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02-01-2008, 03:55 PM #1
transformer/battery charger wiring
I bought a used 24 volt battery charger that is only the trasnsformer. no pocase etc.. Since it is 35 amp output I assume the large leads arte output small leads are input. No labels at all. Problem is it has two pairs of leads out of each part of the tramsformer. I have not used a VOM on it yet.
I am am wodering if you think I just use one set on each side. I was told it was 120 volt input. Would there be two coils in seres for 240 in? I figure the output may give a choice of 12 and 24 volt? Ps this is supposed to be a Lister brand. It is old enough the coils are wedged in with wood wedges.
02-01-2008, 04:42 PM #2
I'm no Tesla , but .
How about this:
Isolate all leads / wires.
Should be able to indentify the Input, either by apearance,
or by position in the skeem of the thing.
Apply 120 ac;
Then check for pairs that give a logical reading.
You may still get some odd voltage from sampling a lead from each intended out-put.
Now; if you got 6v & 12v you have to decide if that should be 12v & 24v.
hence 240 V.
02-01-2008, 05:16 PM #3
Or maybe this
Take all 4 leads and identify the 2 pairs by measuring resistance with an ohm meter. Once you have the 2 ea windings identified measure the resistance of each winding. The lower resistance winding will be the primary winding/120v input.
But I’m a little confused, you say this is a transformer only. It takes a rectifier of some type to derive DC from AC.
02-01-2008, 05:22 PM #4
Yeah, but don't apply 120V, rather apply maybe 6.3VAC from an old [vacuum tube] "Filament Transformer" with a resistor in series to limit current.
Better yet, one of the high-impedance "signalling transformers" used to supply a doorbell is a good source of low-voltage AC for testing unknown transformers. IIRC they are about 16VAC output with a high ESR. (Equivalent Series Resistance)
I think the high-impedance transformers which are used in HVAC control systems would be another good choice, although those are usually 24VAC, which might be kinda high for working with unknown transformers. (Feed a 240V-to-24V transformer backward with 24V, and you are going to see 240V on the input, so watch out!)
02-01-2008, 05:45 PM #5
SouthBend has the right idea. What I like to use is a 12V transformer, because this divides approximently 10:1 from 120V
Around here, I have several old HVAC transformers, but they are 240V primary. This is great, because I can use them on 120V, and have 12V output
(What you have should be simple, wait till you start screwing with 5KV plate transformers)
You need to isolate all windings with an ohmeter, there IS NO OTHER WAY.
Some transformers have tricky tapping and so on, so you MUST at least figure out which windings are interconnected.
On "fast" chargers and other high current transformers, both windings are such low resistance, that it would be impossible to tell what is the low and high taps.
Generally, the primary will be smaller, and there could be two of either primary or secondary, or two of each. Sometimes, for example, there might be two primaries, which are tied together to make one center tapped winding.
(Remember--that a "center tap" configuration usually uses a "full wave center tapped" rectifier, which gives you approx 1/2 the entire tranformer voltage, in the DC output.)
On some of them, you can carefully look at the way the winding taps enter the paper, with a combination of looking and using your meter.
Then, with a combo of first putting your 6-12V transformer into the primary, and measuring the secondary, and playing with taps (if any) you should be able to figure out what are the high and low taps.
If you want to be more sure of what you have--and be VERY careful--then hook your transformer to the secondary, and see what you have on the various primary taps. You will now see approx line voltage at the primary, so BE CAREFUL
When you first hook it up, put a 100-200 W incandescant lamp in series with the primary to limit current.
When you finally hook it up "for real" hook it up with a separate low amperage fuse, say, 5A-10A, with no load on the secondary. Listen for excessive buzzing and quick heating, which would indicate either
shorted turns in one winding
short to transformer frame.
Speaking of which, when you do all this MAKE ABSOLUTELY SURE that you have the frame of the transformer hooked to a good ground. There normally should be NO connection electrically to the frame (core) but if a winding shorts, it could short to the core, making it "live."
