Few transformer questions

[jeff]

1)  I need a heater supply: 12.6V@450mA. My choices from Radio Shack are a 12.6V@450mA or a 12.6V@2A. If I get the 450mA is that cutting it too close and if I get the 2A will my voltages be higher than 12.6V?


2) If I use two 120V:6V transformers(120V:6V-6V:120V), how do you calculate the availible current?
For example, I have two 120v:6v@300mA back to back how much current can I draw from the 120V out?

[Leevi]

I need a heater supply: 12.6V@450mA. My choices from Radio Shack are a 12.6V@450mA or a 12.6V@2A. If I get the 450mA is that cutting it too close and if I get the 2A will my voltages be higher than 12.6V?

Some manufacturers give a parameter "zero load voltage" (not sure about the English term) which tells how much the secondary voltage is over the nominal at zero load situation. When knowing that you can approximate how much the 2A secondary will be at the full heater load.

If I use two 120V:6V transformers(120V:6V-6V:120V), how do you calculate the availible current?
For example, I have two 120v:6v@300mA back to back how much current can I draw from the 120V out?

I removed my reply since I had totally misunderstood your question

/Leevi

[sluckey]

1)  I need a heater supply: 12.6V@450mA. My choices from Radio Shack are a 12.6V@450mA or a 12.6V@2A. If I get the 450mA is that cutting it too close and if I get the 2A will my voltages be higher than 12.6V?

The small transformer is cutting it too close for my comfort. I'd use the bigger transformer and not worry about what the actual voltage is. Sure, with a light load, the voltage will be a little higher than with a heavy load, but I would not be concerned.

2) If I use two 120V:6V transformers(120V:6V-6V:120V), how do you calculate the availible current?
For example, I have two 120v:6v@300mA back to back how much current can I draw from the 120V out?

First calculate the power rating. Your 6V@300mA is 1.8watts (P=E*I).
Now calculate the current rating for the 120V winding. 120V@1.8W is 15mA (E=P/I).
 

[jeff]

1)Thanks, wasn't sure if 12.6V@450mA meant when you draw 450mA the voltage is exactally 12.6V, (meaning it is intended to draw 450mA) or if 450mA was the max rating. Got it now.

The small transformer is cutting it too close for my comfort.

How much of a margin between the max rating and the actual current draw is comfortable?

2)Curve ball : You have (in)120V:6V-6V:120V(out) but you use the 120V for B+ AND the 6V for heater. How do you calculate that?
Example: Two 120V:6V@1A, drawing 300mA from the 6V.
Would you just subtract the .3A from the 1A leaving .7A. Then figure 6V@.7A=4.2W  so  4.2W/120V=35mA?

[stingray_65]

Jeff,
without knowing your tube chart i can't even begin to know how much current you need.

A 12AX7 draws .15A at 12V and a 12V6 draws .225, so a 12V Champ would be safe with the .45A tranny.

But anything bigger than a preamp or that simple amp you would likely be under current.

I seem to recall that RS has a 25V CT tranny @ 2A, that would be easy to run the secondaries parallel to get 12.6 @ 4A.

Ray

[jojokeo]

Man, I dislike even going into a RS these days. Not the same as it used to be. I wouldn't be surprised if they stopped selling actual parts one of these days?

[kagliostro]

Would you just subtract the .3A from the 1A leaving .7A. Then figure 6V@.7A=4.2W  so  4.2W/120V=35mA?

To me that is right

Kagliostro

[phsyconoodler]

What are you building that needs a complete heater supply?

[jjasilli]

1)Thanks, wasn't sure if 12.6V@450mA meant when you draw 450mA the voltage is exactally 12.6V, (meaning it is intended to draw 450mA) or if 450mA was the max rating. Got it now.

But now I'm confused!

1.  I thought that "12.6V @ 450" states BOTH the max current draw, AND the voltage under load at that current draw.  Hence, you exceed the stated current draw at your own risk; and can expect the stated voltage at that current draw.  In my experience Radio Shack trannies run hot, which indicates that their ratings are probably optimistic.  Still, I wouldn't hesitate to use their 450mA tranny for a 450mA load.

2. For 2 identical trannies wired back-to-back, like for an isolation set-up, wouldn't the total current rating be the same as for 1 of those trannies? 

[DummyLoad]

12.6V@450mA

it means exactly that. with 120V RMS on the primary (or whatever the primary is rated at) you will measure 12.6V RMS +/- at the rated RMS current draw. some manufacturers give the no-load spec as well - hammond does - look on the pdf. cut sheets.

--DL   

[Willabe]

2. For 2 identical trannies wired back-to-back, like for an isolation set-up, wouldn't the total current rating be the same as for 1 of those trannies? 

