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Help - Mag Chuck Rewire - Sanford Benchtop SG48 Surface Grinder


Nov 19, 2021
Hey Guys,

I picked up a Sanford SG-48 Benchtop Surface Grinder. It has the original Mag Chuck, but the wiring was taken apart by the previous owner. I have done some research and found a upgrade with a wiring diagram and components.

Full Wave Bridge Rectifier - 250PIV, 10A

Resistor - 35 Ohm, 50W

Capacitor - 1000uf, 250V DC

Sanford Wiring.JPG

My MAIN question... I want to know if its possible to go a step further... and add a "Variable Holding" setup, like the link below.

SCV Series Electromagnetic Chuck Control - Walker Magnetics

Is it as simple as adding a potentiometer to the above setup? I can follow the diagram, but am not a electrical expert when it comes to adding things. Seems easy to a simple minded fellow lol.

Thanks for your time and help!
What's the wattage on your mag chuck? You'll need to get one big enough to handle that.

I am not sure, there are no markings. Any way to test? I am not great at electrical.

Would the 10A from the rectifier and 50W from the resistor be the maximums? Anyway to size it off of those?

Full Wave Bridge Rectifier - 250PIV, 10A
Resistor - 35 Ohm, 50W
There probably is a way to calculate it but the easiest way would be just to build your power supply circuit and see what current it draws when it's fully on. The voltage coming out of the rectifier will be rippling up and down a bit. The capacitor will smooth that out somewhat but I'm not sure to what level exactly, it's been a while since I've messed with calculations for one of those circuits. The voltage that it levels out at times the current that the resistor limits it to will be your wattage. However, you don't really need to concern yourself with that if you don't want to - you can just measure what current the 240VAC circuit draws from the wall while the magnet is on and use that. I see you are using a 240VAC (or whateverish) circuit, I would be surprised if you need even a 250V, 5A Variac, don't know if they even make smaller 250V ones. Bigger won't hurt you, just will be excess of what you really need.

After you build the circuit just replace your 240VAC input to the rectifier with the Variac. (240VAC in to Variac, Variac out to rectifier). Make sure the Variac you use is rated for enough amps to handle the max load. Varying the AC input voltage with the Variac dial will vary the DC output and the strength of the magnetic field.