Re: Motorcycle flasher problem.
- From: steve <stevesemple@xxxxxxxxx>
- Date: Fri, 28 Dec 2007 08:30:46 -0800 (PST)
Well I'm back with some results Bob. There seems to have been a bit of
discussion in my absence. Unfortunetely I'm not Electronically
intellegent enought to add anything into any of the debate. However I
can tell you my results on the tests you suggested on your circuit.
I have put them in context of your text below. I have done 7 tests.
Please see them numbered.
You show around 2 volts on the collector - that is not enough voltage
to turn on the mosfet - you want 4 or more volts there.
With it turned off (lamp in the off state), the voltage at the emitter
should rise to greater than 4 volts, the voltage at the base should be
clamped to around 3-4 volts - if you can remove the transistor - where
the emitter connects the voltage should be ~12+ volts,
** 1. Connector 14.70volts (basically the AC adapter output) So this
seems to be OK.
the base should be 3-4 volts,
** 2. Base 1.6 -- This is too low based on what you are saying.
However I tried two other transistors one the exact same and another
that has a little lower Hfe (because I had it on hand) and it really
didn't change.
and the collector (with no transistor in the circuit), should be 0 V
** 3. Collector is 0.0
I'd suspect a bad PNP transistor (or wrong transistor or connected
wrong). Or just a transistor with poor gain - the gain should be on
the order of 100-200 in that circuit.
** 4. I tested the transistor using the little thing on my Volt meter
I get HFE 189
Second suspect is the timing cap being leaky - in order to get 4 volts
out you have to have >4 at the emitter - and you don't. With the
transistor removed if you got 12+ Volts there after about a second for
the cap to charge - the cap is good.
** 5. I tried this, removed transistor and I get voltage of >14 volts.
I try to discharge the cap and then try it again and it seems to very
quickly go from low 0 to >14volts.
So that seems OK. I changed the cap just in case. Same result.
The other thing that is likely cause, assuming the other things
checked out, is if the current to the gate of the mosfet is way too
high. That would pull down the other voltages.
A good mosfet has almost no gate current - the gate is an insulator -
that 100 K gate to ground is for the purpose of discharging the gate
capacitance. That has to be 100 K or so, you don't want a lower value
there, so check that. (band next to the gold or silver is yellow)
** 6. Tested all resistor they are all right.
To check the mosfet - disconnect the gate (and you may want to
temporarily ground it so you don't destroy it with static) - with the
gate disconnected, the voltage at the collector of the PNP should rise
to >4 volts - if it does and does not when connected, the mosfet has
already encountered static.
** 7. I get about 1.88volts. Interestingly the Light dimmly lights up
untill I put my probe on the gate. I also tried to test the mosfet
using the following ...
http://www.uoguelph.ca/~antoon/gadgets/mostest.htm
It seems to test ok.
Im sorry to be such a dolt on this. Its a simple circuit and yet there
is something Ive done that just isnt right. By the way on my original
schematic I put in the orientation of the transistor and Triac are
they right?
eg Transistor 2N3906 Holding legs from bottom legs pointing down,
looking at flat part of TR. with round part at the back EBC
Mosfet IRFZ34 Holding legs from bottom Looking at "front" of mosfet
Heatsink to top
GDS.
I have replaced the Mosfet and as I mentioned above exchanged the
transistor, tested all resistors to make sure they are the right R
value.
I even am using a different DC adapter. instead of 12v 300 mA Im now
using a 12v 800mA thinking maybe there just was not enough current. No
real change. I have hooked it up in the following way, Using two
diffierent lights. One is a test light that works up to 50v and the
other a 12volt car test light. Both work out of the circuit.
Negative ----- Circuit --- Lamp ---- Positive
Im not sure what is considered before the mosfet.
If the voltage is <4 V at the collector of the PNP with the gate
disconnected (and the 100 K is indeed 100 K) the problem is before the
mosfet.
Or do you think the mosfet specs I used are Ok. Im not sure where to
look next.
Again it is probably user error but I seem blind to what it is. Could
some of the resistors be lowered to increase power ??
Regards
.
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