Can I apply a 24v DC current to an analog voltmeter graduated between 8 and 16v for a few seconds?

Question

I have an analog voltmeter that is used to monitor 12V batteries, with a needle going between 8V and 16V)

In a specific situation, I have a series/parallel switch which turns two of my batteries into a 24V battery, and I want to make sure that this complicated switch is indeed working (notably, it won't set if there's not enough current in the batteries, which happened to me once in the past). In other words, I don't care whether I have 24V or 26V there, I just want to make sure that I have, say more than 15V.

I can think of many ways to achieve this, but since I already have a voltmeter with all the cabling in place, it would make sense to use this if at all possible. Of course this would just be very intermittent.

Will the needle go through the roof, or would my voltmeter die a painful death ?

Unfortunately, I can't easily access the rating information of the voltmeter.

Rather depends on how the voltmeter was designed - are there internal safety circuits, e.g. — Carl Witthoft, Nov 03 '17 at 13:10
It would be a very poor meter that couldn't cope with such a moderate overload. — Laurence, Nov 03 '17 at 13:09
I put my analogue multimeter across 240VAC once, accidentally, when it was in 'Current' mode. It made quite a good bang. — SiHa, Nov 03 '17 at 15:47
@SiHa :D Well more like a short circuit when in current mode. — The_Vintage_Collector, Nov 03 '17 at 15:59
@LaurencePayne Or perhaps a very dead meter that couldn't cope with such a "moderate overload". If this is a cheap import, it would not surprise me to find 20V or even 16V rated components. They have to cut costs somehere! — Technophile, Nov 03 '17 at 22:33
@SiHa - Did the same thing with my brand new VOM, back about 1965. Blew out a 22.5 ohm 1% resistor, and it took me about a year to find a replacement. Didn't do any other obvious harm, and I still have the meter. — Hot Licks, Nov 04 '17 at 02:59
(Generally speaking, a decent quality general-purpose meter should be able to take a 10x overload for a second or two, and a 2x overload indefinitely.) — Hot Licks, Nov 04 '17 at 03:01
can you "short it out" with 9 red LEDs in series? they shouldn't affect anything under 18v... — dandavis, Nov 04 '17 at 06:40
@SiHa, I did that on a movie theater ballast 30V DV @ 100 amps... it was ... bright and loud and I do not wish to do that again... never replaced the fuse... — Grady Player, Nov 04 '17 at 16:59
Oh, EVERYTHING'S a 'cheap import' these days. But anything continuously rated at 16v will be happy with a momentary 24v. I'm wondering if some of the replies here are from people with ANY bench experience! — Laurence, Nov 05 '17 at 14:28

score 25 · Answer 1 · answered Nov 03 '17 at 07:57

25

You could use a couple of resistors to half the voltage seen by the voltmeter. You would then just need to double the reading. This would protect the voltmeter from damage.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

answered Nov 03 '17 at 07:57

HandyHowie

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That's the way I'd do it. Drop the voltage with resistors and measure the voltage drop over each one, then add them up – MCG Nov 03 '17 at 08:14
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Just make sure the voltmeter's internal resistance is large compared with 10k – Carl Witthoft Nov 03 '17 at 13:12
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This works because the precision of the reading is not important in this case. – lucas92 Nov 03 '17 at 13:45
@CarlWitthoft doesn't every half-decent voltmeter satisfy that by far? – Pedro A Nov 03 '17 at 15:52
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@Hamsteriffic many analogue meters don't have a very high input impedance. – Chris H Nov 03 '17 at 16:28
"shunt" is the word missing from this answer. Will also help search engines. – Criggie Nov 03 '17 at 22:43
That circuit is waaaay too complicated! Leave out half of the components (R2) and just use R1 as a multiplier. – Transistor Nov 05 '17 at 09:40

Transistor · Accepted Answer · 2017-11-05T09:38:42.267

Will the needle go through the roof, or would my voltmeter die a painful death ?

It sounds like you have an analogue meter of the moving coil type. You could damage the mechanism by hitting it hard against the end-stop.

One solution is to halve the sensitivity of the meter and double the readings or replace the scale.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The addition of a "multiplier" resistor, R1, allows the range of the meter to be extended.

Measure the resistance of the voltmeter using a multimeter.
Add a series resistance of the same value.
Double the readings.
Addition of SW1 allows you to make a correct reading once you are happy that the voltage is below half full-scale.

This method uses the voltmeter itself as the bottom half of the voltage divider and avoids the parallel resistance problem presented if an external voltage divider is used.

Note that most analog multimeters were 20 kΩ/V so I'd expect yours to work out at about 16 x 20k = 320 kΩ if a standard meter movement is used.

schematic

^{simulate this circuit}

Figure 2. A Zener-limited voltage follower limits the maximum voltage applied to the meter movement.

If you can tolerate the 0.5 V drop that Q1 causes then the circuit of Figure 2 might suffice.

Figure 3. The results of a DC sweep from 0 to 26 V.

How it works

Q1 is configured as a voltage follower. The emitter voltage will be about 0.5 V lower than the base voltage.
When the voltage exceeds 16 V the 15 V Zener and LED will start to turn on. This will clamp the base voltage at about 16.5 V and the meter voltage will max out at 16 V. Meanwhile the LED indicates "overvoltage" getting brighter as the voltage increases.

It may be possible to adjust the meter coil via the front-panel adjustment screw, if fitted, to recalibrate the meter and remove the 0.5 V drop.

Thanks a lot, this is exactly what I was looking for (and more) — Brann, Nov 04 '17 at 06:43
"Measure the resistance of the voltmeter using a multimeter" The voltmeter itself should be turned off at this point, right? Otherwise the voltmeter will influence the multimeter. — Mast, Nov 04 '17 at 14:13
Yes. Multimeter resistance measurements are always taken on unpowered components. The meter works by applying a fixed small current to the resistor under test and measuring the voltage drop across it. Connecting a meter in resistance mode to a powered circuit could destroy the meter. Which solution are you going to try? The transistor can be any small-signal NPN. The 2N3904 is the default on the CircuitLab app. — Transistor, Nov 04 '17 at 14:18

Can I apply a 24v DC current to an analog voltmeter graduated between 8 and 16v for a few seconds?

2 Answers2