Re: Clues on controller circuit problems
From: Pete (lansma_at_bigpond.com)
Date: 08/21/04
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Date: Sat, 21 Aug 2004 16:28:17 GMT
Thanks very much John. I will do just that. I received a post from Kim
off list also suggesting that i fried the transistor. I gave her some
additional information regarding the extra demands I was placing on the
circuit by trying to control the heat generated by a a number of
resistors in parallel. I've reposted seeking a more robust design under
"Need heater strip circuit" above because Kim reckons this design isn;t
going to work for that purpose.
-- Clear and dark skies! Pete ------------------------------------------------ Home Page: http://www.users.bigpond.com/lansma/ Lat: 42° 53' S; Lon: 147° 19' E John Fields wrote: > On Sat, 21 Aug 2004 15:05:21 GMT, "Pete" <lansma@bigpond.com> wrote: > >> Hi, I'm not sure whether anyone can help with this problem but I >> though I would post on the of chance because I haven't a clue. I >> have made a 12 V speed controller/light dimmer circuit that came in >> kit form. It uses a LMC555/7555 CMOS timer and BUZ71A power mosfet >> transistor. A 10K pot controls the motor speed. I was testing the >> circuit tonight and it worked flawlessly then suddenly the pot >> stopped controlling the speed. It now operates at full current >> irrespective of where I set the pot. The pot seems to work alright >> tested with a multimeter, so I'm anticipating the problem is >> elsewhere. The transistor gets very hot (it has a heat sink >> connected) as does a big 3A diode. There are no obvious signs that >> anything has blown at all. Any clues? > > --- > Could be you've toasted the MOSFET or the diode. To find out, > disconnect the diode and measure its resistance in one direction and > then the other using your meter. If you don't have a DIODE TEST > function, then get 1.5V cell (battery) and a 100 ohm resistor and hook > everything up like this: > > > +--[-BAT+]---[100R]---[DUT]---[DMM (20mA)]--+ > | | > +-------------------------------------------+ > > If the DUT (device under test) is OK, when you have it in the circuit > in one direction the meter will read about 8mA and when you reverse it > the meter should read zero milliamps. It might show a few microamps; > that's OK. > > If you've popped the MOSFET, you can find out by connecting the gate > to the source and measuring the resistance between the source and the > drain. It should be very high with the drain connected to the + > output of the ohmmeter, and it should look like a diode with the > ohmmeter connected the other way around. If it reads like a low > resistance both ways, it's gone. > > You can also test the 555 by disconnecting everything from its output, > setting your DMM to DC volts and measuring the output of the 555 as > you turn the pot. It should be low at one end of the pot's rotation > and high at the other. > > Depending on your meter, though, you might have to do something like > this: > > > 555OUT>----[1N4148>]--+------+ <--------+ > | | | > [0.1µF] [1K] [DC VOLTS] > | | | >> ------------------+------+ <--------+
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