Velleman PMLED power supply puzzle

Discussion in 'General Electronics Chat' started by Rich Purdum, Nov 16, 2013.

  1. Rich Purdum

    Thread Starter New Member

    Nov 9, 2013
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    I have installed this panel meter on my 220 volt wood lathe to provide an external indicator to the frequency being output from a Teco single- to three-phase VFD. The VFD allows me to vary the motor RPMs using a potentiometer.


    I have installed this switching 9 volt "wall wart" power supply (there is only 220 volt power in the lathe base) to power the meter. The signal from the VFD to the meter ranges from 0 to 10 volts DC and I have installed the proper resistors on the PMLED (thanks to a topic posted on this forum).


    Everything works as expected if I power the PMLED from a 9 volt battery. For example, if the VFD is putting out 30 cycles I get 5 volts (i.e. 30/60). However, if I attach the output from the 9 volt power supply the meter reads “1” and when I turn on the lathe it goes immediately to 19.99 which I guess is the top of the 20 volt range. If I attach a VOM to the power supply output I get 9 volts. I have confirmed all voltages with two separate meters.


    I can only guess the power supply is "overdriving" the meter but I have no idea why this is happening and am posting this is hope someone can point me in the right direction. I am a neophyte to this so apologies if the answer is obvious.
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The PS only has three reviews on Amazon, and two of them are negative. In addition, SMPS supplies are more noisy than linear supplies. I suspect it is noise on the power supply line that is the problem. The load on the PS is also very light, and some SM power supplies don't regulate well with light loads.

    You might measure the voltage on the power supply and see how close it is to 9 volts. If you have a scope, you might look for noise on the power. You could add some filtering to the output, and maybe add a power resistor to load it down a little.

    ETA: I see that you have checked the voltage, so I am back to noise as the cause.
     
  3. KJ6EAD

    Senior Member

    Apr 30, 2011
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    I agree with tracecom. The A/D converter in the meter is probably being inundated with extra signal edges from high frequency noise. You could try wrapping the 9V power lead a few times through a ferrite ring or snap a split ferrite onto it.

    http://en.m.wikipedia.org/wiki/Ferrite_bead
     
    Last edited: Nov 16, 2013
  4. Rich Purdum

    Thread Starter New Member

    Nov 9, 2013
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    tracecom

    Didn't notice the reviews...my bad. One reviewer mentioned that the unit was not regulated. Found this unit at Radio Shack which regulated and filtered. It was in stock locally so I picked it up. It read 9.0 dead on vom (the other unit was 9.4+) so I was hopeful. Hooked it up and got a .02 on the panel meter which matched the battery. When I powered on the vfd the meter went immediately to 1_.__ and when I put it in "run" the meter read 1.666 (instead of 1.999 with the other power supply.)

    KJ6EAD

    You posted while I was out getting the improved power supply so I headed back to Radio Shack to pick up this choke. Came two to package so put one on...no change...then the other...no change.

    If the Velleman panel meter is so sensitive, maybe I should look for another unit? I only really need a 3 digit display...0.00 to 10.0 would actually be better. Don't need to two decimal precision for my application. Any suggestions?


    ...or stick with the 9volt alkaline solution...
     
    Last edited: Nov 17, 2013
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    You may have better results going with an old style dArsonval movement. They won't respond so well to what seems to be a noise problem for you. Also, it would be very easy to make a custom scale to indicate RPM directly.
     
  6. Rich Purdum

    Thread Starter New Member

    Nov 9, 2013
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    In working with Velleman tech support I tried the following experiments:

    Power the meter using a 9 volt battery and measure the power supply voltage. Works fine and meter reads 9.02.

    Power the meter using the power supply and measure a 9 volt battery voltage. Works fine and reads 9.02 as well.

    Power the meter using the 9 volt battery and measure VFD voltage at 30 hz...should read around 5 volts and the meter read 5.17...close enough.

    Power the meter using the power supply and measure VFD voltage at 30 hz...reads 16.66.

    Seems to me that somehow the meter's power and measurement circuits are sharing something that can handle anything other than dead nuts stable but I'm just guessing.

    To my pea brain, it shouldn't be this hard!

    I did find three meter alternatives which have the right specs. A PM-1029A which has the usual resistors on board and is configured by solder bridges. A CX102A which has an add on board with the usual resistors and solder bridges. And a Sure Electronics ME-SP037 (or ME-DV31423) at Parts Express that can be somewhat configured for scaling using Windows software and an USB or COM1 interface. The vintage on the Velleman PMLED is from 2004 so maybe one of the other units is more forgiving?
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Just on lark, put a .1uF ceramic cap and a 100uF electrolytic cap across the power leads on the meter. Don't forget the polarity on the electrolytic.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    How is the power supply isolated from vfd and vfd output? (distance and lead placement)

    The capacitors across the power input on meter should help.

    Vfd's and leads to motor are powerful noise transmitters.
     
  9. Rich Purdum

    Thread Starter New Member

    Nov 9, 2013
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    0
    It's a 25 mile round trip to the nearest Radio Shack so next time I go down the hill I'll try the caps...they are both in stock. The VFD and 220 power are in the base of a Powermatic 45 wood lathe together with a 3/4 HP 3 phase motor. The mains come in to a box with a switch and a plug. The switch controls power to the VFD and the plug is for the power supply. All of the control signals are run through some 18 gauge sprinkler control wire to give me "remote" on/off, speed control and emergency stop. The voltage signal to be measured is in the same cable. All if this terminates in an aluminum project box mounted to the lathe cabinet. In short I can't reposition things without impacting usability.

    I tried running the panel meter with the power supply and measuring voltage from a 9 volt battery, all while the VFD was powered up and in run mode. Worked fine.

    I'm about to come to the conclusion that the Velleman is too sensitive to noise on the measurement inputs for my application. When I spoke with tech support at Teco, they indicated that this 10 volt signal is used all the time in their units and never has a problem.

    The Velleman unit design is from 2004 so maybe a more modern design will be less sensitive, or I can do as Bill suggested and go with a old style analog meter?

    At some point I need to get this behind me and start making sawdust!
     
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