Power Supply voltage dropping - faulty supply ?

I have a home automation and security system running on a mega 2560. All been working well for over 6 months with no changes.

Nothing changed recently on the components list of things attached to the Arduino.

Power is from a 220V Power supply that is rated output 12VDC 5A with a 12V 9Ah battery in the power supply box ( battery was changed this morning to rule it out of the equation ).

When I power up the Arduino, relay boards, sensors, etc, the voltage from the supply is 11.47V

Over a period of 2 hours, nothing is used on the system. No relays activated, no sensors triggered, no inputs change.

My Hall sensor on the main power line is reading 1.03A and remains reasonably constant ( between 1.00 an 1.05A )

But the voltage measured from the power supply drops to around 9.3V over a 2 hour period.

Would it be correct to assume that the components in the system, pullup resistors, etc should hold a steady 'state' and their drain / effect should remain reasonably constant, and that their effect on the system voltage should be almost instant after power is applied ?

My last guess is that I have a faulty power unit - we did have a power surge a few weeks back after a power outage, and that destroyed my PC, and that was on the same circuit as the power unit in question.

Any other ideas ?

and that their effect on the system voltage should be almost instant after power is applied ?

Not too sure what that means.

that I have a faulty power unit

Yes that would be my guess as well. Just remove all the load to check but it sounds lie a faulty supply.
Of all the parts in a system the power supply is nearly always the most unreliable.

Does it get hot?

That's bad, a power surge and PC destroyed.
You don't know what else is damaged.
I suggest to use a multimeter and check everything step by step.

Hi Mike

Grumpy_Mike:

and that their effect on the system voltage should be almost instant after power is applied ?

Not too sure what that means.

that I have a faulty power unit

Yes that would be my guess as well. Just remove all the load to check but it sounds lie a faulty supply.
Of all the parts in a system the power supply is nearly always the most unreliable.

I am assuming that any effect that any components of the system could possibly have, should be reasonably constant and should exist from shortly after power is applied, and not degrade the power like this over a period of time.

I have removed ( switched off with toggle switch ) the load and the power supply is now reading 11.5V

fungus:
Does it get hot?

Yes, the casing that the power supply and battery are in, is warm, but not too warm that I can't keep my hand on it.

Caltoa:
That's bad, a power surge and PC destroyed.
You don't know what else is damaged.
I suggest to use a multimeter and check everything step by step.

I have done all the steps that I can think of :

disconnected each part of the system from the 12V supply when the reading was down to 9V, one at a time. In each case, there was no noticible change to the 9V reading.

At the end, I was left with a totally disconnected Mega 2560 ( all wires are on male header pins, so were physically removed ) and still no change.

So I suspected the Arduino and ethernet shield.

Removed the shield - no change.

Disconnected the Mega and connected a spare, brand new, Mega - same reading - so unlikely that the Mega was damaged and causing the drain.

DaveO:
My last guess is that I have a faulty power unit - we did have a power surge a few weeks back after a power outage, and that destroyed my PC, and that was on the same circuit as the power unit in question.

If it worked before that and doesn't work afterwards then I think we found the problem.

fungus:
If it worked before that and doesn't work afterwards then I think we found the problem.

My problem with that statement is that it would also be correct to say : If "the complete system as a whole" worked before that and doesn't work afterwards then I think we found the problem. - we still can't say for certain that it is the power supply - although suspicion and logic says it is the most likely candidate.

The Amp sensor is not detecting any real change in the current draw by the system, so is it reasonable to assume that, once any load, no matter how large or small ( like the brand new un-used Mega ) is connected, the transformer or circuitry of the power supply could start it's gradual loss ?

DaveO:
is it reasonable to assume that, once any load, no matter how large or small ( like the brand new un-used Mega ) is connected, the transformer or circuitry of the power supply could start it's gradual loss ?

Yes.

Sort of.

A very light load might not cause the problem. eg. A single Arduino on a 5A supply isn't going to draw much. The only way to find out is to try it.

Me? I'd disconnect the power supply and apply a bigger load, see what happens. Draw a couple of amps from it and see how it holds up. Have you got some high-power LEDs or anything like that? A bunch of high-wattage resistors?

Update :

OK. I have another similar power supply ( 12V 3A with 17Ah battery ) in another room for a non-critical function.

Have re-routed that to the problem system.

Before power-on, the supply is 13.19V
After switching on all components of the home system, dropped to 12.91V and registering 1.2A
Has remained constant for 30 minutes now.

So I guess that if it remains stable for the next hour, I can safely assume my old power supply was faulty.

Thanks to all for the input. Really appreciated.

Update : 2 hours and no change at all :slight_smile:

At the end, I was left with a totally disconnected Mega 2560 ( all wires are on male header pins, so were physically removed ) and still no change.

So I suspected the Arduino and ethernet shield.

Removed the shield - no change.

Disconnected the Mega and connected a spare, brand new, Mega - same reading - so unlikely that the Mega was damaged and causing the drain.

Disconnected the Mega and connected a spare, brand new, Mega - same reading - so unlikely that the Mega was damaged and causing the drain

This is where your logic slipped, you have everything disconnected and output still 9V, so supply is U/S.

Tom........ :slight_smile: