Hi All!
I have an old automotive battery charger that I have, for many years, used for electrolytic rust reduction. I never have had a problem until this week.
It is the non-automatic type (in other words, there is no circuitry to stop charging the battery when the battery has sufficient voltage). The other day, the unit failed so I opened it for diagnosis. That's where my confusion began! I should know the answers to my questions, but maybe I am having a 'slow brain' day today.

The failure was that one diode shorted cathode-to-anode in both directions. I think I must have accidentally shorted DC (-) and DC (+) by placing my anode & cathode too close together in the electrolytic bath (or used too much washing soda, the conductive chemical, in the water or had too large of an anode load in the bath).

I eliminated the bad diode and used the device again successfully. A day or so later, I had a second diode fail by shorting anode-to-cathode. I eliminated it. The DC voltage was lower than after only one diode was skipped, but the charger worked. (I don't know what the voltage was when all four diodes were in the circuit since I never measured it before the first failure.)

Note that the thermal circuit breaker shown in the attached diagram did not open upon diode failure. Instead, a melting link in the primary that is tucked into the transformer body opened. I replaced it with a piece of Sn 63 solder for the time being. It did its job and melted when the second diode failed. (Reasonably close in melt temp to the original, but not exact)

The device has a pair of SPDT switches that allow the user to select between 2 amp, 20 amp or 50 amp charging. The owner's manual says the 50 amp is limited to ten seconds of use. I assume that the 2 and 10 have 100% duty cycles.

At first glance, I thought that the transformer was center tapped because there are three terminals on the secondary. After some meter testing, it appears that this is not the case. The center terminal goes directly to the DC (-) and the two outer ones go to four diodes. I believe it is not center-tapped because the voltage between the two outer terminals is near zero (about 20 mV) no matter the position of the SPDT switches.

The four diodes are wired in parallel and line 2 of the transformer's secondary goes directly to DC (-). I think this arrangement is called Half Wave rectification because the transformer is not really center tapped and no diodes are on Phase Two of the transformer. I do get confused, however, because the frequency across DC (+) & (-) is 120 Hz. It seems to me that a half-wave rectifier circuit would show 60 Hz and Full-Wave & Full-Wave Bridge would show 120 Hz.

As far as amperage changes, all I can figure out (or guess) is that the switches select different amounts of primary windings, increasing the voltage, thus the available wattage & current. Why do I think this crazy stuff? Because I get different voltages between either of the outer terminals and the center terminal as the switches are closed or opened. For example, it will change from 9.45 vac to 11.73 vac when switching between 2 and 10 amps. It jumps to 13.88 when 50 amps is chosen. (With the two bad diodes bypassed.)

The Grand Plan is to remove the diodes and install a Bridge Rectifier (and maybe a smoothing capacitor just for fun). I'd connect it as explained in the lower left of my attached drawing. Part Two of the plan is to provide protection for the transformer.

My questions are:
A) Am I wrong & this really is a center tapped transformer? If so, why is the voltage between the outer terminals near zero?

B) How does the device provide more charging amperage to the battery being charged? Just by increasing the voltage output by selecting more or less primary windings?

C) Is this parallel diode arrangement called "Half Wave" since the two outer transformer taps are the same phase? (But why then is the frequency at the DC side 120 HZ, like a full-wave or bridge, instead of 60 HZ, like half-wave would provide?)

D) Do you have a guess of what the VA of the transformer is? There are no markings. My guess is something about 120 VA (10 amps x 12 nominal volts)

E) If I install a bridge rectifier and want a smoothing capacitor(s) across DC (+) and (-), do you have an idea of the micro farad required? I tried some on-line calculators, but get many different answers. (Form 2,000 mf to 100,000 mf)

F) I would like to protect the transformer with a fuse or breaker on the primary. I am having trouble deciding on a fuse size. At first, I thought a 2 amp fuse would be OK because the rating plate shows 1.87 amps at the 10 amp setting. But the trip curves for fuses & breakers show less than one second for a 50 amp load to open the fuse. The fuse's purpose is to stop the melt link from opening if (when) I goof up again. (If necessary, in order to protect the transformer, I am more than willing to eliminate the 50 amp selection. I don't use it.)

I apologize that the attached drawing is rather rough. I did it this quickly during this morning's cup-of-tea break.

Thanks for your suggestions & thoughts about this device & my questions. I sure appreciate your help!

Enjoy This Day!
Paul

 

Your transformer is a Centre tapped full wave circuit, it uses both halves of the transformer in parallel per half cycle, giving twice the current rating, the Diodes are in parallel to boost the current drain.

Putting a smoothing capacitor on it will just increase the voltage output and may damage your battery cells.

 

Thank You very much Dave for your explanation & for the drawing.

When you said it uses both halves of the transformer in parallel per half cycle, is half cycle why I read zero volts across the two outside transformer taps instead of double the voltage of either tap to the center tap?

For some reason, I expected the upper set of windings to be energized 180 degrees away from when the lower set was energized, thus giving a higher voltage reading 'tap-to-tap' than either 'outer tap-to-center tap'.

