Hey Folks,
I have a 36-volt Lestronic II battery charger that is not working. I do have a slight background in electronics so I've done the diagnosis which resulted in some questions. Here is the situation. When I plug it in (120 volts AC) and turn it on, the transformer draws 8.9 amps even though there is no load on the secondary (36 volt) side. I removed the transformer from the case so that it is not hooked up to the rectifier or anything else. I attach a 120-volt cord, plug it in and it still draws 8.9 amps from the primary side. I noticed that it hums very loud as if a secondary load is on and the secondary windings get hot within 20 to 30 seconds. The primary windings stay cool. My first thought is that I have some shorted secondary windings so I check the secondary to the base of the metal transformer and there is no short. So, I ma lost. But, there is one more thing that I find interesting I'd like to run by you. There is an AC capasitor that is part of the secondary windings. The secondary winding has a double lead without a tap wire. So, there are a total of 6 wires coming off the primary side: 2 black wires which are joined and connected to the + cable of the 36 volt output, 2 white wires which are attached to the rectifier which has a 3rd wire that goes to the - cable of the 36-volt output and a pair of wires (which makes 6) which are attached to the AC cap. My questions are:

1.) What is the purpose of an AC cap in this configuration?

2.) If the cap is removed from the transformer or it was bad, could it cause this condition that appears to be a short in the secondary? I tested the cap with an ohm meter and it appear to be acting normally (i.e. meter jumps up and then drops down to infinity).

Any takers on these questions?

Thanks so much for your time and sorry about the long intro to the question but I wanted to be as accurate and clear as possible.

Opppps. I meant to say that the number of wires off the SECONDARY was 6 wires. Sorry.

The transformer is a "constant voltage transformer" or "ferroresonant transformer". It's output is somewhat of a square wave.. There are large losses at no load.

Since the load is non-sinusoidal, only a true-RMS meter will give the correct value of current. Most meters are average responding. They take the average voltage and multiply by a constant. The average of a sine wave is much less than the average of a square wave. This constant assumes that the input waveform is sinusoidal and will make he meter read the RMS value of a sinusoid. If the input is not, all bets are off. So, you need a more expensive meter.

Yes, they are noisy.

This should answer your questions.

Voltage regulator - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Voltage_regulator)

The transformer should not get hot to the touch. They are usually massive, so warm and noisy is OK. The chassis may also be a heat sink for the transformer.

The cap could have changed value, which could be the reason for it being hot. I can't remember what happens when you leave the cap out.

Look for voltage between each winding and the transformer iron. There should not be any.

This might help a little too:

Sola/Hevi-Duty (http://www.solaheviduty.com/support/faq.html#Q5)

Additional info on this transformer problem. The output voltage of the transformer should be around 36 volts but it is more in the area of 5 volts. Shouldn't even my cheap meter give me a better voltage reading than 5 volts if the transformer is suppose to put out 36 volts? Thanks again!

If anything it should be higher.

so, the question is?

Is the cap bad or has it changed value or does the transformer have shorted turns?

I'm betting on shorted turns. You might be able to get a cap on the surplus market for about $4.00. Otherwise, they may cost about $25.

Cap value can be found with a capacitance meter. Transformers can be tested using a ring test. Basically you give it a pulse and make sure the pulse doesn't die quickly.

Me, I guess, learning the haed way, I had a 70 V CT, 18 amps constant voltage transformer that I tried using for an audio amp. The hum drove me nuts.
I had great bass though.

I then went for a custom 35 V * 4 @ 3A toroidal transformer. It should have been a little higher in amperage.

K-I-S-S,
Thanks for the info. I really appreicate your time.

Your welcome.

Can you disconnect the secondary winding and measure the Voltage?
Isn't that capacitor used to smooth the AC Ripple?

If I remove the transformer from the circuit, the secondary is putting out about 5 volts but the primary is drawing nearly 9 amps which leads me to believe I have a shorted secondary. The secondary gets hot within 30 seconds with no load and there is no short between the secondary windings and the iron core of the transformer. So, I feel there may be shorted windings, shortd among themselves, if that makes sense. Comments? Thanks for the help!

It does sound like the Secondary IS shorted.
Would be nice to compare the resistance against a known good transformer.
Still likely shorted. Take Care

A secondary short means nothing. The primary current high does mean something.
It means:
The primary or the regulating winding is shorted -or- the cap has changed value and is causing the transformer to oscillate at aa non-resonent frequency. Meaning the core is saturating too early.

What's the value of the cap? I'll see if I can find one cheap on the net.

