Circuit to convert MV to V and drive 25ma

Discussion in 'General Electronics Chat' started by snav, Jul 12, 2014.

  1. snav

    Thread Starter Active Member

    Aug 1, 2011
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    We currently have an AGM/SLA 4A PS-battery charger which has a self powered 2wire voltmeter for monitoring output and charging voltages via momentary spdt pushbutton(default is output). The self powered meter draws about 20ma.

    The charging circuit is regulated to <500ma while the output is rated 12/24 @4a. Depending on the output voltage required, the PT supplied voltage is either 16.5(19v) or 24v(28v) @ 100va.

    We currently use an external DMM to measure across a .1ohm 1% resistor in series with the charge leads to measure charge current but would like to add a button to allow using onboard meter rather than External DMM. We would add second meter to board but there is no room while still maintaining adequate circulation for passive cooling.

    I was looking at an LM324 but not sure how to measure near zero as the series resistance is selected to have very low drop from high load during power fail.
     
    Last edited: Jul 12, 2014
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Here's how you could do it. For convenience the output is 1V per sensed Amp, i.e. 4A gives 4V out.
     
  3. snav

    Thread Starter Active Member

    Aug 1, 2011
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    Maybe I'm not understanding this but isn't there an offset, or minimum input required before you get readings, of 3mv? So I have measurements from >30ma across the .1ohm shunt?
    I could derive a negative voltage while there is AC and measurements during power fail are not important. The current limiting for charging is bypassed during current fail so the battery(s) can provide full output.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    With a 500mA charger are you really interested in charging current as low as 30mA? If so, and opamp offset is a problem, you could bias either the inverting or non-inverting input (depending on offset polarity) slightly with a high value pull-up resistor to the positive rail. No need for a negative supply. Be aware, though, that the LM324 output does not go all the way down to ground, so you may have to use an opamp with rail-to-rail capabilities on both input and output.
     
    Last edited: Jul 13, 2014
  5. snav

    Thread Starter Active Member

    Aug 1, 2011
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    Would you recommend a suitable op-amp, Our issue is not really the charger but the float, we seem to be having some 'cooked' batteries and don't know if it's the new batteries or new chargers. We used to see this when our panels were set at factory for 14.4v but now we routinely readjust them to 13.63 before turning them over.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    14.4V to a 6Cell SLA left on indefinitely will ruin it (by eroding the plates and drying out the electrolyte) in a few days. The correct long-term float voltage is ~13.7V (adjusted for ambient temperature). During recharge, you can go to 14.6V or higher (check the battery makers data), but you must revert to the float voltage after the battery stops accepting charge...
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I've never had cause to use one specified as rail-to-rail for both input and output. Others here may be able to recommend one. If not, you could do a parametric search on the web-site of any major opamp manufacturer/stockist.
     
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