Solving RV Power Converter Issues with EE Skills

[natemoore]

Hi. I love it when one's college degree has real world applications :-) It has been 25 years since I got my B.S. in Systems Engineering from USNA. However a detour in the Marine Corps, then 15 years as a remodeling contractor has left my EE skills practically non-existent.

I have an RV with a faulty converter. The RVs lighting is 12V DC, but the outlets are 120V 60 Hz AC. The converter supplies DC voltage to the lights. The problem is that the lights are dimming and flickering. AC input is confirmed good. I measured the DC output voltage with all the lights on. It started out at 13.6V (normal according to specs), but then slowly bled down to about 7 volts then stayed there. You can hear the cooling fan spool down as the voltage drops, as well as see the lights dim.

Here's what I remember from my EE courses. Start with 120 volts, run it through a step down transformer to get 14-ish volts AC, then send it through a rectifier, then a capacitor, then a voltage regulator, voila!, 13.6 volts DC. Obviously, my converter is much more complicated than this. Plenty of caps, a great big inductor wrapped around what looks like a ring magnet, some resistors, and a circuit board controlling the cooling fan among other unknown things.

My recollection is that electronics generally either work or do not work. So this behavior is perplexing. Voltage starts off good, then bleed down to half what it should be. Reset the breakers, turn off all the light, then turn them all back on, the cycle repeats.

The only thing I can think of is that Vr--the reference voltage for the regulator--is being drained, whereby driving the output down.

I examined the circuit board and all components. I see nothing unusual. All soldier joints are good.

Any ideas before I pop another $150 on the RV?

Thanks!

 

[vk6kro]

As a first step, could you try this converter on a 12V battery in another vehicle?

You need to be sure the problem is not in the current vehicle.

If it does prove to be in the converter, you will need to track down a schematic for it, at least.

A brute force method of fixing it without a schematic would be to replace any electrolytic capacitors in the converter.
You may be able to find faulty ones by looking for bulging cases or white powdery deposits near the ends of the capacitors. Or, they may feel hot.
Capacitors can be faulty without any obvious outward signs of it, though.

This is a bit of a gamble as you will be replacing some capacitors that don't need it. However, compared with the cost of a new converter, it isn't much of a risk.