K-I-S-S,
Thanks again for all the help. The cap is a 6 mfd, 660 VAC cap. Not sure why the voltage rating is so high. It's only a 36 volt secondary. I seem to get an OK reading on the cap with my ohm meter but I also realize that a simple test like that can be misleading. I'd like to try another cap but I also hate to invest a lot especially if the transformer is gone. If you can find one cheap, let me know. Thanks again for everything! Also, any idea as to where to get a 36 volt transformer? The ones that I find all seem to have low amperage.

Kiss, I don't understand, a secondary short means nothing, Too me it means it shot, shorted and will have low outputand need replaced, Do you mean something else?

There is a 6 uf at 370 VAC here for a couple of bucks. That should be OK.

Surplus Center (http://www.surpluscenter.com/sort.asp?numrec=20&lastrec=100&sort=1&keyword=EMRC&catname=electric&UID=2008100908240216)

The cap is part of the input circuit and not the output circuit. Look at some of the diagrams from earlier posts so it has to be rated about 2x the line voltage or about 300 V. Just like AC power cords are rated for 300 V when used for 120 V service.

If it works, I would replace it with the correct voltage rated cap.

I never could figure out why you cannot, typically find on a manufacturer's page what 300 V really means. Is it AC or DC. I finally called one and they do mean RMS.

What I am saying is that a secondary short will not cause high current in a normal transformer. Shorted primary turns would.

A major shift in resonance, I believe, would cause extremely high currents because the link below says the transformers are not tolerant of frequency variations.

Lessons In Electric Circuits -- Volume II (AC) - Chapter 9 (http://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_9.html#xtocid236805)

Note that the cap here is connected directly to a secondary winding. The Sola design has some odd things going on with it.

What I would try for fun and giggles is to take a 25 watt bulb. Place it in series with the transformer. Remove the cap and take a look at the voltages on all the windings. Look, especially at the ratio of the primary to secondary voltages and compare with 120/36 or whatever it is supposed to be.

The bulb acts as a current dependent resistor. At low currents, it's nearly a short. At very high currents it just causes the bulb to glow, thus limiting the power to 25 W in my example.

The transformer is designed to operate very close to saturation, so it will not act like a normal transformer.

The lamp in any event will prevent damage, so there is no harm in trying but just look at ratios.

Strat:

What I am saying is that a secondary short will not cause high current in a normal transformer. Shorted primary turns would.

CVT's are very different animals. It's essentially a resonant circuit, so if the LC circuit of the winding and the cap moves it away from 60 Hz, all hell breaks loose. Voltages and currents are out of phase and the PF goes up. The transformer overheats and the transformer is past saturation. The result is very high current.

These things have a minimum of three windings. 1) the primary, 2) One with the AC rated cap across it and 3) One or many secondaries.

I may still have mine, just don't have the time to play.

I say let's get a different cap in there and see what happens. I'm searching for one now. Thanks for the lead!

This was my suggestion:

There is a 6 uf at 370 VAC here for a couple of bucks. That should be OK.

Surplus Center (http://www.surpluscenter.com/sort.asp?numrec=20&lastrec=100&sort=1&keyword=EMRC&catname=electric&UID=2008100908240216)

You can actually make a NP cap, by taking two polorized ones and using diodes so that the cap only sees the right polarity.

Just purchasd the 6 MFD cap off the surplus site. Only $2.49 - not bad (that's the good news). Unfortunately, it was $12.05 for shipping and handling. (That's the bad news.) Oh well. I'll let you folks know the results after I get it and get it installed. Until then, stay tune. Thanks to all for the incredible education. It is truly appreciated!!

So, the capacitor is not a Filter(Polarized)?
I canabilized a Motorized DVD Rack and noticed the 2 polarized capacitors were wired in series, with either Both positives tied together in the center or both negatives, I forget, I assumed that was to make a Non polarized Capacitor?
Kiss, I understand 2 diodes in series, with both Cathodes tied together will block AC but Pass RF?
I used to use Non polarized Capacitors to Pass RF, but Block AC?
Can you provide any input, I didn't go to College, and Most training has been self taught? Thanks for any info.

Kiss, I understand 2 diodes in series, with both Cathodes tied together will block AC but Pass RF?

Not really, what you were seeing was voltages below about 0.6 V for silicon would pass right through. So, that' not a frequency dependence. Just the magnitude of your RF signal was small compared to say 60 Hz AC.




I used to use Non polarized Capacitors to Pass RF, but Block AC?
Can you provide any input.

