Is this also a DC voltmeter?

norbss · Jan 7, 2019

I attached a 1.5 volt AA battery to the said RPM meter and the needle moved a little as suggested by ronsimpson. I tried it with a 13.8 volt regulated dc power supply and the needle moved much bigger(probably 50 rpm). attached is the back view and the whats inside the meter. I also tried to get the reading on the terminals of the rpm meter. It read approximately 27 on X10K . I hope these data will help determine if I can connect these to M+ & M- .

JonSea · Jan 7, 2019

Can you trace the circuit? It should be

[Terminal]--- [resistor]---[pot]---[meter coil]---[terminal]

The order may be changed, but everything will be in series. Replacing the resistor and pot with an appropriate resistor and you can make the full-scale voltage be anything you desire.

Nigel Goodwin · Jan 7, 2019

JonSea said:
Can you trace the circuit? It should be

[Terminal]--- [resistor]---[pot]---[meter coil]---[terminal]

The order may be changed, but everything will be in series. Replacing the resistor and pot with an appropriate resistor and you can make the full-scale voltage be anything you desire.

Are you not getting the pictures? (they appear to be hosted here), the second one clearly shows how it's wired internally, there's a 150K between one terminal and the meter, and a 100K pot between the other terminal and other meter connection.

I'm more bemused about the scaling of the meter, and wondering what rev counter uses a 160V output?

MrAl · Jan 7, 2019

norbss said:
I attached a 1.5 volt AA battery to the said RPM meter and the needle moved a little as suggested by ronsimpson. I tried it with a 13.8 volt regulated dc power supply and the needle moved much bigger(probably 50 rpm). attached is the back view and the whats inside the meter. I also tried to get the reading on the terminals of the rpm meter. It read approximately 27 on X10K . I hope these data will help determine if I can connect these to M+ & M- .

Hello again,

If 13.8v makes it move to 50rpm, then to get to 1800rpm you would need 496.8v but that sounds too high.
With the 270k resistance measurement, that would make it approximately 270v to get to 1800rpm.
With the internal picture, we see a 100k resistor i think and a 100k pot. If the pot was adjusted to around 60k then the meter would read 1800rpm with 160 volts.

So there are a few different views so far. Could the meter also be defective in some way or is it sure that it works properly?

unclejed613 · Jan 7, 2019

Nigel Goodwin said:
Are you not getting the pictures? (they appear to be hosted here), the second one clearly shows how it's wired internally, there's a 150K between one terminal and the meter, and a 100K pot between the other terminal and other meter connection.

I'm more bemused about the scaling of the meter, and wondering what rev counter uses a 160V output?

yeah, that's a really weird way of scaling a tachometer...

norbss · Jan 7, 2019

Hi MrAl, I better check again today on that data I gave regarding the 13.8v making a move to around 50 rpm. sorry..

MrAl · Jan 8, 2019

norbss said:
Hi MrAl, I better check again today on that data I gave regarding the 13.8v making a move to around 50 rpm. sorry..

Hi,

Ok no problem. If you can get a more accurate reading we can make a better prediction about what it takes to get a full scale deflection.

norbss · Jan 8, 2019

these are the new data. 1100 rpm= 85 dc volts, 1200 rpm= 93 dc volts, 900 rpm= 75 volts. I forgot to mention that my DC motor is connected to a speed reducer which turn a round rubber drum .

Nigel Goodwin · Jan 8, 2019

MrAl said:
Hi,

Ok no problem. If you can get a more accurate reading we can make a better prediction about what it takes to get a full scale deflection.

To be fair it's pretty obvious, it's a 1mA meter, and the series resistance is so high as to totally swamp the internal resistance of the meter (so that can be ignored), so it should be 150V FSD with the pot at one end, and 250V FSD with the pot at the other, +/- the tolerance of the resistors.

This takes me back to when I was a kid, we were fairly poor, I couldn't possibly afford to buy resistors or anything (never mind a meter), I used to get all my components from old TV's at a local tip - I used to write down the values I wanted, along with the colour codes of them, and go to the tip with a pair of cutters and a screwdriver, hunt for the values I wanted and cut them out.

Anyway, we did about meter movements in physics at school, and shunts etc. - so I thought if I could find a meter I could make a simple multimeter. The only one I could find was an ammeter in an old car, so I though 'no problem take the shunt out, and use the bare movement' - nice idea, but when I took the meter to pieces there was no shunt, just a very thick 'coil' which passed the +/-30A directly, and a moving iron movement.

norbss · Jan 8, 2019

I believe the reading on the rpm meter is totally incorrect but it serves my purpose since the rpm meter is just a guide on up to where the potentiometer should point during operation. I just hope the meter does not burn or blow up..

