# RPM meter - how does it work?

Discussion in 'General Electronics Chat' started by EB255GTX, Jul 10, 2016.

1. ### EB255GTX Thread Starter Active Member

Apr 30, 2011
54
2
HI folks,

i am looking for a nice big analog meter for a project, i want to run it from DC volts - the exact magnitude is not too important as i can scale either the meter series resistor (assuming a standard permanent magnet voltmeter) or the voltage itself with circuitry if needed.

I see this nice big meter on ebay, the question is- what is the input to an RPM meter likely to be? Yes, i have asked the seller but no response yet and they may not know anyway.

I know automotive RPM meters generally take a pulse - if that's the case, is the meter RC filtering the pulses to get a DC level and then the mechanism is just a standard voltmeter or is there a more complicated mechanism around?

The seller does say int he listing "can be used as a standard analog meter"

thanks for any tips!

2. ### KeepItSimpleStupid Distinguished Member

Mar 4, 2014
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511
It could very well be a basic movement. The deal is with a series resistor, you get a DC voltmeter. Shunts combines with a DC voltmeter give you an ammeter.

50 uA FS used to be a standard. You really won't know until you take it apart.

Without any resistors, it's a basic movement, In order to find what resistor you need to deflect full scale, you have to measure the resistance INDIRECTLY. Use a small battery, a potentiometer (set at max resistance). You;ll have to use trial and error to use the potentiometer to deflect exactly 1/2 scale. Also measure the voltage at 1/2 scale. Remove the resistor and measure it, It will be the resistance of the meter movement.
A=2*V/R(pot) will be the FS current of the meter.

From those numbers, you can make it read voltage or current with external resistors.

3. ### EB255GTX Thread Starter Active Member

Apr 30, 2011
54
2
Thanks for the info - it will come in useful if i buy the meter and adapt it for my project

What do you think the chances of it being a normal meter movement are - it's the fact that it is an RPM meter that is worrying me, i don;t want to spec \$100 and find that it only works on pulses or something....

Hopefully the seller responds and it's just a normal voltmeter-i don;t care what the scale says as i will replace that with a custom one but my driving circuit can only output a variable DC level (filtered PWM from a micro).

thanks!

4. ### Sensacell Moderator

Jun 19, 2012
2,017
633
Since there is really no such thing as a "standard RPM meter" I would be wary of buying it without concrete technical specs.
Since you are replacing the scale anyway- just buy a standard analog meter? Then you know what you are getting.

5. ### KeepItSimpleStupid Distinguished Member

Mar 4, 2014
2,919
511
He did say "tested", so he had to test using something, right?

The old tach/dwell meters had that sort of movement.

There should be plenty of resources for calculating the series resistor needed for a particular movement. Finding the resistance value of the movement was well known in my day as a kid. 50 ua, 1 mA and -1-0-+1 mA were common "movements".

6. ### EB255GTX Thread Starter Active Member

Apr 30, 2011
54
2
Because those ones are frickin' cool looking! I haven't seen anything similar out there and i want a real retro look

Jul 18, 2013
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Check out the LM2917 IC digital to analogue (moving coil meter).
Max.

8. ### #12 Expert

Nov 30, 2010
18,082
9,650
I'm here to tell you that you don't have much to worry about. The age of those meters tells me they aren't crammed with digital processors. In addition, a pulsed input will be averaged by the mass of the meter movement. Just be really careful about testing it to find its specs. These are in the microamp range for current. I'm thinking a 1.5V battery with 30 thousand ohms in series with it will cause enough current to be in the danger zone. Try figuring for 25 ua, then sneak up on that amount of current. The answer might be as high as 1 milliamp, but don't bet on it. Start with 1% of 1 milliamp.

EB255GTX and JWHassler like this.
9. ### EB255GTX Thread Starter Active Member

Apr 30, 2011
54
2
So -

I purchased the meters, and they work just fine for what i want to do i.e. 0-5V by way of PWM from a micro.

interesting thing, they have a small (in value) inductor internally as per the attached pic. Even with the inductor shorted, the meter movement is quite damped, i am not aware of physical means to do this, does anyone know of some trickery in this area e.g. some sort of damped permanent magnet meter movement?

Its not physically binding, the needle moves freely and there are two identical meters that behave the same way.

Very interesting....

Anyway i post this comment in order to followup - thanks to all who replied

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10. ### jpanhalt Expert

Jan 18, 2008
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Is the meter movement damped with the inductor out of the circuit (open)?

John

11. ### Techno Tronix Member

Jan 10, 2015
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Sine wave. max. frequency 10 kHz
Square wave. max. frequency 30 kHz
Contact closure. max. frequency 20 Hz
Open collector with 20 μsec minimum

12. ### ci139 Member

Jul 11, 2016
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... tere are also too many ways to frequency to voltage

13. ### BillB3857 AAC Fanatic!

Feb 28, 2009
2,461
378
Damped meter movements, at one time, simply used an aluminum bobbin to wind the meter coil on. The aluminum, being conductive, acted as a shorted turn. When moved in the magnetic field of the meter, the current created in the bobbin set up an opposing field which damped the movement.

GopherT likes this.
14. ### dannyf Well-Known Member

Sep 13, 2015
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There is typically a spring that pretorques the needle. The needle moves but not completely freely.

15. ### wayneh Expert

Sep 9, 2010
14,970
5,439
There is also liquid damping, but probably not in a tachometer.

16. ### debe AAC Fanatic!

Sep 21, 2010
1,102
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Looking at the picture of your meter, its just a analogue meter. I have built several Tachos using the IC, LM2917 as suggested by Max. They only use a few parts & simple to get going & work great..

17. ### EB255GTX Thread Starter Active Member

Apr 30, 2011
54
2
Hey.... might be something along these lines-thanks for the tip!

18. ### ian field AAC Fanatic!

Oct 27, 2012
6,252
1,131
There was the TI SN76810 speedo/tacho chip - but its getting hard to find even archived datasheets nowadays.

Basically you feed a moving coil meter with a gated constant current source. That is gated by a fixed on period monostable.

The pulse shaper depends on what pickup transducer - you just want narrow (spike) pulses to trigger the monostable.

19. ### ian field AAC Fanatic!

Oct 27, 2012
6,252
1,131
The speedo company I worked for manufactured their own meter movements, damping was a drop of thick silicone oil on the pivots.

Using an off the shelf moving coil movement - a big fat foil capacitor across the terminals is much easier.

Not a bad idea to make sure its discharged first though!