Diode's on pre III relay board

[bob orbell]

Hi all, had two diode's go short circuit on my pre input board, one last year and the other yesterday, what a bummer to get to, checked the voltage and all is OK, 4.90 volts at the relay, would there be any problems if I was to remove all 5 diode's, any thoughts on this please. BOB

[bikerhifinut]

Bob,
I am not completely sure but my belief is that the diodes are to reduce or eliminate the back emf from the relay coils when the supply to them is cut, this could create a spark or other issues across the switch. Bit like the principle of the induction coil, think oold fashioned car ignition circuit.

If I got it wrong a wiser member will correct me.

Andy

[Richard]

Yes as Andy says. The diodes listed are 1N4148 with 75V rating. I don't know what the step up from the coil might be but wonder if small 1000V diodes such as UF4007 etc would be better?

[bob orbell]

Thank's for the suggestions and comments Andy and Richard. Just out of interest on this, would a small value capacitor not do the same job? say something like 0.1uF 400 volt, falling that I will get some high voltage diodes, and may be another PCB and phono connectors, both don't take too kindly to assembly and disassembly many time's. BOB

[bikerhifinut]

Bob,
Another thought occurred to me.
As I drag up the memory of the article I read up on these diodes with small relays, I seem to remember its mainly for protection of semiconductor switching circuits where a reverse voltage spike could do a bit of damage to the output device, usually a small switching transistor or a device on a bespoke chip. So your idea of a small high voltage capacitor may work to damp the spike.
The other thing that springs to mind is that as far as I can make it out, these diodes are across the solenoid coil so you might be able to bodge replacements from the switch to ground, thus avoiding fiddly dismantling as long as you can snip the offending diode out.
Richards suggestion of a Higher PIV rated diode also makes sense to me, I don't know what the voltage generated by the coil could be when the magnetic field collapses when the coil is switched off but I suspect 75V could be achievable under certain conditions.
Once again I invite the experts for their opinion on my musings.



Andy.

[Phil Y]

I also think Richards suggestion of higher voltage diodes is the way to go. Apart from possible damage to the switch from arcing, might you not get clicks/bangs in the audio circuit (RF pickup)if the energy is not dissipated safely?

Regards, Phil.

[bob orbell]

Hi guy's, thank's again for the comments, Andy's idea of putting the diodes at the switch seems good, I think that is what I will do using the high voltage diodes Richard suggested and remove all on the PCB, I will report back if I have any problems. BOB

[A Stuart]

At risk of being shot down:
I always understood the point of the diodes (if fitted correctly!) was to be momentarily forward biased into conduction in order to dissipate the energy from the coil. To my mind this surely makes the reverse voltage rating unimportant.
Alastair

[John Caswell]

At risk of being shot down:
I always understood the point of the diodes (if fitted correctly!) was to be momentarily forward biased into conduction in order to dissipate the energy from the coil. To my mind this surely makes the reverse voltage rating unimportant.
Alastair.
Correct!
I cannot see any reason why these diodes should have failed.
Putting them at the switch end is not a good idea as they are far removed from where they will be the most effective and more likely to introduce radiated spikes. If you are unhappy with the 1N4148 then by all means fit 1N4004/5/6/7 all will all be fine, not necessary to go to the UF types.

John

[bob orbell]

Thank's again for all the feedback on this subject, John, one diode went almost short circuit last year on the phono relay as I was listening to a record, suddenly the music stopped, found the problem and sorted it and just a few days ago I went to play a tape and no sound, found that the tape relay diode had gone completely short circuit I have tested everything and all is fine, am I just unlucky? BOB