I salvaged a transformer from an old alarm clock (120V~ to 9V~ apparently) but it has four wires on the secondary. How can I determine how they're connected without having to plug the transformer into the wall?
Do you own a DVM?
Doc
Yes, I do. I'd rather not hook it up to the wall outlet though, because the alarm clock was broken and I suspect it was a short in the cable I would have to use to plug it in. Which, I suspect, would be bad.
xolroc:
Yes, I do. I'd rather not hook it up to the wall outlet though, because the alarm clock was broken and I suspect it was a short in the cable I would have to use to plug it in. Which, I suspect, would be bad.
So - assuming the transformer is good, do you intend to replace the primary cable (that is, the dual-conductor cable that plugs into the wall)? Is that the part you suspect has a short? Have you tested it for a short with your DVM? That should be your first task.
If it seems ok (that is, if you measure a resistance, rather than 0 ohms, which is what a dead-short would look like - measure it at the plug, then measure again at the transformer itself, if possible), then that wasn't the issue. However, if you do measure 0 ohms, then cut the cord off (ideally, if you can desolder it from the transformer, this is best), and remeasure the resistance at the transformer. If you are -still- seeing 0 ohms, then the transformer is shot and worthless (unless you want to salvage the wire or the core for some reason, which isn't easy no matter how you look at it); throw it away.
If it passes those tests, then it would be time to move on to the secondary outputs.
Likely, with four wires, you are looking at either dual secondaries, or a multi-tapped singular secondary (I suppose a triple secondary with a single common might be a possibility as well, among other options). The best way to handle this would be to make a table and take some measurements. Say you have the following colors for the secondary wires:
red
green
white
black
Then you can have the following distinct combinations of connections to measure:
red/green
red/white
red/black
green/white
green/black
white/black
Take the resistance measurements down. Then post what you find here. Also - you may want to post some clear pictures of this transformer, input cord, output wires, etc.
Ultimately - if you get this working and figured out - you'll want to put a fuse in-line with the primary input cord (likely something small - 500-750mA max).
Are you certain the four wires are on the secondary. If the clock was manufactured for the "international market" it is possible that the primary comprises a pair of 120 volt windings which would be wired in series for use on 240 volts.
If you can actually see the wire gauge used for the windings, assuming it is a step-down transformer, the primary windings will be much finer than the secondary.
Without "exciting" the windings there is no correct way of determining what the various tappings actually do since wire gauge (resistance) will be more prominent in assessment than actual output voltages. For example a high voltage/high current winding may actually have a lower resistance than a low voltage/low current winding.
One method I have frequently used on unknown transformers is to excite the primary with a relatively low ac voltage (say 12 volts AC) and measure the secondary outputs. This will give the respective winding ratios, from which you can deduce the actual output voltages under mains operation.
Are you certain the four wires are on the secondary. If the clock was manufactured for the "international market" it is possible that the primary comprises a pair of 120 volt windings which would be wired in series for use on 240 volts.
Possible, but more likely the secondary. As this is likely an audio device (most "alarm clocks" are clock radios) it is more than likely it requires a dual rail power supply at say + and - 9v. The dual secondaries is the simplest and cheapest way of achieving that.
A lot of those early radios had 2 secondaries one for the Led's in the display and one for the Radio this was done for display noise and cost (no linear regulator), few had dual voltage primary's. If one found all the wires that had continuity with each other. one might find 3 sets of wires. The resistances of the pairs might look like (Primary ~10 - 50 ohms , Secondary 1, 2 - 5 ohms and the last might be 1 - 5 ohms the exact readings are unimportant it is the ratio that is as the lowest resistance pair should be the ~5V (possibly yellow) supply winging, the next higher reading pair should be the ~9V winding (possibly red or blue) and the last and highest will be the primary winding 110 Vac (possibly black in color). for now just the resistance readings are important, once we have those it is likely that your transformer can be completely identified. IMO
Doc
Here are the measured resistances then, starting with the plug.
Plug: 33.6 kOhm
Plug at transformer side: 32.7 kOhm (apparently there is not a short then, thankfully)
red/white: 62.3 kOhm
red/blue: large
red/yellow: large
white/blue: large
white/yellow: large
blue/yellow: 70.0 kOhm
So I take it this means there are two near-identical secondary windings, right?
I'd like to point out that this is a radio purchased and produced within the last five years, by the way, and it was also a clock radio.
I think that if you check again you will find that the secondaries measure 62.3 and 70.0 ohms, not kOhms. That would be about right for a transformer rated at 0.5VA to 1VA with dual 6v or 9v secondaries.
Strange, because I'm still getting kOhms. Since it's an inductor though, I believe it'd depend on the voltage you're measuring with?
And indeed it does; when I have my multimeter set to 2MOhm range I get somewhere in the vicinity of 50-60 kOhms, but when I change the range to 200kOhm, it's closer to 15 kOhms. When measured in the 2kOhm range I get around 750 Ohms.
