IRF3205 -- string trimmer?

Every time I turn around I have to repair (or attempt to) some piece of crap. This time a Troy-bilt string trimmer (20V Lithium battery).

Battery is good, motor is fine but what I thought might be a switch is a rather sophisticated circuit the has an 8 pin IC, quite a few passive components and an IRF3205 mosfet. This "switch" has a low resistance across the battery so it's shot.

What is going on here with this circuit? A regulator of some sort? Any ideas on something basic I can use to replace just to get it in action again? Don't feel like building a circuit (obviously) to cut my damn weeds. What are the consequences of using just a switch and limiting the use of the trimmer? Any simple series thing I could do between Batt+ and the motor?   THANKS.

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

On Mon, 22 Jun 2020 08:33:55 -0700 (PDT), mkr5000 <mikerbgr@gmail.com>
wrote:

>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.



-- 

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

  Claude Bernard
  

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 jla...@highlandsniptechnology.com:
> On Mon, 22 Jun 2020 08:33:55 -0700 (PDT), mkr5000 <mikerbgr@gmail.com>
> wrote:
> 
> >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.
> 

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 current.

The feature of putting FETs in parallel is that they can share current evenly since they have a PTC or positive tempco unlike BJT's.  This feature also becomes a faault if you run the FET's too hot by constantly start-stopping the mtor with a duty cycle that prevents cooling down of the FET yet conducts up to 10x the power heat dissipation loss under duress of starting with 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) = 8.0mΩ

The usage of frequent start stops or the heatsink and possible or lack of fan cooling it might not be adequate.  Inspect it for grass air flow restriction and plan on running it with fewer start/stops. Free spinning in air might 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 the 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? Battery is 20v. 6ah.

On 6/22/20 4:10 PM, mkr5000 wrote:
> 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? Battery 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.

tirsdag den 23. juni 2020 kl. 17.40.02 UTC+2 skrev Bill Martin:
> On 6/22/20 4:10 PM, mkr5000 wrote:
> > 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? Battery 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.

with a single  mosfet it is not brushless

On Monday, 22 June 2020 19:10:12 UTC-4, mkr5000  wrote:
> 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? Battery is 20v. 6ah.

I understand your frustration, but I can guarantee that a 15A switch much faster 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 is 10x the rated current where the RPM slows down perhaps 50%. 

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

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