12v rail smoothing?

[Doomguy42]

Hi,
My new company were using relays on a project: 5v coil with 12v line. When the relays closed the circuit and the 12v circuits were complete it was causing the 12v rail suddenly become erratic for a few seconds... a sensitive device also connected to the 12v rail was reseting because of this. I took my oscilloscope in so we could see what was happening to the device supply.
The switching power supply was relatively rubbish and I guess that's what we are seeing on the scope.
I'm hoping to design like a break out board further down but what would be nice would be to try and regulate the 12v supply to the sensitive device on the same board.
I've just transfered from high end joinery to engineering- electronics repairs are my main interest so I'd like to have a go and solving the problem. I'm used to fixing what's there not thinking what to use.
Initially I'm thinking electrolytic cap but that is just from what I've seen elsewhere or use a precision voltage regulator. I don't really know how to work it out if anyone could teach me what a proper electronics engineer would do or how to approach it.
Thanks

 

[Nigel Goodwin]

For a start post the FULL circuit, so we can see exactly what you're doing.

 

[Doomguy42]

It's like a small automotive style power rail circuit driven off a 12v power supply. Realtively low draw. Its more like radio audio/led/controller 12v rather than heater or any big 12v draw.
I'll ask my boss if I can draw it out and share it. The device, I think it has anti tamper built in so i think it has a tight supply tolerance Or it resets. The power supply was supposedly upto 27amps but can't seem to stay flat when the relays activate momentarily.
I've got it running OK with an independent power supply. I'd rather just learn something and do it from one rail if possible.

 

[Papabravo]

It's like a small automotive style power rail circuit driven off a 12v power supply. Realtively low draw. Its more like radio audio/led/controller 12v rather than heater or any big 12v draw.
I'll ask my boss if I can draw it out and share it. The device, I think it has anti tamper built in so i think it has a tight supply tolerance Or it resets. The power supply was supposedly upto 27amps but can't seem to stay flat when the relays activate momentarily.
I've got it running OK with an independent power supply. I'd rather just learn something and do it from one rail if possible.

Words are a very poor substitute for a drawing. You've been asked once. Please humor us.

 

[Pommie]

5V relay on 12V supply?

Mike.

 

[rjenkinsgb]

When the relays closed the circuit and the 12v circuits were complete it was causing the 12v rail suddenly become erratic for a few seconds...

That sounds like a bad 12V power supply plus inadequate decoupling.

[For clarity: 5V controlled relays, switching 12V circuits, and the 12V side is unstable with load variations?]

 

[Pommie]

An obvious solution would be an inductor/capacitor filter on the power supply to the relay circuit.

Mike.

 

[Doomguy42]

Thanks very much for getting back to me... unfortunately my boss doesn't want me to share the whole schematic
I'm still keen to learn what I can though.
The relay coil is activated by a 5v circuit from your generic miniature pc device I/O (400ma) It's switching the 12v circuit via the relays.
The original power supply was absolutely awful... it looked like a big ripple on the scope.
Our device +12v is not decoupled or filtered at all... its the dips in the 12v that I think are resetting it...
When you say filter caps did you mean an electrolytic, poly box cap, or usually ceramic type decoupling caps? Or what sort of inductor. I know it's a bit of an ask but if I can improve it at all it's going to go down well. Thanks.

 

[Diver300]

If you have some loads on the supply that are large, and others that are sensitive to fluctuations, then it could be worth separating the supply for the two types of load.

27 Amps is a lot for a load that is sensitive to fluctuations and it is also a lot to smooth out. You would need a huge capacitor to smooth it out. On the other hand a motor or a heater won't mind a supply with glitches. The solution could be to put a diode in series with the the sensitive loads, and a capacitor in parallel with the sensitive loads. That way the capacitor only has to support the sensitive loads during a glitch and if the sensitive loads take much less current than the main loads, it will only need a much smaller capacitor to smooth the sensitive load than the whole 27 A

 

[Nigel Goodwin]

Thanks very much for getting back to me... unfortunately my boss doesn't want me to share the whole schematic
I'm still keen to learn what I can though.
The relay coil is activated by a 5v circuit from your generic miniature pc device I/O (400ma) It's switching the 12v circuit via the relays.
The original power supply was absolutely awful... it looked like a big ripple on the scope.
Our device +12v is not decoupled or filtered at all... its the dips in the 12v that I think are resetting it...
When you say filter caps did you mean an electrolytic, poly box cap, or usually ceramic type decoupling caps? Or what sort of inductor. I know it's a bit of an ask but if I can improve it at all it's going to go down well. Thanks.

Sorry to be negative, but it doesn't sound like your 'design' has actually been 'designed' at all, which is why it doesn't work.

Having the schematic 'expertly' critiqued, and getting suggestions for improvements, is what's required - not random 'bodges' with no idea of what the circuit does, or how it does it. Pictures of the construction would be useful as well, as that could be part of the issue - but mostly it sounds like there's no one there who knows anything about electronics?.

 

[danadak]

If you are working in a vehicle environment you need to address load dump :

Its an ugly issue to address but not rocket science.

https://www.ti.com/lit/an/snva681a/snva681a.pdf?ts=1683122330275&ref_url=https%253A%252F%252Fwww.google.com%252F

https://m.littelfuse.com/~/media/electronics/application_notes/littelfuse_tvs_diode_meet_automotive_load_dump_standard_application_note.pdf.pdf

https://www.st.com/resource/en/application_note/an2689-protection-of-automotive-electronics-from-electrical-hazards-guidelines-for-design-and-component-selection-stmicroelectronics.pdf

Regards, Dana.