You have a defective part or there is something left out here. This is a real transformer right? with an iron core (stacked little metal plates/bits) a plastic 'bobbin' (former) with tape wrapped around it? with the wires coming from under the tape? does it have a little heft?. From what I see so far I would ask you if it was working before you removed it as that primary resistance will not allow real power to flow. You didn't return the information I suggested you test for so it is hard to go farther in attempting to 'decode' your little black box (puzzle). you need to use the ohm meter to discover which leads have continuity and what the relative resistances are. Make a little square on apiece of paper, start measuring all the leads to find those that are under 1K ohms or so and group them together on your drawing. I think you have an idea of what a transformer symbol looks like... get your meter and use it to discover what is connected to what and how much resistance is there. Fill in the blanks. The highest resistance ( between any two wires and under 1K max more likely under a hundred ohms) will be the primary. The next wire set will be secondary wires one set possibly with a tap or mid point connection would be the LV or LED supply and the intermediate value would be the radio supply. There is one possible exception where the secondary might have 4 leads with 10 - 20 ohms of resistance and that is a secondary winding that interconnects all the supplies. It might look like 0Vac - 7Vac - 7Vac (led supply) 12Vac Radio supply referenced to 0V. So in that case you would have a half-wave bridge 7 - 0 - 7 led supply and 0 - 12 for the radio where the common wire is in fact the ground or return wire for all the secondary winding, the hints are common colors.Wire color coding is usually body color / stripe color so common colors can be part of a single winding or maybe in your case a multiple interconnected winding. This was my experience from the 60's and 70's when I actually repaired radios. Any resistance above a hundred ohms is just as likely to be an indication of a defect as it is a valid/ok winding. Back in the day... Line surges frequently killed power transformer like that and a lot of times there was simply no way to find out any sources of replacement parts like that so one had to be a little clever and in some cases redesign the PSU again to make the radio work. The old JC Pennys stores and Montgomery Wards in the US were Notorious for that.
Doc
you should be able to search the specs on the transformer by googeling the numbers stamped on it with the word transformer added
in any event most stepdown transformers found in radios are a 10 :1 ratio with an output of 12 volts
if its output is 9 volts its a 10:1.25 ratio
the one you are describing has dual secondary windings
and i agree with Docedison
continuity readings should be low but not 0
remember that low DC resistance winding's are quite different when AC is applied. XL = 2 X Pi X f (hz) X l(hy). Are your fingers touching the meter probes when measuring the transformer windings? don't that, might well be the reason for the totally unreasonable information you have returned. Use clip leads if you have them. You should have two sets at least, likely three of LOW resistance wires by sets I mean 2 or 3 wires that have less than 100 ohms continuity between them, if not there are three possibilities,
- it isn't a transformer. 2. it is defective, seriously so. and 3. you aren't testing any lead to any lead for continuity but rather arbitrarily grouping them before you measure them, when you identify a conducting pair twist them together physically not electrically and move on to the next combination that produces low values of resistance and do the same until the pairs are all made, then check between pairs for more continuity... none, good make a drawing, some, good make a drawing and examine your drawing, make sense, Good, Not LOOK AGAIN...
This is what I would do to solve a "Black Box" puzzle which is really all you have to do. It's Easy... really it is.
Doc
Okay, you have a point, and I'm just being stupid. I don't have wire clips, but I was touching the leads.
I've figured out the way it's connected now, and I already know it's 120V to 9V by looking at the voltage ratings of some capacitors on the board (10V, but 9V is a more standard voltage, and running at exactly the limit is a bad idea anyway).
Actually, it could be 6V, perhaps... the 20:1 ratio is a bit more logical, and I for one wouldn't use 9v with a cap rated at 10v anyway.
Thanks for your help!
YAY...You got it. Sometimes it's hard, the first try, really. But in the end well worth it. Besides it was fun trying to figure YABB... Yet another Black Box. Thank you for some really silly entertainment. If I was hard on you forgive me, please. I had to get you to think. I knew from the 27K readings that it was likely your skin resistance you were measuring. It Still can be confusing even for me at times... but everyone says I'm confused anyway... and I know where to place that...
Good work finally by the way, one last tip use the biggest electrolytics you can and highest voltage too. They ALL dry out after a while. Bigger ones just take longer... Hopefully.
Doc
Well, those that weren't produced in Taiwan between 1999 and 2007, anyway. 
No young man they were, really cheap stuff from Taiwan and China too, cheaper stuff really mostly crap. Panasonic and Craig ans Sony were available as well as a host of others, some of premier quality Nakamichi... over the past 10 the manufacturers paradigm has changed in many ways. Quality over all even considering the ROHS requirement approaches 98 % over all. Thin\gs, new fail so infrequently that they (complex electronics) electronics boards get a cursory examination and then they are tested, module by module and in many cases these few fall outs are easily repaired those that aren't are crushed as waste or in some cases sold as scrap... I'm sorry this is a hell of answer... Sorry I'll leave it but it really wasn't the answer. Yeah they were hard to find, the american ones costing half of the price of the radio and not fitting very well mechanically or physically. It's late and I digress too far, think I will cut it short...
Doc
I was talking about the capacitor plague, from a faulty electrolyte formula that was popular among many Taiwanese manufacturers. It caused many capacitors to fail within about two years of use.
Yeah I do remember that still causes me to 'hesitate' when someone asks me to fix or restore anything from that Era, sure made me a lot of money back in the day... I used to go from TV shop to TV shop repairing small items piecemeal... $100.00 each and I usually could do 2 or 3, 2 or 3 times a day. Typical net in 1972 was $100/day... and got unemployment as all the money was 'under the table' Great times...
Doc