LED Voltmeter current draw?

lookforjoe

True Classic
I found out today, driving with the OBDLink MX BT app connected to the Honda EMS that the system voltage is perfect - 14v when running. The problem is my (generic, eBay/Amazon) LED voltmeter show 1-1.3v below system voltage. The wire feeding the meter is an existing switched feed from the fusebox with other consumers on the circuit. I ran a new wire straight off the main battery feed to the fusebox, and now it reports the charging voltage accurately.

So, of course now the volt meter is live all the time. I can wire the feed through a relay if I have to, but if the LED current consumption is low enough I'll leave it be.

Does anyone know about these? @bbrown? I can't find the info searching.
 
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Do you have a description of the meter you are using? It should have a spec for the power supply current. The current draw on the sensing terminal is normally extremely low - microamp level. Can this meter be connected so that the power terminal can be hooked to a switched source and the sensing terminal be connected to the battery?

The other concerning issue is why it was reading low at the fuse box. Is the reading wrong or is there that much voltage drop at the fuse box? Certainly, the meter is unlikely to draw enough current on its own to drop the voltage that much unless the resistance between the battery and fuse box is ridiculously high. In that case, you would be seeing much bigger issues.
 
Do you have a description of the meter you are using? It should have a spec for the power supply current. The current draw on the sensing terminal is normally extremely low - microamp level. Can this meter be connected so that the power terminal can be hooked to a switched source and the sensing terminal be connected to the battery?

The other concerning issue is why it was reading low at the fuse box. Is the reading wrong or is there that much voltage drop at the fuse box? Certainly, the meter is unlikely to draw enough current on its own to drop the voltage that much unless the resistance between the battery and fuse box is ridiculously high. In that case, you would be seeing much bigger issues.

I can't find a (digital) receipt - it's just one of those generic 3-position boards with no casing. I can add a relay to switch it if need be. I ran all new wiring for all the systems I have added for the Honda swap, of the things I've changed it's now only the cluster that was using existing power supply/fused sources. I had just assumed that a 1 volt drop was pretty average for the stock wiring/circuitry, based on all the other times I had to rewire to overcome problems created by the stock layout (switched power to ignition coil was a big one with the old LH2.4 setup).

IMG-20200219-112004.jpg


I just used 3M double stick tape to locate in on the faceplate

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Not sure about the spec's of the meter you're using but if you need a steady power to the device itself and you only need low current to run it, you can use this: https://www.ebay.com/itm/373184978979/ it will supply the voltage you need to the device from a lower (or variable) source and supply a steady voltage to the device itself while allowing you to monitor the desired source. At least this way, you can hook it back into the intended voltage feed without running special wiring back to the battery....
 
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@lookforjoe, this makes me wonder if any of your OTHER gauges are also inaccurate - due to not getting proper voltage? And I think it does support the known issue with the stock wire harness not supplying good current to things.


you can use this: https://www.ebay.com/itm/373184978979/ it will supply the voltage you need to the device from a lower (or variable) source and supply a steady voltage to the device itself while allowing you to monitor the desired source
Bob, I'm fascinated by this little device, but really do not understand. What exactly does a "automatic buck-boost power supply" do?
 
I suspect it either drops or raises the voltage from a fixed or variable input voltage, bucks/boost
 
@DrJeff, Well, it does what it says ... Takes in voltages anywhere from 2.5V to 15V and turns it into a regulated 12V supply. Like magic! :)
 
@lookforjoe, this makes me wonder if any of your OTHER gauges are also inaccurate - due to not getting proper voltage? And I think it does support the known issue with the stock wire harness not supplying good current to things.

The other 'regular' gauges are close enough -for example, I've compared the coolant temp gauge with the EMS values, and with the BT app, the results are consistent. Same for oil pressure. Gas gauge, who knows - it's never seemed that accurate, but it's always been close enough.
 
Not sure about the spec's of the meter you're using but if you need a steady power to the device itself and you only need low current to run it, you can use this: https://www.ebay.com/itm/373184978979/ it will supply the voltage you need to the device from a lower (or variable) source and supply a steady voltage to the device itself while allowing you to monitor the desired source. At least this way, you can hook it back into the intended voltage feed without running special wiring back to the battery....

Thanks Bob. I wouldn't use that for the voltmeter, since I want it to display the (variable) battery charge rate.
 
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