Resistors for dummy load?

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Originally posted by wintermute
That's not a bad Idea, I might go to the local auto shop and see what I can get, non halogen globes should be pretty cheap too.

Precisely my thoughts. While you're at it, also look for holders or mounts of some sort, so that you can set all them bulbs up in some way.

I suppose what all this really comes down to is whether or not the idea is to do accurate measurements of freq response, power output etc, or simply a way of loading the amp to check for things like oscillation, PS ripple, clipping behaviour etc. For me (at least at the moment) it's more the later.

Precisely. I too wanted to build dummy loads basically to test (i) whether my power amp PSU (specially the transfi) heats up after an hour or two of full-load use, (ii) whether my power amp behaves, (iii) how hot my power amp heatsink gets. I think that this sort of test is a must if you want to trust the reliability of the poweramp for many years to come. In such situations, I don't want to vary the signal frequency much (it'll either be 50-100Hz or something "standard" like 1KHz), and I don't want to take any but the grossest of measurements like output current, rail voltage, temperature, etc. So a bit of inductance or capacitance doesn't bother me, as long as the power amp doesn't get into oscillation.

For precise bandwidth measurements, I might not even want to take things to full load, and if I do, I'll think about pure-resistive loads then. Presumably, I'll be rich enough by then to buy a couple of hundred linear resistances with very low tempco and low L and C.

Tarun

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Originally posted by AuroraB
Something doesn't stick here.....
First- there is massive discussions on the nonlinearities of power resistors, inductance and capacitance included....
Then the suggestion is to use light bulbs,--- a highly nonlinear resistor in response to temperature.

I too was confused.

However, even light bulbs can be treated as quite stable impedances, if you choose bulbs which will be driven to "full load" by the amp. Hence my choice of auto bulbs, which are designed to glow brightly and reach a stable thermodynamic condition with just 12V. A few such bulbs in series can be good test loads for amps at full power and low frequency, provided you choose the number of bulbs in series to match the voltage output by your amp at full load.

Trying to use incandescent lamps, and then fussing with them to ensure they remain cold, is too unstable an approach, I feel. You must drive them so that they glow at full brightness.

One assumption here is that you'll be pumping a steady signal of whatever frequency but at unchanging amplitude into your amp. Varying the output power rapidly will allow the bulbs to cool and re-heat, thus sharply changing their impedances. It all depends on what kind of tests you want to do.

And to find out the impedance of the bulb, it's of no use measuring it with an Ohmmeter when cold. The best approach, it seems to me, is to take the rated wattage and voltage of the lamp, and calculate --- not measure --- the impedance, and then realise that this impedance is valid only if the rated voltage is applied on the lamp from a low-impedance high-current source, i.e. a power amp (not an Ohmmeter).

This auto-headlamp approach was suggested to me by The Great Audio Guru Angshu.

Tarun

[AuroraB]

I agree in principle to using lamps as loads, - this has been done. However- during my army days in the early 70s, we used som special bulbs as RF loads, but I cannot recall what it was......

Personally I think there is only two easy choices- wire wounds or paralled compositions. Otherwise- no way around the expensive stuff...

Actually, -some years ago, I needed to test a 24V / 30 A PSU in a hurry,...
I soldered a handful of "Dales" at hand to two wires and hung it from my desk.....after a while there was some strange thumps...looking closer, I found the resistors on the floor.....

Heatsinks are absolutely recommended....!!

[wintermute]

Originally posted by AuroraB

I soldered a handful of "Dales" at hand to two wires and hung it from my desk.....after a while there was some strange thumps...looking closer, I found the resistors on the floor.....

Heatsinks are absolutely recommended....!!

Originally I asked the question about inductance, simply because I didn't know if there was some nasty thing that it could do, which would make my amp look like it had problems when it didn't.

I've wondered a few times whether some of the nasty things I've seen were in fact due to the cheap jaycar 5W resistors I currently have (only two in parallel so 10W, although good for 100W for about 20 seconds before they start smelling!)

Seems like the only real concern is whether or not you want absolute perfection in the measurements, which at this stage I don't. It's certainly been an interesting discussion though!!!!

Tony.

[JOE DIRT®]

this is the resistor I was referring to in my previous post