Bypass RF maybe. Usually this is a result of RFI, where you want frequencies above a certain value to bypass to ground, thus keeping the system free of RF induced noise or am I missing something?

Their was some one down here that wanted ESPN about 15 years ago, Cable Company said it was too far(1000 ft), So I got permission from the Cable Company, and ran 1000 feet or RG 6 from the house to a Utility pole, I had an Isolation Transformer in the house, feed voltage down the Coax, to a 35? Db Amp(on utility pole), cut off the AC power cord, used chokes and capacitors to separate and pass the AC and RF.
Basically an Antenna Preamp on Steroids. (Voltage up, amplified signal back down)

So, the capacitor is not a Filter(Polarized)?Filter has many meanings low pass, notch, bandpass. A DC filter is really a ripple filter. It dampens AC oscillations.

AC caps are used in crossovers to pass high frequencies to the tweeters. They are also sold as "bass blockers".

In this transformer, no. I dug out a transformer design book which is really tough to understand and they just touched on CVT's. With come design notes.

(2*PI*f)^2*L*C = 0.76

And then later they offered a way to reduce the value of C by adding another winding because generally C is too large, but as usual, they don't do a good job explaining things. This compensating winding seems to connected in opposite phase with the winding that would normally be connected to the cap. So, with the compensating winding, the value of Cc cannot
be determined by the equation above.

Your also used to Constant Current Transformers. They are known as ballasts.

Wat they did mention though, as I interpret it is that the voltage across the cap is usually 1.4 to 1.6 times the input voltage.


Back to NP caps. Yes, I do believe the connection method you chose will work too, but you have to pick which circuit is appropriate. To get the lowest ESR (Effective Series Resitance) you would want to steer each half cycle through a polorized capacitor. Not good for audio applications, for instance.

The cap in my situation is not used as a filter. It is non-polarized and in series with the secondary windings. Hope that makes sense.

Do you have:
1. primary (120V)
2a: secondary Nc
2b. Secondary Nc'
These two windings are series opposed and the combination connected to a cap with value of C or 6 uf @ 600 V
3. A third winding for your battery charger?

Does the AC voltage to the charger circuitry come from (3) or across the cap?

K-I-S-S,
Here's the detailed story on this troublesome transformer:
1.) There is a single primary whose two wires are fed by 120V directly from the plug.
2.) There are at LEAST 2 secondary windings because the one side of the secondary comes to a 3-prong rectifier (2 prong have the secondary wires and the 3rd is the DC output).
3.) There MAY be 3 windings on the secondary because the capasitor attached to the secondary appears to have a smaller gauge wire. It seems that the cap wires are 20 gauge while the power output wires from the secondary are 16 gauge.
4.) At first, I though that the cap was in series with one of these secondary widings but now you got me thinking and I do remember the cap wires being of a higher gauge wire - interesting.. .
5.) The AC voltage to the charger circuitry does NOT go across the cap. Both wires to the cap go directly into the transformer secondary windings.

Thanks again for everything!

The CAP should either be attached to a secondary winding by itself with nothing else. -OR-

There are two secondaries that the cap is attached too, but it's still one winding. One of the windings is out of phase. That is a compensating winding so that the value of C can be smaller. Connections can be internal.

The power output wires should be the largest wire.

So, it sounds about right. Your making me want to PLAY with mine.

The cap and input wires will probably be about the same.

So, it's probably somewhat important to measure the input voltage and the voltage across the capacitor. I would bet, that if you excited the transformer with a lower voltage (with bulb) without the cap, you could get an idea of the winding ratios. I do have the possibility of doing that at 3A variable (0-120V) isolated and 10 A fixed isolated.

Hey folks,
Not sure about the rest of you but I'm getting one heck of a great lesson on transformers! I have an update for you folks, complete with pictures. Hopefully, I will figure out how to include them. Here's what I found.
First, the secondary winding that provides the output voltage is a 16 gauge wire and is the inner winding of the secondary. Next, the outer winding of the secondary is a 20 gauge wire that is attached to the 5 MFD 650VAC cap. Finally, the pictures, if I can figure out how to include them.
The first picture, Transformer 1, shows you the size of the transformer. The second picture, transformer 2, shows you a side view of the secondary winding with the double layer of coils (one for the cap, one for the output voltage). Let me know what you think.
I ordered the cap from the surplus site you folks gave me. Thanks and, again, thank for the great lesson!

