A pcb we've been asked to look at has two 15k 1W resistors in parallel with a 15A AC motor. Silly as it sounds, we're a little stumped why they're there. It's probably bloomin' obvious, but we can't settle on an answer. I don't get out much and certainly don't associate with the wild AC motor crowd. So...braking maybe? Some kind of protection for when the circuit breaker opens? Or to keep racket away from the micro side? Or just decorative? They do look fun and festive.


simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Aren't they too high for braking? \$\endgroup\$
    – DKNguyen
    Feb 21 '19 at 21:56
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    \$\begingroup\$ At 15k, those resistors won't do diddly for braking. More likely to discharge any capacitors on the motor when you shut off the power. But, you don't show any capacitors so ... Hmm. \$\endgroup\$
    – JRE
    Feb 21 '19 at 21:58
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    \$\begingroup\$ I'd also suspect capacitors. The motors is a black box and many single phase AC motors types a phase shift capacitor for either start or run, either of which would be in-circuit at rest. \$\endgroup\$ Feb 21 '19 at 21:59
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    \$\begingroup\$ 15A at 120VAC is way more than a couple of one watt resistors could handle - and to do braking you'd need to handle nearly as much power as you use to drive the motor. And, if they were for braking they'd only be switched in when you want to stop or slow the motor - not full time like yours. \$\endgroup\$
    – JRE
    Feb 21 '19 at 22:01
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    \$\begingroup\$ Two of them because 30k across 120VAC would be too close to the 1 watt rating for a single resistor. \$\endgroup\$
    – JRE
    Feb 21 '19 at 22:03

I'm going to summarise my comments into an answer.

  1. 30k is not going to do any good for braking. To brake the motor, you would have to be able to absorb about as much power as you put into the motor. 30k as a load is a teensy tinesy fraction of the driving current. Besides which, you would want to switch a braking load into the circuit to stop, and out to run. There are no switches or relays in your circuit for braking. Also consider: The motor takes in over 1500 watts when running. How much of that could 2 one watt parts absorb?

  2. Since the resistors are always in the circuit, they are continuously dissipating power. 30k at 120VAC is about 0.5 watts. They probably split it into a couple of 15k, 1 watt resistors so as to keep them from running too hot.

  3. The only thing that makes sense is for those resistors to be used as a discharge path for capacitors in the motor. Without the resistors, any capacitors in the motor that are across the 120VAC input can stay charged after you shut off the motor. That can give you a nasty shock if you touch the motor connections - even though the power is off. Universal motors like the ones in kitchen mixers have capacitors across the AC supply to reduce intereference - and they are required to have a resistor in parallel to bleed off the charge. Larger motors like yours may have a start and a run capacitor. These can also remain charged when you shut off the power, making a bleeder resistor also be a good idea for these motors.

Bleeder resistors are usually in parallel to the capacitors rather than being external in a controller board. So, yours aren't really in the place you'd expect them to be.

But, its the only usage that makes sense.

Maybe the motor itself doesn't have bleeders, and the folks designing the controller decided they'd rather have a bleeder after all.

  • \$\begingroup\$ Bleeding makes sense, but there are no capacitors. The "microcontroller and other discrete stuff" is a mess of over-complication and unnecessary parts. It looks like maybe some are due to over-caution, but some look like circuits lifted out of something else and shoehorned in. Wondering if they think the resistors prevent spikes from being passed to the linear transformer and discrete components when the motor is switched off (there's a relay after the circuit breaker to switch the motor on/off). Or they were on a design with motor caps and they just added "because". \$\endgroup\$
    – Drone601
    Feb 22 '19 at 13:53

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