SMPS wall wart failure.

Could it be a VTS, ie designed to go short circuit in overvoltage situation? Perhaps the ovelay component character set does not allow for VTS .

** Never seen a rectifier diode and zener in *parallel* before.

Only purpose I can think of is the main diode is a Schottky type and needs protecting from excess reverse voltage spikes.

Schottys range at about 30V up so maybe try a 27V zener.

... Phil

THat is a protection device.. could be a MOV but it sure is a protection device design to clamp if voltage exceeds, that is, if it is truely across the output.. The caps are bad and caused higher than desired voltage from the switching signal, thus shorted the protection device. Basically, it did its job. You may be able to get by using a 5 watt zener but you still need to replace the caps.

btw, it seems a very common problem for the switching wallwarts to fail in this manner for routers, I replaced my 3 times, now I have a transformer 60hz type and works great...

Jamie

Apologies folks. Looks like I misread the pc traces. The anode of the power diode is only in common with the secondary of the transformer. The cathode connects to one end of the zener diode, the filter cap (which I replaced), then goes to a choke, then to the output wires.

Checking further, I've discovered that the shorted zener diode was in parallel with the filter cap. That explains the low, in circuit ESR reading. The negative side of the filter cap and zener are also connected back to the secondary of the transformer. I imagine the zener is used to protect the cap and output circuit from voltage spikes?

Thanks for your replies.

--
David Farber 
Los Osos, CA

Hi Jamie,

I did replace the faulty cap. As far using a 5 watt zener diode, the original one is 3mm long and 1.4mm in diameter. That would seem to be in the under 1 watt category. Also, please see my corrections to my original post. The zener diode is paralleled by the filter cap, not the power diode.

Thanks for your reply.

--
David Farber 
Los Osos, CA

Basic crowbar zener. Meant to protect the device powered by the supply in case the supply runs away.

If the zener is there strictly for protection, the power supply should work normally without it. Have you tried using it without the zener?

If it's in parallel with the output, it's likely some voltage slightly over 5V, such as 5.6v or 6.8v. If the zener is expected to short upon overvoltage, I would guess(tm) that a low power zener would be more appropriate than one that can handle the power.

I've had my share of 5v 2A power supplies blow up. I don't recall if they were made by Jentec, but when I replaced the usual bulging capacitor(s), most would not recover. I never bothered to troubleshoot further and just recycled them. Thanks for the hint about the zener.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

Hi Arfa,

I agree with you regarding the phrasing. Your way does sound better and it is splitting hairs. (-:

I was wondering what would happen if the choke were placed before the capacitor and zener diode. Wouldn't the choke filter out the spikes and then make the zener diode unnecessary or at least less likely to short?

Thanks for your reply.

--
David Farber 
Los Osos, CA

** A choke would present a high impedance in series with each current pulse from the switching tranny - causing a big drop in the voltage appearing on the electro cap.

** The zener (probably 6.2 V) conducted heavily and failed short when the e lectro went high ESR and caused the peak output voltage to go high.

An electro that has developed high ESR cannot smooth the current pulses bei ng delivered by the switching tranny and diode, so the output wave has cont inuous high peaks with a low *average* value. The control loop responds to the low average and tries to correct it by making each current pulse strong er, which only makes things worse.

In short, the output electros in a SMPS are critical to it operation and in many cases there in nothing to prevent the output voltage going high when they wear out.

I have seen serious damage done to 5V logic when this happens.

... Phil

Hi Phil,

I was forgetting about the pulsing action. I guess I was thinking about a standard AC transformer operating in the 60 Hz range. I imagine it would hold true that whatever the transformer pulse/sinusoidal frequency is, the choke belongs at the end of the line.

Thanks for your reply.

--
David Farber 
Los Osos, CA

Hi Jeff,

As you suggested, I installed a 5.6v zener and it works fine now. The output voltage is a solid 5.17v.

Thanks for your reply.

--
David Farber 
Los Osos, CA