Re: Motorcycle flasher problem.
- From: default <default@xxxxxxxxxxxxx>
- Date: Sun, 23 Dec 2007 10:46:26 -0500
On Sun, 23 Dec 2007 05:26:33 -0800 (PST), steve
<stevesemple@xxxxxxxxx> wrote:
After changing from incandecent lamps to LED's the existing flasher
did not work on the motercycle. Presumably the led's didn't draw
enough current to work.
I asked for help from this group and was one kindly given a schematic
to resolve the problem. I built it and sadly it did not work. I
believe the person who gave me the diagram had built it for himself so
I'm sure it works.
I'm not an electronic person per say. So what I'm hoping is that if I
provide the diagram with the various voltages that I have tested for
found someone could help detect what is wrong. I have tried to go over
it over and over to make sure its right and isn't some silly mistake.
The one thing I had to "guess" on was the Mosfet. The OP didnt say
what Mosfet to use just gave some specs. I tried to buy it based on
that. Perhaps thats my problem?
Here is the my diagram.
Im testing it with a 12volt AC/DC 300ma adapter that seems to put out
15.11-12 volts. I'm using a testing light that lights up from
1-50volts. I have provided I hope all details. The numbers in ( )
brackets are the values I have found when testing with my volt meter.
--------------------------------------------------------
+o----------------------,
| + |
(15.12v) | |
.------------.-|<-.----o--------------o------------------. |
Lamp
| 1N4148 | | | .-.
| / - \ | | Mosfet | ( X )
| /------\ | | IRFZ34 | '-'
o | T R | | | 60v 30A | |
| |2n3906| o | 0.050 Ohm | |
|(14.6v) '------' | | | |
.-. | | | .-. | Front | |
| | 100k | | | | | 220k | .--o---. | |
| | E B C | | | | | | |
'-' '-' | | | | |
|(1.97v) | | '------' | |
o-----. o | | | | | |
| | TR 2N3906 | | | | | | |
| | | | | | | | |
| | | | G D S |
'--.
| | E | | |
|
| (Q1)>| (1.35v) | (Q2) |(15.12v) |16Vz
|
| |-----B--------o o---o |
1N4744|
| + /| | Mosfet | | |Zener
|
--- | C | VF5408 | D | |
|
--- |(1.97v) | ||-+ - /--/
|
10uf |(0v) | 10k (1.79v)| G||<- ^ ^
|
/16v | '----|___|-------|----o------||-+ | |(0v)
|
Elec | | | | | |
|
trol | | .-. o---o |
|
ytic | 3.4Vz | | |100k | S |
|
| 1N4728 /--/ | | | |
|
| Zener ^ '-' |(0v) | +
o
| |(0v)|(0v) | |
o----------------------o----o---------o------------------'
| - -
o------------------------o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
------------------------------------------------------------
Here is the OP description of the circuit.
------------------------------------------------------------
The electronic turn signal flasher works over a load range of about 20
milliamps to 10+ amps (will flash a pair of standard auto headlamps)
Theory of Operation:
Initially, the turn signal is in an off state but provides voltage to
the signal flasher circuit in series with the lamp(s). The 10 uf
electrolytic capacitor begins to charge. When the capacitor voltage
exceeds the base voltage by six tenths of a volt Q1 starts conducting
and puts the capacitor voltage on the gate of the N channel power
MOSFET Q2, turning on the mosfet. The initial conduction of the mosfet
causes the base voltage on Q1 to drop, which, in turn, causes Q1 to
conduct harder and drive the mosfet harder . This action keeps the
mosfet from spending time in its linear region, turning it on hard,
minimizing power dissipation in the mosfet.
The capacitor discharges through the 100 K resistor. When the voltage
on the gate of the mosfet falls below its conduction threshold, its
"on" resistance begins to rise. This causes the voltage on the emitter
of Q1 to rise, and since the base of Q1 has dropped it tends to turn
off quickly, allowing Q2 to turn off more etc., until the circuit
switches back to its initial state turning the series connected turn
signal lamp off.
I don't know the part number of the mosfet but the specifications are:
Voltage source to drain 60, Resistance in the "on" state is .04 ohms,
maximum current 30 amps, TO220 package. No heatsink is required with
up to about 15 amps or more. The entire circuit was mounted on 1" X 1"
piece of perf board. All resistors 1/4 watt. I put the circuit in a
small empty dental floss container and filled it with epoxy, running
the wires out to a pair of 1/4" male "quick connects," that plug into
the turn signal flasher connector. The on time and off time can be
adjusted independently by changing the value of the 220K charging
resistor (off) or 100K discharge resistor (on), or changing the value
of the capacitor (both off and on).
---------------------------------------
Here is the original link to the discussion ....
http://groups.google.com/group/sci.electronics.basics/browse_frm/thread/4ecf37b3d76f74ba/83fb8a294a4deff6?lnk=gst&q=motorcycle+flasher#
Regards
Steve
Hi Steve
That's my schematic. There appears to be a problem in the area of the
PNP transistor. The base should be clamped to around 3 volts (while
the thing is in the off state - which is what you have stays off (?).
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, the base should
be 3-4 volts, and the collector (with no transistor in the circuit),
should be 0 V
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.
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.
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)
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.
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.
Once it is wired into the circuit - the mosfet gate is connected to
something and won't be sensitive to static electricity - but the bare
part can be easily destroyed. You can read about proper handling of
static sensitive parts - the object is to avoid a voltage differential
greater than ~50 volts on the gate and other terminals - easy to get
on a dry day, room with a carpet, or leakage in a soldering iron.
Good luck
Tell me what you find, or any questions on the troubleshooting I've
outlined.
I'll be out of town tomorrow but will check the group if my wife
doesn't have other plans.
bob
--
.
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