Dummy Load - Question

[kagliostro]

If I remember correctly, if I've four 50W 4R resistor I can put:

all in series for 16R Load

two in series and then in parallel for 8R 4R Load (see Sluckey and PRR post)

two in parallel and then in series for 4R Load

But allways I've a 200W Dummy Load

is that correct ?

Thanks

Franco

[sluckey]

two in series and then in parallel for 8R Load

This is not correct. This would give 4Ω.

Everything else you said is correct.

[kagliostro]

Right ! Thanks Steve

Franco

[PRR]

> two in series and then in parallel for 8R Load

Won't make 8.

[kagliostro]

Yes, my brain wasn't much collaborative 





Franco

[shooter]

In your parallel example, I like "see" them as 2 4ohms in parallel, (both ends twisted together) X 2, then joined together in series for 2ohm + 2ohms = 4ohms.  not sure there's a real difference from your drawing though, all the 40yr old theory in my brain is filled with holes!

[xm52]

Often, non-inductive resistors are used for dummy loads.


Heat dissipation is an important consideration. Some resistors require air flow around them, others are designed to be mounted on a plate or heatsink.


It's a good idea to offer different a variety of input connectors connected in parallel such as a ¼" jack, Speakon, BNC (for connecting a scope), and even banana jacks.

[xm52]

It might sound silly but people have used hot water tank elements for dummy loads.

I've used a couple of these: Camco 02283 3500W 240V Screw in Water Heater Element. Some hardware stores sell them or they can be found on amazon. They do a good job if you need a high wattage capacity. They are low inductance.
The problem is, they need to be cooled. Folks mount them in a can filled with water or mineral oil to absorb the heat. Years ago hams used a popular Heathkit product called a Cantenna. Basically the same thing in a paint can.




8 Ohm Load For Vacuum Tube Power Amplifiers - YouTube




   

[kagliostro]

Thanks for the info, cool link

Just to clarify, I worked something like 24 years at wholesale supplies for plumbers 

I'm not a Radio Amateur but the most part of my friends are, it is since 1968 I know them, at that time I was CB amateur 

So I thank you, but this solutions I know 

May be you are interested on this

http://powerelectronics.com/power-electronics-systems/testing-power-converters-using-liquid-rheostat-dummy-load

http://www.nessengr.com/techdata/liqresistor/liquidresistor.html

---

Good idea to put on it various connector types

I've also seen one with a small speaker just to be able to listen the signal without the use of a speaker cabinet

http://www.audiovalvole.it/carico_fittizio_amplificatori_audio_di_potenza.html


AP means Altoparlante = Speaker

--

I don't require a so high Dummy Load, 200W I think it will be enough for me

I've find (on the cheap) 4R and 8R 100W resistors so I'll order 2 x 4R and 2x8R

this way I can have

2 x 4R in parallel = 2R 200W

2 x 4R in series = 8R 200W

2 x 8R in parallel = 4R 200W

2 x 8R in series = 16R 200W

So, with 2 x 4R + 2 x 8R and a pair of switch I can have 200W 2R - 4R - 8R - 16R load

each resistor is less than 1.50€

Oh, effectively I've think to put the resistor on an heat sink and then inside a can filled with mineral oil to increase dissipation

with 200W dissipation on air I think I can obtain at least a 1500W dissipation (for some minutes) if positioned under oil

Thanks

Franco


EDIT: I think that a non inductive load is necessary for RF but not indispensable in AF

[xm52]

I have a number of dummy loads, it allows me to have more than one amp going at a time. It's always nice to find inexpensive resistors for amp work as these things can get pretty expensive.


Below is an image of a lab unit with expensive resistors. It uses a pair of 300W Arcol NHS300 8 ohm 1% resistors.

[kagliostro]

Very nice (professional) resistors, 2 x 8R 300W mean you have a 8R 300W Load and a 600W 16R plus a 600W 4R, not bad 


Franco

[xm52]

Groove tubes used to sell a rack mount unit called a speaker emulator. Basically a dummy load with a speaker motor and a tone control. The idea was to connect this to the output of an amp, it produced a line level output that could go into another amp or for recording. A good idea, there are a number of similar products on the market today.


The first product that I can recall was put out by Garnet amps in the late 60's called a Herzog. It was basically a Fender Champ with a build in dummy load. The output was typically plugged into the input of another amp. It saturated the input producing a harmonically rich tone. The Firezog was put together as a stand alone unit that could work with different amps. The volume reducer was another variation on the line out.

[xm52]

> EDIT: I think that a non inductive load is necessary for RF but not indispensable in AF

Yes, people do use regular inductive type resistors for dummy loads with amps. The results are often good enough.

