"Inverse" VDR

[benta]

Hi:
I'm looking for a component that probably doesn't exist, but I'd like your input all the same.
Application is a tachogenerator for a motor with a wide rpm range.
The TG is driving two optocouplers with antiparallel LEDs for isolated mesurement (forget about the secondary side).

The problem is, that at low rpms the output voltage from the TG is only 3 Vpp, meaning that I need a rather low resistor value for the LEDs.
However, at max. rpms, voltage from the TG is 60 Vpp and the chosen resistor value pulls a lot of current from the TG.

Question is, does something like an "inverse" VDR exist, meaning a resistor the increases in value as the voltage over it increases?

Thank You,

Benta.

 

[nbucska]

Use a 100K and 10K voltage divider clamped with a diode
to GND. For 3 to 60 V it will give .3 to .7V .
the output of the comparator can turn on the LED.

Use hysteresis.


Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[itsmoked]

Very Nice benta. Thanks for the come back!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[benta]

Thanks Keith.
I have the opinion that a forum like this is only really valuable if the results of previous questions/problems are fed back into it.

Benta.