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 Post subject: Battery Charger - 2 Half Bridge Button Diodes - Scope Trace
PostPosted: Dec Tue 14, 2010 1:18 am 
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This is a battery charger (test gear of sorts), a Schmmacher SS-51A-PE, has two diodes, button types. I'm guessing it should be full wave and this is half wave trace. Not sure the battery needs to be connected to check. There are no diodes but there is a automatic control circuit, a SCR, Circuit Breaker and a meter for charge amps and LED for charge complete. I'm thinking I have a bad diode. I checked them with a DMM and they seemed OK.

Image

The button diodes are held in a plastic holder so the cathodes (-) of both diodes are against a metal plate. Two leads from the transformer go to the anode (+) of the diodes. I think this is a circuit (not sure guessing).

Image

I must have a bad diode or put one or both in backwards when I took it apart. I took the diode holder apart to check them. They both test OK with a multi meter. The plastic holder was cracked and causing intermittent contact. I repaired that.

A few weeks ago I put the battery leads on the battery backwards. I got an expected spark on the clips at the battery, but also saw a flash in the charger. I think it was one of the diodes arching. I thought it was still working. I don't have schematics and need to find that and replacement diodes. Never worked with these button rectifier diodes.

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 Post subject:
PostPosted: Dec Tue 14, 2010 1:51 am 
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SOUNDS TO ME THAT YOU HAVE THE DIODES BACKWARDS.
THE "+" SHOULD GO TO THE PLATE AND THE "-" GOES TO THE TRANSFORMER WIRES.

i WOULD CHECK THE DIODES AGAIN WITH THE DMM. DO YOU HAVE A SYMBLE OF A DIODE AS A POSITION ON YOUR DMM, IF YOU DO PUT IT IN THAT POSITION. THE METER SHOULD READ INFINITY IN ONE DIRECTION AND .7 IN THE OTHER DIRECTION, IF THE DIODES ARE SHORTED IT WILL "0".


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 Post subject:
PostPosted: Dec Tue 14, 2010 2:22 am 
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ertzc2020 wrote:
SOUNDS TO ME THAT YOU HAVE THE DIODES BACKWARDS.
THE "+" SHOULD GO TO THE PLATE AND THE "-" GOES TO THE TRANSFORMER WIRES.

i WOULD CHECK THE DIODES AGAIN WITH THE DMM. DO YOU HAVE A SYMBLE OF A DIODE AS A POSITION ON YOUR DMM, IF YOU DO PUT IT IN THAT POSITION. THE METER SHOULD READ INFINITY IN ONE DIRECTION AND .7 IN THE OTHER DIRECTION, IF THE DIODES ARE SHORTED IT WILL "0".

I will flip them but that plate is not a "ground" it is kind of a heat sink and looking at the diagram above the cathode tie together.. The DMM gave me a voltage of .438 (or something like that) and zero in the opposite. There is no markings, but half wave is not right. I'd like to order new diodes. I called schmmacher technical help was useless. They told me where to buy parts. They sell the whole plate with the holder and diodes for $20.

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 Post subject:
PostPosted: Dec Tue 14, 2010 3:37 am 
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The problem with things like that is they're not made to be repaired. Like a lot of modern products, they are made to be thrown away when something goes wrong. Your scope trace definitely looks like only one of the diodes is conducting. But without knowing how the regulator circuit and SCR are tied in, it's not so easy to say if that's due to a bad diode or something else.

If the regulator circuit is fried, it may not be repairable, so it would probably be best to find out if it works before investing in new diodes. This could be done by substituting a pair of generic silicon diodes for the two on the metal plate. As long as you stay within the current and voltage ratings of the "temp" diodes, you should be able to determine if the rest of the charger works. Do not draw more current or leave it on longer than necessary for testing; the "temp" diodes may heat up pretty quickly without a heat sink. Epoxy diodes have been known to sometimes blow apart when severely overloaded, so take reasonable precautions when doing this kind of testing.

