IRF3205 -- string trimmer?

and it isn't any sort of "speed control" it runs full blast or off.

Maybe it's a current limiter? You could probably live without it.

that's what I was thinking. I guess I should measure the current draw of the motor and maybe add some series resistance and just use it with shorter intervals.

mandag den 22. juni 2020 kl. 18.51.32 UTC+2 skrev snipped-for-privacy@highlandsniptechnology.com:

or a cut off if the battery runs low

Most likely the switch would burn out like the FET and is not rated for the abuse of constant start-stop with inrush currents that may be approx. 10x the steady-state rated current under load and 50 to 100x the no load curren t.

The feature of putting FETs in parallel is that they can share current even ly since they have a PTC or positive tempco unlike BJT's. This feature als o becomes a faault if you run the FET's too hot by constantly start-stoppin g the mtor with a duty cycle that prevents cooling down of the FET yet cond ucts up to 10x the power heat dissipation loss under duress of starting wit h full acceleration.

A smarter design would use a PWM soft start to reduce the surge current yet take longer to get up to speed.

The choice of FET is good. RDS(on) = 8.0m?

The usage of frequent start stops or the heatsink and possible or lack of f an cooling it might not be adequate. Inspect it for grass air flow restric tion and plan on running it with fewer start/stops. Free spinning in air mi ght only take 10% of the full load current and 1% of the start current and it iwll run cooler and last longer. Maybe plan on using it for less than th e total battery time of ~30 minutes to extend both the FET and battery life . Low SoC is known to reduce overall battery life.

When in doubt , call tech support at the factory.

Well, don't want to spend too much time with it so I'm going to try using a standard 120v 15a switch on it and see what happens. I'm past the days of spending way too much time just proving to myself "I can fix it" (although I miss those days). I'm too old. At least the 120v switch is robust? Batter y is 20v. 6ah.

Is the motor brushless or simple brush type armature commutation? If it's brushless you are screwed...a switch will not work.

ng a standard 120v 15a switch on it and see what happens. I'm past the days of spending way too much time just proving to myself "I can fix it" (altho ugh I miss those days). I'm too old. At least the 120v switch is robust? Ba ttery is 20v. 6ah.

with a single mosfet it is not brushless

a standard 120v 15a switch on it and see what happens. I'm past the days o f spending way too much time just proving to myself "I can fix it" (althoug h I miss those days). I'm too old. At least the 120v switch is robust? Batt ery is 20v. 6ah.

I understand your frustration, but I can guarantee that a 15A switch much f aster than the 50k cycles it might be rated for at 15A.

The IRF3205 FET is rated for = 110A and it failed.

I tried to explain why they fail with the frequent start surges and that i s 10x the rated current where the RPM slows down perhaps 50%.

If you buy another, gas might be better, although they have their reliabili ty issues too with fuel clogging.

Maybe, hire a kid on the block with his own tools. Train him for your expec tations. It's like motivating them for the future.

Yes it's just a power-ON FET buffer for the low current 15A switch.

ng a standard 120v 15a switch on it and see what happens. I'm past the days of spending way too much time just proving to myself "I can fix it" (altho ugh I miss those days). I'm too old. At least the 120v switch is robust? Ba ttery is 20v. 6ah.

I understand your frustration, but I can guarantee that the 15A switch wil l fail "mucho" faster than the 50k cycles often used for power switch ratin gs.

The IRF3205 FET is rated for = 110A and it failed.

I tried to explain why they fail with the frequent start surges and that is 10x the rated current where the RPM slows down perhaps 50%. If you buy another, gas might be better, although they have their reliabil ity issues too with fuel clogging. Maybe, hire a kid on the block with his own tools. Train him for your expe ctations. It's like motivating them for the future.

Just use a switch and go for it.

the motor and maybe add some series resistance and just use it with shorte r intervals.

Sure not much to lose.

Actually it's not the 10x surge current alone that burns the contacts but t he turn off arc voltage that creates ionizing air with 5000'K flaming air t emperatures that may conduct partially to the contacts, when they open and the device slows down. RC Snubbers are used to protect power contacts by d iverting some current.