Pommie said:What was the question? I can't identify one.
Mike.
House0Fwax said:The way I see it is, there are two criteria;
1. The tachometer itself.
2. ( the big one ) the power supply.
I once took a 'drag cup' tachometer apart and stuck a reed switch near the spinning magnets. This signal I then fed to a very inacurate frequency counter that I'd made out of some 7 segment led displays, some 4026's a 4017 and a 555. Then I tried to calibrate it using a pcb drill.
Utterly futile attempt, but great fun.
... So that's part one sorted.
As for part two, all I can come up with is charging a capacitor and using some kind of regulation.
**Goes to back of class**
Indeed yes. Moving coil was what I was told to use. Low pass filters are far too finicky, and there is afar better way to achieve a defined pulse...JimB said:OK now that you have defined the indicator as a moving coil meter, my first idea would be to rectify the 3-phase signal, giving a pulsing DC at 3 times the original signal frequency.
That signal can then be used to drive a pulse shaper giving a defined pulse width and amplitude.
Low pass filter the pulses to drive the MC meter and the deflection will be proportional to the frequency (and RPM).
The part was actually a 2" OD cockpit can. Honestly, I am not trying to be obtuse, I just did not count on so many different directions. It actually had to be in calibration at 2-80RPM as it replaced existing units in thousands of cockpits.Generating the power for the pulse shaper is a bit more problematic at low speeds.
Does the indicator really NEED an 80:1 turn-down ratio? ie if fullscale is 8000rpm, do we really need a sensible indication at 100rpm? or, can we make the bottom end of the scale say 200 or 300 rpm where we will probably have plenty of volts to use for a supply?
As I recall I did indeed use 200V germaniums since HV schottkys did not exist at the time...At 2v peak, when we rectify, we will lose two "diode drops" from the supply, so we would have to use germanium or schottky diodes to maintain as high a supply voltage as possible.
So we need to make our pulse shaper (probably a monostable multivibrator type circuit) run reliably from about 1.5 volts.
We could use a step-up transformer to give us some more volts to play with. But the problem here would be with the low frequency, it would need a lot of iron to work at 1hz, but we don't need to transfer a lot of power because our circuit is inherently low power so we may get away with a reasonably sized transformer.
JimB
I was thinking in terms of a simple RC network, probably not needed anyway as the MC meter is inherently a low pass filter.cadstarsucks said:Low pass filters are far too finicky,
Ok then, what is it?cadstarsucks said:and there is afar better way to achieve a defined pulse...
Some zeners to limit the high voltage would be a reasonable idea, but we need to be aware of the series resistors which will steal some of our supply volts at low RPM.cadstarsucks said:...but nothing would stand up to 100V...
JimB said:I was thinking in terms of a simple RC network, probably not needed anyway as the MC meter is inherently a low pass filter.
Ok then, what is it?
Some zeners to limit the high voltage would be a reasonable idea, but we need to be aware of the series resistors which will steal some of our supply volts at low RPM.
We could use a modified "series regulator" where the pass transistor is biassed hard on at low input voltage and turns off as the supply voltage rises.
JimB
Ok...House0Fwax said:Yes please.
Ah that makes sense.Hero999 said:I don't think anyone noticed that you edited your post.
Actually the silicon was, the part I use was the same die in a TO92 package, but they appear to have discontinued that variant.My only question is what sort of current did your circuit end up drawing?
I'm much too young to remember from first hand experiance but I've been told that old MOSFETs can't dissipate much power and I'm certain the MOSFET in the datasheet you posted wasn't around back then.
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