I have several old battery chargers kicking around. I decided to look at this one because it has no labels left and so it's a bit of a mystery.




Searching around, it looks like it might be a Schauer K412. It's in working order, so I figured I'd clean it up a bit and see what's inside.




Looks like a center tap transformer that reads approximately 13V-0-13V. The diodes are the old selenium type. Oddly, they are on the negative side. The battery positive connects to the center tap. The battery negative goes through what I'm guessing is a thermal breaker, and then through the diodes back to the transformer. Would this be an example of a positive-ground circuit?




Anyway, I'm trying to decide if I want to restore it like it was, modernize it, or maybe turn it into a general purpose power supply. I think at the very least the power cable should be grounded since the case is metal.

 

Looks like a center tap transformer that reads approximately 13V-0-13V. The diodes are the old selenium type. Oddly, they are on the negative side. The battery positive connects to the center tap. The battery negative goes through what I'm guessing is a thermal breaker, and then through the diodes back to the transformer. Would this be an example of a positive-ground circuit?

Hi rebel,
As the transformer provides isolation from the mains supply, you can choose which terminal is the reference Common.
The metal case should be connected to the mains Earth wiring.
If you change the Selenium diodes to say Silicon types, the rectified DC will be higher, as the Selenium types have much higher forward voltage drop.

If you do upgrade to a bench PSU, I would add a DC On/Off switch.

Also, you could use a bridge rectifier across the transformer secondary ends, you would get a dual +/- DC option with the centre tap as Common.

E

 

If you are going to use it as a bench DC supply, with updated rectifiers, you should be aware that the older type of 12V simple charger use the forward resistance of the rectifiers plus the resistance of the transformer secondary winding to limit the current. The output voltage will be very dependent on the load current.

 

If you are going to use it as a bench DC supply, with updated rectifiers, you should be aware that the older type of 12V simple charger use the forward resistance of the rectifiers plus the resistance of the transformer secondary winding to limit the current. The output voltage will be very dependent on the load current.

Using a repaired battery charger as a DC supply is quite reasonable. Certainly I would replace the selenium rectifiers with adequately rated silicon diodes, maybe adding some heat sinking. If the anodes oof the newdiodes are on the threaded post ens then they can be mounted on heat sinks tied to the case, and the case will be the negative output connection. If you want voltage regulation then it is quite a bit more work, also if you want filtered power you wwll need to add a filter circuit.

 

As it's build, the charger isn't all that great. It will keep charging a battery up to 16V, or maybe even 17V without any kind of limitation other than waiting for the battery to match the output voltage. So I think instead of just refurbishing it, I'll convert it into a benchtop power supply.

I wasn't able to identify the transformer, so I've been doing a bunch of tests and measurements to see what it's capabilities are.





I think it could continuously output 5-6 Amps. The thermal breaker is rated for 8A, and the similar chargers I've seen online look like they're rated for 6A. At 4A the transformer gets warm but not hot. The old selenium diodes sure get hot though. They only drop the voltage by about 1V as far as I can tell, but I'm going to replace them anyway. They don't seem to be able to handle the current.

I think I have a full bridge rectifier somewhere that can handle the Volts and Amps. Just gotta find it. I don't have any capacitors that could smooth out the resulting ripple though. I'll have to order that.

 

Which way are you leaning: +/- outputs, or a single output?

Also, any output regulation, or is this going to be a "bulk" supply?

