Vexing problem

[les45]

I'm having a strange problem with my DC lighting system.
My DC system is fairly simple. It is a computer power supply plugged into an AC outlet. The AC outlet is the only line on a 15 amp mini circuit breaker.
The DC system provides power to two overhead incandescent lights, one LED reading light over the bed, an incandescent porch light, and a Fantastic Fan.
The two overhead DC lights are in parallel on the same feeder circuit and a single DC fuse of 4 amps.
Each of the DC circuits goes through a DC fuse ranging from 4 to 6 amps.
The problem comes when I turn on the rear overhead light and it causes the circuit breaker to trip. None of the DC fuses are blown.
I can be running all of the other DC fixtures with no problem but when I turn on the rear overhead light, it trips the breaker.
It also trips the breaker if I only have the front overhead light on and nothing else running.
Now, the vexing part. When I get it home to work on it it is working fine with no problems.
I took the rear light apart and can see no obvious problem with loose or shorted wires.
I'm not an electrician but I don't understand how a single DC light can cause the AC breaker to trip but it does not blow the DC fuses.
And the fact that it is an intermittent thing that seems to come and go.
It did this one time on a previous camping trip but it was near the end of the trip and I could find nothing wrong when I got it home.
On my latest trip, it did it early in the trip and stayed that way for the last two days of the trip until I get it home again and now it's working fine.

Here is an early pic of my system. The black wire on the left is a temporary AC power supply feeding the breaker box. The two wires going off to the right are the circuit that feeds the two overhead lights. The DC fuse block is in the middle between the AC breaker box and the computer power supply.

[OP827]

Did you check camp site voltage? Is the power source working properly under maybe lower campsite voltage, ie not causing the AC breaker to trip, since its own max current consumption should be lower than 15A at a maximum load on 12V side, correct? Since there is no other loads on AC side except this computer power source I would first check that part.

[Pmullen503]

What do those over head lights draw? There still may be a short somewhere. It's possible I suppose that they overwhelm your power supply so fast the fuses don't have time to blow.

[saltydawg]

We call that a gremlin. No answer except I see multiple breakers, swap them around and see if the problem follows the breaker.

[troubleScottie]

Maybe it a heat problem -- the power supply when running for awhile starts failing. Hence when you get home -- it has cooled, no problem.

When it happens again, try just touching the components to see if they are warm. A good thermometer or IR devise would be safer.

The fan in the housing should be sufficient, but maybe not. Or the airflow is constricted and you are just recirculating the same air which gets warmer over time.

[les45]

Maybe it a heat problem -- the power supply when running for awhile starts failing. Hence when you get home -- it has cooled, no problem.

I thought about that but if I turn off the rear light and re-set the breaker, it will continue to run all the DC fixtures (including the fan at high speed) all at the same time with no problem.

[les45]

I swapped out the breaker for the DC power supply with one that only covers one interior outlet. Everything is still working as it should so I guess I'll have to wait until I take it out again to see if that helped.

[wannabefree]

I know this is an old thread and maybe your problem is fixed, but if it persists consider replacing your computer power supply with a 12V-only power supply. Computer power supplies are designed to provide multiple voltages, typically 3.3V, 5V, and 12V and they expect a load on all of them. The 3.3V output can pump out a lot of current relative to the 12V output and has much tighter regulation requirements. What this means is they can take some shortcuts in the design in which they only regulate the 3.3V output and expect the 5V and 12V to follow along with the leader (while you may be thinking that is really cheesy, I built a power supply for a satellite that worked this way - size and weight matter when things fly). This can result in your 12V output being substantially lower or higher than 12V because the 3.3V output is unloaded. Some supplies may use remote sensing, and if you didn't hook that up you will get no regulation at all. Better power supplies will regulate all outputs, but if you buy the high priced supply then you are not saving any money, though it may be tempting to use the 5V output for USB charging. Check the electronic surplus outlets and you will find reasonably priced power supplies under $30 capable of providing 20 to 30 amps, more than enough for the average tear.
While you're thinking about it, no, the 12V power supply will not charge your battery. It isn't designed to do that. You will need a separate charger for that job. While you can buy all-in-one charger/shore-power combo units, reports are they don't work very well and they are more expensive than a charger and a power supply. I wired my system through a 30A automotive relay so that when I plug in shore power the relay clicks over to light my trailer from the power supply, and when shore power goes away it clicks over to the battery. Just use the 12V from the power supply to energize the relay coil, run the normally open connection to the 12V power supply, the normally closed to the battery, and the center pole of the switch to your fuse panel.

