I have a repurposed power supply “Converter/Battery Charger” I salvaged out of an RV that I have used for my electrolysis tank for a number of years that I use to remove rust for my projects. I won’t need to use it again until warm weather as the tank is outside, but thought this would make a good post for the new forum. Have used it with this setup for a number of years until it lost output last summer. Input is 120V AC 60hz and output is 12V DC 40 amps (has a full wave rectifier) when it’s working correctly. I get 22VAC across the output windings of the transformer. It has a large cap that connects to a winding on the output. Doesn’t appear to be a filter cap the way it is connected. It has 2 rectifier diodes and they both test fine. Pulled a leg off the cap and discovered it is shorted. Left the lead off the cap and the output voltage of the supply goes up to 5.5VDC . The only info I could find about the circuit is that it is “ferroresonant”. The cap is 6.9uF 600V. I can find caps in that capacity range ( up to 200 or 300 volts) but none that go that high in voltage. Does the cap need that high of a voltage rating? Can anyone explain how this circuit works with the cap on the separate output winding?
Its made by Triad model # Tu-440-2 . Couldn’t find specifics on that model, but I’ve posted everything I have on it here:
https://drive.google.com/drive/folders/1duXTYs5pHP2gq6Ba3nWpXR5Xrb8ixTc8?usp=drive_link
The schematic shows 4 diodes and 3 caps, where mine only has C1 and 2 diodes, but otherwise is the same.
Thanks
I consider myself a fresh learner when it comes to electronics, so take everything I say with a big block of salt 🙂
The first two thoughts that crossed my mind were that the cap is either meant to be ballast, or it’s used to condition the frequency on the secondary side.
Ballast would be odd, since it’s usually in series with the load itself.
Frequency conditioning is also odd, as I’d expect trying to force a frequency on the secondary side would also effect the primary… OTOH the manual in your linked google drive does say that it expects the primary to be driven from a generator and in the “What is it?” section bullet point 10 does state “Frequency is more important than voltage”. Can you measure the inductance of the secondary to which the cap is attached? Using the f=1/(2*pi*sqrt(LC)) formula for inductance-capacitance resonant frequency I’d expect something along the lines of 930mH to 960mH.
Regarding replacing the cap itself: since it’s directly attached to the transformer, I’d expect it to be a non-polar type of cap, probably a foil cap of some sort. Digikey and Mouser have caps in the range from 6uF and 8uF caps rated for 660V AC available. Depending on the space available, you could also try hooking up e.g. two 3.3uF caps in parallel to get close to the original 6.8uF, but I’d be careful to add some insulation to any exposed leads and make sure any cableing you’re using is rated for 600V+.
Again, this is just a shot in the dark. Looking forward to see if anyone knows that that cap is really used for.
@srd This is a large metal cap in a can like you would see in a motor or an appliance. I posted a pic of it with the other docs on google drive.
I have a feeling that the voltage rating for the capacitor is just extra cautious. A 400V cap would probably be fine. I don’t think the capacitance is super important either.
I have come across battery chargers like that in the past. They are called “Ferroresonant” supplies. Here is a link that explains their operation . If that doesn’t work, just google “ferroresonant” and you’ll find lots of links. https://www.sunpower-uk.com/glossary/what-is-a-ferroresonant-power-supply/