Retro-fit speed control to battery grinder?

[Robert Atkinson 2Participant @robertatkinson2]

Can't see that video without registering. Not going to do that…

[Pete RimmerParticipant @peterimmer30576]

Posted by Robert Atkinson 2 on 20/11/2022 19:29:30:

Can't see that video without registering. Not going to do that…

Oops my bad I didn't realise I had to give it an 'all audiences' rating. Should be OK now

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Pete Rimmer on 20/11/2022 19:39:10:

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Should be OK now

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Works a treat, Pete

MichaelG.

[Pete RimmerParticipant @peterimmer30576]

Posted by Michael Gilligan on 20/11/2022 20:24:53

Works a treat, Pete

MichaelG.

Thanks Michael. I found out if I don't rate the video as safe you have to log in to view just in case it's unsafe for kids.

I've had a lot of fun doing this conversion this weeekend, had to make a spacer plate, a new 6mm dia 6 tooth drive pinion for the motor and matching 38 tooth plastic gear plus a load of measuring and fitting up but it's worth it to recover the use of an old Biax.

[Pete RimmerParticipant @peterimmer30576]

Well, I ended up junking the soft start and bought a couple of cheapo 15a speed controllers from Amazon (£13 for two). I stripped down my spare grinder and wired the motor into the speed controller, it ran perfectly at all speeds. The only problem was, the circuit board is much too large to fit inside the grinder housing.

So I removed the heat sinks and ran the test again, it still ran fine. After that I removed the terminal block and cut the board down to a size that would fit.

And this is where it all went wrong. I installed the board and hard-wired the connections by soldering. Somewhere along the way I messed up and when I test-ran the motor it blew the internal fuse in the battery.

No problem, I have two so I fitted a new fuse in the battery and wired in the second unit. That too worked perfectly so this time I carefully un-solderded the heat sinks and instead of hard-wiring the lead sI soldered the terminals I had removed from the first board onto the second one and once again wired in the motor. This time it did nothing until I turned up the speed pot at whic time it sprang into life at full speed and I found that using the speed controller pot had no effect.

So it seems that either I am cooking components whist removing the heat sinks (unlikely as I was un-doing the tab screw first) or when soldering tabs on the board, or it's simply that the first test without a heat sink was fine but the transistor was on it's way out already and the second test finished it off.

I'm not done yet, I have ordered a different style of speed contoller which hopefully I will be able to shoe-horn into the casing with much less modification.

[Michael GilliganParticipant @michaelgilligan61133]

Nice try, Pete

The moral of the story being … if the manufacturer of a product made ‘down to a price’ feels obliged to fit heat-sinks, it’s a fair bet that they are necessary.

At worst … you could try using heat-pipes, to put the fins somewhere more convenient.

MichaelG.

[Pete RimmerParticipant @peterimmer30576]

True that, but I figured that if the single soft-start fet that was on the grinder originally managed without a heatsink then maybe these would too 😀

Truth is I could probably keep two of the three so if this other-design controller doesn't work out I'll buy a couple more and keep experimenting. I can keep two of the thhree sinks I think and cut one side off the third, it just makes situating the speed control pot a little more tricky. At the death of it I could simply 3d-print an extension housing and put it in there.

[SteviegtrParticipant @steviegtr]

I fitted a speed controller to the x axis power feed mondeo wiper motor on the milling machine. It did not work well, until i realised it was current limiting not voltage. It was 20A unit. I removed this & fitted a lower i think 10A unit & now it controls the speed perfectly. The 1st one looked very much like the one in your picture.

Steve.

[Pete RimmerParticipant @peterimmer30576]

Continuing on with this I bought a different speed control unit comprising of a pair of RU6888R mosfets, a LM517 voltage regulator, NE 555 timer and MBR20100 shottky diode along with various reisitors, diodes and capacitors. I trimmed back parts of the PCB to make it fit, removed the Shottky diode and hung it on a glying lead so I could re-locate it, shoe-horned it into the casing and was well pleased. After hot gluing it all into place and buttoning it up, the unit ran perfectly at all speeds one time then the next time fired up at full speed and now runs flat out regardless of where I set the speed pot.

One of the original controllers did this same thing and I found one leg cracked off the Mosfet, I suspect that this one too has blown the FETs too though I can find no apparent sign of distress on any of the components. For sure the voltage regulator is working and the diode must be intact because all of the power goes through it. Here's the unit, beofre I attacked it to fit:

Getting a bit frustrated now due to my lack of electronics knowledge. I wish I knew what was killing off these mosfets I bet it's something very basic.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Pete Rimmer on 07/12/2022 19:16:05:

[…]

though I can find no apparent sign of distress on any of the components […]

.

I’m perfectly happy to be wrong, Pete … but those PCB tracks look like they have been rather hot

MichaelG.

[Pete RimmerParticipant @peterimmer30576]

Trick of the light Michael. The soldering is dire but that track is shiny and smooth in good light. Not that it matters since I chopped the corner of the board off and ran wires to that component

[Pete RimmerParticipant @peterimmer30576]

Well, to wrap this up I have to say that I have abandoned the idea of retro-fitting a speed control and headed off on an entirely different tack.

So far I had spent £15 on a grinder body (not including the battery), £20+ on various Chinese speed controllers all of which failed plus quite a few hours work. Instead I have chosen to use a grinder body which already has speed control built in. So I bought one of these Katsu Fit-Bat grinders from Amazon for £34. They can use Makita batteries which are about the most plentiful, or the cheap copies of those.

Another bonus is that the new grinder body is much more ergonomic to handle and a much cleaner fit to the existing scraper front end.

I had to make a new pinion to suit the Biax and turn out the armature on the grinder motor, but that was not a difficult task.

Thanks for all the constructive suggestions from all who replied.

Pete.

 

Edited By Pete Rimmer on 11/12/2022 12:55:44

[Michael GilliganParticipant @michaelgilligan61133]

Well-played, Pete

MichaelG.

[Pete RimmerParticipant @peterimmer30576]

Well this is all redundant now because th project is happily finished and working like a charm on a different type of power unit, but I do find it ironic that Big Clive has done a video this week where he reverse-engineered this very circuit board for his youtube channel. Not that it would have answered my question, but it would have 1) saved me asking it and 2) informed me that what I was hoping to do was hopelessly beyond my capabilities

Here's the link, if anyone is interested/bored enough to want to watch it.

Edited By Pete Rimmer on 29/12/2022 22:07:20

[Michael GilliganParticipant @michaelgilligan61133]

I was sufficiently interested/bored to watch it, Pete … and am very impressed !

I’m quite sure that in some markets, that cleverly-designed and well-constructed controller would cost more than you paid for the machine.

MichaelG.

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