Hi,

I've got a rather large lead-acid charger laying around, which is capable of outputting 12v/24v at 50a, and also has a 300a jumpstart function. It's ancient so it weighs a ton, it's missing the wheels and part of the casing is gone.
So basically as-is I have no use for it. I'm wondering if I can somehow modify it to work as a 12/24v high current power supply.

As it's ancient it has no load detection circuit so it's basically 'always on'. The output is actually 16/32v measured at full load (hooked up to a 24v 250a motor), and for some reason 34/76v under no load (this part makes NO sense to me at ALL)

In any case, I was looking for a way to adapt this unit directly, turning it into a regulated power supply. If that is not possible or feasible, taking out the transformer, and using that to make a linear power supply.

I'm sure I forgot some crucial information, I just can't seem to think of what. If you miss anything please ask
All input appreciated, thanks in advance!

 

I'm sure I forgot some crucial information, I just can't seem to think of what. If you miss anything please ask

Here's a crucial question: Why?

Really! I'm more than a few decades old and I've never used one of these, so I wonder what you plan to do with it. Once you know what you want to do with it, you can think about how to make that happen and whether it's worth the time, effort and expense. If not, sell it for the copper and move on.

 

Here's a crucial question: Why?

Really! I'm more than a few decades old and I've never used one of these, so I wonder what you plan to do with it. Once you know what you want to do with it, you can think about how to make that happen and whether it's worth the time, effort and expense. If not, sell it for the copper and move on.

Mainly? Because it's possible! I'm extremely interested in this kind of stuff. Right now I've got a 300w lab psu which is great for powering small projects and testing, but I need something slightly bigger.

As for what I want to power: I acquire broken electronics from cars frequently. I try to repair them and resell. I also own a small shredder that runs off a starter motor. That thing pulls 2.4kW at 12v as well.

Now I could just get a new car battery (+charger unless I get 500Ah), but that'd get pretty expensive too.

Now time and effort are currently not really issues, expense may or may not be.

 

OK, so you primarily want a 12-15V beast supply, minimum 20A , but 50A would be better.

I'd be tempted to take some turns off the secondary to reduce the unloaded voltage. You don't need 76V!

 

Interesting and definitely in my area of high power gear.

The 16/32 volt DC output would be about right for a high current 12/24 charging systesm and I am suspecting that given its age that it either used selenium plate rectifiers which is why the transformer has a consdierably higher AC voltages or you are reading the two ends of the center tapped secondary winding and it used a half bridge rectification system.

As for turning it into a large linear power supply it's no different than making a small one. You just use bigger components.

 

Exactly.. though I don't know if it's supposed to be this way. Maybe something's broken, I'll have to check later.
If it can maintain +-24v instead of anything between 36-76v I can use it to power 'medium-sized' projects too, that take more than 10a without fear of spontaneous combustion because of overvolting.

 

Interesting and definitely in my area of high power gear.

The 16/32 volt DC output would be about right for a high current 12/24 charging systesm and I am suspecting that given its age that it either used selenium plate rectifiers which is why the transformer has a consdierably higher AC voltages or you are reading the two ends of the center tapped secondary winding and it used a half bridge rectification system.

As for turning it into a large linear power supply it's no different than making a small one. You just use bigger components.

There you go

 

Exactly.. though I don't know if it's supposed to be this way. Maybe something's broken, I'll have to check later.
If it can maintain +-24v instead of anything between 36-76v I can use it to power 'medium-sized' projects too, that take more than 10a without fear of spontaneous combustion because of overvolting.

I doubt it's broken being a broken transformer typically only does one or two things. Growl badly and/or smoke.

Picture wise I can't tell what its got exactly. It almost appears to be SCR based rectification which if so then there should be a voltage control board to stabilize the output voltage which would make turning it into a large regulated power supply even easier.

 

I doubt it's broken being a broken transformer typically only does one or two things. Growl badly and/or smoke.

Picture wise I can't tell what its got exactly. It almost appears to be SCR based rectification which if so then there should be a voltage control board to stabilize the output voltage which would make turning it into a large regulated power supply even easier.

How do I find out if it's SCR based? I can't seem to find any transistors there (though I'm not sure what those white things on the first and last plate are). There are buttons on the front which control the current and the voltage, but with all of them set to low output is still 60v instead of 24.

 

If you want smooth DC then you will need a (very) large filter capacitor since battery chargers don't normally have (or need) filter capacitors as the battery provides the filtering function and batteries don't care if the charging current is not smooth.

Also the voltage will likely vary significantly with current.
If you want voltage regulation than that will require some high power electronics with large heat sinks to dissipate the power lost during regulation at high currents.
If it's SCR controlled as TT suggested than that would greatly help and perhaps eliminate the need for additional regulation circuitry.

 

How do I find out if it's SCR based? I can't seem to find any transistors there (though I'm not sure what those white things on the first and last plate are). There are buttons on the front which control the current and the voltage, but with all of them set to low output is still 60v instead of 24.

What are the blue and white wires going to? They don't look like normal rectifiers and they are obviously not selenium plate rectifiers either which leads me to believe they are some sort of SCR devices which if so means those blue and white wires go to a control board of some sort.

 

What are the blue and white wires going to? They don't look like normal rectifiers and they are obviously not selenium plate rectifiers either which leads me to believe they are some sort of SCR devices which if so means those blue and white wires go to a control board of some sort.

The blue wires go directly into the main power switch, 1 being from the power cable and 1 from the device mounted on the rectifier's back plate. The white wires connect the device on the back plate to the one on the front plate. The front plate device also has a brown wire going in.
So I suppose the white devices are diodes of sorts, and that the blue wire coming out of the back plate's should be brown as it's directly connected through to the live wire

 

Also, off to bed.. will check again tomorrow. Thanks for the replies so far!

 

Okay, I see. Those are thermal switches that shut the power off if the rectifier plates overheat.

Given that, I have to suspect that it uses normal old style press fit button diodes.

Pictures of the whole thing from different angles and possibly a wiring layout would be nice.

 

Okay, I see. Those are thermal switches that shut the power off if the rectifier plates overheat.

Given that, I have to suspect that it uses normal old style press fit button diodes.

Pictures of the whole thing from different angles and possibly a wiring layout would be nice.

There are diodes pressed into the plates as one of the pictures will show. I hope this helps - I haven't been able to actually find a wiring layout, but if required I'll try making one later today.

 

So, after taking another look at the rectifier.. I'm now rather confused as to how to draw such a thing. Seeing as how it's really just one component, but it contains components - diodes, caps, thermal switches... I'm not sure where to go from there.

 

Google for an image of a full bridge rectifier, or a half-bridge rectifier. Note that diodes are directional.

Or, you might search for an old-fashioned battery charger schematic. There might be one drawn up somewhere that resembles yours. No use starting at zero - it's easier to edit an existing work.

 

Here's a schematic for a slightly larger charger from the same company. Mine does not have a timer, and has 2 more switches for boosting the current. Other than that I believe they're mostly the same.