DC Power Supply and Multimeter

Thread Starter

Tom1962

Joined Sep 10, 2012
19
I have a power supply that plugs into a 110 volt household outlet (a/c) and produces a 24 volt (dc) supply. I plan to use the 24vdc supply to power the heater bed on a 3D printer. The heater bed has about 6 ohms resistance, and is little more than copper traces that heat up when the power is supplied.

I have a nice Fluke multimeter and am a bit confused by its readings. When I put the positive probe on the 24v positive terminal and the negative probe on the negative terminal, it shows 24 volts. Perfect. However, if I leave the positive probe on the power supply and take the negative probe off the power supply completely, the multimeter still shows a small voltage (maybe a .1 volts); and if I touch the negative probe to anything big (like a nearby metal rack) the multimeter shows over .5 volts. Why is that?

What I'm trying to figure out is whether I can safely turn the heater bed "off" completely by switching the negative wire from the dc power supply? If .5 volts is still getting through somehow, I don't want the heater bed to be staying warm all the time even when I try to turn it off...
 

wayneh

Joined Sep 9, 2010
16,162
The meter is extremely sensitive. Don't worry about it. If there is even a tiny "load", like a 100kΩ resistor, your voltage reading will be drawn down to zero.

You're low side switching should be fine, but I'm not so sure your power supply can handle that heater load. Be careful.
 

crutschow

Joined Mar 14, 2008
24,109
Your meter is likely picking up an AC signal from the power lines and some of that registers as DC on your meter. Turn the meter to AC under the same circumstances and you will likely see a larger voltage. Nothing to be concerned about.
 

adamclark

Joined Oct 4, 2013
472
On my bench supply I have now (0-30vdc 500ma) it does the same, but watch your meter, if your is anything like mine you touch the negative lead of your meter with your finger it should zero out.
 

bwilliams60

Joined Nov 18, 2012
1,246
What your seeing is a "ghost voltage" that the meter is picking up from its surroundings as others have said. I have meters that read 0.2-0.3 VDC no matter where I put them with nothing attached. Carry on but others have said, be careful with the power supply and your load. It may not handle it for long periods of time
 
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