RV AC/DC converter overcharging issue

[Corliss]

Hi everyone new to this forum, need a little guidance on a 12 volt converter made by "systems monitor inc" Here is the sequence of events I experienced with my RV. I installed some 12 volt LED bulbs in the fixtures, within the first hour of one being left on I returned to the RV to find the bulb I left on had exploded. First thought was to check polarity and voltage to the fixture. I found the fixture voltage to be 16.1 volts, I unplugged the RV from the 110 supply, remeasured the fixture voltage to find 13.1 this confirmed to me the converter to be the cause. I removed the converter from the RV, took the cover off to do a visual inspection, nothing burnt or obvious. I am assuming the voltage regulator is the culprit, a search of voltage regulators was confusing. Here is a photo of the converter if anyone can with direction or help I would appreciate it.

 

[d123]

Hi,

If you haven't located the fault yet, could you put together a schematic of how it was all wired together when the LED bulb broke? It's a bit hard to suggest anything without knowledge of at least the actual converter and the bulb, maybe links to the converter manual and bulb datasheet would help.

 

[BradtheRad]

Is your converter made to charge a battery? That may be the reason it produces 16V.

Evidently some (but not all) automotive items can stand up to 16V. If you could drop merely 1 or 2 or 3V, then your led could receive a safe voltage. You'd need to install an led, or resistor that achieves such a voltage drop. Not easy to do.

If it were me I would turn a screw, or anything that looks like an adjustment. (And keep track of anything I do, so I can return everything to its original state.) Have caution because such tactics can ruin the converter.

Sometimes the adjustment is in the form of a fixed resistance which was installed at the factory. In other words, it's a component which affects the control section somewhere, but it can't easily be changed. If you can locate it, then you could experiment with its value, and see whether output voltage is affected.

Looking at your photographs, I notice the bare coils of wire, simply because it's accessible and substantial enough that it looks like it carries power, in a manner that adjusts power. As it turns red its resistance goes up, possibly performing a current-limiting function. So perhaps by shortening it you could cause output voltage to drop? Or would it cause output voltage to rise?
Suppose you were to clip a power resistor across its terminals? 100 ohms? 10 ohms? That would reduce its resistance. Would it affect output voltage? This is a tactic that could ruin the converter, so I should emphasize 'If it were me'.
Or else its resistance may need to go up, if you want output voltage to go down. This creates a challenge. It's merely speculation on my part, of course. You may get a reply with more helpful advice.

 

[Corliss]

Thanks Brad, I've been reading about the transformer to find the number of windings is what changes the voltage. Could the transformer have failed creating the higher voltage?

 

[betwixt]

Transformers like that fail if they overheat and the insulation between turns of wire breaks down. When they do that the result is (hopefully) a blown fuse but the chances of one producing too much voltage is almost zero.

The voltage it produces SHOULD be higher than battery voltage to enable it to 'push' power into the battery so it charges but the regulator unit should limit the power it can produce so that for example a completely discharged battery can't overload it. As far as I can tell from the pictures, the transformer is center tapped and the two ends go to rectifiers on the heatsink and then to ground. The DC is taken from the center tap to a relay (the black square with the blue disk in it) and the relay switch contacts either connect or bypass the current limiting resistor (the coil of wire on the small board).

This kind of circuit produces 'dirty DC' and it relies on a good battery being across it's output for it to work properly. If the battery is bad or disconnected, the voltage will rise substantially but more importantly, it will not be steady, your meter will show the average because it can't react to the fast variations but it will in fact go from zero to about 25V about 120 times every second. The high voltage peaks may be enough to damage your LEDs. Incandescent lamps are more forgiving and the higher current they draw will also drop the voltage somewhat. That regulator isn't like the ones used to stabilize a fixed voltage (which is what you need), it just kicks in under overload conditions to limit the output.

When you tried this out, was the battery still connected? If it was, please measure the voltage directly across the battery terminals when the charger is switched on and tell us what it is.