Note that on standard diodes, the banded end is the cathode. This connects to the positive (+) side of the charger circuit. The unmarked end of the diode (anode) connects to the transformer. This is no different than a vacuum tube rectifier, where the plates of the tube connect to the power transformer and the cathode (or filament) connects to the + terminal of the filter capacitor.

If replacement diodes are needed, there are any number of good Schottky and standard types available in TO-220 and other "flat" packages. Schottky diodes, which have less forward voltage drop, and therefore greater efficiency in low voltage, high current applications, may have been used originally. You could attach a pair to a metal plate of your own with screws and nuts for a lot less than the price of the original replacement rectifier.

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 Post subject:
PostPosted: Dec Tue 14, 2010 4:12 am 
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Chris108 wrote:
The problem with things like that is they're not made to be repaired. Like a lot of modern products, they are made to be thrown away when something goes wrong. Your scope trace definitely looks like only one of the diodes is conducting. But without knowing how the regulator circuit and SCR are tied in, it's not so easy to say if that's due to a bad diode or something else.

This is a pretty heavy duty charger. It actually works and charges a battery still, but I think it's doing it at half efficiency. Those button diodes are 50 cents... no big deal. They are like automotive ones that are plugged into some alternator rectifiers.

ImageImage

Quote:
If the regulator circuit is fried, it may not be repairable, so it would probably be best to find out if it works before investing in new diodes. This could be done by substituting a pair of generic silicon diodes for the two on the metal plate. As long as you stay within the current and voltage ratings of the "temp" diodes, you should be able to determine if the rest of the charger works. Do not draw more current or leave it on longer than necessary for testing; the "temp" diodes may heat up pretty quickly without a heat sink. Epoxy diodes have been known to sometimes blow apart when severely overloaded, so take reasonable precautions when doing this kind of testing.

I am a little more optimistic the auto charging board is working since the auto/manual switch, meter and charge light seems to work and it charges a battery. It's just the trace is ugly (or may be it's suppose to be a pulse DC?). Good idea trying some diodes temporarily to experiment. If the SCR is bad there is a part number on that. I am going to give them a call tomorrow and see if I can get some info. The guy I talked to in tech support today was clueless. A new charger equiv to this one is $70; I am pretty sure it's the diode. I have another fancy charger for AGM batteries, microprocessor with digital read out, but this one is a work horse. It's fun repairing stuff. I had it apart once, it is reparable. In fact you can buy a new charge controller board for it for $16, new meter, new diode assembly, but that comes with the plate and SCR for $19. I think I'll try $1 for two new diodes. I just need to confirm which way they go in and their specs.

http://www.centurytool.net/SS_51A_PE_Sc ... /30835.htm

The guy I talk to today on the phone from Tech support, when asked what type diodes the were, said Oh I'd have to go get the schematics... (meaning you want me to do work). He said go to radio shack. I asked what spec, volt amps... it was a joke.

Quote:
Note that on standard diodes, the banded end is the cathode. This connects to the positive (+) side of the charger circuit. The unmarked end of the diode (anode) connects to the transformer. This is no different than a vacuum tube rectifier, where the plates of the tube connect to the power transformer and the cathode (or filament) connects to the + terminal of the filter capacitor.

Button Diodes have no markings except may be the raised metal pads, one sticks up a little higher. This is what I think I have:

Transformer (+) anode ----->|------- Cathode (-) to the plate which as the SCR bolted to as well.

Quote:
If replacement diodes are needed, there are any number of good Schottky and standard types available in TO-220 and other "flat" packages. Schottky diodes, which have less forward voltage drop, and therefore greater efficiency in low voltage, high current applications, may have been used originally. You could attach a pair to a metal plate of your own with screws and nuts for a lot less than the price of the original replacement rectifier.


Schottky diodes are great. On the other hand it would be easier to replace them with button diodes. If I have them in backwards that might solve it. I need the schematics or at least them to tell me what way they go. When I took it apart they few out. It's a 2/10/15 amp charger so I'm sure they are 30 or even 50 amp, which is common for these button diodes.

