Circuit/Project Advice

Hello All! New member hoping for some assistance. I am a Tech Ed instructor at a small Wisconsin high school. I'm trying to build a small electromagnetic table for my students so they can process small cnc plasma items they are doing for a fundraiser. I currently have a 10 amp, 12 volt power supply (110v input) wired in series to two matching microwave transformers with the secondary windings removed to use as the electromagnets. Each one provides about 1 ohm resistance. I'm getting good magnetic pull on both but in order to keep the supply from overpowering I've had to add two 1 ohm, 100 watt resistors. That keeps the power supply happy but now my resistors are overheating, Surface temp on them reaches 280 degrees F. I don't think they will last too long at that temp, but maybe I'm wrong. Looking for any advice on how to keep everything balanced and happy. I'm attaching pics and specs of the supply and resistors, and I'm using 14 awg stranded wire between supply and magnets.
Thanks a ton in advance for any assistance!

Use a lower voltage.

I've seen transformers hold a piece of steel strongly enough with a single AA battery connected to the point a person couldn't separate them.

Have you got the two transformers in series - should draw ~6A? No need for any resistors then.

Mike.

Power resistors can run hot for long periods of time. It is normally stuff that is nearby that suffers from the heat more than the resistors do. The maximum surface temperature for those is probably around 175 deg C or 350 deg F

A 1 Ohm resistor will generate 100 W when it has 10 V across it. A 100 W resistor of that type is only rated to 100 W on a significant heat sink, and will probably be rated to 50 W with no heat sink.

You can just have more resistors or larger resistors, as long as the combination is giving you the resistance that you want. The same power spread over more resistors means that they will run cooler.

Alternatively, find a lower voltage power supply.

Wow! Good stuff guys, thanks. I am running them in series, but when I tried it without resistors the green light on the power supply would flash on and off and you could hear the power supply cycling on and off, so I assumed it was not sensing enough resistance and was cutting itself out. I think I will try adding a heat sink and cooling fan (already have those on hand) and see how it runs. I will post back with results. Thanks again!

If you have the two transformers in series then it should be 2Ω which will take 6A from a 12V supply. If you put a 1Ω resistor in series with the two transformers then it should be 3Ω and take 4A. As your supply can handle 10A then there should be no problem. Something doesn't add up.

Mike.

When I get to school I'll take a picture of my setup so you can take a look at it and see what you think. Thanks again for the reply!

Here's a pic

The 100W resistors are only rated 100W when mounted on a large heatsink. Without a heatsink, they handle a lot less power, somewhere between 25% to 35% of its rating. Some better brands will handle 50W, but still run very hot, over 100C. (up to 120C)
Also, your "1 ohm" measurement of the transformer may be in error if using a standard digital multimeter. Those are not accurate at low ohm ranges.
Assuming the transformers are actually low resistance, like 0.1 ohm (large size primary wire), you would be passing about 6A through both resistors. That is 36W of heat in each one, making them very hot. It may not damage anything, but will run hot regardless.
Otherwise, the setup looks ok to me.

Ok, thanks for the feedback. I'm going to mount 2, 120mm 12v fans in the 12 x 6 x 4 project box, one drawing in, the other exhaust to get some air moving past those resistors.

raygussy said:

Ok, thanks for the feedback. I'm going to mount 2, 120mm 12v fans in the 12 x 6 x 4 project box, one drawing in, the other exhaust to get some air moving past those resistors.

Click to expand...

You could do with getting them on heatsinks, even just bent aluminium sheets, as that will make the fans far more effective.

Something still doesn't add up. Your picture shows the transformers and resistors connected in series and that should have a total resistance of 4Ω which should result in a current of 12/4 = 3A (I=V/R). A 1Ω resistor carrying 3A will dissipate 9W (I²R). With only the resistors the current should be 6A and the power (per resistor) 36W. From the figures you've stated, you shouldn't need the resistors at all. Can you measure the current that is flowing?

Mike.

Measure and post the voltage across each of the four pieces in the circuit.

If the electromagnets are indeed 1 ohm, then all of the voltages will be about 3 volts. But If they're not, the numbers will tell us a lot about what's going on.