Resources for identifying damaged, unmarked components?

[leadsled]

Hi all,

      I've been searching for methods of identifying my mystery transformer for quite awhile and this is likely my last resort. There are no markings to speak of and the transformer is faulty so I cannot get an output voltage. What I do know is that it plugs right into the wall (in the US) so presumably 120v. I'm getting infinite Ohms on the primary and 2.3ish Ohms on the secondary. I'm getting wild Ohm-age readings between the secondary leads and the core but nothing between the primary leads and the core. It's out of a battery charger "Snap-On model CTC572". No obvious way of mounting the thing unless plopping it in there is an accepted engineering strategy. I don't believe I've made any mistakes taking my readings and I've poked this thing pretty much everywhere and in every way. I'm almost tempted to cut the thing in half and count the windings. I've done various searches on the supplier websites the wiki provides and I've seen some that were kind of similar but not close enough that I feel like it would help if I compiled a list to post here.


If someone has an answer to my problem or a good place for me to look for one I would greatly appreciate it. I'll apologize in advance for any rules I may have broken, obvious posts I've missed, or how incomprehensible this may be. I've been looking for this thing for hours and it's like 3am so I don't exactly have the rules memorized. Thanks for your help or your patience or both.

[SMdude]

Can you post some photos?

Also is it a wall wart type transformer or a transformer mounted in a device? If it is a wall wart type, just try and find another with the correct voltage and current requirements.

[mikerj]

The first thing to do is check for a thermal fuse in the transformer primary, it will almost certainly have one and is likely responsible for the open circuit.  With care these can often be replaced, though they usually need to be crimped rather than soldered for obvious reasons.  These chargers used to run pretty warm even when working perfectly, so any lack of ventilation or charging packs with shorted cells could cause overheating.

It's a charger for 7.2v NiCad packs used in cordless tools, with an output current rated at 0.8A.  If (as is likely) the charging the circuit is some kind of linear regulator this gets you at least some way to the required transformer specs. 

A 7.2v pack has 6 cells, and each a NiCd cell can peak at around 1.55v during charging.  This gives a maximum pack voltage of 9.3v, so the rectified output of the transformer must be at least this high, plus whatever the drop out of the charging circuit is under maximum load.  More inspection of the charging circuit will be needed to nail down the requirements, e.g. is the rectified output smoothed (these old chargers often used very low value reservoir caps so lots of ripple).  How is the current limiting circuit designed e.g. a simple power resistor or a constant current circuit?  These old chargers tend to be fairly simple, so reverse engineering the relevant parts of the schematic shouldn't be difficult.

[Haenk]

Hires picture of the PCB (esp. close to the transformer) certainly would help, with the components readable.
If there is a voltage regulator (likely), the transformer does not have to be an exact match, an old wall wart might be good enough as donor.

[leadsled]

Thanks for all the replies,

    I've attached a few pictures that I believe are relatively concise. On boardFront.jpg In the top right hand corner of the image and above the four diodes, there are the two empty holes where the transformer leads were soldered. I replaced the aforementioned diodes because the tan colored adhesive that was originally applied to them appeared to have burned to a dark coffee color. I inspected the transformer looking for a thermal fuse but I don't see anything obvious. If it is located deeper in the device I don't see a non-destructive way of reaching it. I'm sorry to say that boardFront.jpg doesn't show component markings very well. I would have to take individual pictures of each and I'd hate to clutter the post with a dozen images if they end up being unnecessary. I'm more than happy to take them if requested or I can type up a parts list because some of them would be difficult to read through a camera lens anyway. Thank you all for indulging me. It may not seem complicated to anyone here but this process is teaching me quite a bit.

[Gregg]

Did you check the primary transformer winding resistance at the plug or where the wires are soldered to the transformer winding?

There is a good possibility that the plug wire is broken where it went through the strain relief; if so, shortening the cord a couple of inches and re-soldering carefully to the thin transformer primary wires may fix it.

