Repairing Lilliput touch controller / Fried Zener

**rando** · 08-01-2004, 02:32 AM

I fried my Lilliput touch controller. Looking at it, there is a diode across the +5v rail and ground that got fried. When i removed the diode, the controller worked again. Accoring to the board silk screen, it's a foward biased zener. I'm not sure I understand what that zener would be doing there. If forward biased, would that essentially short the USB +5v rail?

In any case, does anyone know what it is? I assume it was 5.1v zener (reverse biased despite the what the silk screen says) there to protect against an overvoltage. This is supported by the theory that my t/s controller started working again once I removed the fried part.

**Seth** · 08-01-2004, 03:11 AM

When forward-biased, zener diodes behave much the same as standard rectifying diodes: they have a forward voltage drop which follows the "diode equation" and is about 0.7 volts. In reverse-bias mode, they do not conduct until the applied voltage reaches or exceeds the so-called zener voltage, at which point the diode is able to conduct substantial current, and in doing so will try to limit the voltage dropped across it to that zener voltage point. So long as the power dissipated by this reverse current does not exceed the diode's thermal limits, the diode will not be harmed.
Sounds forward-bias mode to me. Might be there for reverse current protection ? (That explains why the controller works without the zener)
Which one is it?

**rando** · 08-01-2004, 03:32 AM

Your board is a little different than mine. I'm guessing that zener up in the top left corner is the corresponding part. My board doesn't have those nice connectors on the input/outputs (just wires soldered to the board) and so the diode in question lies just about where your input connector is.

I still don't understand why you'd want it forward biased across the power rails. As you said, the foward zener drops 0.7V yet the power rails are at 5v. Sounds like a recipe for creating magic smoke. In reverse mode, it would provide (some) overvolt and reverse voltage protection, wouldn't it?

In your picture, I'm assuming that the top trace (also the top pin on the input connector) is the +5v USB supply and connects to the top of the diode. The 2nd pin from the top is GND and connects to the bottom of the diode. Can you tell me what markings appear on that diode? Also, is it

pin1 -|>|- pin 2

or

pin1 -|<|- pin 2

Thanks

**rando** · 08-01-2004, 09:37 AM

I've circled the damaged part and labeled the traces.

EDIT: The labeling is backwards. The diode is actually reverse biased.

**rando** · 08-02-2004, 12:29 AM

bump ... come on, soooo many Lilliputs around here! Somebody can help me out and read the codes off that zener for me, right?

**Seth** · 08-02-2004, 01:30 PM

rando first of all this is not my screen's controller. It's the controller Armen sells in his site. I just posted it for reference

. I believe the diode is there probably for overvoltage and/or overcurrent protection. Most of the SMD components unfortunatelly don't have product codes written in them but I believe it's a 5.6 V / 0.5 Watt zener. You should ask for Armen's help in this also.

Hope you figure it out

**Bravellir** · 08-02-2004, 01:56 PM

Rando
I don’t know the difference between a diode and well.. anything else, but I have my lilly in pieces in the garage so..
My controller has the component in the exact same place as the picture, but as yours, the cables are soldered. But I’m afraid I couldn’t find any markings in the diode except the green, red and brown circles.

Brave

**digitalww** · 08-02-2004, 02:01 PM

It just says 5V on it.

This what you board looks like?

Originally Posted by rando

bump ... come on, soooo many Lilliputs around here! Somebody can help me out and read the codes off that zener for me, right?

**rando** · 08-02-2004, 02:56 PM

Originally Posted by digitalww

It just says 5V on it.

This what you board looks like?

Yes, that's the one. I believe the 5V is the label for the input pin (5V from USB), not for the zener. Are you guys saying there are no markings on the zener (could be between the zener and the board)? I was under the impression that they are usually labeled (at least) by their zener (breakdown) voltage. I'd expect it to have something like 5v1 or 5v7 etc on it.

Also, I'd like some clarification on which way that thing mounts in there since the silk screen indicates that it is positively biased across the USB power rails which would make it shunt the power rails under normal operation (doesn't make any sense to me). I'm probably confused about something here, I just don't know what. Thanks in advance for any help.

... Maybe that 5V isn't the label for the pin in which case this zener may actually NOT be foward biased (a good thing). I guess I better double check this when I get home. It was like 1AM when I looked at it. argh.....

**digitalww** · 08-02-2004, 03:56 PM

I can probably take this component from one of the bad controllers I have and send it to you, for a $1

What do you think?

Armen

Originally Posted by rando

Yes, that's the one. I believe the 5V is the label for the input pin (5V from USB), not for the zener. Are you guys saying there are no markings on the zener (could be between the zener and the board)? I was under the impression that they are usually labeled (at least) by their zener (breakdown) voltage. I'd expect it to have something like 5v1 or 5v7 etc on it.

Also, I'd like some clarification on which way that thing mounts in there since the silk screen indicates that it is positively biased across the USB power rails which would make it shunt the power rails under normal operation (doesn't make any sense to me). I'm probably confused about something here, I just don't know what. Thanks in advance for any help.

... Maybe that 5V isn't the label for the pin in which case this zener may actually NOT be foward biased (a good thing). I guess I better double check this when I get home. It was like 1AM when I looked at it. argh.....