How to double reading on analog voltmeter? - Page 1
Forum section: electronics / beginners

author: primegaps
author_rank: Contributor
timestamp: April 15, 2017, 10:12:39 am
content: I have a few of these old power/mate corporation analog needle gauges that i took from some old power supplies. I have a few of the power supplies left as well, and they work a treat, but a couple were broken, and I salvaged them for parts. Mostly just took the pots and the banana connectors, as well as the gauge you see here.When I hook the back labeled as + - with alligator clips to + and - on a power supply, the meter reads exactly half of the actual potential being put across the meter.I cranked up the supply till the pictured gauge read 6V, but the supply was putting out 12. And when I put the supply at 6, the gauge read dead on 3.Is there a way for me to double the reading on the free gauge? This would mean I need to double the voltage across the gauge at a given input.I'm thinking this is not possible? I'm sort of familiar with the concept of a voltage divider, but from what i understand, you can only use that to have the meter read even less.But before I removed the gauge from the broken power supply, it had a few resistors, and i think a paper capacitor wired across some off the terminals. Perhaps there is a way to have 6 volts go to the gauge while my supply only puts out 6? Some sort of "voltage multiplier."Thank you for reading.

author: Rerouter
author_rank: Super Contributor
timestamp: April 15, 2017, 10:24:28 am
content: Behind that back cover you will likely find a series resistor,Analog volt meters are actually just current meters with a resistor inline to define the range, as you want it to read higher, you will likely need to halve the value of that series resistor, then trim in until your reading correctly,

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2017, 06:59:34 pm
content: I agree with Rerouter; remove the two screws holding the scale in place and you should find a resistor inside the meter which sets the full scale sensitivity.

author: Circlotron
author_rank: Super Contributor
timestamp: April 16, 2017, 10:52:20 am
content: Get your digital multimeter out and measure the resistance between those meter terminals. This the total value of both the internal resistor and the meter movement in series. What you need to do is put in a lower value resistor that makes the total value half of what it is now.

author: Farley
author_rank: Regular Contributor
timestamp: April 16, 2017, 02:40:29 pm
content: Alan, W2AEW, has a couple of good videos illustrating how to change the range of these types of panel meters:

author: 128er
author_rank: Regular Contributor
timestamp: April 16, 2017, 09:32:20 pm
content: Quote from: Farley on April 16, 2017, 02:40:29 pmAnd not to forget, the post apocalyptic inventor as well:

author: alsetalokin4017
author_rank: Super Contributor
timestamp: April 17, 2017, 01:16:09 pm
content: I guess simply making and calibrating a new scale for the meter face is too obvious.

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 17, 2017, 06:27:27 pm
content: I think you are all missing one point here: it was taken from a power supply. So although I agree that you can simply adjust the series resistor that is probably hiding within what puzzles me is: how could it have been working? If you set the power supply to 5 volts there's not a secret 10V connection to the meter is it?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 17, 2017, 06:55:54 pm
content: Quote from: PA0PBZ on April 17, 2017, 06:27:27 pmI would assume that the meters were salvaged from a supply with a matching voltage output. I have quite a few supplies from several suppliers that have different output ranges. The internal meters look the same except for a different scale. Back when analog meters were the standard it was a simple matter to specify the desired range on the purchase order. If you were buying in production quantities the cost of this customization was negligible. A part afternoons work for the artist, some photo work, a few minutes for an engineer to specify the resistor and boom.

author: Seekonk
author_rank: Super Contributor
timestamp: April 17, 2017, 09:10:32 pm
content: A lot of screws on the back of that meter, could be other resistive taps. I find it hard to believe they are amplifying a 1A shunt to 36V.