Just to be clear, what is the open circuit voltage across the secondary?

ak

 

Good questions in post #6. I have reconditioned a few old chargers and used the first one for many years. The recondition consisted of replacing the selenium rectifiers, cleaning the switches, and new cords for both the battery side and the line cord. On several units since then the main failure has been the output cable and the battery clamps. The #20 wire suffers a flex-fail breakage at the clip end. So that cable gets replaced with #16 lamp cord, much better flex protection at the clips, and soldered attachment electrically. The rectifiers also get replaced if they are not silicon diode types, or if they fail a diode test. And the 6 volt/12 volt switch gets either cleaned or replaced. Some chargers switch the primary feed to select the output voltage, others have switched connections on a small circuit board that seems to include either a transistor or some arrangement to select secondary taps.
One of them got a Variac to adjust the transformer output, and terminal posts, along with a filter capacitor. That is a handy non-regulated DC supply for checking assorted stuff. The output can be as high as 17 volts, no load, the current up to 5 amps. (Thee Variac transformer was salvage with a damaged case.)
And always the case gets a cleanup and coat of spray paint, which does make it look so very much better.

 

I gave it some thought, and I don't really need the option for multiple voltage outputs. I prefer to use adjustable buck converters. I imagine setting this charger up with one that also has adjustable current limiting.

The open voltage on the transformer is 26V. Keeping the Transformer output at 5A or less, means 130W of power or less. After rectification and smoothing, I'd expect the DC to be around 35V. Which means a buck converter that can handle more than 35V, 130W input. I'll have to shop around and see what's available.

There are computer power jacks available that are grounded and have an internal fuse holder. I may get one of those and a matching computer power cable to replace the old power cable. There are no switches to replace, so I'm thinking of adding a basic power switch to the front. I have several tools that have banana jacks for the DC output. I'll probably use those instead of a built in cable.

Something like this:
Except the fuse should probably only be 2A, not 10.



Many buck converters come with a built in display. Maybe I could get one of those and not need to add my own. The charger's box is not very big.

 

The one change that I would make to that circuit would be to put the thermal overload device in the feed from the transformer to the bridge rectifier. That would allow it to also protect against a failure in the rectifier bridge, such as a shorted diode.

 

Good idea. I've remembered though, that the thermal breaker is rated for 12V. The new configuration will be around 36V. I may look for something rated for that voltage.

The capacitor I ordered is 8200µF. I calculated that there should be a 5V variation in the output once the capacitor is in place. My oscilloscope is no good for measuring that, so I have to make due with my multimeter which says about 36V. Hopefully that will be good enough for a buck converter.

 

Most of the case parts are clean and de-rusted. Looks like there will be room for everything, though I probably won't be able to put the power socket in like I wanted. It'll be just a cord and gromet through a hole in the back to save room.

Here's a picture of the new capacitor temporarily connected:




A new oscilloscope is on my list of things to order, but it still won't be one of the nice expensive ones. Should be better than the one I have though.

I added a ground wire and so far all the parts are having continuity to it. I'll have to make sure that's still the case once I paint everything. The capacitor should sit nicely in the back, and I found a spot for the new rectifier.




I should have plenty of room for the buck converter, and it looks like the two holes in the front will be perfect for banana jacks. I found a heat sink that fits on the back of the case pretty good. I may have to drill a couple holes later, but that's fine.




I have a panel mount fuse holder that I'll add to the back somewhere. The buck converter I'm looking at has a programmable maximum current limit that operates independently from the adjustable current limiter. It also has a built-in temperature sensor, and I can set a maximum operating temperature.

 

I just examined the transformer of an older charger redone into a regulated supply. Using the standard of "100 watts per square inch of center core" , it is only good for 75 watts, (1 Inch thick by 3/4 inch wide). So that would be about 8 amps at 100% load. I have run similar supplies at that load for hours and they do get rather hot. The chargers were marked as "6 Amp" chargers when they had the old rectifiers in use. The 10 amp silicon bridge has less drop. I use it as just a full wave, not as a bridge. That would be for 24 volt charging, which i do not need .
In the same package it should be possible to have a much larger transformer and provide up to 20 amps. But it would want a small blower for air cooling.

 

My transformer core is about 2.4x3.8cm. The 100W per sq in. rule would suggest about 140W.

