Changing the scale of an analog meter

[jorpec]

Hi

I want to change the scale of an analog meter, to monitor the charge of an li-ion single cell, so the needle at the end would point 4.2 volts and the 0 on the meter would point 2.5 volts or so, i tried to add diodes or a zener in series with the meter but the changes of the ambient temperature also changes the voltage value on the meter
I wonder how i can accomplish this so the meter would be precise at any temperature ?

Thnaks

[eccerr0r]

Re: Changeing the scale of an analog meter

Though all diodes are temperature sensitive, forward bias diodes are especially temperature sensitive so don't use that for accuracy. Diodes are current sensitive, so you need to keep the current constant. So it looks like you need an amplifier to do this....what's your budget in terms of complexity, as an amplifier needs to be powered?

[stj]

Re: Changeing the scale of an analog meter

meters dont show voltage, they show current.
you need to convert the voltage to a varying current somehow.

[redwire]

Re: Changeing the scale of an analog meter

It is called an "expanded scale voltmeter". They are common for lead-acid batteries as well, wanting say a 11-16V scale.
You would use a TL431 instead of a zener, for good precision and you can adjust the low end of the scale, if you wanted 2.5V or 3V etc.

[jorpec]

Re: Changeing the scale of an analog meter

what's your budget in terms of complexity, as an amplifier needs to be powered?

Must be something that wont consume much current or would drain the battery

It is called an "expanded scale voltmeter". They are common for lead-acid batteries as well, wanting say a 11-16V scale.
You would use a TL431 instead of a zener, for good precision and you can adjust the low end of the scale, if you wanted 2.5V or 3V etc.

Thanks, i will look into it

[redwire]

Re: Changeing the scale of an analog meter

I would start with this circuit I drew. R2 is to keep minimum 1mA flowing in the TL431 with a low battery.
You could add a trimpot to set the low voltage scale up to say 3.0V or more if you wanted.

[jorpec]

Re: Changeing the scale of an analog meter

I would start with this circuit I drew. R2 is to keep minimum 1mA flowing in the TL431 with a low battery.
You could add a trimpot to set the low voltage scale up to say 3.0V or more if you wanted.

Thanks for the schematic, works perfectly no variation on the meter even heating the TL431 with a heat gun at 100ºC, but even the total current consumption of this circuit isn't much, around 4.5mA at 4.2V will slowly drain the battery, using only the analog meter it consumes 0.22mA at 4.2V. I wonder if there is a TL431 equivalent that needs less than 1mA to work, or i just add a switch when not using it

[Per Hansson]

Re: Changeing the scale of an analog meter

You can look at the ATL431 for example.
Read this linked section of the Wikipedia article on the TL431:
https://en.wikipedia.org/wiki/TL431#...nd_derivatives

[jorpec]

Re: Changeing the scale of an analog meter

You can look at the ATL431 for example.
Read this linked section of the Wikipedia article on the TL431:
https://en.wikipedia.org/wiki/TL431#...nd_derivatives

Thanks
Yes the ATL431 is ideal it needs a very low current to work 0.035mA