Electrolytics across relay coils

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

pucho812

Well-known member
Joined
Oct 4, 2004
Messages
14,832
Location
third stone from the sun
Not sure where to post this.

A couple of relay cards on our desk have 47uf/25 lytics across what looks like the coil pins  In parallel. We have some 40 of these cards in total and only a couple have this while others don’t.  Would there be a reason for this? Any ideas?



 
pucho812 said:
Not sure where to post this.

A couple of relay cards on our desk have 47uf/25 lytics across what looks like the cool pins  In parallel. We have some 40 of these cards in total and only a couple have this while others don’t.  Would there be a reason for this? Any ideas?
At Neve we fed relays via a small series resistor followed by a an electrolytic to ground. This acted as a local energy store and prevent current spikes getting onto the B+ line when the relay was operated (because there was no separate relay supply in 70s Neve consoles) but we never had caps directly across the coil.

Cheers

ian
 
ruffrecords said:
At Neve we fed relays via a small series resistor followed by a an electrolytic to ground. This acted as a local energy store and prevent current spikes getting onto the B+ line when the relay was operated
It seems to me that because relay switching and holding current is different, this could be designed to reduce overall current consumption because the cap would supply the necessary switching current and the resistor would supply the necessary holding current which could be quite a bit less.
 
squarewave said:
It seems to me that because relay switching and holding current is different, this could be designed to reduce overall current consumption because the cap would supply the necessary switching current and the resistor would supply the necessary holding current which could be quite a bit less.
yup


JR
 
It seems to me that because relay switching and holding current is different, this could be designed to reduce overall current consumption because the cap would supply the necessary switching current and the resistor would supply the necessary holding current which could be quite a bit less.
This is quite well explained on this series of articles about relays : Relays
( Fig 6.2 link is broken, here's the low-Q file I found on google image preview :
1703421281668.png )

But I think Pucho is describing something that looks more like this configuration ( this is something I've retro-engineered from a high-end and serious studio gear manufacturer product ) :

1703420879909.png

Note that the capacitor's polarity follows the flyback current
 
Last edited:
Also from Rod Elliott's article :
"I also tested the circuit shown with a 100nF ceramic capacitor in parallel with the coil. The flyback voltage measured 86V, and the relay released in 1.23ms. That's a good result, but the voltage is higher than desirable and the cap needs to be a high-reliability type to ensure a long life. This makes it more expensive than other options, but there may be situations where this turns out to be the best choice for the application, with or without a series resistor."
 
So Pucho's questions's answer might be here, the capacitors' purpose could be to lower the flyback voltage while keeping the switching time fast.
Concerning the circuit I've retro-engineered, I'm a bit perplex because the capacitors are still facing a massive voltage inversion when the relay is on, maybe the fact that the electrons have a direct path through the coil preserves the cap, I don't know, but this didn't seem to cause any problem on the device in question for a long operation time, or maybe my reverse-engineering is just wrong, but I don't think so..
Otherwise the purpose of the cap, combined with the diode, might be to fasten the switching time while keeping the flyback voltage low.
 
Last edited:
This is quite well explained on this series of articles about relays : Relays
( Fig 6.2 link is broken, here's the low-Q file I found on google image preview :
View attachment 119345 )

But I think Pucho is describing something that looks more like this configuration ( this is something I've retro-engineered from a high-end and serious studio gear manufacturer product ) :

View attachment 119344

Note that the capacitor's polarity follows the flyback current
I would suggest a small resistor in series with the entrance to the Relay circuit, as the cap will take a sec to charge and could arc the switch contacts over time.
 
Back
Top