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Last edited by gmcjetpilot on Dec Tue 14, 2010 2:24 pm, edited 2 times in total.

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 Post subject:
PostPosted: Dec Tue 14, 2010 4:28 am 
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Location: Baltimore, MD, USA
You may have more luck testing a diode with an analog meter. I have had problems with a DMM when testing diodes. For that matter, many DMMs have a diode test function.

However I may be blowing smoke :wink:

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 Post subject:
PostPosted: Dec Tue 14, 2010 5:15 am 
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A couple comments.

If one or both the diodes were reversed, there most definitely would have been some tangible result. A blown fuse, a loud hum, or smoke signals would be be at the top of my list. Simple things like these are pretty brute force, and easily become catastrophic.

On your scope trace, though its not really clear, it does look as if one of the diodes is history, but it also looks like there may be something oscillating as well, because of the thick trace at the upper portions of the voltage crest, perhaps because there isn't a load.

Take a few minutes and construct your own schematic.

GL


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 Post subject:
PostPosted: Dec Tue 14, 2010 1:56 pm 
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Joined: May Sun 09, 2010 3:43 pm
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Take a look at this. Had to root around on the web some (need a shower now). This fellow has a schematic for your charger and a description of the circuit:

http://www.en-genius.net/site/zones/des ... col_101308


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 Post subject:
PostPosted: Dec Tue 14, 2010 2:24 pm 
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Wow, that's impressive (the page, AND your finding it).


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 Post subject:
PostPosted: Dec Tue 14, 2010 2:30 pm 
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rollei35guy wrote:
Take a look at this. Had to root around on the web some (need a shower now). This fellow has a schematic for your charger and a description of the circuit:

http://www.en-genius.net/site/zones/des ... col_101308


Thanks a million, yep that is it. I googled the heck out of it, but I give up about three pages of hits down. I'm absorbing the info. according to the schematic, I put the diodes in correctly, if I identified the Anode/Cathode properly (with the DMM). However they are not just plain old diodes, they are double Motorola SR4355 Schottky diodes. The author says obscure.... figures. He also suggest a substitution as Chris suggested TO-220 or -247 package.

The author laments:
"Schumacher needs to upgrade their charger designs to switchers
if they hope to stay in this market for very much longer"


They do, they have multi step microprocessor logic controlled switching power supply chargers. I have a SSC-1500A which weighs next to nothing, no transformer, a switching power supply. The other charger has a big transformer, weights a brick, simple for simple charging duties. The BIG transformer 60 Hz (as he calls them) chargers are still used in big shops.

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Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


Last edited by gmcjetpilot on Dec Tue 14, 2010 2:48 pm, edited 1 time in total.

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 Post subject:
PostPosted: Dec Tue 14, 2010 2:45 pm 
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Joined: Jul Wed 22, 2009 8:32 am
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Mikeinkcmo wrote:
A couple comments.

If one or both the diodes were reversed, there most definitely would have been some tangible result. A blown fuse, a loud hum, or smoke signals would be be at the top of my list. Simple things like these are pretty brute force, and easily become catastrophic.

On your scope trace, though its not really clear, it does look as if one of the diodes is history, but it also looks like there may be something oscillating as well, because of the thick trace at the upper portions of the voltage crest, perhaps because there isn't a load.

Take a few minutes and construct your own schematic. GL

You are right if I put them in backwards or one opposite it would
make smoke.

The "ringing" at the top does not bother me. I think that is normal
circuit harmonics. There are no capacitors in this thing. The battery
is the damper. I zoomed into it, and it's not AC, it is high freq and
really quite small and very pretty. The picture is blurred. I am not
totally ignoring it, but so far that is my guess. When I get a FULL
WAVE rectifier (not cap) wave form, connect it to a battery, it that
ring is there, I can worry about it then.

Your last comment is spot on. With rollei35guy's link I should sit
down and write it out. I am switching from just repair to learning
how this works. Who knows I might modify it.

