Timer board

Sorry...yes put the diode across the coil as in the diagram.
When the relay contacts open the solenoid connection to the relay goes several hundred volts positive forward biasing the diode allowing the current to harmlessly re-circulate around the diode/coil parallel circuit.
 
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Across the coil would be the norm. It's the 'back emf' across the coil which can damage contacts etc. Putting a diode across the contacts would send that voltage/current 'somewhere else', where I imagine it might possibly damage electronics.

Kind Regards, John
Sorry John, when you say coil, do you mean the coil in the timer relay, or as the other posters suggest across the maglock
 
Sorry John, when you say coil, do you mean the coil in the timer relay, or as the other posters suggest across the maglock
Ah, yes, there is some potential confusion here (at least partially due to me - apologies!) because there are two coils - the relay coil and the (I presume) solenoid coil of the maglock.

There would usually be a 'reverse diode' across the relay coil, to protect the driving electronics from the back emf when the relay coil was de-energised - but that should already be present on your timer board (if they deemed it necessary).

To protect the relay contacts from possible effects of the back emf created by the maglock solenoid when it is de-energised, you need the 'reverse diode' across the maglock (as suggested by others). As has been agreed what you do not want to do is to put a diode across the relay contacts.

Kind Regards, John
 
Thanks again, that is exactly where i was getting confused, I will put it in the maglock.


images
images


Though must admit, i need to read up what that Diode is doing and how.
 
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Thanks again, that is exactly where i was getting confused, I will put it in the maglock.
You're welcome.
Though must admit, i need to read up what that Diode is doing and how.
It's simple enough, so allow me to expand a little on what Gasguru explained, and maybe I can spare you that reading! ...

If you suddenly cut off the (DC) current flowing through a substantial inductor (like a relay or solenoid coil), the the collapse of the magnetic field results in a large 'back emf' appearing (very briefly) across that coil - potentially large in voltage and in opposite polarity to the current flowing through the coil when it is energised. In the case of a relay coil, that high 'back emf' can damage the driving electronics, and in the case of a solenoid coil it can result in arcing at the switch (or, in your case, relay) contacts.

If you connect a diode the appropriate way around across the coil (either type), during normal operation it does nothing, since it is 'reverse biased'. However, when the 'back emf' (of opposite polarity) appears, the diode acts almost like a short circuit, so that current due to the 'back emf' just flows 'around in circles' (through the coil and diode) until it (very quickly) dies away, thereby keeping that potentially damaging 'back emf' away from the electronics or switch/relay contacts.

Does that help at all?

Kind Regards, John
 
I think it still read as being unclear..

Just to make it clear - THERE ARE POTENTIALLY TWO entirely separate locations where a diode might be needed in the circuit, but only the one will need to be added, the one across the relay's output contacts to deal with the back EMF and prevent those contacts welding themselves up.

If a diode is needed across the relays own coil, to protect the electronics, then maker will have designed one in.
 
the one across the relay's output contacts to deal with the back EMF and prevent those contacts welding themselves up.

The correct location for a back EMF diode is not across the relays contacts. The correct location is as close as possible to the source of the back EMF which in this case is the coil in the lock mechanism.

Putting the diode across the relay contact will cause the EMF energy to pass through the power supply and this could be detrimental to the components in the power supply unit.
back emf diode location.jpg
 
Putting the diode across the relay contact will cause the EMF energy to pass through the power supply and this could be detrimental to the components in the power supply unit.

I do agree the diode is best placed across the solenoid coil, but usually a PSU is more able to withstand the back emf than the contacts. The relay contacts are not so robust, they can burn and sometimes even weld themselves up.
 
Relay contacts are more likely to weld on closing than on opening. They can burn on opening and become resistive when closed and as a result they can over heat and eventually weld on closing. But the initial damage is due to the arc on opening.
 
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I think it still read as being unclear.. .... Just to make it clear - THERE ARE POTENTIALLY TWO entirely separate locations where a diode might be needed in the circuit, but only the one will need to be added, the one across the relay's output contacts to deal with the back EMF and prevent those contacts welding themselves up. If a diode is needed across the relays own coil, to protect the electronics, then maker will have designed one in.
I thought that, with one exception, that's exactly what I wrote in post '#18, which I thought (and still think) was about as clear as it could be :)

As for, the 'one exception' (emboldened in quote above), as I said, and bernard has agreed, I thing that appropriate (better) place for the 'diode to be added' to be added is across the coil of the lock solenoid, not across the relay contacts - for the reasons that bernard has explained. Why expose the power source to the consequences of the back EMF when one doesn't need to?

Kind Regards, John
 
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I do agree the diode is best placed across the solenoid coil, but usually a PSU is more able to withstand the back emf than the contacts. The relay contacts are not so robust, they can burn and sometimes even weld themselves up.
Maybe, but if one puts the diode in the 'correct' place, then neither the relay contacts nor (as I've recently written) the PSU (or anything else) will be exposed to any significant back EMF at all.

Kind Regards, John
 
The correct location for a back EMF diode is not across the relays contacts. The correct location is as close as possible to the source of the back EMF which in this case is the coil in the lock mechanism.

Putting the diode across the relay contact will cause the EMF energy to pass through the power supply and this could be detrimental to the components in the power supply unit.
View attachment 233139
when the contact opens the back EMF from the inductor is of the opposite polarity (Hence the reason the the diode is fitted reverse biased) I've corrected your drawing such that the diode fitted across the contact can pass the back EMF current.
upload_2021-5-10_17-0-59.png


:D
 
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Well spotted and it really is not my day.......

That is why a diode across the switch is no good. It bypasses the switch and keeps the lock permanently energised
 
Well spotted and it really is not my day....... That is why a diode across the switch is no good. It bypasses the switch and keeps the lock permanently energised
You're not alone ... that very fundamental problem had not occurred to me (probably because, had it not been suggested here, I would probably not have dreamed of putting a diode across the switch/relay contacts!), either (and it seems that the penny didn't drop for SUNRAY, either!).

In any event, that's a pretty good reason for putting the diode in the 'correct' place (across the coil) :) .

Kind Regards, John
 

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