You are right about inductance playing a more important role at high frequencies.  The advantage of non inductive is that the resistance is pretty constant at all audio frequencies and the results will be more accurate. How bad an inductive resistor can be depends on the resistor, some are better than others. If you are using an audio analyzer for example to sweep the bandwidth of the amp and are measuring power output, you don't want the dummy load to be a weak link. If the non-inductive resistor costs just a little more, why not do it right.

Some people use these wire wound cement resistors. The nominal resistance value of these devices does not have a  tight tolerance. But as long as you know what the resistance is, you can calculate power output: http://www.parts-express.com/Search.aspx?keyword=dummy%20load&sitesearch=true


BTW, attached are some typical test readings of the two Arcol resistors with frequency. Not bad.

[xm52]

Franco, I like that big heatsink on the site that you linked to above (http://www.audiovalvole.it/carico_fittizio_amplificatori_audio_di_potenza.html).

[pompeiisneaks]

I did a video on my dummy load but basically it followed one of the schematics here:

http://aikenamps.com/index.php/dummy-loads

and his schematics: http://aikenamps.com/images/Documents/DummyLoad.pdf

It's pretty well documented set of schematics that help make a ton of various options for how to build one.

Mine was one with 1 8ohms, 3 4 ohm and 1 2 ohms, that ends up getting 2, 4, 8, and 16 ohms in one unit, and it is for 100W if you use all 50W, I used all 100W so mine is 200W like you wanted as well. 

~Phil

[PRR]

> non-inductive resistors are used for dummy loads.

I think this is worrying over trifles.

At least for guitar amps.

I suspect somebody measuring Hi-Fis, long ago, observed a small difference in THD at 20KHz with an extra inducty resistor, and started urging non-inductive, and the un-thinking masses followed.

> test readings of the two Arcol resistors

Thank You for Hard Data.

The difference one to another is puzzling?

Taking a worst-case: 666nH at 20KHz is another 0.083 Ohms total impedance. Yes, enough to nudge a THD meter up one digit. But notice that the nominal "8 Ohms" is already in-error by about the same amount: 0.08 on one, 0.05 on the other. About 1%.

We never know our audio numbers to 1%. Wall-voltage varies +/-5% or more. Device (tubes, transistors) vary +/-30% and even with tricks we rarely control result to 1%. And our AUDIO (the vital ingredient) varies by 1,000:1 or more (nominally 63,000:1 for "CD-quality audio").

And face a real world. Small speakers have 0.2mH coil inductance. Large speakers with no crossover filter (guitar speakers) may have over 1mH inductance. That is 200,000nH to 1,000,000nH!! (Check my work, these zeros are slippery.) Saying that 666nH in a dummy load is a problem is saying that any loudspeaker is a BIG problem.

There may be "worse" resistors out there.

But some thinking suggests that it is "hard" to "incidentally" get enough inductance to matter in a 8 Ohm resistor at 20KHz. If we want to cut-off a midrange speaker, we need a BIG yet compact coil of wire. A voice coil is smaller but has a massive magnetic core. A resistor winding would typically be spread-out to shed the heat, and no iron... less accidental inductance.

Another real-world issue: all your wires have inductance.
https://ampbooks.com/home/amplifier-calculators/wire-inductance/
30 Feet (10m) of light speaker wire is 19uH or 19,000nH. Just 20 inches (50cm) makes 666nH. (Were those Wayne-Kerr numbers corrected for lead effects?) However this is for single wire. If you put two wires, out and back, together, some of this cancels. I'm not finding data for amp-size cable.

The stray inductance "can" matter and even at 60Hz power frequency, even in cable. My home power is nominal 240V 100A 60Hz. This is a very low 2.4 Ohm load, if I actually pulled 100A. I don't because the lights dim. The wire from the street is very long. I've worked the numbers. Just on pure resistance, my numbers are about 10% off. When I look-up the inductance of this type cable, it adds about another 10% impedance at 60Hz. The residual error is well within the accuracy of my length measurement and the cable-factory tolerance. (Wire gets bigger from start to end: the dies wear.)

[drgonzonm]

PRR,
When you say 240v, this is the three wire system entering your main panel, where hot-hot is 240 and hot-neutral is 120v (which is a single phase center tap transformer), rather than a 240 hot-neutral? 
By the way, the downtown area in Albuquerque NM is 208v hot-hot and 120v hot-neutral. (3 phase wye system.)

 

[tubenit]

I saw this posted by Martin Manning on TAG.  Note PDF attached.   I am probably going to move this thread to ARCHIVES since this issue comes up every so often.  However, I will edit the thread some before doing so.

With respect, Tubenit