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Last edited by gmcjetpilot on Dec Tue 14, 2010 2:55 pm, edited 4 times in total.

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 Post subject:
PostPosted: Dec Tue 14, 2010 2:53 pm 
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I do know that automotive battery chargers aren't filtered worth a darn and just put out a pulsating DC. I'll look at my Schumacher charger on a scope tonight and see what the waveform looks like.
It's one of the big transformer types.

-Mark-

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 Post subject:
PostPosted: Dec Tue 14, 2010 2:56 pm 
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Location: Long Island
Just a quick "oh by the way" when testing Schottky diodes. They like to have a minimum of about 1 mA of current flowing through them to exhibit their forward resistance correctly. An analog VOM on the X1 scale should do this without fail. The diodes should be disconnected from the circuit, otherwise the resistance of the transformer winding could trip you up.

In the forward direction, they will look almost like short circuits. The forward voltage drop is on the order of only 0.15 volt or so, as compared to 0.6 or 0.7 for a conventional silicon diode.

In the reverse direction, they will have a measurable back resistance, but it will be significantly higher than the forward resistance (like thousands of Ohms). This is considerably different than what you find with ordinary silicon diodes, which have nearly infinite resistance in the reverse direction.

The reason for "double" diodes is presumably because the PIV of Schottkys made some years back was not very high. This should be considered when selecting replacement diodes (if necessary).

One other observation: The output waveform of a battery charger is usually pretty rough. They usually do not contain any filtering. It has been documented in many places that batteries tend to charge faster and last longer if charged on pulsating DC, rather than "pure" straight-line DC.

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 Post subject:
PostPosted: Dec Tue 14, 2010 3:02 pm 
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Joined: May Sun 09, 2010 3:43 pm
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gmcjetpilot wrote:
... according to the schematic I put the diodes in properly. However they are not just plain old diodes they are double Motorola SR4355 Schottky diodes. The author says obscure.... figures. He also suggest a substitution as Chris suggested TO-220 or -247 package.

The author laments:
"Schumacher needs to upgrade their charger designs to switchers
if they hope to stay in this market for very much longer"


They do, they have multi step microprocessor logic controlled switching power supply chargers. I have a SSC-1500A which weighs next to nothing, no transformer, a switching power supply. The other charger has a big transformer, weights a brick, simple for simple charging duties. The BIG transformer 60 Hz (as he calls them) chargers are still used in big shops.


I always cringe when I see obscure parts in a circuit. But as long as a substitution can be found it's ok.

I have an older Schumacher charger/starter, a manual 2/10/40 amp (i think) charger with 200amp starting. More akin to welder i think. It's version of overcharge protection is a mechanical timer.

Yours looks to be a pretty nice charger. Good luck with it.


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 Post subject:
PostPosted: Dec Tue 14, 2010 4:37 pm 
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Image

I flipped the diodes around and this does not look right. It is
putting put negative volts, however it looks more like a full bridge.
I had it right the first time. The question is, does a pulse or half
bridge DC voltage of the first make sense? I don't think so. I'm
confused why the Freq? Scope is at 2 mS, each division is
500 Hz. Why is the Freq not 60 or 120 Hz? (Edit: corrected picture,
makes sense with help, thanks)


I flip the diodes again, and it looks the same as before (no
surprise). I need to try it on the battery with the scope, see if it
mellows out. Hate taking the scope out to the garage, but that will
tell the story. I called Schumacher again. They had no idea what
type of diodes they were, "it's not on our schematic".

Measuring the diodes with the DMM diode test they measure .493-
.500 volts. The red test lead is on the Anode (+) and the black test
lead is on the Cathode (-) (per sketch below). The scope above is
now with both anodes on the plate. I used a white paint pen and
marked the diodes, since they look almost symmetric and have no
markings.

Image

They do sell just the diodes and holder.... for about $16.00. I'll pass on that.

http://www.centurytool.net/Rectifier_Di ... p/rr50.htm

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Last edited by gmcjetpilot on Dec Tue 14, 2010 11:18 pm, edited 3 times in total.

