rp

Relay bypass control
« on: July 02, 2020, 10:35:08 AM »
I'm trying to implement relays bypass, which I don't have much experience with. I'm not sure what best practice is.

I came up with the circut below.

My thinking is:
- The indicator LED in series with the relay coil piggy-backs onto the current required to energize the relay.
- The Mosfet makes it so very little current flows through the bypass switch.
- The voltage divider at the gate of the mosfet reduces the "on" gate voltage input to a comfy 6v. I think I could get rid of it though, as the bs170 has a gate-source voltage of 20v.

Does it make sense to do it this way?


rp

Re: Relay bypass control
« Reply #1 on: July 02, 2020, 10:36:06 AM »
Alternate version with no voltage divider:

Rochey

Re: Relay bypass control
« Reply #2 on: July 02, 2020, 10:50:46 AM »
I'd be cautious about putting the LED in series with the relay.
Better to put it in parallel with it's own current limiting resistor, both switched throught the low side FET. (or NPN if you really want to pinch pennies)

/R
Expat Audio Home: http://www.expataudio.com

JohnRoberts

Re: Relay bypass control
« Reply #3 on: July 02, 2020, 10:59:18 AM »
I'd be cautious about putting the LED in series with the relay.
Better to put it in parallel with it's own current limiting resistor, both switched throught the low side FET. (or NPN if you really want to pinch pennies)

/R
+1... typical LEDs drop around 2V forward, leaving 10V for your 12V relay... Check the relay data sheet that might still work.

To be clever and a little green you could hang an electrolytic cap across the LED so relay gets full 12V until that cap charges up...

I have done similar tricks back in the 70s-80s because relays need lower hold voltage/current than initial latching voltage/current.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Rochey

Re: Relay bypass control
« Reply #4 on: July 02, 2020, 11:06:09 AM »
also, don't forget your diode in parallel with the relay to avoid back-emf spikes.
Expat Audio Home: http://www.expataudio.com

rp

Re: Relay bypass control
« Reply #5 on: July 02, 2020, 12:20:07 PM »
Thanks for the replies!

I'll use a separate LED with current limiting resistor. That makes sense. I imagine keeping the led and relay independent would be easier to troubleshoot if one or the other failed.

I breadboarded an LED inline with the relay coil just to see if it would work, and it does. The relay is only getting 10v though. If I'm reading the datasheet correctly, it should work down to 9v (pickup voltage = 75% of 12v).

I wonder if it would work to keep the led-relay coil series arrangement, but take account of the led voltage drop, add a resistor, and use a 15v supply?


 

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