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
    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
    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
    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).

  4. Sensacell


    Jun 19, 2012
    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
    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
    Because those ones are frickin' cool looking! I haven't seen anything similar out there and i want a real retro look :)
  7. MaxHeadRoom


    Jul 18, 2013
    Check out the LM2917 IC digital to analogue (moving coil meter).
  8. #12


    Nov 30, 2010
    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
    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 :)
  10. jpanhalt


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

  11. Techno Tronix


    Jan 10, 2015
    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


    Jul 11, 2016
    ... tere are also too many ways to frequency to voltage
  13. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    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
    There is typically a spring that pretorques the needle. The needle moves but not completely freely.
  15. wayneh


    Sep 9, 2010
    There is also liquid damping, but probably not in a tachometer.
  16. debe

    AAC Fanatic!

    Sep 21, 2010
    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
    Hey.... might be something along these lines-thanks for the tip!
  18. ian field

    AAC Fanatic!

    Oct 27, 2012
    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
    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!