Power Supply - Isolated supply for ammeter

Discussion in 'Power Electronics' started by Standisher, Aug 2, 2016.

  1. Standisher

    Thread Starter Member

    Jan 16, 2015
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    I'm building a simple low voltage/current DC power supply (with the initial input coming from an AC/DC 15v wall wart) and want to incorporate in the supply output a cheap, 4 wire digital ammeter which measures current drawn by the power supply load in the range 0-999ma. The wiring diagram for the Ammeter posted by the eBay seller shows 2 ways of wiring and I think I would prefer the 'Independent Power supply' option. I would however want the independent power supply to be disconnected when I powered down the supply.
    My question is whether a DC-DC converter isolated power module, such as this one http://www.ebay.co.uk/itm/DC-DC-Con...928240?hash=item464cf66a70:g:AYAAAOSwYmZXHKda could be connected in to the power supply on/off switch to provide an isolated independent power supply for the ammeter?
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    The 15V wall wart must a regulated type as the maximum input voltage for the DC-DC converter is 16V and an unregulated one is very likely to exceed that.
    The 15V input to your power supply can be connected in parallel with the DC-DC converter and connected by a switch to the wall wart to switch on/off (obviously switching off the wall wart will switch off everything)
     
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  3. OBW0549

    Well-Known Member

    Mar 2, 2015
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    The DC-to-DC converter could be powered from the incoming 15 VDC coming in from your wall-wart through the ON/OFF switch, but the particular converter you've chosen appears to only tolerate input voltages up to 16 volts. From my experience with DC wall-warts, most are not regulated and their rated output voltage only holds true at the specified maximum load current; at low currents, their output voltage can be as much as 25-30% higher which would likely be more than your DC-to-DC converter could handle.
     
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  4. Standisher

    Thread Starter Member

    Jan 16, 2015
    47
    3
    Thank you both for your replies. Sorry for the delay in replying. I agree that the wall wart (Panasonic) delivers a voltage which would normally be a little too close for comfort. That said, without load it has delivered a constant 15.04V without fail (even immediately on switch on) and no higher. Taking into account your advice however (and with future planning should I need to change the wall wart), I have looked for an isolated DC-to-DC converter with a wider/higher rating at a reasonable cost. ....but I'm struggling, despite scouring a number of datasheets to find something in the UK (or indeed via cheap postage) that can accommodate voltage input between 15v and, say 18 or 24v whilst giving an output of between 5-12 volts and not requiring a significant load to operate effectively. Perhaps I'm just not very good at searching products :(
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    If it has a no-load voltage of 15V then it is a regulated supply and should be fine driving the DC-DC converter.
     
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  6. Standisher

    Thread Starter Member

    Jan 16, 2015
    47
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    Thanks. I'd still like to find an isolated DC-DC converter with a wider spec in case I need to change the supply at a later date but it looks like I can go with the one in the OP in the meantime.
     
    Last edited: Aug 2, 2016
  7. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Sounds like it might be regulated, then. One way you can verify that it is regulated, is by monitoring its output voltage while connecting and disconnecting a load resistor (say, 150Ω or thereabouts); if the voltage stays constant or changes by only a few dozen millivolts, then it's almost certainly regulated and therefore safe to power your DC-to-DC converter.

    I'd call this combination "too close for comfort" if you were making a commercial product and planned on producing hundreds of units; in that case, more safety margin would certainly be called for. But as a hobby one-off, I'd call it safe provided the wall-wart is indeed regulated.
     
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  8. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I use those types of meters too, so...

    Why do you want an isolated power supply for it when its negative power lead and the load are internally connected?

    It makes no sense. You gain nothing from it but unnecessary complexity that serves no purpose in your application.
     
  9. Standisher

    Thread Starter Member

    Jan 16, 2015
    47
    3
    You'll notice I said in my opening post that "I think I would prefer the 'Independent Power supply' option". I said that because, perhaps in my ignorance, I thought that connecting the positive "thin red line" positive lead (to power the ammeter) to the positive of my supply may impact the voltage available to the output of my supply (is that not the case?). I do not have voltage to spare so did not want any impact on the supply. As I say, perhaps I've misunderstood something here so would be grateful if you could educate me.
     
    Last edited: Aug 2, 2016
  10. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I see.

    Those little digital meters are pretty efficient. The ones I have I doubt use more than 40 - 60 milliamps peak so unless this is tiny sub ampere power supply I doubt you need to worry about their drain on the system.
     
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