(Another) analogue voltmeter does not go back to zero

[remzo]

Hi all

I'm using an ESP32 DAC pin to control an analogue voltmeter as a display. The voltmeter was designed to measure voltages up to 300V. I removed (what I think were) the resistors and connected the ESP32 pin output. The output produces a max of 3.3 volt. This moves the needle a little bit, but cannot move the needle to the end of the scale. Around 15V is needed to do so.

Also the voltmeter did not go back to zero. It stayed in a position around of third of the scale even if the ESP32 was disconnected.

I've 2 questions:
1. Is it possible to make this voltmeter work (as in control the needle around the scale) with 3 volts? Or should I increase the voltage of the output?
2. Is there a setting to 'reset' the needle to zero? I've tried the adjustment screw, but that is for much finer control.

 

[Ramussons]

1. Is it possible to make this voltmeter work (as in control the needle around the scale) with 3 volts? Or should I increase the voltage of the output?

Normally, about 50 - 100 uA is what is needed for full scale deflection, if it is a DC meter. Maybe there is another resistor in series with the coil which may have to be removed.

2. Is there a setting to 'reset' the needle to zero? I've tried the adjustment screw, but that is for much finer control.

Zero setting is normally Mechanical. Is there some mechanical damage? Bent Needle or dislocated Coil?

 

[For The Popcorn]

I recommend downloading a copy of The ARRL Handbook, 1975 Edition from this link and looking at the section starting on page 506 on descaling analog meters. This is an in depth discussion of everything you need to know.

Also, analog meters are precision instruments and won't withstand much (any) abuse, especially when out of their case.

 

[rjenkinsgb]

That looks like it may be a "moving iron" AC voltmeter, rather than the precision moving coil type in your other post.

Moving iron ones are not linear, and putting DC through it may have left some residual magnetic field which causes the zero shift?

I just looked at the scale photo - not at all linear so 99% sure its a moving iron one.

 

[JimB]

The meter in the previous "Does not return to zero" thread was a moving coil meter.

The meter in this thread is not a moving coil type, I am not sure what it is, maybe it is a moving iron type, especially as it is specified for AC on the scale.

As for the scale, look at it, it is non linear, typical of moving iron type meters.

Why this meter has not returned to zero I do not know.
My best wild guess is that passing DC through it has given the mechanism a bit of a magnetic offset.
Try running some AC through it and see if the offset clears.

JimB

 

[JimB]

What?
Jenks has written the same as I have?
What is this telepathy or something?
Are we twins separated at birth or what?

JimB

 

[remzo]

Ramussons: I do think that there is another resistor, but I'm hesitant to open up the meter even further as I approach the point on which I'll be unable to put it all back together...

For The Popcorn: This is a very nice and nostalgic book. As I understand the recommended pages, this information is useful when you want to measure higher voltages and it describes how to add resistors. But I want to measure smaller voltages.

rjenkinsgb: If magnetic residue is the cause, then it's not possible to drive the meter with an ESP32 I guess?

JimB: I do think you are a twin. If that is your real photo. It looks like somebody else, maybe your brother. Anyway, I like that you have given a possible solution and now I'll try to find some low voltage AC!

 

[JimB]

I do think you are a twin. If that is your real photo. It looks like somebody else, maybe your brother.

Yes, that is my real photo, taken nearly 40 years ago.
I definitely do not have a brother (or a sister).

JimB

 

[aliarifat794]

To control the voltmeter fully with 3.3V, you can use an op-amp to amplify the DAC output to the required voltage level (around 15V).

 

[MrAl]

What?
Jenks has written the same as I have?
What is this telepathy or something?
Are we twins separated at birth or what?

JimB

Yeah that's weird
Maybe it's the old "great minds think alike" syndrome.

I've never seen an analog meter do this and I have seen a LOT of analog meters because I came from the age where that was just about all you had, besides those green "eye" tubes for adjusting things (which were cool BTW).

I've had one meter though that had a problem where you had to lay it on it's back to get the reading to go to zero and be accurate too. That's about it.

There is one problem that does come up sometimes where the jewel movement gets 'stuck'. That means the movement itself has to be adjusted not the screw on the back. It's been so long that I do not remember how this is done but if you examine the jewel movement itself you can usually see what is going on.

I suppose that the needle itself can be bend and getting caught on the back face. I've seen this happen with an analog clock (not a meter per se') where the second hand would get stuck on the back face.

 

[alec_t]

I've had one meter though that had a problem where you had to lay it on it's back to get the reading to go to zero

The present meter has an instruction to use it horizontally. That's another indicator for a moving-iron type.

 

[rjenkinsgb]

This is an old AC moving iron meter; the biggest clue that one is a moving iron type is the very non-linear scaling, with the ends compressed compared to the divisions near mid scale.

eg. The 0-5 area is tiny compared to 5-10, just like the 0-50 vs 50-100 on the OPs meter.

 

[MrAl]

The present meter has an instruction to use it horizontally. That's another indicator for a moving-iron type.

It worked fine when new, then one day something went wrong I can't remember what it was this was many years ago. It was the Radio Shack meter with the "range doubler" feature. It had a nice big face too. I eventually got rid of it though. It also had an FET input stage using matched FET's.

 

[schmitt trigger]

The photo shared by the TS clearly shows AC VOLTS, and thus it is a moving iron type that will not work for your particular project

It is a shame, because it is a very nice looking vintage meter.

As others have mentioned, you do require a 50uA DC or a 100 uA DC meter. These are still widely available devices at very reasonable prices, both at Amazon and Aliexpress.

They are also very affordable, from $US 2 to $US 25 depending on size and accuracy.