Re: Clues on controller circuit problems
From: John Fields (jfields_at_austininstruments.com)
Date: 08/21/04
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Date: Sat, 21 Aug 2004 11:19:00 -0500
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]
| | |
GND>------------------+------+ <--------+
--
John Fields
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