need suitable trimmer to calibrate led voltmeter

Thread Starter

sgabriel

Joined Jul 18, 2018
5
hi,
i'm building a voltmeter using led readouts,i require two readouts for two separate batteries and one for total voltage,the circuit is easy enough,the problem I have is that I need to calibrate the led displays that I have bought so both read what the batteries are,what im looking for is the difference between the 2 voltages,i need the correct size trimmer pots to calibrate the displays.any helpers?
Gabriel.
 

Hymie

Joined Mar 30, 2018
819
The total display in your diagram will not detect any voltage because the two batteries are isolated from each other.

If you connect the positive of the lower battery to the negative of the upper battery – that will work.
 

Thread Starter

sgabriel

Joined Jul 18, 2018
5
The total display in your diagram will not detect any voltage because the two batteries are isolated from each other.

If you connect the positive of the lower battery to the negative of the upper battery – that will work.
sorry forgot to put that line in,there is a metal bar connecting both batteries,,but well spotted.what size trimmer would I need to calibrate from dedicated power supply?
 
What is your dedicated power supply? Do you want to make sure the meters are accurate? What do you mean by trimmer? Usually it is a small variable resistor--what do you want it to trim?
 

Thread Starter

sgabriel

Joined Jul 18, 2018
5
What is your dedicated power supply? Do you want to make sure the meters are accurate? What do you mean by trimmer? Usually it is a small variable resistor--what do you want it to trim?
yes pcb variable resistor,need to calibrate the meters by 10th of volt,batteries I am using are 12v-13v,problem is that batteries must drop equally within 0.2v any more then machine shuts down,so need to monitor both voltages at same time.
 

Hymie

Joined Mar 30, 2018
819
Could you clarify what you are trying to measure?

Your diagram shows one voltmeter measuring the total voltage across the two batteries and a voltmeter measuring the voltage across each battery.

If you subtract the voltage across one of the batteries from the voltage across the other – this will be the difference between the two battery voltages.
Or do you want a meter to display this difference between the battery voltages (without you having to do the calculation)?

You also state that if the batteries discharge at a rate of greater than 0.2V (between the battery voltages), then the machine shuts down. To prevent his, you could regulate the voltage from each battery, or investigate why such a small voltage difference is critical to the operation of the machine (and redesign the circuit).
 

ebp

Joined Feb 8, 2018
2,332
[EDIT] see #14

R11 is clearly a potentiometer. It might be a calibration trim or perhaps something to do with the display brightness. If the IC is marked, you might be able to find a datasheet that shows a typical circuit.

Regardless of what you do, you must expect at least one count of uncertainty that will be virtually impossible to trim out. It might be possible to trim it out initially, but small variation in temperature and parts aging would likely "restore" the uncertainty fairly quickly.
 
Last edited:

ebp

Joined Feb 8, 2018
2,332
At first I though R11 was a through-hole part mounted on the display side of the board, but after looking more closely at the photo, I now think it is an empty place for a surface mount pot.
 
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