MrAl · Jan 8, 2019

Nigel Goodwin said:
To be fair it's pretty obvious, it's a 1mA meter, and the series resistance is so high as to totally swamp the internal resistance of the meter (so that can be ignored), so it should be 150V FSD with the pot at one end, and 250V FSD with the pot at the other, +/- the tolerance of the resistors.

This takes me back to when I was a kid, we were fairly poor, I couldn't possibly afford to buy resistors or anything (never mind a meter), I used to get all my components from old TV's at a local tip - I used to write down the values I wanted, along with the colour codes of them, and go to the tip with a pair of cutters and a screwdriver, hunt for the values I wanted and cut them out.

Anyway, we did about meter movements in physics at school, and shunts etc. - so I thought if I could find a meter I could make a simple multimeter. The only one I could find was an ammeter in an old car, so I though 'no problem take the shunt out, and use the bare movement' - nice idea, but when I took the meter to pieces there was no shunt, just a very thick 'coil' which passed the +/-30A directly, and a moving iron movement.

Hi,

Oh ok good, i thought that resistor was 100k but now i see that one band is green not black making it 150k which of course means the adjustment range is 150k to 250k or in terms of voltage 150v to 250v.

From his new readings i get 139v to 150v full scale without changing the pot setting, whatever that is at.
So i assume that the pot is to be adjusted to get 160v full scale, in whatever application that was intended for.

That takes me back to a time when we used to go out at night and collect discarded electronic devices like old TV's and radios and what not just for the parts. I had a big collection of tubes back then from old TV's. People would leave their old stuff at the curb to be collected by the town that week so we had to get to it first.

MrAl · Jan 8, 2019

norbss said:
I believe the reading on the rpm meter is totally incorrect but it serves my purpose since the rpm meter is just a guide on up to where the potentiometer should point during operation. I just hope the meter does not burn or blow up..

Hi,

Ok so it looks like you can adjust the full scale with the pot and it can probably allow full scale of 150v to 250v approximately.
The lowest of your data shows it could be as low as 139v but that's probably not as accurate.

Nigel Goodwin · Jan 8, 2019

MrAl said:
That takes me back to a time when we used to go out at night and collect discarded electronic devices like old TV's and radios and what not just for the parts. I had a big collection of tubes back then from old TV's. People would leave their old stuff at the curb to be collected by the town that week so we had to get to it first.

Ours was actually an old quarry, right next to the road - I don't know if people were allowed to dump stuff? (probably not), but as kids we used to play there a LOT - we used to get old bikes and make up 'things' we could ride, and pram wheels etc. to make carts - lot's of wheels, and lot's of kids on them, riding down the 1 in 6 road

unclejed613 · Jan 8, 2019

norbss said:
I forgot to mention that my DC motor is connected to a speed reducer which turn a round rubber drum .

so there's a DC motor acting as a tach generator? it would be a simple way of doing it, but i would think something like that could be more easily done with lower voltages.... unless it was a situation where the company was swimming in motors.

Nigel Goodwin · Jan 9, 2019

unclejed613 said:
so there's a DC motor acting as a tach generator? it would be a simple way of doing it, but i would think something like that could be more easily done with lower voltages.... unless it was a situation where the company was swimming in motors.

I'd like to think it was doing something else, and not just feeding a tachometer?.

rjenkinsgb · Jan 9, 2019

It is quite common on industrial machines to have separate tachogenerators for either feedback, monitoring or both.
Some are built in to motors but others are quite a large separate unit, added to the end of a motor or mounted separately.

Newer ones tend to be something like 20V per thousand RPM but older ones were often 60V or possibly 90V per thousand.

eg. See the lower motor in this photo - the lump on the back is a tachogenerator.
https://media.exapro.com/product/20...81f197/462x340/stanko-2a622-i-p80622233_4.jpg

And they do often look pretty much like a motor:
https://shop.pagus.eu/images/product_images/popup_images/8832_0.JPG
**broken link removed**

edit - missing machine photo link added in...

Nigel Goodwin · Jan 9, 2019

rjenkinsgb said:
Newer ones tend to be something like 20V per thousand RPM but older ones were often 60V or possibly 90V per thousand.

Interesting to hear about that.

unclejed613 · Jan 9, 2019

i'm used to seeing tach sensors being either a toothed wheel and a pickup coil, or an optical sensor with either a toothed wheel or interrupter disk, but that's on small motors

rjenkinsgb · Jan 9, 2019

They are not so obvious on newer designs of motors; eg. a big DC motor like this:
**broken link removed**

typically has a tachogenerator something like this built in to the back end:
**broken link removed**

Is this also a DC voltmeter?

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