Brian.

 

[BradtheRad]

Thanks Brad, I've been reading about the transformer to find the number of windings is what changes the voltage. Could the transformer have failed creating the higher voltage?

A transformer is durable, not likely to be the failing component. Perhaps a regulating device failed and the converter reverted to its maximum voltage. On the other hand perhaps the converter really is behaving normally. It looks robust like it was made to power an RV appliance, refrigerator, pump. Or charge a vehicle battery. Etc.

 

[betwixt]

Note the absence of capacitors Brad. It produces DC pulses with an average of ~16V (based on meter reading) so it relies on the battery as reservoir. The peak voltage for LEDs is probably excessive and if the LEDs have their own built-in current regulator they wont like the pulsed supply.

Brian.

 

[BradtheRad]

Note the absence of capacitors Brad. It produces DC pulses with an average of ~16V (based on meter reading) so it relies on the battery as reservoir.

I believe you're right. No smoothing capacitors. It looks like a battery charger. Your post #5 gives 25V as the peak of the waveform. This can destroy devices, unless we hook up a battery to bring it down to 12 or 13V.
And then wouldn't a battery get charged all the time it's connected? Or perhaps that is the designed intention with this power converter? There's a gap in my knowledge about RV appliances and RV electrical systems.

 

[betwixt]

I'm no expert either. I don't own an RV and my own vehicle is one of the smallest Diesel engined cars on the market - but then its 7 years old with only 16,000 miles on the clock.

I think this is just a big battery charger and guessing by the size of the transformer, continuously rated at maybe 6A or so. The coil resistor is probably only about 1 Ohm and the output current itself will pass through the relay coil to pull it into service. If the battery is good, it will work, if it isn't absorbing charge or the batterry is disconnected it would wreak havok on the whole electrical system. It certainly isn't a substitute for a battery if thats how it was used. I would guess other electronic items such as radio or TV would be at risk of damage too.

Brian.

 

[Corliss]

Hello Brian and Brad thanks for your thoughts. Yes it is basically a battery charger. its purpose is to recharge the rv batteries and power the 12 accessories in the rv when plugged into 110 volt. Most chargers for this purpose that I've seen produce 13.2 volts to a max of 14.4 volts, some step the voltage to prevent sulfation of the batteries. Brian I did perform the test you asked, plug in the rv with it connected to a 12 volt battery, the transformer output was 17.87 volts. Not connected to the battery it was 18.01 volts. My battery voltage was at 12.54 so I guess it is why the voltage dropped to 17.87?

 

[BradtheRad]

14.4V is typical end of a charge session.

12.6 or 12.8 is normal resting voltage with little or no activity.

13.2V is reasonable maintenance. About right for putting a continual trickle charge into the battery. Provides small current to the clock, security, etc. Ensures the battery won't become depleted. So I can picture an RV sitting hooked up to your charger continually, while the battery is maintained at a healthy 13.2V.
Your 12V led will be exposed to 13.2V peak. Whether this is advisable depends on its internal protection.

 

[betwixt]

its purpose is to recharge the rv batteries and power the 12 accessories in the rv when plugged into 110 volt

That's what concerns me. It will work as a battery charger but not as a substitute for it. If you use it without the battery it WILL damage other electrical items. It needs the battery to act as a reservoir, sinking the high voltage pulses and feeding the dips.

When you say it produces 17.87 Volts but the battery is 12.54V are you saying there is 5.3V drop in the wiring between the two?

The other worrying point is how are you measuring the voltage, especially with the battery disconnected. The unit doesn't produce a steady output voltage, it is unsmoothed DC, the polarity doesn't reverse like AC but the voltage goes between zero and about 25V at the rate of 120 times per second. You meter is only telling you the average because it can't react fast enough to see the rise and fall, you need an oscilloscope to do that. The risk to anything electronic, including LEDs, radios and even vehicle instrumentation if you feed them 25V is very high, they will not respect the average voltage!

Brian.