LED dimmer problems

This is a last ditch since i haven’t been able to solve this myself, only getting more confused.
This is for a car’s dash lights (LED’s) so I’m switching the supply side not the ground side.

I have a mini 05 driving a BC557/547 pair. This output drives a test LED in proportion to a pot as expected.
I put a NPN P36NF06L transistor on it (G to +5v signal, D +12v, S to LED’s) and it seemed to drive a test
LED ok.
When it was installed, the dash LED’s are barely on with the pot to the stop.
So i changed the P36 to a PNP TIP137, same connections, and something started smoking.

What do i have stuffed up?


A P36NF06L is an N-channel MOSFET and not suited to 'high-side' switching. Connected as you had it, with the drain to +V and source as output, it's output could only ever get as high as the (5V) gate voltage minus the gate-source threshold voltage.

Regarding the TIP137 PNP transistor, you can't have connected it using the same connections as the N-channel MOSFET. It doesn't have a gate, drain and source.

Draw a schematic so we can see exactly what you did.

The ideal setup would probably be a P-channel MOSFET with an NPN small signal transistor switching it's gate to ground. (Plus a couple of well-placed resistors.)

Schematic added

I have to go and cook my dinner right now, but if no one else has helped by the time I've cooked and eaten it I'll draw up a new diagram for you. That isn't suitable. And the reason the TIP137 released it's magic smoke is that it was connected in reverse-polarity. And a couple of other things are wrong with that circuit. Don't use it.

Meantime, what type of LEDS are you driving, and how many? ie How much current does the circuit have to provide? (I assume that the current-limiting resistors are already attached to the LEDs.)

10 LED’s @ 20mA each, all parallel. Resistors already in place.

These circuits will have no problem with that current. I only asked about the current consumption in case they were some sort of fancy, high-current LEDs.

This first one uses a TIP137, since you already had one I thought you might have another.
A Darlington isn’t really that important for the current you’ll be switching.
(Just in case you didn’t know, a TIP137 is a Darlington-pair transistor, and actually has two transistors and two resistors in the one package.)
Rint1 and Rint2 are the TIP137’s internal resistors. You don’t need to add those.
There will be up to a 2.5V drop across the C-E of the transistor, but at 200mA it shouldn’t need a heatsink, (10 LEDs x 20mA).
Dash LED Dimmer - TIP137.JPG

This circuit uses a P-channel MOSFET instead. It’s the better of the two circuits, and there’ll be a lower voltage drop across the MOSFET than the circuit above. A heatsink definitely won’t be needed for this one. Almost any P-channel MOSFET will do. It doesn’t have to be “logic-level”, since it’s being driven by 12V.
Dash LED Dimmer - MOSFET.JPG
Note that you can’t use your P36NF06L for this, since it’s N-channel.

And for whichever circuit you use, make sure you connect the Arduino ground and 12V ground together.

Many thanks for those. I have ordered a P MOSFET. If I get it together before it arrives, I'll use a TIP137 then replace it. Andy

andy120: Many thanks for those. I have ordered a P MOSFET. If I get it together before it arrives, I'll use a TIP137 then replace it. Andy

No worries. Let us know how you go.

Success at last. Worked on a test LED but was full on only when installed. Puzzled me. It dawned on me that maybe the chassis earth I was using wasn't good enough, so I added a new earth from most of the LED's to the board and a new chassis earth. That did it.