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

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

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

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

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

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.