Hey Folks,
Just an update on the transformer problem. I replaced the cap but pretty much got the same results. This is what I found:
1.) The output continues to be 5 volts, no change
2.) I figured out that it is the outer secondary winding, the one that is in series with the cap, that is heating up. The inner secondary winding that feeds the rectifier for output voltage stays cool.
3.) The draw on the 120 v side used to be 8.9 amps but is now 8.4 amps. Don't think this has any significance.

Conclusion: I have no idea what the he** is going on. Why is the output voltage on 5 volts when it is suppose to be 36 volts? What is causing this transformer not to perform?? It's killing me. Any ideas? Thanks for the continued help!

I think the winding where the cap goes is shorted.

What's the voltage across the cap?

Time for the light bulb trick. Take a 40 W light bulb and solder wires to it, if you can't find a socket. Put it in series with the AC line.

Take out the cap.

Measure the voltages including the voltage at the primary. Where the cap goes and the secondary.

Hey K-I-S-S,
Thanks for hanging in there with me on this transformer problem. Here's the latest. I did what you asked and here are the results:

1.) I installed a 55-watt light bulb (didn't have a 40) in series with the transformer's primary and put the juice to it. The light bulb lit brightly.

2.) The output at the secondary windings, the windings that goes to the rectifier, was less than 1 volt AC on both secondary windings. (Remember, I had two secondary windings that power the rectifier.) It didn't matter whether the cap was in the circuit or not. The voltage was the same - less than 1 volt.

3.) The voltage at the cap wires with the cap removed was 5 volts AC. Again, this winding is the outside winding on the secondary coil.

So, I hope you have some magic here because I certainly don't. Again, thank you. Your efforts are really appreciated!

I Still think secondary has shorted windings.
Know anyone else with a charger like that?

You didn't quite do the right measurements.

You missed:

What is the primary voltage when the winding where the cap was 5 volts and the bulb was in series with the primary?

And...

What is the voltage across the cap in the normal configuration (No bulb)?

What I want to see is the ratios of those two voltages. The bulb will drop the voltage on the primary.

OK, I think I got it now. Here are the answers:

1.) Voltage across the primary is 10.2 volts.

2.) Voltage across the cap is 4 volts and it doesn't matter if the cap is disconnected. With or without the cap the voltage is 4 volts on the cap winding.

3.) The output of the secondary winding that goes to the rectifier is less than 1 volt, if that is needed.

Again, these readings were taken with a 55-watt bulb in series with the primary winding. Let me know if you need any other info. Thanks again for everything!

Well, you missed one, but I don't think it matters. It would have been the voltage across the cap with no bulb.

It sure looks like you have a shorted primary. This was probably consistent from the get-go because of the high current at no load. Without any experience with actual measurements of a good CVT under these odd conditions it was hard to tell.

A 1:10 reduction isn't right and neither is a 10:4. Since both of these are wrong, the primary has to be shorted. The small gage wire is where there is likely to be problems anyway.

The CVT is a great way to get regulation cheaply for low current and that's why it was used.

I told you about the experiments with a stereo I made with the CVT and bass performance. I also purchased a 500 Watt sine wave AC voltage reguator and with a torroidal transformer, the sustained bass performance improved because of the better regulation.

That transformer will be Expensive. Your only real hope is a transformer on the surplus market.

For a 36V battery charger, you will be looking at a secondary that's not 36V. It does depend on whether a half wave or full wave bridge is used.
Hammond Greeting Cards & Promotions (http://www.hammond.com) has the sizing equations.

You can estimate the secondary current by the wire gage and the specs of the charger.

A little education for both of us.

Shouldn't you read 120 at primary, not 10.2 volts?

Yep, but not when the transformer draws 9 amps at no load. I you do a little math like R=120/9 and 55=120/R, you get about 4 V. Not exactly right, but in the right ball bark.

If the winding was a "Normal" transformer, the bulb would not glow and you'd measure 120 V. The bulb limits the power to about the bulb wattage. Sometimes better than fuses. Remember when, there were glass fuses and you could screw in a light bulb and keep pulling stuff out until the light went out. When the light went out you found the shorted appliance.

Since the transformer is trash anyway, maybe he can try measuring the output with no capacitor and no bulb?

Don't know if this info helps:
http://www.lesterelectrical.com/techservice/servicedocs/transformerhum.PDF
If you can Install their language pack, you may get some good info, would try:
Lester Electrical Home (http://www.lesterelectrical.com)

Mmmm. Lots of info in the newsletters. They seem to do a poor job advertising their products and a bang up job advertising support.

Looks like the option is calling them for a replacement xformer.