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Shift register > Relays - inconsistent results when load is attached to relay (discuss.tchncs.de)

submitted 1 year ago by ryanomite@discuss.tchncs.de to c/askelectronics@discuss.tchncs.de

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First of all, I apologize for the lack of a "formal" schematic... I tried to draw one out, but got buried in the nuances of it :)

Second, I've done enough research to have some idea of what my problem is: namely, I need a decoupling capacitor in the mix. Problem is, I'm new enough to this that I'm not 100% sure where to put it, nor what capacitance I should use.

So the setup: -I'm using a Wemos(like) D1 Mini to drive 8 relays via an HC595 shift register -The D1 Mini microcontroller, HC595, and relay module are all powered by the same 5V DC and ground -The microcontroller addresses the shift register via 3.3VDC data pins -The relays power a water valve via a separate 24VDC power supply and transformer - plugged into the same AC outlet

And the problem: everything works just fine UNTIL I actually add the 24VDC power load. Once that's done, the results are very erratic... usually turning on the correct valve correctly, but rarely turning it off in the same way. As I mentioned, I intuitively suspect noise in the data caused by the sudden power to the valve... but it doesn't make too much sense since everything is on different power supplies.

Any guesses as to where I need to put some capacitor(s)? Thanks!!

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[–] pneumapunk@lemmy.sdf.org 4 points 1 year ago (2 children)

That's interesting, so you can flip the relays all you like without trouble as long as the 24DC supply isn't connected? If that's true then your problem presumably isn't the typical inductive kick from the relay coil. It looks like your relay board has stuff on it which is presumably drivers and snubbers so let's assume all of that is adequate to the job.

So, if it's inductive kick from the valve solenoid it's being coupled all the way from there, back through the 24DC supply to the outlet, then forward through the USB supply to your shift register, which is impressive! But not implausible.

Anyway, three places I'd add some stuff:

The main thing you need is a snubber network across the valve solenoid coil itself, ideally physically close to the valve (you want to minimize the area of the loop formed by the valve coil - wiring - snubber). Something as simple as a freewheel/clamping diode would probably help a lot. This will also improve the lifespan of your relay contacts which are probably arcing a little.
Small decoupling cap on your breadboard, say 0.1µF on the power supply rails, to keep your logic happy.
Larger decoupling cap on your 24VDC rails (the bus on the left), just to eat any transients the snubber doesn't deal with. Maybe 1-10µF or so?

[–] ryanomite@discuss.tchncs.de 1 points 1 year ago (1 children)

Thanks for your thoughtful response... and sorry it took so long to get back to you. I tried different combinations of capacitors, a diode on the load lines, etc... nothing worked. And then I put a 0.1uF capacitor directly between the power leads on the valve itself... and everything started working fine. Admittedly, I'm not 100% sure why... but I won't complain :)

[–] pneumapunk@lemmy.sdf.org 1 points 1 year ago

That makes sense, it forms a simple snubber network. A capacitor in series with a low-value resistor might work even better. Did you try a freewheeling diode directly across the valve leads?