Relay Driver Circuit

Thread Starter

fastbike

Joined Dec 29, 2020
56
I've just found the following circuit on a board I'm repairing. There are 3 N channel mosfets and 1 P channel mosfets driving the relay coil.
The signal is provided by two 5V micro pins (labelled IO7 and IO8 on the schematic).

1615876563690.png

I'm kind of struggling to understand what makes it tick.
 

Thread Starter

fastbike

Joined Dec 29, 2020
56
I've done a bit more working through the circuit and think a pulse from IO8 would latch the circuit on, keeping the relay conducting without the micro having to constantly output.
And then a pulse from IO7 would turn the relay off ? I can't figure this out - time to look at how P channel mosfets work.
 

Thread Starter

fastbike

Joined Dec 29, 2020
56
LOL, I've just noticed that. For some reason my schematic program has inverted them. I will amend and repost ... soon.
Thanks :)
 

ericgibbs

Joined Jan 29, 2010
12,199
hi,
OK,
Check up on your N and P type MOSFET's ..... ;)
P have a Neg voltage on the Drain wrt the Source and N types have a Pos voltage on the Drain.

E


Update:
Are you sure the relay code is correct, ie: Non latching.???
 

Thread Starter

fastbike

Joined Dec 29, 2020
56
Thanks for the help.
I will check the relay code tomorrow. In the meantime I have taken another look at the devices and had transposed my N and P channel devices, so now the circuit schematic looks like this

1615886430907.png
 

Thread Starter

fastbike

Joined Dec 29, 2020
56
Hmmm, the relay is marked Finder 40.61 on the side, but on the top is then marked 12V BS. My guess is BS=bistable -> latching relay. So the correct replacement part will be FINDER_40.61.6.012.0000.

So one IO is the On pulse and the other is the off pulse.
It's getting late here, I'll go to bed and sleep on it and see if it all makes more sense in the morning.
 

Thread Starter

fastbike

Joined Dec 29, 2020
56
So my reintepretation of the circuit in post #10 above would be something like this
1615921508380.png

I just need to size resistors R7/8 to match the current requirements of the relay coil.
 

Thread Starter

fastbike

Joined Dec 29, 2020
56
hi fast,
Here is the function of the circuit, I would for sure the Relay is Latching.
Note the current reversal.
@fastbike
E
View attachment 232924
Thanks for the simulation- it looks like our posts crossed. I'll take another look at your simulation.
My question is why are 4 mosfets required in the board I'm repairing when other circuits I have found only show 2. What advantage does the extra component count bring ?
 

Thread Starter

fastbike

Joined Dec 29, 2020
56
So part FINDER 40.61.6.012.0000 seems to hard to find in stock.
Would Omron G5RL-U1-E-DC12‎ work as a substitute?
It has the same footprint, capacity, coil voltage etc.
 
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