Completed Project Why does the output of a transformer increase when connected to a battery for charging?

Thread Starter

Rahulk70

Joined Dec 16, 2016
504
I have a 110VAC- 14VAC 5A transformer with me. I have the output rectified and it shows like 13.8VDC. But when I connect it to my 12V SLA for charging it output rises to 15.8V instantly and I hear a lot of fizzing from the battery. WHy does this happen?
 

R!f@@

Joined Apr 2, 2009
9,734
15.8V across a 12V SLA will damage the battery.

Is your transformer a CT ?
Are you using a 4 diode bridge with a capacitor ?
 

Dodgydave

Joined Jun 22, 2012
9,161
Because your battery is acting as a capacitor and smoothing the rough DC to make it higher.

To charge a Sla you need a Constant Voltage of 13.8V dc.
 

#12

Joined Nov 30, 2010
18,210
Pretty much what The Dodger said, but in a different way: Your charger is not delivering DC voltage. You try to measure it on a DC range and the meter can't produce a truthful reading of the DC voltage because it isn't DC, it's pulses. Then you connect it to a battery which sustains the DC voltage between pulses and your meter gets a better measurement.
 

crutschow

Joined Mar 14, 2008
24,963
You should use a bridge rectifier at the transformer output.
A single rectifier (half-wave) will cause a large DC current int the transformer which can cause the core to saturate and overheat.

A 14Vac transformer will generate a peak DC output of about 18V with a bridge rectifier and a filter (in this case the battery acts as a filter).
You need to add a regulator to reduce the voltage to about 13.8V as DD suggested.

The charge current should also be limited to whatever the suggested value for the battery you have.
Above that, the battery can be damaged.

The circuit you need depends upon how much charge current you want for the battery.
 

tcmtech

Joined Nov 4, 2013
2,867
Typically for a basic lead acid type battery charger I try to use a transformer with a 10 -11 VAC output or take off windings for one that a bit higher to get to that point.

For a cheat if you are not running higher amperages just putting some diodes in series with the output to use their combined forward voltage drops to get down to the voltage you need works too.

Personally when I build custom battery chargers I aim for a ~1 amp output at ~ 14.4 volts on a near fully charged battery which in most cases with a good capacitor in place of the battery the charges runat an open circuit voltage of ~ 15.5 - 16 volts which at a light load hits that target voltage and current nearly perfect.
 

tcmtech

Joined Nov 4, 2013
2,867
You should use a bridge rectifier at the transformer output.
A single rectifier (half-wave) will cause a large DC current int the transformer which can cause the core to saturate and overheat.
A Center Tapped secondary with two diodes is the same as a four diode full wave bridge as far as the transformer sees the load.

Also being the input power to the primary winding is what's is alternating the transformer core can't go into DC saturation due to an unbalanced load on it secondary. The windings will burn up long before the core has any flux saturation overheating issues.
 

crutschow

Joined Mar 14, 2008
24,963
A Center Tapped secondary with two diodes is the same as a four diode full wave bridge as far as the transformer sees the load.
But the output voltage will by 1/2 as compared to a bridge connected to the ends of the winding.
Also being the input power to the primary winding is what's is alternating the transformer core can't go into DC saturation due to an unbalanced load on it secondary. The windings will burn up long before the core has any flux saturation overheating issues.
Yes, it would appear that core saturation is not generally an issue with a half-wave rectifier.
According to Hammond the maximum DC current should be limited however:
Hammond recommends an output DC current of 0.28 times the RMS current rating of the transformer for half wave rectification and 0.62 times the RMS current rating for full wave bridge rectified current.
but that's due to the high RMS peak current of the rectified waveform into a filter capacitor.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
504
As noted, that will give half the voltage output.
If the transformer is a 14Vac output with a center-tap as the op says he has, then the filtered DC output of that full-wave circuit will be a little over 9Vdc.
Yep. You are right the output from the supply using the above confiuration using centre-tap gives me about 10.11Volts.
 

Dodgydave

Joined Jun 22, 2012
9,161
The ts said his transformer is 14V, so that should give him about 18V dc across the smoothing capacitor with a two diode bridge, otherwise he needs a four diode one and ignore the CT.

14v- 0.7 =13.3v x 1.414 = 18.8V

Or is the transformer a 7-0-7v ??
 
Last edited:

tcmtech

Joined Nov 4, 2013
2,867
The ts said his transformer is 14V, so that should give him about 18V dc across the smoothing capacitor with a two diode bridge, otherwise he needs a four diode one and ignore the CT.

14v- 0.7 =13.3v x 1.414 = 18.8V

Or is the transformer a 7-0-7v ??
That was what I was assuming too. It was a 14- 0 - 14 battery charger transformer not a general purpose 7 - 0 - 7 one.

If it is wound with the secondary on the outermost part of the coil set it may be possible to unwind enough turns to reduce it down to a 10 - 11 volt unit. I've done that cheat countless times to make odd transformers into special application ones for unregulated DC power supplies and such applications.
 

#12

Joined Nov 30, 2010
18,210
If it is wound with the secondary on the outermost part of the coil set it may be possible to unwind enough
That's easy on torroids. You can just add turns going to higher or lower voltage by running some wire through the center and never risk damaging the original windings.
 

tcmtech

Joined Nov 4, 2013
2,867
That's easy on torroids. You can just add turns going to higher or lower voltage by running some wire through the center and never risk damaging the original windings.
I've done it on E/I cores many times and it's not that bad really.
 

#12

Joined Nov 30, 2010
18,210
I've done it on E/I cores many times and it's not that bad really.
I over-wound a choke for a HPS ballast about 20 years ago. It has been running 10 or 12 hours per night for 20 years, so I know it can be done successfully. I just wanted to mention that over winding torroids is a walk in the park.
 
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