Electronic load questions.....

[cowasaki]

I am looking at buying an electronic load for testing my own power supply, testing power supplies in things I am upgrading and modifying and for use as a speaker substitute.

The last item is where I have questions.....

1) Obviously speakers are measured by their impedance and take an AC input.  Do electronic loads run AC for a constant impedance load?
2) Is there a better alternative than using an electronic load?  I currently have a box of resisters to mimic 2 (rare), 4, 8 and 16 ohm speakers
3) If the electronic load does work with the AC from the amp will it display the power or current?
4) Any suggestions as to a suitable one would be welcome (or an alternative) - I would like a load for PSUs anyway and thought that it would be nice to use it for another task.  I repair a lot of amplifiers and build some so it would allow me to justify it more :-)

I do not need silly accuracy as I'm not measuring battery capacities and stuff like that.  10mW accuracy is probably more than my current needs require.
I have a lot of amps with 100watt outputs BUT they often bounce above that to maybe 150watts for short periods and some are just more powerful than they say anyway.  Today I had a 350w bass amp on the bench but highly unlikely I will get something capable of taking that without spending a lot and it's rare I see anything like that.

[PKTKS]

Cheap simple loads are fine for battery things like
10A to 20A and bench PSUs tests.

https://www.aliexpress.com/item/32848001849.html

But will not fit AMPs in particular BASS amps.
For that unless you will discard the real speaker..
you need a relative huge RESISTOR.

That will fit the REAL POWER (COILs will mask the VA power)

That load above also has higher rated versions..
ALL POLARIZED, will not suit AC signals from AMPS
Paul

[nctnico]

Since the load resistance is fixed I'd go for fixed resistors.
100W panel mounted wire-wound resistors are not very expensive. You could make a casing with 4mm banana sockets allowing to wire the resistors in series / parallel to get 2 Ohm / 4 Ohm / 8 Ohm. A small (low noise) fan can help to push the heat out.

An speaker power meter does the rest. Random Google find:
http://zerocapcable.com/?page_id=221

[PKTKS]

[TheUnnamedNewbie]

Having helped on a local hobby-class where students build their own amps, I think you should consider also trying with speakers. We saw some amps becoming unstable because speakers can present a very, very messy load w.r.t. frequency (EG, some amps were fine into an 8 ohm dummy resistor, but were all over the place when we hooked up an 8-ohm rated speaker).

What I have seen people do to 'break in' speakers, is to take two of the same type, wire them 180 degrees out of phase (IE, hook one up 'backwards). Then point them at each other and move them close together.

They should cancel out most of the sound of each other, and while not perfectly silent, they will be much quieter than they would be when not facing each other (can always put them under a pile of pillows to really shut them up).


For initial testing, I have 2 big 10 ohm resistors.

First step: Do a no-load test, and just measure the DC/AC voltages with a multimeter (and/or scope if you know what you are doing).
Second test, if the first doesn't show crazy stuff: Hook up the dummy resistors and short the input of the amplifier. Again, measure with meters and bring up slowly.

Once I am confident they are not doing something crazy (like putting VCC or VSS on one of the outputs) I can hook up speakers to test fully.

[Shock]

The US company lowes.com sells item #362342 the Utilitech 240V Universal (water heating) element (it was $9 last time I checked) see this video explaining usage.

They just happened to be 16 ohms which is perfect to put a 16/8/4 ohm set together depending on how many you have. About the cheapest 3.5kW resistor you will find.

[cowasaki]

I’ve been repairing amps for a while and have two 8 ohm 100w resistors which gives me 4, 8 or 16 ohm but was wondering if I could use one as a resistance.

[Shock]

Really your problem with resistors is heat dissipation. Unless it's rated to stand in free air you need to provide cooling. The resistors I linked above you can use oil or even water if needed, as long as your not dumping huge amounts of power into them for too long. If doing that you would require an additional solution for cooling.

[cowasaki]

There are some 1Kw 8ohm resistors on eBay for about £25. I just thought it MIGHT be a simple case of setting the resistance and just using the electronic load instead. If I can’t it’s simple enough.

I don’t run them on the resistors for long but when I’m using the signal generator a speaker is not ideal.

[Shock]

The general idea is the resistance needs to be non reactive to amplitude and the resulting increases in temperature. So that regardless of the power you are disspating through the load your measurements aren't skewed. If your are exceeding the resistors rating including temperature don't expect them to survive for long.

[nctnico]

There are some 1Kw 8ohm resistors on eBay for about £25. I just thought it MIGHT be a simple case of setting the resistance and just using the electronic load instead. If I can’t it’s simple enough.

An electronic load can only deal with DC. There is probably fancy equipment out there which can act as an AC load (with a specific impedance) but it is going to be expensive and overkill. Either way an AC load will need to dissipate the energy just like a resistor. A resistor will be cheaper and much more rugged compared to an electronic load.

[eevcandies]

An electronic load can only deal with DC

Depending on the needs, adding a high power bridge rectifier can do the trick...then the load sees a positive rectified wave.  Of course, the load has to be capable of responding at the signal frequency & there is distortion as you get close to zero.  Used this trick for some reversible motor driver loading testing to simulate accel/decel.  Probably not so good for precision audio work.

[cowasaki]

I bought a load of variacs (see sales) and hidden away in there is a monster 100ohm rheostat rated at I’d guess about 300w I’m probably going to stick it in a metal box and use that just checking the resistance before each test to match it. I just need to check the wattage but it certainly works.