Looking to make some dummy adjustable loads. (up to 15A at 50VDC). Just looking for suggestions on where to start (mosfet and sense resistor or something else). I've got tons of large resistors that I have been using but wanted something adjustable via a pot. Also keep in mind I would like the ability to parallel these units so I can get up to say 150A at 50VDC.

 

15A at 50V is 750 Watts. I suggest large wirewound resistors in a horse trough filled with distilled water.

150A at 50V is 7,500 Watts. This would work well to keep an Arctic swimming pool nice and warm.

 

Water heater elements. You can get them pretty cheap and they are usually around 1500w
at 120v. So you may need 10 of the units to cover your 7,500 watts. I would use the water trough with this.

http://www.amazon.com/gp/product/B0..._m=ATVPDKIKX0DER&pf_rd_r=01CXSTVCQNEDAK7Y705V

For $9, it may be worth getting to see what the 50VDC will do for you.

This is probably the lease expensive way to get that about of power out of your circuit. This is a standard solar, or wind dump load.

 

Another approach is a large bank of transistors, each with a large heat sink and fan. Electronic loads are a standard piece of test equipment.

Here is a lightweight I've been contemplating building.

 

You might use power MOSFETs to control the ground path of the resistors, and apply PWM to get an average load. However, you are going to have to dissipate a LOT of power, as already has been mentioned.

The PWM circuit won't be very complicated. Keeping the MOSFETs cool could be complicated.

A LARGE inductor in series with the load would help to keep the current sink fairly constant - something like an arc welder transformer.

 

DO you think something like this would work for my levels if I simply size Q1/R10 correctly (or just parallel mosfets and the op amps to get my ratings)? Can I parallel this easily? I do not like dropping resistors into salt water or mineral oil. Something about electricity/water/liquid just doesn't mix IMO http://electronicdesign.com/article/power/resistive-dummy-load-draws-constant-current-from-1.aspx
http://archive.electronicdesign.com/files/29/4056/figure_01.gif

 

large heat sinks and fans will def. be part of this device of course.
I envision simply making 1 device that would accomplish my 15A at 50V then stack 10 on top of each other and bus them together with a copper bar to get my 150A rating. Doable?

 

Actually, mineral oil works quite well. The local technical college has a PC motherboard/supply/etc in an aquarium filled with mineral oil to keep the temp stable. They keep it running 24 hours a day.

 

Think large connectors throughout. Really large. With each unit independently settable, it sounds doable.

This is a project best done in the winter, you are going to heat your house up something fierce.

 

I am fully aware of the heat generated.. We have 2 large miller load banks (that get run at 600A at 50V each) that will run for 3 hours during a test... It heats up our whole building and will suck any moisture from your body in minutes. Lube up I'm going in the lab.

My plan was something like 1/4" studs protruding out the back for the load connections so I can attach a ring terminal/compression lug or slap a bus bar over to bus them together.

 

You might consider the types of cables/connectors the military is using for 28v systems; for example jump-starting a HMMVEE. The plug looks something like a very large RCA connector.

Either that or the types of connectors they use on welders.

1/4" won't be enough.

 

I remember a story something similar my bro told me. His company received a brand new power supply for a burn in chamber, something like 50V at 100A or more (special AC wiring required). A manager insisted Jim (my bro) test it out. Jim tried to tell him this wasn't trivial, but the manager was adamant. So he rigged up two hefty poles with parallel strands of wire between them, about 20 or so. Then they were all glowing bright cherry red at about 1000W with Jim asking if he wanted it brighter and the manager relented.

Anyone knows anything about the subject can tell there is a element of smart alac in this, I'm told the engineers watching the demonstration were laughing their assets off.

 

1/4" won't be enough.

1/4" studs for each 15A box is overkill actually (I just chose that 1/4" because its easy to find ring terminals with a 1/4" hole). I envision 1/4" studs (1 for -load connection and 1 for +load connection) sticking out of each 15A box. Then like I said I would stack 10 boxes and run a large copper bus bar connecting each of the 10+ terminals together and another copper bus connecting the 10- terminals to allow me to attach to each of those 2 copper bus bars (-bar and +bar) with larger wire and compression lugs. Like 2 or 0 awg min.

 

But if one failed, you are going to start popping like corn.

And I would also consider the low oxygen super stranded welder cable.

Here is a good supplier on what you may want to use.
The connectors SgtWookie mentioned are available here:
http://www.fastenal.com/web/products.ex?N=999600994

 

If you were nearby, I've got a nice little pot you could borrow. It's adjustable from 0.3 Ω to 17.5 Ω and rated at 1.6 kW continuous. Would make a good heater in winter...

 

I would love to see a photo of that 1.6kW pot.

You could make the ultimate GEEK space heater. (Just dont try to adjust the temp )