Ryobi trimmer mod

Thread Starter

Jw76

Joined Apr 12, 2020
2
My Ryobi trimmer got wet a couple years ago and I was wondering if I could replace the speed control board with a toggle switch. The battery tests fine so this is my best guess on the problem.
 

Thread Starter

Jw76

Joined Apr 12, 2020
2
Thank you for the answers. I tried it and it still didn’t run so now I’m wondering 81F8B7A6-7C8C-462D-81A7-45918FC5A971.jpeg3C70FA4C-67BE-4854-9057-70C02D820E6A.jpegif the motor is the problem. That’s usually the last thing I suspect and I can’t for the life of me find my multimeter. Anyway here is the speed control...
 

MrAl

Joined Jun 17, 2014
7,891
Hi,

Based on my experience in the past with Ryobi i would say bypass the entire product by throwing it squarely into the garbage can and going out and purchasing the same product by just about any other manufacturer.
:)
 

MisterBill2

Joined Jan 23, 2018
7,728
UNtil we can see the motor and the motor connections there are only guesses. I did miss the M+ and M- markings until Max mentioned them. With two ICs and at least three power devices there is a lot happening on that circuit board. And it may be that the motor itself has failed for some reason. OR the battery may be totally dead. That does happen. And since I see no battery charging connection that will be an issue as well.
 

Dodgydave

Joined Jun 22, 2012
9,864
Looks like the Motor is Negative driven with two Mosfets using a Common Source mode, possibly pwm controlled to give a slower starting, you could link the motor across the battery with a heavy duty switch,..
 

ebeowulf17

Joined Aug 12, 2014
3,275
Looks like the Motor is Negative driven with two Mosfets using a Common Source mode, possibly pwm controlled to give a slower starting, you could link the motor across the battery with a heavy duty switch,..
I'm intrigued by the layout.

It looks like one MOSFET is in parallel with the motor, with drain and gate tied together. I'm guessing that's there as a flyback diode for back EMF, as there appears no way to turn it on, nor any good reason to turn it on.

The next two MOSFETs appear to be in series with each other. I'm not sure why they're both needed. I noticed there's a shunt resistor - is it possible that one MOSFET is configured as a current limiting device and the other works in a simple on/off (possibly PWM) mode?

I can't trace enough of the circuit to understand why there are two MOSFETs in series, but I'm very curious about it.

Regardless of my above curiosity, I agree that the board is not a BLDC controller, and that the motor could simply be switched with a suitably robust mechanical switch.
 

MisterBill2

Joined Jan 23, 2018
7,728
OK, it is not a BLDC motor controller. For many devices with any capacity to damage errant fingers there is a braking function so that when the switch is released the motor goes into a panic stop mode with terminals shorted. At least my last 3 AC powered mowers had that feature until I removed it. Full dynamic braking puts a large electrical stress on the motor, much more that the across the line start does. So it is a good feature to remove. Just remember to never stick your hand in a running mower.
Of course some fools will do that, it seems.
 

Dodgydave

Joined Jun 22, 2012
9,864
I'm intrigued by the layout.

It looks like one MOSFET is in parallel with the motor, with drain and gate tied together. I'm guessing that's there as a flyback diode for back EMF, as there appears no way to turn it on, nor any good reason to turn it on.

The next two MOSFETs appear to be in series with each other. I'm not sure why they're both needed. I noticed there's a shunt resistor - is it possible that one MOSFET is configured as a current limiting device and the other works in a simple on/off (possibly PWM) mode?

I can't trace enough of the circuit to understand why there are two MOSFETs in series, but I'm very curious about it.

Regardless of my above curiosity, I agree that the board is not a BLDC controller, and that the motor could simply be switched with a suitably robust mechanical switch.
One is a Dual Diode, the two in series are Mosfets.
 

MrSoftware

Joined Oct 29, 2013
1,959
OK, it is not a BLDC motor controller. For many devices with any capacity to damage errant fingers there is a braking function so that when the switch is released the motor goes into a panic stop mode with terminals shorted. At least my last 3 AC powered mowers had that feature until I removed it. Full dynamic braking puts a large electrical stress on the motor, much more that the across the line start does. So it is a good feature to remove. Just remember to never stick your hand in a running mower.
Of course some fools will do that, it seems.
Not to get off topic; but two stories we were told in the engineering class about liability; (1) the guy who tried to trim his hedges by picking up his lawn mower and using it like hedge clippers. Of course he lost his fingers as soon as he wrapped them around the deck to pick up the mower, but he sued and won. This was the days before releasing the handlebars stopped the blades, and probably a contributor to that new feature. (2) a woman who would bathe her tiny dog then put in on the door of an open oven to dry him off faster. Well, when microwaves first came out she got one and tried to dry the dog in it. Poor poor dog.. but she sued and won. It's so hard to make things idiot proof, the idiots just keep getting better and better.
 

MisterBill2

Joined Jan 23, 2018
7,728
Not to get off topic; but two stories we were told in the engineering class about liability; (1) the guy who tried to trim his hedges by picking up his lawn mower and using it like hedge clippers. Of course he lost his fingers as soon as he wrapped them around the deck to pick up the mower, but he sued and won. This was the days before releasing the handlebars stopped the blades, and probably a contributor to that new feature. (2) a woman who would bathe her tiny dog then put in on the door of an open oven to dry him off faster. Well, when microwaves first came out she got one and tried to dry the dog in it. Poor poor dog.. but she sued and won. It's so hard to make things idiot proof, the idiots just keep getting better and better.
Indeed it is amazing. And the courts all seem to do their very best to fight natural selection. Nature can certainly be brutal. My feeling is that the folks should be fined for being so stupid.
 

MisterBill2

Joined Jan 23, 2018
7,728
Not to rain on anyone's parade, but those both appear to be urban legends.

https://www.snopes.com/fact-check/the-microwaved-pet/

https://www.snopes.com/fact-check/hedge-fund/
Possibly, but also, you would not believe the levels of stupidity that I have seen and read about in the news. One example was weed killer sprinkled on pastry because it was the same color as the sugar sprinkles. Another one still happens in other parts of the world, somebody lighting a candle so that they can continue to see the pooled gasoline from a wrecked tanker that they are scooping up. The last instance of that was just a few months back. So there is no reason to doubt that somebody tried to trim a hedge with a rotary mower.
 
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