[fzabkar]

It looks like a switchmode charger. What are the markings on the two ICs?

If it is switchmode, then the transformer specs can be less stringent.

[leadsled]

Sorry for the delayed response,
I did check the wires up close to the transformer. Not as close as I potentially could have but well past the expanded plastic grommet device or whatever the technical name is. The thing that protects the wire from fatigue. The markings on the ICs are not legible. From the orientation of the images I posted earlier the upper one appears totally blank. I can barely make out a "G" and a "7" on the lower IC. As always I really appreciate the help.

[bdunham7]

Can you flip the transformer over and take a clear, focused picture of the primary side as closeup as possible?  And what is the voltage rating on the big capacitor??

[Ian.M]

+1!

A really close look at the transformer primary would be helpful. 

If you can locate the thermal fuse, which will be *somewhere* under the blue tape, (and if you are lucky its pins will be accessible once the tape's removed), you may be able to bypass it and run the transformer long enough to measure its unloaded output voltage, and its loaded current and voltage.   You'll need to secure the mains lead before you remove the tape e.g. by cable-tying it to a coffee stirrer glued to the bottom of the core, as it also anchors the wire ends and it would be far too easy to snap off the very fine primary wires.

[connectTek]

What's the voltage rating on the big electrolytic Capacitor?
I would suspect the output is not far off this value.

[leadsled]

     I just tried out your suggestion Ian, and it appears I just found the numbers that I was looking for. To insure my readings are correct I'd like to double check and explain how I took my measurements. I bypassed the blown out thermal fuse and re-soldered everything in it's proper place. I then plugged in the cord and measured the voltage across the two secondary leads. I took one measurement without a battery in the charger at 19.3v, then another measurement with a battery in place at 16.9v. I then cut a transformer lead and measured the amperage at 3.2mA. The charger did recognize that a valid battery was present and I'm very sure that it needed the juice. I didn't notice until I began typing, but the close-up image of the board somehow manages to barely show any identifying markings on the components. The largest capacitor located adjacent to the transformer leads reads" 1000μF 35v". This is fantastic progress and you're all amazing people. Thank you.

[Ian.M]

So you need a 17V transformer with 14% or better regulation, VA as yet to be determined.

It cant be 3.2mA AC from the secondary, as that would take months to charge the pack!  From the charger's nominal 800mA output rating I'd expect more like 1.3A  AC RMS, provided the pack is sufficiently discharged for the charger to go to full current. 

Are you sure you had the meter on AC A not DC A?

Are there any markings on the thermal fuse?  If so, I'd be tempted to obtain a same temperature replacement, fit it, patch up the insulation, and see how long it lasts . . .   N.B. you *MUST* heatsink between the joint and the body when soldering thermal fuses, and solder them as quickly as possible to avoid them blowing from the heat of soldering them!

[bdunham7]

Can you post the physical dimensions of the transformer?  I'll look in my pile of stuff...

You will find that they have designed this charger right on the ragged edge of the design limits of almost everything, which is why the thermal fuse cooked.  You need to measure that output current properly, 3.2mA isn't right--and I suspect your voltage will drop a lot more under a full load, which would be in the middle of a charging cycle.  If it is for your own use, a large, external wall-wart type AC (or even DC) supply might be a better way to fix it. 

[leadsled]

      It looks like I forgot to hit a button on my multimeter, you are correct. New measurement is .55A while charging a battery. The dimensions are 4.5cm x 4cm with a height of 3.5cm. I managed to loose the old fuse as well so I'll have to properly hunt it down. There were makings on the head and I'm sure it would be much cheaper to replace just the fuse (assuming I don't cook the new one). If they're cheap enough I may order a few spares and I have some big brass wedges I could work off of. If it doesn't work out I do have a few scrap chargers laying around I could maybe salvage a transformer from. Thanks for your patience, I got a little excited when I started getting results.