, 470 would be nice.  EDIT: for B+, a box with high voltage uF cap values of, say, 8, 10, 20, 47 and 100 would be nice. Combined in parallel they'd give lots of useful values.
« Last Edit: March 02, 2017, 08:34:56 pm by jjasilli »

Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #29 on: March 02, 2017, 08:41:25 pm »
Ok, I have tested and testing is certainly simpler than the math, and testing does not completely leave us in the dark.  I agree, the unit is not really for filtering and filtering has a ground reference.


It seems if the cap is in series as in a plate coupling, even if using a scope measuring voltage, the cap will block DC, however reading the link posted by JJasilli obviously the caps do receive an initial voltage.  I do not own a test instrument that will show any.


The reason for my interest in the ranger specifically, to build switching cap box with the multitude of values would not be feasible.  Sure, I have boxes with the standard values we use as coupling caps and another with pf values, but I have always felt it a royal pain to go back and fourth.  Even then, a lot of times I find one cap passing more than I want and the next one not quite enough.


For instance, I do not have a switching cap box with a .33uf and when working on a power sections in Marshall topologies like the JTM 45 the .1uf from the PI to the grid of the power tube combined with a 56K slope makes the bass touchy and if increased a little begins to sound woolly.  Even more gets into woof and flab hence the treble booster.


To find what I was happy with took a lot of reworking of the circuit as I also do not have a cap box containing a 270pf.


Currently I am beginning a large project and again building another workbench.  This time I would prefer to have as much functionality from each piece of test equipment as possible.  I find this will work for me very well, but others certainly have their methods whic is preferred.  I really do not know, I might switch back, but I cannot know what I do not know.


BTW, I find I prefer a 33k Slope and .33uf and even increase the treble cap from .02uf to .03uf with an increase of the stock 270pf ceramic to a polystyrene in a 510pf.  This tool, if used correctly could have helped.  I am not sure about how much time other look for their tools, but I know I waste a lot of time doing so.  During this time I get interrupted and often even forget what I have and have not tried so I retrace steps and I am tired of doing this.  It is very inefficent.


It is a simple reason, I just had no idea of the complexity of series caps and when asked Sluckey he suggested I make a post on the subject.  Here is that post.


So now I have tested 10uf in series with 1uf following the 10x rule so to speak.  And yes, it is not the complete answer to having any cap value available at my finger tips, but I do have 1 piece of test gear and drawers in-front of which I would have anyway, containing caps 10x the size.


Yes, I would like to understand more and will continue to research.  Like currently I cannot find how a cap having a ground reference does increase measurable voltages across common filter cap even if the voltages vary some, but it does and why when used as coupling the first cap seems to block.


I have even tried the other way with putting the smaller volt first in the series and still no DC present.


May be old hat to you fellers who have been around the block a time or 2, but to me this is fascinating.


Thank you for indulging my curiosity and please do not hesitate to correct me.  I know I have a lot of this wrong.

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Re: Series caps differential voltage
« Reply #30 on: March 02, 2017, 08:51:57 pm »
HBP,
Attached is a photo of the cap ranger.  It looks as if it does indeed have as you asked.  Could you please let me know if this is what you meant ...

I had hoped for only 2 binding posts, or a plain-metal 3rd post.

I suspect the Black post is attached to the case.

One of the Red posts is probably common to one side of all caps.  The other Red post is probably common to all switches (on the other side of the caps).

If it were me, I'd add my blocking cap to the first of those Red posts (common to all caps) and conspicuously mark that post to always be attached to the highest voltage.

However, the Cap Ranger looks like a small case.  Will a 1µF 630v film cap fit inside?
Yes a 1uf 630 Solen will fit easily.  Actually if you wanted you could actually convert it into a 600V box except the stackpole switchse are rated at .5a/250v, which I consider conservative, but certainly not 600.


Right now with the large cap in front the cap ranger is only passing AC.  I actually now have injected a sine and square wave to view possible distortion.  Very clean.


This unit is extremely nice and I had no idea how nice until I looked at the price of the thing on Allied's website.

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Re: Series caps differential voltage
« Reply #31 on: March 02, 2017, 10:11:17 pm »
For instance, I do not have a switching cap box with a .33uf . . .also do not have a cap box containing a 270pf.


You could build a box with as many individual values as you want.  But with the examples I gave in the post above, by switching-in just a few caps in parallel, they add, so for a filter cap box:  8uF + 10uF = 18uF which is close to 16uF; 20 + 10 = 30 which is close to 33.  For a pF box: 220pF + 47pF = 267pF which is close enough to 270pF.  I.e., one box for pF's.  One box for cathode bypass caps; one box for signal caps; one box for filter caps.  With just 5 or 6 caps in a box they can add to a lot of values.
« Last Edit: March 02, 2017, 10:13:20 pm by jjasilli »

Offline HotBluePlates

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Re: Series caps differential voltage
« Reply #32 on: March 02, 2017, 10:58:56 pm »
... When connected series i actually simply put a cap 630v ... no measueable voltage INSIDE at the base of the post. Nothing! ...

How is the large blocking when in series?
...
I still not cannot wrap my mind a series cap blocking when when used in filter positions we depend upon increased voltage. ...

Yes, what I think ... is that the result is dependent upon voltage, frequency, and the position of the cap in a series string of caps; and that this relationship is complex.  ...

Guys, you're making this harder than it is; partly that's because we misunderstood Ed's question & goal at the outset.

What is a cap?  2 conductors separated by an insulator.  In ideal terms it is ∞Ω to d.c.