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 Post subject:
PostPosted: Dec Tue 14, 2010 5:10 pm 
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Location: Santa Rosa, CA
Quote:
In the forward direction, they will look almost like short circuits. The forward voltage drop is on the order of only 0.15 volt or so, as compared to 0.6 or 0.7 for a conventional silicon diode.



It is unlikely that Schottky diodes would be used in a charger. They look like conventional Motorola-style button diodes... usually rated around 30 Amps and 100 V. The "button" style was developed for car alternators because it was cheap and could be cooled from both sides of the chip.

A Schottky power diode usually has a forward drop voltage around 0.3 to 0.4 V. A conventional pn diode will read around 0.6 V to 0.7 V when measured with a low current meter.

Schottky diodes tend to be more susceptible to voltage spikes and their high switching speed isn't needed in charger or alternator applications.

I can't find a data sheet for SR4355.

Rich


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 Post subject:
PostPosted: Dec Tue 14, 2010 5:15 pm 
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Chris108 wrote:
One other observation: The output waveform of a battery charger is usually pretty rough. They usually do not contain any filtering. It has been documented in many places that batteries tend to charge faster and last longer if charged on pulsating DC, rather than "pure" straight-line DC.

The first wave form does NOT look like a typical half wave rectifier.
It has hard square wave OFF and the Freq of 2000 Hz. Hum? Looks
like a pulse.

I am learning, here is WIKI on PULSE CHARGING.....

Pulse
Some chargers use pulse technology in which a pulse is fed to the
battery. This DC pulse has a strictly controlled rise time, pulse
width, pulse repetition rate (frequency) and amplitude. This
technology is said to work with any size, voltage, capacity or
chemistry of batteries, including automotive and valve-regulated
batteries. With pulse charging, high instantaneous voltages can be
applied without overheating the battery. In a Lead-acid battery,
this breaks down lead-sulfate crystals, thus greatly extending the
battery service life.

Some chargers use pulses to check the current battery state when
the charger is first connected, then use constant current charging
during fast charging, then use pulse charging as a kind of trickle
charging to maintain the charge.

Some chargers use "negative pulse charging", also called "reflex
charging" or "burp charging". Such chargers use both positive and
brief negative current pulses. There is no significant evidence,
however, that negative pulse charging is more effective than
ordinary pulse charging.

_________________
Tube: AM/FM Zeniths, RCA, TrueTone table tops; Transistor: Kaito KA1103, TenTec RX320D, Pioneer SX780


Last edited by gmcjetpilot on Dec Tue 14, 2010 6:22 pm, edited 2 times in total.

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 Post subject:
PostPosted: Dec Tue 14, 2010 5:15 pm 
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There is a lot of info on the internet regarding car battery chargers, and the trend now has been moving away from charger designs that have output with pulsating DC and turning to a cleaner DC output. Apparently having high levels of AC component in battery charger output has been causing damage to sensitive electronic devices in cars today that are under power from the car's battery at all times. These new chargers of course come with higher price tags. It will be interesting so see what my old "welder" looks like on the scope.

-Mark-

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 Post subject:
PostPosted: Dec Tue 14, 2010 5:52 pm 
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I must first admit to being a dunce, but it seems to me that if you extended the curved portions of the wave form in the first scope display, it would approximate the proper waveform of a full-wave rectifier.

Also, the 'off' or zero volts portion of the wave form would have a period of 1/2 cycle plus the time period taken up by the 'square bottom' of the wave form if one diode was not conducting. I would deduce that both diodes are rectifying properly and that some wave shaping is possibly being done by the SCR. This might explain the strange results from the flipping experiment.

I have not looked up the schematic and I am sorry for inventing 'technical terms' but I hope you follow my drift. That being said, I'm probably way off base.......

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 Post subject:
PostPosted: Dec Tue 14, 2010 6:13 pm 
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gmcjetpilot wrote:
Image



What program do you use to annotate your pictures? They look really good.


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