Power issues locking up circuit board

Thread Starter


Joined Aug 26, 2019
Good evening all. I'm new to the forums and dont claim to know much on the actual circuitry side of circuit boards. I'm on the HVAC/control side and deal with them on a daily basis. I have run into an issue that I'm hoping someone on here can assist me with.
I have 6 large rooftop air conditioning units that are having random lockups due to what now I'm thinking to be power issues at the site. Losses/spikes of less than half a second. I have a communication board in this unit that basically freezes up and once power is removed and reconnected all is good. Until next time that is...
The board is being fed from a 115VAC to 12/5 vdc transformer. The 12VDC feeds main board and 5vdc feeds the board in question. The machine has a phase monitor with a normally open set of contacts but only inputs as a status point on main board. Does not kill power to either board. Was thinking about installing a timing relay to kill 5vdc in case of phase issue but I'm afraid that the loss/spikes are less than the minimum of 2 seconds response time of the monitor. Any ideas a way to protect the 5vdc from any power glitches coming into it and make sure it stays clean with no losses/spikes? Thanks in advance..


Joined Jan 23, 2018
If you have a programmable extra output you can add a routine to reset a 10 second timer every few seconds, and as long as the system is running it will not time out, but if the system locks then the timer would time out and do a reset. That is a work-around, not a fix. The fix is to first discover just what causes the lockup and how it gets in to the board. It may be coming in on the power leads, or on the communications lines, or on general I/O connections. You could try using a different AC power circuit for the dual voltage supply by running an extension cord to a different power feed circuit. That would be simple and easy, I hope. Motor starting and stopping does cause line disturbances.


Joined Jan 8, 2017
You could try adding a very large capacitor before the 5 volt regulator that was large enough to maintain the supply for a few seconds. You could also feed the input to the 5 volt regulator via a diode from its original unregulated supply and also feed it via another diode from a suitable battery. This could be a temporary setup just to prove that the problem is with the 5 volt supply. Another thing you could do is to have a LED (With current limiting resistor.) connected between the anode of a small SCR (Such as a 2N5060.) and the 5 volt supply (Or it's unregulated source.). You would initially trigger the SCR on. If the power had been lost when you came to check the LED would not be lit. If you put it on the unregulated supply adding a zener diode in series would make it better at detecting the voltage dropping without getting close to zero volts.



Joined Jan 23, 2018
Using a temporary connection to a remote power source is still a method for discovering the path of the disturbance without digging in to the circuit board and risking damage not covered by warranty. And if a remote connection to power is not easily available, you could also use a fairly small UPS unit that includes a well filtered output, which some UPS packages provide. Really, a UPS might even be a permanent solution, if it solves the problem. It will add the benefit of keeping the control board unmodified, so you don't need to document all of the details.
It may also be that the manufacturer of the system is aware of the problem and has a fix available.