Hi

I'm trying to reverse engineer a quad channel dimmer
The output stage comprises a pair of TK16E60W mosfet per channel.
There are 2 devices connected in series between the Drain and Gate of the mosfets, they look like little dark grey glass diodes.
They are about 4mm long, and have a metal cap at each end which is soldered to the PCB.
One device is marked "1", three times in a band at the end. The other is marked "2" in a similar manner.
I do not have my phone with me so can not manage a photo at the moment.

Any help would be appreciated. Otherwise I'll see if I can get some photos tomorrow.

 

Is the mosfet drain connected to the load?

1N111 and 1N222 would be valid numbers for diodes, but both are through hole devices and probably obsolete. 1N111 is germanium to boot. It would seem odd to have them connected between the Drain and Gate.

A picture would help.

 

A picture would help.

Thanks for your suggestions, although I'm thinking they are probably some type of Zener diode ?.
I've grabbed a picture which is attached. The devices appear more brown than grey in this photo, and are the pairs attached to the G and D pins marked.

 

Are you sure about the MOSFET pinout? Zener diodes between the gate and source would make more sense than between the gate and drain.

Bob

 

Are you sure about the MOSFET pinout?

This is what the pdf shows


The mosfet is attached to a heatsink as in this photo

so the pins are in the same position as the diagram above. From L to R they are GDS and the "diode" marked with "1" is attached to pin 1 and the "diode" marked with "2" is attached to pin 2. The ends of the diodes not marked are joined together.

 

Your photo doesn't convince me they are "in series" between the gate and drain. Series diodes are sometimes used to create a known voltage drop. That is, two, typical silicon diodes in series will produce a 1.4V drop. Germanium and silicon will be bout 1.1V, but that still seems odd to me.

How do you know they are between the gate and drain? In circuit testing can be deceptive.

 

I've done a visual inspection and am reporting what I can see. The PCB is a double sided, and not particularly hard to follow.
The schematic of what I observe, assuming these are zener diodes, is like this

 

Your photo doesn't convince me they are "in series" between the gate and drain.

Here's another view of the pair attched to the mosfet in the heatsink photo. Pins from L to R are GDS and the traces clearly show the diodes in series between pins 1 and 2.

 

I don't think that is correct. Look at Q1 connecting line to ground!

I am not sure of the nomenclature. I would not call those diodes "in series" but rater connected anti-biased or something similar.

 

ook at Q1 connecting line to ground!

Connected to a floating ground to provide return circuit for the gate current i.e. not protective earth.
Schematic amended to make this clearer.

 

I just came back to edit my prior post and notice you have posted in the interim. Zener diodes could be connected like that, but they usually have a resistor in series to create a voltage drop when they breakdown at their zener voltage.

 

usually have a resistor in series to create a voltage drop

That would make sense. My replacement circuit will use the FL5150 driver IC. This provides over current sensing and protection.
I was trying to understand the existing circuit to see if there was any function these devices were providing that I would need to replicate.
As this thread is making clear this appears to be an uncommon arrangement for mosfet protection.