My concern is how many Amps to put through the transformer wire. I never have found a strait-forward answer to internal transformer ampacity. Just lots of conflicting "rules of thumb" and crazy equations with variables that I don't know.

I got a bunch of drilling, grinding and filing done today.




An old computer power cable has been sacrificed to provide a grounded cable for the input. Also, since it is now polarized, the hot wire will go through the switch and fuse. The switch and the bezel for the voltage regulator didn't turn out quite as good as I'd hoped, but they will work.

 

I see the connectors on the front panel and I wonder if those are the power out connections. They appear to be the type to mate with the guarded banana plugs on meter probe leads. The current rating is unknown, but probably not very much.
In the photo the panel and back look good, and it appears that the meter will be digital, not the traditional analog type found on common chargers. How would the digital meter be powered? some of them need an isolated supply.

 

According to the advert the banana jacks are rated for 35A. But your right, the cables I use will need to be able to handle at least 5A, so regular multimeter cables won't be sufficient. The meter is indeed digital, though it is actually a voltage regulator and current limiter. It will run directly from the rectified transformer output.

 

The meter sounds like an interesting and useful product for one of my applications. Could you provide any information about the model and the source?
And for the transformer, the "100 watts per square inch" was for the capability based on the portion thru the center of the coil, and 60 Hz power. . " My transformer core is about 2.4x3.8cm. The 100W per sq in. rule would suggest about 140W." With the possibility of a 130 watt load, mentioned already, that is good.

 

I bought this fellow from Amazon: https://www.amazon.com/gp/product/B0BQBXMH67/ref=ppx_yo_dt_b_asin_title_o00_s02?ie=UTF8&th=1

Hopefully it works ok. I haven't tried it yet except to verify that it turns on. I chose it because it has the voltage and current range I'm looking for, and it has a bunch of safety features built-in. The one I'm most interested in is the programmable maximum current setting.

Looks like the weather isn't going to cooperate for spray painting for a while, so I'm looking in my stash of regular paints to see what will look good for this project. I think the black paint I've used on the base has gone bad. It's supposed to be semi-gloss, but it's drying super powdery. I mainly want to protect the metal from future rust, but I'm not sure this paint will do the job.

 

I bought this fellow from Amazon: https://www.amazon.com/gp/product/B0BQBXMH67/ref=ppx_yo_dt_b_asin_title_o00_s02?ie=UTF8&th=1

Hopefully it works ok. I haven't tried it yet except to verify that it turns on. I chose it because it has the voltage and current range I'm looking for, and it has a bunch of safety features built-in. The one I'm most interested in is the programmable maximum current setting.

Looks like the weather isn't going to cooperate for spray painting for a while, so I'm looking in my stash of regular paints to see what will look good for this project. I think the black paint I've used on the base has gone bad. It's supposed to be semi-gloss, but it's drying super powdery. I mainly want to protect the metal from future rust, but I'm not sure this paint will do the job.

YESTERDAY I did try to spray paint something outside. No breeze at all until I started to spray. Mother Nature can certainly be grouchy some times.

 

Finished! Painting was definitely the hardest part.





I borrowed a set of output cables from my other power supply to try it out.





The bulb got hot and started melting the plastic box right after I took the picture. But the power supply had no issues.

The Chicago Electric charger earned its keep again. This time it jump started my lawn tractor after several months of disuse. I really thought I'd only ever use the charger function and not the starter function on that guy, but it's come in very handy.

 

If that bulb is the same as in post#5, it may be a backup light bulb, which are VERY HOT indeed. a much smaller bulb would be appropriate as an indicator, I don't think it is a circuit element.
Have you experimented wit varying the output voltage with a triac light dimmer?

I just acquired a 1KW microwave transformer and I am thinking of rewinding it into a low voltage high current supply transformer. I might be able to cut out the welds and then rewind it much more easily. If I use 3 strands of #12 wire it should be good for 50 amps really and 60 amps in theory. That will depend on how many turns per volt it produces.