Capacitor only on transformer secondary 1

[goofy173]

I am looking at a Lestronic II 12 volt battery charger. Looking at the circuit diagram it has 3 secondaries on the transformer. 2 feed thru diodes to supply the 12 v output but the 3rd goes directly to a 3.0 MFD cap. I'm just a tech so I figured I'd ask my uncle, an electronic engineeer for a very long time and he has no idea what that cap does in that curcuit.

The wiring diagram can be seen here

http://batterychargerdepot.com/BatteryChargers/13810-74.pdf

So what gives with this capacitor. Thanks for you replies.

 

[Compositepro]

I has to be there to smooth-out the wave form of the power drawn from the line. The capacitor and transformer form a resonator that will store power. Batteries will only draw current when the supplied voltage exceeds the battery voltage. The transformer output voltage is a sine wave. So charge current to the batteries occurs in pulses at the peak of the sine wave. Without any filtering these pulses can be seen in the power line. I'm sure others will have a much more mathematical explanation but I would say the capacitor/transformer resonance acts like a flywheel in an engine.

 

[Comcokid]

What the battery charger is using is called a ferroresonant transformer or Constant-Voltage Transformer (CVT) design approach. This allows the battery charger to have a (somewhat) constant voltage output regardless of input voltage and load.

 

[Skogsgurra]

Yes, a CVT it is.

The transformer is made with a large leakage inductance which increases impedance so it gets a built-in current limit.

The winding plus capacitor then forms a series resonant load, which draws so much current that the transformer core goes to saturation and the transformer cannot deliver any more voltage.

The voltage produced when core is saturated is constant (same volt-second area regardless of input voltage) and that means that the output voltage is constant regardless of the input voltage - within a range. And that is how constant voltage is achieved.

Compositepro is absolutely right in saying that the resonance circuit works as a fly-wheel. In fact, many CVT:s are used to bridge short interruptions if the grid is unruly. Typically, it can keep voltage up for 50 - 100 milliseconds, if lightly loaded.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

this is a constant voltage xfmr; cap makes voltage on charge secondaries constant for constant VOLTAGE charging; very common on old fashion gold cart battery charges.

 

[goofy173]

Thanks for the info!