DC Power Supply and Multimeter

Discussion in 'General Electronics Chat' started by Tom1962, Aug 14, 2014.

  1. Tom1962

    Thread Starter New Member

    Sep 10, 2012
    19
    0
    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...
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    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.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,986
    3,224
    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.
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    Make sure that 24V power supply can "supply" more than 4 Amps (24V/6ohms =4A).. Should be a 5A or greater rated power supply.
     
  5. adamclark

    Member

    Oct 4, 2013
    472
    6
    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.
     
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    Its normal.. multimeters leads are basically antennas..
     
  7. Tom1962

    Thread Starter New Member

    Sep 10, 2012
    19
    0
  8. bwilliams60

    Active Member

    Nov 18, 2012
    722
    88
    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
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    That supply is marginal for your load. Make sure that you have unrestricted air circulation to the supply...
     
Loading...