Paul

 

The Centre to Outside winding should read half of the Voltage between both the outsides, so if your transformer is a 12-0-12v AC you should be getting 12V AC between centre and any outside winding, and 24v AC between both outsides.

Only one Diode will conduct per half cycle, the other will conduct on the opposite cycle.

 

Hi All!
I have an old automotive battery charger that I have, for many years, used for electrolytic rust reduction. I never have had a problem until this week.
It is the non-automatic type (in other words, there is no circuitry to stop charging the battery when the battery has sufficient voltage). The other day, the unit failed so I opened it for diagnosis. That's where my confusion began! I should know the answers to my questions, but maybe I am having a 'slow brain' day today.

The failure was that one diode shorted cathode-to-anode in both directions. I think I must have accidentally shorted DC (-) and DC (+) by placing my anode & cathode too close together in the electrolytic bath (or used too much washing soda, the conductive chemical, in the water or had too large of an anode load in the bath).

I eliminated the bad diode and used the device again successfully. A day or so later, I had a second diode fail by shorting anode-to-cathode. I eliminated it. The DC voltage was lower than after only one diode was skipped, but the charger worked. (I don't know what the voltage was when all four diodes were in the circuit since I never measured it before the first failure.)

Note that the thermal circuit breaker shown in the attached diagram did not open upon diode failure. Instead, a melting link in the primary that is tucked into the transformer body opened. I replaced it with a piece of Sn 63 solder for the time being. It did its job and melted when the second diode failed. (Reasonably close in melt temp to the original, but not exact)

The device has a pair of SPDT switches that allow the user to select between 2 amp, 20 amp or 50 amp charging. The owner's manual says the 50 amp is limited to ten seconds of use. I assume that the 2 and 10 have 100% duty cycles.

At first glance, I thought that the transformer was center tapped because there are three terminals on the secondary. After some meter testing, it appears that this is not the case. The center terminal goes directly to the DC (-) and the two outer ones go to four diodes. I believe it is not center-tapped because the voltage between the two outer terminals is near zero (about 20 mV) no matter the position of the SPDT switches.

The four diodes are wired in parallel and line 2 of the transformer's secondary goes directly to DC (-). I think this arrangement is called Half Wave rectification because the transformer is not really center tapped and no diodes are on Phase Two of the transformer. I do get confused, however, because the frequency across DC (+) & (-) is 120 Hz. It seems to me that a half-wave rectifier circuit would show 60 Hz and Full-Wave & Full-Wave Bridge would show 120 Hz.

As far as amperage changes, all I can figure out (or guess) is that the switches select different amounts of primary windings, increasing the voltage, thus the available wattage & current. Why do I think this crazy stuff? Because I get different voltages between either of the outer terminals and the center terminal as the switches are closed or opened. For example, it will change from 9.45 vac to 11.73 vac when switching between 2 and 10 amps. It jumps to 13.88 when 50 amps is chosen. (With the two bad diodes bypassed.)

The Grand Plan is to remove the diodes and install a Bridge Rectifier (and maybe a smoothing capacitor just for fun). I'd connect it as explained in the lower left of my attached drawing. Part Two of the plan is to provide protection for the transformer.

My questions are:
A) Am I wrong & this really is a center tapped transformer? If so, why is the voltage between the outer terminals near zero?

B) How does the device provide more charging amperage to the battery being charged? Just by increasing the voltage output by selecting more or less primary windings?

C) Is this parallel diode arrangement called "Half Wave" since the two outer transformer taps are the same phase? (But why then is the frequency at the DC side 120 HZ, like a full-wave or bridge, instead of 60 HZ, like half-wave would provide?)

D) Do you have a guess of what the VA of the transformer is? There are no markings. My guess is something about 120 VA (10 amps x 12 nominal volts)

E) If I install a bridge rectifier and want a smoothing capacitor(s) across DC (+) and (-), do you have an idea of the micro farad required? I tried some on-line calculators, but get many different answers. (Form 2,000 mf to 100,000 mf)

F) I would like to protect the transformer with a fuse or breaker on the primary. I am having trouble deciding on a fuse size. At first, I thought a 2 amp fuse would be OK because the rating plate shows 1.87 amps at the 10 amp setting. But the trip curves for fuses & breakers show less than one second for a 50 amp load to open the fuse. The fuse's purpose is to stop the melt link from opening if (when) I goof up again. (If necessary, in order to protect the transformer, I am more than willing to eliminate the 50 amp selection. I don't use it.)

I apologize that the attached drawing is rather rough. I did it this quickly during this morning's cup-of-tea break.

Thanks for your suggestions & thoughts about this device & my questions. I sure appreciate your help!

Enjoy This Day!
Paul

Bigger rectifiers *MIGHT* survive, but they'd have to be a fair bit bigger to stand being shorted - you're basically relying on stray resistances like the secondary winding resistance to protect the diodes.

I'd put some series resistance in to limit the short circuit current - you might find the right compromise between protecting the rectifier and making the process take too long.

 

Boy Oh Boy Was I Wrong!
I don't know how I did it, but I was completely wrong when I said the voltage between the outer taps on the transformer's secondary was near zero. I took the transformer out of the device and bench tested it. It is, just as Dave said, center tapped.

