Dummy Load Suggestions

Hi as you may have noticed in my other threads, I’m working on a control system for an electric gate. I could really do with a means of simulating the load of the motor. Unfortunately the actual motor is attached to the actual gate some distance away. I’m not particulary concerned about an inductive load or resistive. It needs to be able to handle 300w at 24v…

This can be done with MOSFETs. It is called an electronic load.
Each MOSFET would be used as a current sink and you would
parallel a sufficient number of MOSFETs to distribute the 12.5Amps
and 300W. You will need to get a decent heatsink (possibly fan cooled) or you
will need a lot of FETs.

For an example scroll down to the electronic load section at
http://www.luciani.org/geda/util/matrix/index.html

(* jcl *)


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Well 300 watts at 24v is 12.5 amps. So you require a 2 ohm 300 watt dummy load. That could be two series wired 1 ohm 150 watt resistors which is somewhat costly for just testing, even if you find a surplus buy on e-bay or such.

What we use to use when testing high current DC power supplies (up to 30 amps or so) at a refinery shop was to use long lengths of copper wire calculated to the correct length for the resistance required. For a 2 ohm load you could purchase 200 feet of 20 gauge AWG wire and spread it around in a circle on concrete or other surface that’s not tool heat sensitive. The wire will get hot over time but will handle the load current long enough to test with.

You might also consider using two high wattage 12vdc auto head lamps wired in series, if you can find two 150 watt rated lamps.

Lefty

Considered using bulbs - found some 24v 300w ones pretty cheap. Could do with topping up my suntan too! (might need to put them in some sort of box…)

If the motor is 300w at 24v, then you might go with standard light bulbs (cheap and reusable). They should have ~5 times the resistance of similar watt 24v bulbs. I’d try ~1500w of bulbs rated at 120v wired in parallel. The heating coils of a space heater, hair drier or similar could be used. The heating element from a stove might might also provide a load in the desired range. Any resisive load rated at ~1500w at 120v should be in the desired resistance ball park. Auto supplies have dummy loads for testing batterys, but they cost $$$ just for doing one time testing.

When testing motor loads we used to couple two similar motors together with a coupling. The leads of the driven motor are connected to a resistor. Just use old jug elements (dirt cheap)in parallel dunked in a tub. (the water will get hot!). Don’t used light bulbs as their resistance varies wildly as a function of current. (short drive pulses will draw more current than you expect) This set up allows a reasonable simulation of a load; certainly for initial testing anyway.

Our dummy loads consisted of some mig wire, stolen from the engineering department wrapped around some poly pipe, looked rough but the handy thing was you could change the load by moving the contact along the pipe, as far as I know they are still using it.