2 or 6 Harbor Freight 6/12 volt battery charger model #45005 2 or 6Amps -has a fried diode?

So my charger made a noise today when I tried to charge car battery. I immediately opened it up and one of the two larger diodes was still warm. Tried to charge again and heard same noise and same diode was very hot. Looks like the diode is getting so hot it expands on the circuit board and makes a cracking sound. The manual states that the manufacturer (Chicago Electric) sells no parts for these. The stamp on diode says BT151. Web search shows several variations of this diode. Not sure of exact differences but some of difference were variances between average amperage(between 7.5 and 12amps), and voltages between 500-800. Voltages are way over minumum specs so should be ok there. Would it be ok put in one of these replacements -I assume a new match set should go in?

Woody

 

Have you considered that there might be another failure and the diode
is merely a consequence of that?
Suggest you buy two diodes.

 

Is there anything else in this charger besides the transformer and
switch, like a regulator circuit? Lenny

 

That's an SCR, not a diode:
http://www.nxp.com/documents/data_sheet/BT151-800R.pdf

- Franc Zabkar

 

If your circuit layout looks anything like the following, then you
should be able to remove the faulty SCR and check whether the battery
charges at half the rate with a single SCR:

http://electroschematics.com/6850/12v-scr-battery-charger/

I wouldn't run it for too long, though.

- Franc Zabkar

 

I ordered two new SCRs on FleaBay. After I ordered, I was thinking that it might be a good idea to mod this battery charger into a bench power supply. Any tips, ideas, comments, etc. on doing this?

Woody

 

Publish the desired features and specs.
The devil is in the details.

 

Not sure what features and specs would make a basic bench power supply, that could be also used for electroplating copper, gold, and silver. My electronics circuit knowledge sucks, but could probably mod charger if had a design.
I have an extra Harbor Freight #45005 car battery charger that has a center-tapped Transformer. On the 6/12 volt, and 2/6 amp settings my meter measured some nice stepping voltages including 8.5, 12, 14, 17.5, 24, 28.5. Notsure if present circuit board could be adapted or not because I don't yet understand it. Or if it should be bypassed completely and only the transformer, metal case, and amp meter be salvaged. Any tips, suggetions, etc. would be appreciated.


Woody

 

The most important part of the project is to decide the result you want.
People give me crap when I say that.
When you call the airline, the first
thing they want to know is your destination.
Then they figger out how to get you there.

I used to have a running argument with the EX.
"you never take me anywhere!"
"OK, dear, where would you like to go?"
"I don't know, you pick..."
"I pick here."

She couldn't decide what she wanted.
But it was definitely my fault.

No specification >>> no project >>>>> no EX.

But I digress.

If you can decide the voltage range and the range of the current limit
and what connectors you want to use and how you want it metered,
ripple/noise/regulation, what do you want to happen when you stuff voltage
on the output, like charging a battery,
and...and...and...
you can get some help on how to create that.

Be aware that most of what you read on the interned has fundamental
flaws. Power supply designs are no exception. There certainly
exist good designs, but average Joe won't be able to tell the
difference until it makes smoke, or blows up his iPAD.

You're gonna find that almost nothing in the typical battery charger
is useful in a general purpose benchtop power supply.
Even the transformer is unlikely to be able to sustain current anywhere
near the current number on the faceplate. And it's probably specifically
wound to have high leakage inductance to make it a better/safer battery
charger.

Don't even think about designing a power supply without an oscilloscope
to test it. Power supplies are well behaved...except when they aren't.
And how you connect the wires between the components matters.
There are subtleties that don't show up as components on the schematic.

A power supply is not a simple project. I've had to fix a bunch of 'em
for engineers who thought they were smart enough to design one.

 

A power supply is not a simple project.


Thanks for ALL of the above informative, - very useful input on pitfalls of moding a battery charger. That's definitely a little more than I want to take on right now.

