Electronic – Single quadrant analog multiplier

analogmultiplier

I wish to control a programmable power supply to deliver constant power (watts) into a heater. The heater resistance varies from unit to unit due to manufacturing tolerances.

  • The PSU has a 'programmable' or remote setpoint input. 0 – 10 V gives zero to maximum voltage.
  • The PSU has two feedback signals – one for output voltage and one for output current. These are 0 – 10 V signals scaled zero to maximum.

I'm wondering about the following approach to provide a solution.

schematic

simulate this circuit – Schematic created using CircuitLab

  • The analog multiplier MUL1 gives an output proportional to the product of the voltage and current.
  • Op-amp OA1 compares the power level feedback with the power level setpoint (R3) and adjusts the voltage setpoint until the output power equals the setpoint.
  • R1 and R2 set the gain of the response.
  • C1 provides some filtering to reduce noise.

I only need a single quadrant multiplier as both feedback signals are positive. This raises the hope that I could find a single (positive) supply multiplier chip that with an output range of 0 – 10 V.

Q1: Are there any obvious problems with this approach?

Q2: I can't find a single-quadrant multiplier. (I've used the AD633 four-quadrant multipliers before.) Does anyone know of any?

Update 1

  • The PSU voltage setpoint has an RC filter with protection zener on it. R is 1k and the time constant is about 1 – 2 ms so C is about 1 uF.
  • The heater will be turned on for a second or so every five or six seconds.
  • A power supply rise time to 95% voltage of 50 ms or so would be adequate.

Best Answer

As others have said you may get an unstable situation so, temporarily forget about the multiplier and check to see that you can control the voltage output of the power supply based on voltage feedback directly to the op-amp. Use your pot to check that you can control it across the levels you might need with the load resistances you might use.

For a simple and fairly accurate multiplier you could turn the current feedback signal into a PWM signal - duty cycle proportional to output current. Then, use the PWM signal and a couple of analogue switches to chop up the voltage feedback signal. Use a filter on the output to give you a dc value proportional to power. Try and use a fairly high PWM frequency so that the extra low pass filter can run at tens of kHz.

Linear tech make the LTC6992 that would suit your needs: -

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It takes a dc input voltage (aka current reference) and converts it to a duty cycle of up to 1 MHz frequency. Here's the idea applied to a triangle wave and note that this is a 4 quadrant multiplier but who cares - it operates in the first quadrant just as well as the other three: -

enter image description here

If the duty cycle were more like 99% the resulting red triangle wave would be much fuller in amplitude and just fractionally less than the original triangle wave.

Finally the block diagram would look like this: -

enter image description here