# Background:

I recently bought a Scr Voltage Regulator (10000w Ac110v 220v 75a) to act as a motor controller for a hobby project (Figure 1). After taking it apart, I found the Voltage Regulator is controlled by a 220k Ohm 2W Potentiometer (Figure 2).

Figure 1: SCR Voltage Regulator

Figure 2: 220 K ohm 2W Potentiometer

# Question:

I'd like to replace the mechanical Potentiometer with a component I can control with a microcontroller. I found the ADN2850 However, this seems like it can only support 1 W.

• Is there another type of chip on the market I should be looking at?

OR

• How would I build a circuit that allows me to use a smaller control current to regulate the speed regulator?

You can't do this easily and should not attempt it. You could easily hurt yourself or damage your computer.

You need a power controller with a DC voltage input that is isolated from the mains. It's far easier to design something like that from scratch than to try to retrofit to a non-isolated phase control. That pot, for example, will be subject to hundreds of volts in operation, far beyond what any digital pot can withstand.

If you connect your Arduino directly to a non-isolated circuit there's a good chance it will be sitting at 120VAC relative to ground and could electrocute you or blow your PC's motherboard to smithereens via the USB programming port.

• Ok, how would I design a power controller with a DC voltage input? – GentlemanS Jul 2 '18 at 14:10
• @GentlemanS You should probably just buy one. You can find designs which use optoisolators to transmit the zero-crossing and opto-isolators to transmit the signal to the thyristor with a microcontroller or other circuit inbetween, but I'm afraid that is too broad a question for this forum (and there are safety concerns). – Spehro Pefhany Jul 2 '18 at 14:12
• – Spehro Pefhany Jul 2 '18 at 14:21
• Thanks for the information. Could you tell me more about why digital pots can't withstand this voltage? Additionally, If I power the Arduino and the digital pot through a different circuit, would that keep my computer save? – GentlemanS Jul 2 '18 at 15:03
• Digital pots can't have any voltage on their pins that exceeds their own supply voltage rails. There needs to be galvanic isolation for safety. – Spehro Pefhany Jul 2 '18 at 15:27

The easy way would be to replace the potentiometer with a motorized pot, and then control the pot with the arduino, being careful to separate the power of the arduino and your SCR.

The real easy way would be to buy the controller you need.

1. Not possible

• Totally incompatible interface VI ranges for a digital low voltage DC pot to a high voltage and power pot for AC biasing 240Vrms SCR phase trigger. (not even close)
2. How would I build a circuit that allows me to use a smaller control current to regulate the speed regulator

Buy a proper pot or PWM controlled full bridge FET driver and supply rated for >5x max load current to handle start/stop controlled acceleration surge current

\$ or cheaper maybe to reduce input voltage with a transformer to reduce speed. 10kW and 10hp motor has high energy storage than must be regulated efficiently.

The most effective means is to buy a VCD motor driver. With constant V/f speed control with high torque current.

However what might be possible with design experience is to use a high current phase controlled current pulse and Pulse Transformer from a driver that can generate such a high current (<1A?) pulse and be immune to interference. But this becomes like a PWM current control to the motor which weakens torque at low RPM from low duty cycles.

This is very sub-optimal for many applications. But unless the entire energy, torque, line f , RPM and reaction times are stated in a design, there will be many issues.

• Thanks for the info. Could you explain to me a bit more about why this isn't possible? What is it about changing the control system from a manual knob to a digital input that makes this impossible? – GentlemanS Jul 2 '18 at 14:49

If this is hobby related and not a professional unit, then I'd suggest you could build your own motorized input to the existing pot very cheaply.
Replacing the pot in your existing unit (assuming it does what you want when used manually, is full of hazards. The likelihood is that the pot is at a mains voltage (deadly), so interfacing this to a low voltage MCU as a remote activation provider would be difficult.

You might consider using something like this servoblock which would be easy to fit to your existing pot shaft:

Most RC servos have an output radial capability of 180deg, which would give most of the coverage of the pot (likely 270deg). It would then be simple to drive the RC servo from an MCU giving you remote control of the unit.