02-01-2008, 09:13 PM #6
You have a step down transformer so the voltage will be lower on the battery/secondary side and the wires will be thicker in diameter for the increased current like you say. It sounds like you have a step down tranny out of a battery charger rather than a complete charger.
Any AC powered battery charger will need rectifiers somewhere to convert to DC. If you look at this transformer the low voltage output will have one of two setups:
1. There will be three wires coming out of the transformer secondary with two of the wires connected to their own individual diodes and two legs of the diodes will be connected to typically be the + terminal with the third wire forming the - terminal.
2. There will be two wires coming out of the transformer connected to four diodes wired as a "bridge rectifier" or a one piece bridge rectifier assembly and the + and - terminals are two terminals of the bridge rectifier.
The mains wires should run to the transformer via a fuse for safety so you should see a fuse on the mains/primary side of the transformer.
02-01-2008, 11:09 PM #7
02-02-2008, 03:27 AM #8
Thanks for the ideas. Yes this is just the transformwer so I need to add a bridge for dc output. I do have a Variac somewhere so I will use that to supply low voltage ac. This transformer has two totally seperate windings. each winding has four leads. One coil is about #10 so that must be the output. then other coil has about #14 so that must be input.
PS: I started to use my VOM and found four seperate pairs. Two input and two output. Then the VOM battery died so I have to wait until I get to a store.
02-02-2008, 06:26 AM #9
You're probably right.
It's a step down transformer, so the primary (where you hook the 120 VAC in) will more than likely have a HIGHER resistance (more ohms). The secondary will be a rather low resistance (less ohmes). THe thicker wire will probably be the output if it's a high current model. Low current transformers will have a low resistance secondary with skinny wires, and a high resistance primary with somewhat fatter wires, but still skinny (about 18 ga). The resistances might not vary by a lot (like 1/2 ohm), but you can still figure out what's what this way with a good meter. You SHOULD test with your ohm meter from ALL input wires to the case/core. They all should have infinite resistance, no connection to the case/core. If you get a connection to the core, it might still be something weird like a midpoint grounded transformer, but those are rare, more than likely the transformer is toast. DON'T use it if it has a connection to the case until you figure out what it is. Put a lightbulb in series with the transformer. If you've got the primary and secondary backwards, you'll have 600v on the output of the transformer!!! Without the lightbulb, it would fry it, and the lightbulb SHOULD NOT LIGHT. If it does, it's either backwards, or shorted. Test the output voltage, CAREFUL, because it might be 600v AC. If it's 24v, you've got it right, so you can use it.
02-02-2008, 06:57 AM #10
With two separate primary and secondary windings it will be necessary to determine the phasing of all four windings by experimentation. Connected the wrong way the windings will cancel out their outputs.
If you control the output with a variable output autotransformer like a Variac, Powerstat, etc. Connect the autotransformer to the primary winding of the stepdown transformer, not the secondary winding Be sure the voltage rating of the autotransformer is equal to or greater then the primary input voltage.
02-02-2008, 11:01 AM #11
Your light bulb idea is a useful tool. But if your light bulb is in series with one or more of the secondaries in any combination of this particular transformer there is no way you would see 600 V from the primary.
For example a 100 W 120 V incandescent lamp within 20 milliseconds will be near 120*120/100 = 144 ohms. For this size transformer unloaded you will not apply a very large voltage to the coil you connect to.
For two reasons.
1. The shunt impedance of the transformer unloaded and assuming no saturation for a secondary on this transformer will be low compared to 144 ohms. Thus, full line voltage would not be applied to the secondary. I have no idea what it would be, but suppose it was 24 V and the transformer ratio is 5 to 1 from one secondary to one primary, then the primary voltage would be 120 V.
2. The second reason is iron core transformers are designed to go into heavy saturation somewhat above nominal voltage for the particular winding. Saturation greatly lowers the average impedance.