oddball question for the electronics guys

So I picked a table top band saw the other day got home plugged it in and turned on. It ran like crap so I pulled the motor cleaned and recut the comm cleaned the brushes and slammed it back in. Ran better but still rough at higher speeds. Pulled the board and long story short it got fried. The board has a few large diodes, a lot of small diodes and resistors a quad op amp and 2 thyristors. I could probably go through and find and replace the bad parts but was thinking it might be easier to do something else. Would I not be able to just use a dimmer (the power label says it operates at 2.3A, 5A dimmers are easy to find) and a full bridge rectifier?

What band saw has a circuit board

If it's a pcb board, it's usually going to be sold as entire unit and would be easier to replace it just as a whole.

That said, you could probably just wire in a switch and call it a day.

That seems like an awful lot of circuitry for a DC motor.
How many big diodes?
Can you post a pic of the board?

Does the motor have a part number of on that'll give you the voltage spec?

If I could buy a board that would be the easy fix but parts are no longer avail. The motor is DC so just throwing a switch on it would not work. Thank you for your input Bob but if things need to be explained you probably shouldn't answer.

You're right, there's no reason I, or anybody, would need more information to tell you whether you can operate it safely without that PCB.

canibalpenguin said:

If I could buy a board that would be the easy fix but parts are no longer avail. The motor is DC so just throwing a switch on it would not work. Thank you for your input Bob but if things need to be explained you probably shouldn't answer.

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Right. You gave us so much information to go on... I mean, god forbid I assume itd a pcb circuit for the safety switch....

Great info and attitude buddy

Bridge rectifier and heavy duty light dimmer.

NiHaoMike said:

Bridge rectifier and heavy duty light dimmer.

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What voltage, what amperage, is the interlock part of the circuit, what size capacitor output, etc etc.

More to this than throwing parts together. Sure would be nice if he'd give a model with this kind of information.

Manufactures don't tend to waste money on unnecessary parts, i would recommend replacing like for like if at all possible. Shortcuts tend to cost more in the long run.

If the band saw controller uses thyristors, whyis a bridge rectifier needed, except to power the electronics on the controller?

Why are 2 thyristors used, instead of 1 bigger thyristor? I hope they don't operate in parallel because I've heard any mismatch between them can lead to destruction of both. If you put in a bigger thyristor, be sure the gate current rating is about the same because thyristor come in regular and sensitive gate versions. It's not unusual for thyristors to be rated 600-1000 volts peak, just to withstand spikes from the motor. Also there may be a snubber network for those spikes, and it may have blown because they're sometimes made with a capacitor that can't stand the full voltage or a resistor that can't stand the power, and when a snubber blows out, other components may be damaged.

Unlike light dimmers, some motor speed controllers actually measure the motor RPM, either through a sensor or the motor back voltage.

If this is a Harbor Freight product, you may be able find the schematic at their website. And if it is something like a Harbor Freight product, it may be wise to replace the original power components with higher rated parts because some may be undersized. For example, my 1500W HF heat gun uses a diode to get the lower temperature setting (very common design), but the diode is rated for just 5 amps continuous, despite 8 amps flowing through it, and this product actually received UL approval. OTOH my ancient American made Sears Craftsman 6 amp battery charger (by Schumacher) uses a thyristor rated for at least 25 amps.