Zener voltage of an 1N4004?

[Fluxed Matter]

While working on a bad lamp timer circuit I noticed that they used a 1N4004 as a zener diode after a bridge rectifier. What would be the zener voltage. When I checked the datasheet it list an rms reverse voltage of 280 volts. Would this be what they are using to determine the zener voltage? If not how would I calculate the voltage?

[Mark Hennessy]

Are you sure? It's so poorly defined that I find it hard to imagine that someone did this purposely...

[grumpydoc]

I thought that 1N4004 was 400V (and the datasheet I just read seems to confirm that; 280V is the RMS voltage.

So the breakdown voltage would have to be higher than 400V.

I've also always assumed that 1N4001-7 are all the same at the point of manufacture and then binned according to the reverse breakdown - which would mean that the reverse breakdown could be >1000V

[Fluxed Matter]

I thought that 1N4004 was 400V (and the datasheet I just read seems to confirm that; 280V is the RMS voltage.

I Read that too. It is being used to convert from 120 AC down to around 24V DC but I find that hard to believe. Curious, because that would be a great and cheap 24V zener diode for just a few pennies.

[sleemanj]

Perhaps you can draw out a rough schematic, or take a couple pictures of the board.  It sounds weird.

[Psi]

I've also always assumed that 1N4001-7 are all the same at the point of manufacture and then binned according to the reverse breakdown - which would mean that the reverse breakdown could be >1000V

I would expect that too, lots of parts marked as 1N4001 may perfectly meet the 1N4007 specs.
Remember many years back when Intel sold a massive number of 450Mhz rated CPUs as Celeron 300mhz chips.
It was an overclockers dream.

One situation where this may not be true is replacement parts for military/medical/automotive/aerospace.
I imagine in those cases they require parts which are exactly the same. Maybe even produced with the same manufacturing processes as the original part (even when that process isn't done any more). 
Which is why parts for these uses are specified as such and carry extra cost.

[AG6QR]

If it's not on the datasheet, don't depend on it.  I share the skepticism of other posters that anyone would use a 1n4004 as a zener, but I just checked the data sheet from here: http://www.fairchildsemi.com/ds/1N/1N4004.pdf

Figure 4 on page 2 shows the reverse characteristics.  It's showing a near-vertical I-V curve at about 122% max rated peak reverse voltage at 25 degrees C, or about 130% rated peak reverse voltage at 100 deg C.  At 150 degrees C, on that graph, the curve never goes vertical, but it's starting to steepen at somewhere between 130% and 140%.

The same data sheet is used for various 1n diodes, each with different rated max reverse voltages; the rated peak reverse voltage for the 1n4004 is 400V. 

122% of 400V gives 488V at 25C. 130% of 400V gives 520V at 100C.

A point of caution is that the data sheet only shows reverse characteristics at up to 1 mA of current.  I'd guess it won't take a whole lot of reverse current before it starts to overheat.

[SeanB]

Just remember that that curve is "Typical", and whatever diode you have will not be on that curve unless you are lucky.

The most common diode there is a 1N4744 15V zener diode, it probably has the same case as the 1N4004's, and the middle 2 numbers are obscured by the board.

[amyk]

I think a more likely explanation is that it is being used as reverse polarity protection.

[sleemanj]

I think a more likely explanation is that it is being used as reverse polarity protection.

If as described it is after a bridge rectifier, then it would do nothing, it's already rectified to the correct polarity.

[KJDS]

I think a more likely explanation is that it is being used as reverse polarity protection.

If as described it is after a bridge rectifier, then it would do nothing, it's already rectified to the correct polarity.

Which is fine until someone pushes volts up its output.

[SeanB]

Then you still have 2 series diodes to do the same job. Slightly higher reverse voltage, but still doing the job.

[Rerouter]

perhaps there is some device that dumps voltage on an input or output to one of the rails through a resistor, and they needed to ensure the rails did not go beneath that magical 0.7V that is quoted in many older op amp datasheets.

[Mark Hennessy]

Or perhaps it's just a mistake. Can happen.

Let's see some pictures and a schematic, please...

[Fluxed Matter]

Thanks everyone for all the comments. It took me some time but here are a few pictures and a schematic that I drew up real quick. I hope I got the circuit right. I circle the diode in question on the pic of the pcb.

[Mark Hennessy]

It's not being a zener, rather it's there to absorb the back-emf from the relay coil 

Mark

[Rerouter]

While i have my suspicions that you have something traced out wrong around the battery (that schematic could only ever charge it), in that case the diode looks to be used for a reverse clamp on the relay when its switched on or off.

edit: did not notice the second page...

[Fluxed Matter]

Rerouter, I thought I had it wrong but that's how the battery traces out. It would kinda of explain why the timer part would not retain power. Let me check it again just to make sure I have it right.ll

[SeanB]

It should be a 24V zener diode, not a 1N4004. It should be a 1N4749 24V zener diode, not an ordinary diode.

[Fluxed Matter]

SeanB I agree with you completely but that not the case in this situation. And that probably explains why the product is a total fail. The battery is suppose to power the clock timer if the is no power but it looks undersized for 5V. I am not looking to make it work, I am just studying the circuit while I cannibalize it for my own purposes.  My curiosity was peaked at the possibility of using a 4004 as a Zener.

[Joule Thief]

Has this timer functioned properly before in your home, or is it an unknown junk box find from a HAM fest, etc?