High Resistance Power Converter

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mbrandon12

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My wife and I recently purchased a 2000 Rockwood Freedom pop-up camper. The previous owner had caused some electrical damage to the popup by powering on the 110V and 12V on the Dometic Refrigerator at the same time. I replaced the burnt out 110V switch on the fridge and everything appears to be working when plugged in. On the DC side, there was a very high resistance (1200 ohms) between the positive and negative battery cables. I traced the resistance to the LP gas detector (900 ohms) and the CS 1200 power converter (the remaining 300 ohms). I suspect they were damaged during the electrical snafu. I also suspect I need to replace both but wanted to know if anyone else has experienced something similar and if there is a cheaper way to fix the problem than buying all new equipment. Any advice or tips wold be greatly appreciated.

Cheers,
Brandon
 
Odds of damage from turning on both 12 and 120 volt switches on a 3way fridge are rather close to zero (In math we would say "Approaches Zero") since the only thing the two heater elements have in common is the control board. and on the control board it is two seperate relays, nothing more.  So the odds are the "Snafu" is not the cause.

Question, What damage did you fix on the fridge.. I can think of several ways IT might be damaged but this would not cause electrical damage.

You said 900 ohms on the propane detector.. let's see 12/900 = 4/300 = 1.66ma  Bit low for a propane detector but... The resistance (Apparent) may well go down when fed with 12 volts instead of 1.2 volts (Due to a diode that may be in the power in lead)

On teh converter looks like you again were measuring the forward voltage of a diode, not resistance.. your Multi-Meter can not tell the difference between those.  One of mine can, but... You don;'t want to know the list price on that one.
 
How did the previous owner manage to power both 12v and 120v at the same time? The control panel doesn't do that, so he must have been messing around with the controller or maybe directly powering the heater elements from an external source. That raises all kinds of possibilities for damage. But I agree with John - merely having both 12v and 120v heater elements active at the same time should not cause a problem.

I'm unclear what you are measuring on the 12v side. Why are you measuring resistance on the 12v source?  Voltage is the interesting value, not resistance. Are you getting +12vdc to the fridge? If not, +12vdc anywhere else? Are you operating from battery or shore power via the CS1200 converter/charger? Is there voltage at the CS1200 output or across the battery terminals? Is the battery charged and can you run ok from battery alone?
 
The previous owner told me he toggled both switches (110V and 12V) on the back of the fridge at the same time. The 110V switch was melted when we bought the pop-up. I replaced the 110V switch and all seems to be fine on that side.

I started measuring resistance because when I attach the battery, I was seeing a voltage drop (13V dropping down to 9V). In my mind this meant I had a short somewhere. I measured resistance across the positive and negative leads and measured 1200 ohms and then started tracing the wiring to find the source of the high resistance. When I found the high resistance at the LP detector and power converter I assumed something could have been burnt up on a circuit board if the previous owner had managed to get 110V on the 12V line.

I posted this thread because I didn't know if having a high resistance on these devices is by design or if they have been compromised. Could it be a bad battery? Thanks for the feedback.
 

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Resistance is inversely proportional to current flow.  High resistance means little current flows through the device.  Low resistance means more current will flow, with zero ohms being a dead short (most current).

Resistance measurements in the 300-1200 ohm range, like John said, means you are only drawing a few milliamps from the battery (1 milliamp = 1/1000th ampere) and sound normal for the devices you measured.

If this makes your battery lose voltage when you connect the wires, you have a bad battery that's not storing any energy.

If you mean the system voltage drops when you connect the battery, that means the battery is discharged and is drawing more current than the converter is supplying.  Wait a bit and see if the charging rate tapers down and the voltage comes up as the battery regains a charge.
 
I don't think you can determine much of anything useful by measuring resistance on the entire 12v load circuit except that it is not shorted (zero resistance). Measuring resistance across the converter/charger output is probably just measuring the output side of a transformer. It would be more interesting to know of the converter/charger is working, i.e. producing 13+ volts with shore power connected. If it is, it is probably fine. The values for the LP detector are probably ok as well.

The resistance measurement that is meaningful is the 12v heater circuit in the fridge. That will be a simple resistance heat device and should have a measurable resistance. If very low, it is shorted and could drag your battery way done when connected. However, a short there is also likely to blow a fuse or burn a wire.

Can you try the battery by itself, with just a simple load across the battery terminals? A battery load tester is the ideal way and most auto parts stores will do it for free. You can do it yourself, though, with a light bulb and voltmeter. You can use a 120v light bulb to increase the load if you want mor eload than a typical 12v bulb (only about 1.0-1.5 amps)..
 
Here's a stupid question. When you guys are saying "converter", are you talking about the 12 DC power supply? I referred to the "lifting mechanism" as a hoist one time and got corrected.  ;D You seem to have your own language.  ;)
 
Yes the "Converter" is the box that "Converts" 120 VAC to what is needed to power your 12 volt systems (13-15 volts depending on conditions).. I also tend to think of them as "power supplies" because... Well, the ones I designed and built were single stage, well filtered, and designed to power one or more radios or computers.  The top line "Converters" uses on modern RV's are better for battery systems. (low end are not much different then my radio supplies).
 
Thanks for all the feedback. I'll have some time on Friday to run some tests. I'll let you all know what I find. Much obliged.
 
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