# Voltage to Frequency with phase controlled SCR (LM331?)

Thread Starter

#### shanemm

Joined Feb 24, 2017
22
I need to add torque control to a motor controller that already has speed control through a tach feedback signal. My thought is to use voltage feedback from the anode of the SCR's and scale it to the same signal level as the tach feedback. The tach ranges from 0 to 360Hz and 0-33Vrms AC signal which is fed to a LM2907 in the following circuit with tach signal coming in on A8 and A9 (ignore the writing):

The SCR circuit:

So my question is: Do you think I can take the voltage from A4 in front of the load coil and use something like an LM331 along with some op amps to generate the appropriate range of signals to feed into the LM2907? Or will the pulsed form of the SCR voltage make it incompatible with VFC's like LM331?

If this won't work my other idea is to use a Hall effect sensor to get an analog voltage and then manipulate it to get the appropriate waveform to feed into the tach circuit.

Any thoughts or comments would be appreciated.

Thanks,
Shane

Thread Starter

#### shanemm

Joined Feb 24, 2017
22
Alright so I got an ACS712 working (the ACS725 didn't have a breakout board...) and hooked it up in line with some motor leads. It gives me 2.5-5 VDC when I change the current. However, I'm not getting any frequency output from the LM331. This is my circuit with all of the cap/resistor values:

I calculate that I should get about 120-240Hz with this set up (from 2.5-5 VDC input and using the equation below the circuit diagram). I take my readings at the cathode of the 10k resistor on the output (pin 3). I'm not sure if maybe I need to hook up +5vdc at V_logic (where I am currently taking readings from) and then take my readings directly from pin 3? Or could it be that I need a high Vs? From that data sheet it looks like I can use anything for Vs up to 40V.

I've tried to find any resource online that isn't using the standard resistor/capacitor values in their LM331 circuit and I can't seem to find any. Eventually I'll need 0-360Hz output from the LM331 but right now I'm just trying to prove the concept out.

Thanks for reading

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#### neuhauser

Joined Mar 3, 2017
6
Looks like a 45/90v dc eddy-current clutch controller. Dynamatic or Louis-Allis?