Help identifying glass diode without markings (parallel to relay)

[Gonçalo Ferreira]

Hi all,

I'm trying to repair a board and have found a short in a diode. I'm having trouble identifying it since there are no markings... I'm not an electronics professional but have some basic knowledge. It not enough thought to know the specs that the diode should have in this application.

The diode is in parallel to the coil of a DSP1a AGP2003 12V relay.
I've found that the relay worked correctly with 12V applied once I removed the diode, so it seams to be the only damaged part.

The same board has another relay/diode mirror pair that is working correctly so I removed the diode to do some tests.

I put it in series with a 220ohm resistance and an unregulated (finished this at home...) 12V power supply. Plugging the '12V' power supply only to the resistance I measured 15,15V.
Plugging the power supply to the diode in series with the resistor I measured 18,05V across the diode and 0V across the resistor, and no current. Changing polarity had the same results.

I attach a photo of the diode.
Its approximately 2,5mm long by 1,75 diameter.

I can't make anything else of the circuit...The relay output goes to a connector and to another board. The controlling signal to the replay passes through a potentiometer and then to an enclosed small board with several ICs but all covered with no way to identify ICs or the tracks. The diode is probably to protect the controlling ICs on relay switching.

 

[Martaine2005]

Hi, this looks very much like a switching diode.
Have a look here at the data sheet.
http://elcodis.com/parts/380917/1SS133T-77.html

Also seems to be very similar to your length measurement too.
Martin

 

[Minder]

I usually keep 1n4007's on hand for this use if through hole mounting, if it is small size and a low power coil then 1n4146 gen purpose.
Not critical for BEMF diodes.
M.

 

[Gonçalo Ferreira]

Thanks for your reply Martin.

The look and dimensions certainly are correct. But it seams there are a few different diodes with the same 'looks'. Like I said in not a professional or did even study electronics, but from that datasheet shouldn't I measure a voltage drop of 1.2V across the diode when connecting it in series with a 220ohm resistance and supplying 12V like I did? On my test there is no current (or there is a VERY small one) when connecting in series with a resistor, in either direction.

I believe this diode must only let current pass on certain conditions. Maybe high voltage?
Is there such diode type?

What other tests can I make to identify conclusively?

 

[Gonçalo Ferreira]

Thanks Minder.

My previous post also applies to your reply.

This board will be very expensive if I'm not able to repair it or happen to further damage it by incorrect repair, so I'd like to replace the diode with a part as close as possible to the original. It is protecting a part of the circuit that I will not be able to repair if damaged...

 

[davenn]

as Minder told you,
He gave you ideal info

the diodes are connected across the relay coils to protect other circuitry from back EMF
if you really think these diodes are faulty ( I doubt it)
just replace them each with a 1N4007 and if things don't work, then your fault is elsewhere


cheers
Dave

 

[Minder]

As already covered, these diodes are not generally critical, they come into play at turn off time of the relay, in order to suppress high voltage BEMF which can cause problems when used in low voltage/logic level equipment.
M.

 

[Gonçalo Ferreira]

Ok, thank you all!

The diode was really faulty as there was a short and removing it removed the short (it is in parallel with the relay).

I was just wandering why the manufacturer placed a (apparently) more complex diode and if placing a general purpose one would not endanger the board and thus cost an additional 6000$ for a new board :/

I'll go with your suggestion and I'll report back once I have any conclusion.

Thank again to all

 

[Gonçalo Ferreira]

I found a 32V DC transformer and some 1N4001 and 1N4148 diodes.

I retested the diode from the board at 32 V and it gave the same results. No current in either direction (the full 32V across the diode and none across a 10k resistance in series). This is consistent with the measurements on the board.

I also tested the 1N4001 and 1N4148 diodes in the same way and these both had a 0.6V voltage drop in one direction and the full 32V in reverse... I'm not doubting your recommendations, just trying to understand what kind of diodes are these that are originally in the board.

 

[davenn]

Back EMF diodes are nothing special
The 1N4007 is a very common one used ... I use dozens of them myself for the same purpose for gear I manufacture

its quite possible that glass one is a 1N400x variety, I have seen them in that style before

Dave

(1N400x = 1N4001 to 1N4007)

 

[davenn]

the 1N4001 and 1N4148 diodes are probably a bit light for that use ... don't have the voltage or current capabilities

pretty much all silicon diodes have a Vdrop of around 0.5 - 0.7V

 

[Minder]

I found a 32V DC transformer and some 1N4001 and 1N4148 diodes.

I retested the diode from the board at 32 V and it gave the same results. No current in either direction (the full 32V across the diode and none across a 10k resistance in series). This is consistent with the measurements on the board.

Sounds like you may be testing with a resistance range rather than a meter with a diode check?
The diode check is actually a voltage drop test and allows enough current to test a semi-conductor.
M.