For a while I was thinking it was a Dual Secondary Transformer (which I had forgotten existed). Way back in the early 1970's we would install Dual Secondary transformers when converting older buildings in Detroit from DC to AC. The primary was 13.2 and the secondary was corner grounded 240 delta. One secondary winding would power the rectifier banks for the items that were to remain DC, such as elevator motors and steam condensate pumps. The second one would power the items that were to be AC powered, such as lighting & power panels. If I remember correctly, the grounded circuit conductor (neutral) from each winding was tied together, so I'd have to make sure I had the phasing correct. I never gave them much thought, as I was just young kid, thinking more about the paycheck than learning.

The primary is interesting in that the amperage switches energize more or less of the primary windings when the switch is moved from 2 amp to 10 amp to 50 amp. I suppose by increasing the secondary voltage in this manner let the manufacturer supply different amperages in a cost effective manner.

Thanks Ian for your thoughts about short circuit protection. That sounds like a very good idea. I'll start experimenting with resistors to find a good compromise.

Thanks Again Dave & Ian! I sure appreciate your help and sharing of knowledge.
Enjoy This Day,
Paul

 

Thank You Again Dave & Ian for your help and advice. It was of great help in saving this battery charger.

I tried a pair of 40HF60 (40 amp 600 volt) diodes that I had in my Box-O-Treasures and the device is working great. I wired it as per the drawing that Dave posted above.

I didn't put a filtering capacitor. My calculations show it would have to be about 67,000 mf. The expense didn't justify it for the intended use of this device. I also don't think one, or several parallel, capacitors would fit in the case.

But, I have a follow-up question, if you don't mind my intrusion:
I'd like to protect the transformer against over-current as well as accidental short circuits of the load. I came up with a couple of plans that I'd like to pass by you guys.

Plan A is to fuse the primary.
The choice would be a 2 amp slow blow fuse (or a circuit breaker). The current draw on the ten amp setting is 1.8 amps at 120 vac, so I chose 2 amps to allow for inrush. However, I only could estimate the inrush.
The 50 amp output setting would be eliminated. (I never use it.)

The current draw on the 50 setting is 7.6 amps and the duty cycle is 10 seconds on. The duty cycle at 2 or 10 amps is 100%. This tells me that the transformer is probably 120 va, so protecting for the 50 amp setting could allow the transformer to be destroyed, thus the 2 amp fuse choice.

Plan B is to protect the secondary
I'd use a 10 amp slow blow fuse or breaker. For this plan, I'd also eliminate the 50 amp output setting.

Which one, if either, do you think would provide better protection for the transformer (and diodes if possible)?

Thanks For Your Help & Advice!
Paul

 

Go with a normal house din rail circuit breaker, 30/45 amp, easily reset.

Search for sites in your country....

http://www.bimblesolar.com/extras/dc-circuit-brakers/32a-din-breaker-tomzn

 

Thanks Dave. That style will fit well in the device's case.
I'm curious how you arrived at the 30/45 amp choice? (trying to learn as much as I can).

Thanks Again,
Paul

 

Thank You Again Dave & Ian for your help and advice. It was of great help in saving this battery charger.

I tried a pair of 40HF60 (40 amp 600 volt) diodes that I had in my Box-O-Treasures and the device is working great. I wired it as per the drawing that Dave posted above.

I didn't put a filtering capacitor. My calculations show it would have to be about 67,000 mf. The expense didn't justify it for the intended use of this device. I also don't think one, or several parallel, capacitors would fit in the case.

If you were doing plating - a filter capacitor would be a bad idea. Rough DC would be better and adding a resistor to simulate reverse leakage in the rectifier would improve the finish.

Whether any of that is relevant for electrolysing corrosion off, is something you're in a better position to find out than me.

 

Thank You Ian for the information about the capacitor vs. plating. I never knew that rough DC was a better choice for plating. I know very little about plating- mostly from reading and a small amount of experimenting.

Simulating reverse leakage with a resistor sounds intriguing. I'll have to try to learn more about how this helps and how to implement it.

I suppose that electrolytic corrosion removal is similar to plating, but only backwards.
If I understand it properly, the process isn't really removing the ferric oxide, it is 'reducing' it. The process will use the oxidation to start a reduction process which takes electrons from the cathode & donates them to the cathode, thus converting the rust back into iron (and leaving a coating of what I think is a mixture of iron and magnetite). It's interesting that only the orange rust is touched by the process and any non-oxidyzed steel is left alone.

I have found that higher voltages leave a much rougher finish on the victim. It will be interesting to experiment with filtering capacitors to see the surface finish changes in both the anode and cathode.

I have a 1 quart sized device made for electrolytic rust removal that I think is from the 1930's. It is called Mighty Midget Electro Chemical Cleaner and is really interesting looking. It will make a perfect device to use for testing. It provides unfiltered DC straight from a bridge rectifier. (It came with a selenium rectifier, but I eliminated that component for safety.) Interestingly, the original cathode is a lead pot.

Thanks Again To Both Of You for helping me out with the charger repairs. Hopefully I'll have a chance to finish the repairs on it today.

Paul