Resistors for dummy load?

[wintermute]

I've been using some very underpowered (2 X 5W|| wire wound) resistors as a dummy load when testing my amp, and have been looking to create a proper dummy load that won't burn the carpet, melt plastic it comes in contact with or imit strange smells .......

I've searched and people gennerally recommend non-inductive types, but for 100W rated non-inductive resistors I'm looking at $40AUS each (and I'd want four... 2 X 15 Ohm wired in parallel). so that's $160!!! (luckily I have a heat sink).....

I can get ceramic wire wound (Obviously inductive) resistors for about $17Aus each which works out at about $68.........

Wouldn't the fact that these resistors have some inductance mean that they are a closer (marginally) approximation to the load presented by a speaker? Is it ok to save some bucks and get the cheap inductive ones, or should I just grin and bear it and fork out for the non-inductive ones?

Tony.

[tubetvr]

Connect multiple low power resistors in parallell, I use metal oxid type 5W and have connected totally 20 in series parallell to get 8 ohm capable of 100W continuous power.

9 39ohm + 1 47ohm resistors is connected in parallell giving very close to 4 ohm, 2 of these combinations are connected in series.

BTW, this is how dummy loads for RF was done long time ago by some manufacturers.

Regards Hans

[JOE DIRT®]

Tony...I did post the resitors I use awile back with pics...Dale part# hl-225-07Z......225 watt and affordable

[wintermute]

Hmmmm that's two options Thanks guys.... I was trying to avoid paralleling up too many resistors, but I guess the advantage of using heaps of 5W is that a heat sink wouldn't be neccessary.

I did think about using 3 X 47R and 1 X 22R all in parallel for 7.6 Ohms at 100W but wasn't sure whether the power disipation would be even or whether the 22R would cop more of a beating since the whole thing isn't symetrical....... that'd work out at a tad under $70Aus.... I would like it to be able to handle about 150W though......

Those are some mutha resistors you've got there Joe I'll see if I can find some BTW why the change on the avatar?

edit: Ummm searched on dale, and dale electronics, but I'm not sure I have found the right site... Got a url?

Tony.

[Bill Fitzpatrick]

I have a few Dale resistors which are ideal for dummy loads.

See at http://www.kbacoustics.com/dale/

Do not reply to the email address on that page - it is obsolete.

[millwood]

how about using a heating element? You can take out the heating wires inside and use it as you wish.

I did that way back in the 1980s but haven't tried it lately.

[tubetvr]

I was trying to avoid paralleling up too many resistors

Why?, because of the increased capacitance? Capacitance is not a big problem when building a dummy load for 8 ohm or similar impedance, the problem with using many resistors is when you connect them in series as it will increase the total inductance and that can have an effect even at audiofrequencies.

For a simple calculation using 20 resistors in parallell:

Capacitance of each resistor ~10pF, giving 100pF for 10 in parallell, 100pF + 100pF in series give a total of 50pF in parallell with 8 ohm. The reactance of 50pF at 20kHz is ~159000 ohm so the effect is neglible.

Regards Hans

[sayang001]

9 39ohm + 1 47ohm resistors is connected in parallell giving very close to 4 ohm, 2 of these combinations are connected in series.

This is exactly how I build my dummyload, I used 20W wirewound resistors... but paid only $6 for the 20 resistors in Indonesia when I was there on holiday.

[wintermute]

Hi Hans,

No I wasn't worried about the capacitance, just the logistics of connecting them all up Ideally I just wanted to bolt a couple of resistors to a heat sink that I have and connect the speaker wires...... Then I saw the price!!!!

These resistors were the last thing I was going to add to an order I'm putting together for various bits and peices and it's allready up to about $140 without them....

Bill, the cheaper resistors I mentioned before look very simmilar to the ones you posted (although maybe your's are low-inductive ones.... I discovered you can get low inductance wire-wound).

Millwood, I think I'd be more likely to burn the carpet if I used heater elements But full marks for the diy element

Maybe I'll just get some of the cheaper 25W ones and mount them on the heat sink, will be better than the 10W power rating I have at the moment . I can allways by some fancy non-inductive types later on, if I feel the need. After all my amp isn't exactly audiophile quality, I just need something that'll make it do some work while testing (which most of the time isn't at full power anyway.

BTW Joe, I found something very simmilar to your Dale resistors at RS Australia, but they were $65 each and 4.7 ohms (although that's not bad considering they are rated at 200W)

Regards,

Tony.

[Christer]

Tony,

Do you have any compponent surplus companys around? They seem
often to have a lot of power resistors in the 5 - 10W range, at least
here around. You will have to figure which of those values they have
to use and how to hook them up in series and parallel to get what
you want, but it can be a bargain and you won't need any heatsinks,
which save a lot of money. It'll probably end up a worthy candidate
for the ugly prototypes thread, but so what.