Hi,

I am trying to build a simple circuit where 8 load resistors dissipate the same energy each (lets say 500mW). I want to buy a cheap DC power supply that can be remotely turned on and off (Mean Well RSP-75-12). I don't want to buy 8 power supplies unless I have to. I plan to put a potentiometer in series such that I can adjust the power dissipated by the load resistors.

My original plan would be to put these load resistors in parallel. One issue with that is I want modularity. For instance, I may only have 4 running at a time, with the other 4 having open leads. I would like a circuit that is indifferent to this, such that I don't have to re-calibrate the power dissipation every time. Are there any cool circuit tricks to accomplish this?

-David

 

I'm having a hard time understanding your description. Could you possibly sketch up a diagram of your idea. It would help a great deal. Thanks a lot

 

Does that help? Each resistor is essentially a heater for an 8 up test. I want each resistor to dissipate 500mW, and hopefully not have to re-calibrate every time I take a unit on or off.

 

If the power supply is set to a constant voltage, and is capable of supplying the maximum current for all loads simultaneously, then disconnecting some or all of the loads will have little or no effect on the voltage to the remaining loads.

Everything changes if you have a pot in series with more than one load. [update]. Yeah, your drawing won't work. You need to adjust the supply voltage instead. A pot to adjust that much power would be expensive and won't allow you to remove loads without readjustment.

 

So, get a bunch of 288 ohm resistors and connect them. 12 squared /288 = 0.5W
Here ya' go. 34 cents each.
http://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0wnbnZ1z0wsz3&Ns=Pricing|0

 

Since the RSP-75-12 is a voltage source, its output voltage will remain constant regardless of the load resistance to which it's connected (until its output current specification is exceeded) so, for the configuration you describe, you may switch loads with impunity.

 

Since the RSP-75-12 is a voltage source, its output voltage will remain constant regardless of the load resistance to which it's connected (until its output current specification is exceeded) so, for the configuration you describe, you may switch loads with impunity.

See his diagram that includes a series pot. Won't work.

 

See his diagram that includes a series pot. Won't work.

I'm hoping the T.S can figure out he doesn't need a pot if he uses the right resistors.

 

I'm hoping the T.S can figure out he doesn't need a pot if he uses the right resistors.

Maybe, but right for what? It's not clear what the goal is.

 

Hi, thanks for the responses. Still not sure what to do.

one other feature that I didn't mention is that I would like to adjust the power dissipation if necessary (from 300-800mW). The load resistors are inside my devices, I cannot change the 200 ohm number for the 8 devices.

 

Why can't you just use a variable voltage power supply?

 

That could work (same circuit, get rid of the potentiometer right?). I need one with "remote on/off", was hoping to find something cheap.

 

The software gal i'm working with likes RS-2332 or apparently with those Mean Wells you just send a 0 or 1 to turn off/on.

 

See his diagram that includes a series pot. Won't work.

Indeed.
My interpretation was that he wanted to use a rheostat in series with each switched load and that all of the loads and their series rheostats would be in parallel, but his schematic belies my belief.

 

I'm hoping the T.S can figure out he doesn't need a pot if he uses the right resistors.

one other feature that I didn't mention is that I would like to adjust the power dissipation if necessary (from 300-800mW). The load resistors are inside my devices, I cannot change the 200 ohm number for the 8 devices.

I was also hoping that the T.S. told us what he needed in the first place instead giving us the run-around.

 

I was also hoping that the T.S. told us what he needed in the first place instead giving us the run-around.

Note his avatar.

 

Part of the problem is I don't know what I want

 

Hold the phone. Realization...If I get potentiometers for each load then this whole problem goes away and I don't need a variable voltage power supply. I'd calibrate each UUT to my 500mW and be done. Sorry for thinking stupidly out loud on the forum.

 

Hold the phone. Realization...If I get potentiometers for each load then this whole problem goes away and I don't need a variable voltage power supply. I'd calibrate each UUT to my 500mW and be done. Sorry for thinking stupidly out loud on the forum.

There are still two problems in that you stated, in post #10, that you wanted to be able to to vary the load's dissipation from 300 milliwatts to 800 milliwatts.

The first problem is that a 200 ohm load with 12 volts across it can only dissipate 720 milliwatts, so your 800 milliwatt specification is unattainable.

The second problem is that you're now stating that the goal is to get each load to dissipate a fixed 500 milliwatts.

What is it you really want to do?

 

I also think you're in for a surprise when you price out 8 pots rated for >1W. They'll need to have low temperature drift, since they'll heat up under load, as will your 200Ω resistors. If you need precision and accuracy, you might need individual current or voltage controllers on each leg.