Yes I believe you are right, as long as they are rated the same.


            Brad         

[jeff]

I seem to recall that RS has a 25V CT tranny @ 2A, that would be easy to run the secondaries parallel to get 12.6 @ 4A.

 I don't think you can parallel them. Because they share the CT and aren't two seperate windings there's only one way to wire them parallel. Relative to the CT, the two 12.5V taps would be out of phase.

2. For 2 identical trannies wired back-to-back, like for an isolation set-up, wouldn't the total current rating be the same as for 1 of those trannies?

Yes I believe you are right, as long as they are rated the same.


            Brad         

I don't know about this.
If:      P(in)=P(out)
Then: V/I=V/I
         120V/XmA=6V/450mA
         X=22.5mA
So a 120V:6V@450mA draws 22.5mA
And two 120V:6V@450mA would make one 120V:120V@22.5mA not 120V:120V@450mA

Do I understand corectally?

[Willabe]

Yes,    --- power in = power out ---   that's why it works.

So 2 PTs at the same ratings, voltage and current,  = the same power in and power out.

Lets say this, 120vac in - to 6.3vac out @ 100mA's, now if we place a second PT with the same rating, back to back, we should get -- the  same -- power in -- and -- the same power out.

If the secondary can pass 100mA's of current @ 6.3 vac, that PT has to be able to handle/carry/pass the same current draw/load on/in from it's primary.

No loss of power. The PT does not care or even know if it's wired forwards or backwards as long as the voltages and current demands are not exceeded and respected. It's still able to handle/carry/pass the same power total.


        Brad        

  

[PRR]

> Radio Shack are a 12.6V@450mA or a 12.6V@2A. If I get the 450mA

I would use the 450mA for a stage-amp; 4 hours a night 4 nights a week and probably do a full re-think rip-out after a while.

I would not trust it for a 24/7 "forever" 450mA load such as home heat controller.

I have not seen a Radio Shed filament transformer fail, even some in service 20+ years.

> if I get the 2A will my voltages be higher than 12.6V?

Up to 20%-25% high. Actually with a 1/4 load (0.45A/2A) perhaps 15%-18% high. This is OK for smoke-testing. Sometime in the first hundred hours you should note the actual heater voltage and add series resistors to get it within 10%-5% of nominal.

> If I use two 120V:6V transformers(120V:6V-6V:120V), how do you calculate the availible current? ... 120v:6v@300mA back to back how much current can I draw from the 120V out?

12V@300mA is like 120V@30mA. So 30mA is safe.

The regulation may be worse than 20%, TWICE. So on AC load at 30mA the "120:12:120" may come out barely over 100V. DC rectifier capacitor really suffers. You may not get over 130V 20mA DC. Or with a doubler, 220V 10mA DC.

It might work best to do the 12V 2A for heaters plus to feed a 12:120. That gets the 2A winding up near half-power, so voltage won't get so high, and it lessens the losses on the way to HV DC.

[jeff]

Just remerbered, my 6.3V bulb. So I'm gonna get the bigger one.
It's OK to ground the CT and run the heaters from 6.3V to 6.3V leg(for 12.6V) AND run the 6.3V from one leg to ground, right?

Thanks for all the help.
   Jeff

[stingray_65]

Just remerbered, my 6.3V bulb. So I'm gonna get the bigger one.
It's OK to ground the CT and run the heaters from 6.3V to 6.3V leg(for 12.6V) AND run the 6.3V from one leg to ground, right?

Thanks for all the help.
   Jeff

#1815 lamp

radio shack PN 272-1118

disregard their claim of 30hrs working life.

it is based on a 14V rating

I've always known it as a 12V bulb with a 3000 hr life.

They are a pretty common bulb in 50's era cars so you may try your local NAPA or Auto Zone

lot easier than riggin' the wiring around IMHO

Ray

[jeff]

 Thanks, but I'd rather use a #47 bulb if possible, only because I'm very forgetful and it's good to have all my stuff take the same bulb. That way I won't have to think about.

Is it possible to use the 12.6V for heaters and use one leg to the CT for 6.3V for the bulb?
If not I'll get the #1815 like you said.

[PRR]

Yes; and if I had a stash/supply of 6V lamps, that's just what I would do.

Actually, I'd do an LED. Who wants to change lamps??

And isn't this a rather petty detail for an amp that's only a thought?

[jeff]

OK thanks, I think I have enough to build a preamp with my spare parts. One last question. I know transformer orientation is important for low noise in output transformers, but what about the orientation of power transformers? Is there any problem with mounting two power transformers on top of each other? The two transformers have the same spacing and I want to use the same holes to mount them both, one above the chassis and one below. Any problem with that or should they be offset by 90^o and apart from each other?
 
          Jeff