Film caps come close to that ideal; if a coupling cap leaked 1µA and had a 1MΩ grid leak following, the next stage's grid would be at 1v instead of 0v.

Electrolytic caps are far from ideal, and might leak in the milliamp range.  In that case, and when the two series electrolytic caps have unequal leakage current, the voltage distribution among them can be skewed to exceed a cap's rating.  Hence the voltage sharing resistors.

... it does not appear that the Cap Ranger is useful for B+ Filtering.  It seems more useful to test for: (i) smaller value cathode bypass cap; (ii) caps in the signal path.  ...

Right.

Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #33 on: March 03, 2017, 08:07:21 am »
... When connected series i actually simply put a cap 630v ... no measueable voltage INSIDE at the base of the post. Nothing! ...

How is the large blocking when in series?
...
I still not cannot wrap my mind a series cap blocking when when used in filter positions we depend upon increased voltage. ...

Yes, what I think ... is that the result is dependent upon voltage, frequency, and the position of the cap in a series string of caps; and that this relationship is complex.  ...

Guys, you're making this harder than it is; partly that's because we misunderstood Ed's question & goal at the outset.


Right.

I have absolutely no doubt I am making this harder than it is.  I always do at the onset when something is new to me.  Eventually the understanding comes as have with this topic.  Even when I poorly describe the reason.

What made it harder is assumptions I make.   For instance, when measured across a series filter cap from Positive to Ground I wrongly thought I was measuring the voltage through the caps simply because I have never considered I was simply measuring the node in relationship to ground.  I have simply always, for instance, blindly put 2, 80uf/350 volt caps in the reservoir  position of say a Super Reverb AB763.

Well it now seem dumb to even consider the caps were passing anywhere near the node voltage, actually in a perfect condition it would be zero.  Just never thought of it because nothing had ever "made" me focus on this area, until this thread.

So now it is much simpler to me and I appreciate everyone's replies and as I mentioned this may be old hat to some, but at one time none of us understood this and I am willing to bet another lurker reading this has learned something they did not know.

As always, thanks for putting up with my ignorance and know that even though the intention was simply trying to extend the use of the test box the results have certainly given me a new understanding.  BTW, me saying my ignorance is in no way self-condemnation, simply ignorance meaning not knowing.

Offline PRR

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Re: Series caps differential voltage
« Reply #34 on: March 03, 2017, 03:30:51 pm »
It is quite simple.

At turn-on, voltage divides as inverse of capacity. The "small" cap gets the bigger voltage.

After a long wait (maybe only seconds in audio), the voltage divides as inverse of leakage. The cap with less leakage gets the bigger voltage.

For two caps with "similar" insulators, leakage increases with uFd. 1uFd likely leaks around 10X more than 0.1uFd. If that is exactly true (never happens) then the initial and final voltage divisions are the same.

See below. Proposed plan in SPICE. 1uFd (notionally 600V rated) and 0.1uFd (notionally a 200V job). A 600V battery is abruptly connected at 1 mSecond, we watch for 100mS.

SPICE complains about the "floating node" at the cap junction. It can't compute a "DC" value here. Inspection shows there is no true DC value. But all caps leak. I have used arbitrary small and equal "leakage" resistors to shut-up the sim and to show what would happen over time.

At bang-on, the smaller cap gets 545V of the 600V, just 10/11. The large cap gets 1/11. As predicted.

Since the intent is to use a 200V cap as the "smaller cap", and it is going to 10/11 or 91% of the applied voltage, it is still going to pop at turn-on.

(Yes, 300V might be more usual. But that still comes to 272V, over the 200V rating.)

If the leakages were in-proportion to the capacitances, the lines would be horizontal (I did run this, boring).

Assuming leakage roughly proportional to C, the small cap always has the higher voltage.

A slower start changes things so slightly that it is no practical help.

I tried some tricks, like shorting the small cap until the large cap is full-charged. Yes, the small cap starts from zero VDC but will rise according to leakage, and the technique is awkward. Adding leakage (shunt resistor) does change the long-term VDC but does not affect the initial spike, and useful values are likely to mess-up the tuning you are doing.

A "200V" cap does not blow-up at 200.1V. There is a safety margin because there are weak spots to allow for. But in this kind of abuse, what can happen is the repetitive over-voltage surges burn-out uFd. Your "0.1u" cap becomes 0.09u, then 0.06u, etc. You won't know what you are really getting.

_I_ think the answer is to get/make a 400V-600V cap box.

Cap Assortment on Amazon has thirty 630V caps from 10nF to 1uF, and twentyfive 400V caps to 2uFd, for $12 free shipping (eventually). Series-parallel the extras of each value to get in-between values. ---Uhh, this may not be the best-bet vendor. 100% feedback, but only 9 of them, one "Perfect to my dog!". Ah, a fair assortment of assorted electronics, but the top product is a dog-leash.

Offline Ed_Chambley

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Re: Series caps differential voltage
« Reply #35 on: March 03, 2017, 04:21:34 pm »
Thank you PRR. I have made a large voltage cap box, I just never considered making a dual switch box to series some values. As a matter of fact, I have amp builds made in this manner, just not a test tool.


I certainly do not want to damage the unit and as of its current condition it is spot on. I think I will use it as intended per your suggestion as I certainly have not learned enough about this as of yet.


I will say I sent a PM to Sluckey thanking him for telling me to make a post of my question. My learning from this has been eyeopening to say the least. This is the type of thing I hope for, but are impossible to plan for. What I mean is you cannot ask what you cannot imagine.


As far as caps, I have on hand a plenty so I do not need to use my prime membership.


Seriously, thank you for taking the time to explain. I know more today because of you and the others who took time to join in.

 


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