[OP827]

Would be interesting to have schematics of using a combination 12 PC power supply and smart charger. While on shore power the 12V Power supply would be taking electrical loads and smart charger charging the battery. While on battery the charger disconnects from battery and battery connected to the trailer. Can the above be done via a relay logic and 12V automotive relays?

[wannabefree]

OK, words did not suffice. Here's a schematic:

I used a 30 amp auto relay. You can see that when there is no shore power the relay is not energized and the battery supplies power. When you plug in, the battery is charged by the charger and is disconnected by the relay, which is energized by the power supply. The trailer now gets power from the power supply. It's very simple, reliable, and keeps charging transients and other stuff from harming your sensitive electronic stuff. Not that I have any. I went this route because chargers are not expecting loads other than the battery they are charging and I did not want to risk interfering with its operation.

Hope that helps.

[OP827]

Thanks, I am a visual guy and this helps. Now, does it make sense to add another disconnecting relay contact for charger, when there is no shore power or there is no concern to have it connected to the battery all the time?

[wannabefree]

It would do no harm, but I don't think it's necessary. Most of these smart chargers, aka battery maintainers, expect to be connected to the battery full time. There really shouldn't be anything in the charger that would drain the battery when the charger is unpowered.

[wannabefree]

Rethinking the OP's problem, there are really only 2 possible culprits: a defective circuit breaker and a defective (or misused) power supply. I'll say again this is not a good use of a computer power supply and my first shot would be to replace it with a 12V open frame power supply like this: https://www.mpja.com/12-Volt-Power-Supp ... /18435+PS/. OP could be experiencing a power factor problem, which happens when voltage and current get out of phase with each other, usually caused by large inductive loads, e.g. switching power supplies.
As far as the circuit breaker goes, I have seen circuit breakers trip when there is no good reason for it to happen, and the problem has gone away when I replace them. Circuit breakers are either thermal or inductive devices. The thermal units have a bimetal spring that gets hot when too much current is drawn, as the metal heats up is bends and opens the contacts. Thermal breakers are not used in breaker panels so far as I know; they respond too slowly and ahve to cool down before you can reset them. The inductive type have a coil of wire inside, that when too much current is drawn generates a magnetic field that pulls open the contacts. (I am a bit hazy on how this actually works, this is based on what I learned as a teenager tearing apart a defective circuit breaker and that was too many years ago, so be kind if I got this wrong.)
In this era of counterfeit parts you can't always even trust the parts you buy in regular retail outlets. My advice would be to replace the power supply because you should do that anyway, then the circuit breaker if that doesn't solve the problem.

[OP827]

It would do no harm, but I don't think it's necessary. Most of these smart chargers, aka battery maintainers, expect to be connected to the battery full time. There really shouldn't be anything in the charger that would drain the battery when the charger is unpowered.

My "smart" charger from Walmart would have a voltage/current/status LED numbers display still continuously on once 120V power is disconnected and battery is still connected, so it must be drawing some power from battery to do that. So having that disconnected is not a bad idea. Sorry for hijacking and thanks for information.

[wannabefree]

Yes, so it would. It should be a very small load, though, not more than a few mA. I've never checked my charger for draw when unplugged, but it doesn't light up and i would expect any load it presents to be very low. It's true enough that anything that remains connected to the battery that does not have an off switch is drawing some power, chargers included. So rather than believe my glib statements it would be wise to measure the current under these conditions and decide if you can live with it. Sigh, I've been wrong before, I expect to be wrong again.