As a quick alternative, people have been moding computer power supplies. I'm sure these PS conversions have limitations as well. If you please, I'd like to here your thoughts on them as well.

I found this easy and quick way to use them for $14.
http://dangerousprototypes.com/docs/ATX_Breakout_Board


Woody

 

I like the name. dangerousprototypes.
The honesty is refreshing.

You steadfastly refuse to specify what you want.
There's a voltage tolerance spec linked
at that site.
If this meets your needs, go for it.

Be aware that, although the spec is written in stone,
the PS vendor is free to "interpret".
IF they decide that, for the intended application,
the 12V output will never be unloaded completely, they
can forgo a load resistor. They saved 3-cents. They don't
care that the open-circuit voltage goes to 18V under some
conditions that will never happen in a computer.

I predict what most people are gonna do.
They're gonna decide that 1.25Amps is less than they need.
They're gonna look at the sticker on the ATX supply and
see 12V 10Amps and 5V 30Amps.
They're gonna short out the polyfuses and be happy campers...
until whatever they're working on explodes.
I'm a smart and careful person. I've never dropped a screwdriver
or had a probe tip slip...pay no attention to that big burned divot
in the probe tip.

There are technical issues. The 9W optional load resistor is
there for a reason.
Typical ATX supply has one transformer with multiple outputs.
Regulation is based on one output, usually 5V. The switcher
puts only enough energy into the transformer to make the 5V right.
Problem is that if you want a lot of current from the 12V output,
there just isn't enough energy to support it. The fix is to
add load to the regulated 5V output until the energy put into
the transformer can support the 12V load current.
The ATX supply is designed as a system component and people
who design loads accept the limitations.

The vendor has determined that their system works as advertised
at 1.25Amps.

If you want a real bench supply, EBAY listings and ham-radio swapmeets
are full of 'em. If you know a ham, call him up and ask him to put
the word out. Most of us have way more power supplies than we can ever use.
Put a want on Craigslist.
Stick your hook in the water and try to snag something cheap.
If you're patient, they're out there.

Designing a power supply is deceptively simple. 99% of it is trivial.
That last 1% comes with a lot of smoked circuitry.
You don't want to learn at the expense of the iPAD you were trying to power
or the battery that blew up in your face when your regulator latched up.

You would not trust a power supply design to a freshly-minted engineer
unless you had a lot of free time to mentor him on this fast-track
learning experience.

 

Mike, this would be for general purposes. New hobby is electronics/repair, and maybe dabbling in electroplating -small items.

You said there is only one transformer in a computer PS. Actually most allcomputer PSs today are switching type power supplies that use three small light weight transformers. Not sure how it all works but supposedly if only one transformer was used it would have to be very large, heavy, and more inefficient because full power from transformer would be always on.

I have an extra computer 350 watt 24 pin PS laying around, and rather than throwing it out will probably mode it so it can take a little more current than the ATX breakout board does. That is of course until I overload, and blow it up charging a battery, or explode dangerous plating chemicals in myface.

Another question is how can I determine the VA rating of a linear transformer when there are no markings on it? As you mentioned, the manufacturer has overated my dual tapped battery charger at six amps and 12 volts. I may isolate this transformer from battery charger circuit, slap a bridge rectifier/heat sink to it with some kind of capacitor, and load test the various voltages just to see what currents it can safely output. Again, your suggestions would be greatly appreciated.


Woody

 

I like that. My resources at home are no very good right now in power
supplies. Some of my home made supplies have broke for one reason or
another, and have not had time to fix. One old supply I'll always cherish
is the old battery eliminator by EICO. I like those for fooling around
charging batteries. They use variac to unregulated supply.

I wired up a computer supply in my little camper for 12vdc via cigarette
plug. That will run a little tire compressor.

Also got a harbor freight charger that it, or I blew out the regulator. I
can still use it, but the auto feature does not work. Got o fix that.

Greg