Frequency (=RPM) measuring via DMM

[domwild]

Hi,

Have a simple DMM which has two frequency ranges. The manual talks about using an "attenuator"; this may be Jinglish. Would I be able to get away with a few loops around one of the wires (phases) coming off the alternator to inductively pick up the Hz and therefore the RPM of the mill??

Am I right in saying that the DMM can only stand a few milliAmps without the magic smoke escaping?

One more question: There are 14 coils (poles) and I would pick up 14 kicks per one revolution or 14 Hz, by timing could I pick up the RPM that way??

Thanks.

[johnlm]

The meter will read the change from one magnets passage to the next, meaning the frequency you read per revolution will be the number of pole pairs (total magnets divided by 2), so the frequency is dependent on the number of magnets not the number of coils.  To get RPM take the frequency reading and multiply by 60 and divide by the number of pole pairs.  Example: if you have 14 magnets, you have 7 pole pairs. If you are reading 20 HZ (cycles per second) then (20 X 60)/7 = 171.43 RPM.

It does sound as if the input to your meter on the frequency setting is not to be used at high voltage.  Your suggestion of a wire loop may work.  If not run the (ac) output into a 1000 Ohm resistor with a 100 Ohm resistor connected to ground at the node that connects to the input to your meter.  This will "attenuate" or divide down the voltage coming out of the alternator by a factor of 10.

Johnlm

[Opera House]

Rather than just an attenuator, you should build a limiter.   Put a resistor in series and four back to back diodes to limit the voltage to about 1 volt.  That is, two diodes in series so the bands face the same direction and then two more diodes in parallel.  This will produce a pseudo square wave.   Resistor can be 100-1,000 ohms.  I would also place an .01 uF capacitor in parallel with the diodes.  This will reduce the pickup of RF from long leads that might drive the meter crazy.

[johnlm]

My mistake, actually the divider ratio of the above example is 11 to 1. Not 10 to 1 as stated. Opera's suggestion is actually a better more failsafe way to limit the output to no greater than a certain level.  One would think that if all the meter had was a very low voltage input range for the frequency input they would have put a back to back clamp inside the meter.

John

[ghurd]

Safe is good.  But all my meters with Hz readings (3?) have been connected directly across a 12v 7ah battery at around 14v and 360hz. They worked. No magic smoke.

G-

[domwild]

Thanks for all the comments.

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[domwild]

Thanks. Find it hard to understand how you calculated first 10 and then 11.

I need to explain it better. There are 42 coils on a F&P, one phase is 14 coils.

One magnet covers all three coils belonging to each one of the three phases, i.e. there are 14 magnets.

In one revolution one phase should show 14 pulses as each one of the 14

magnets has flown over each one of the 14 coils.

Are you right or am I wrong??