problem with my full wave bridge rectifier!!! 1

[garuda]

hello everyone,

i prepared full wave bridge rectifier using 4001 diodes. the input is given from a tranformer 220/12v.
the output terminals r connected to a 4900 microfarads capacitor.now the voltage across the capacitor is 36v.i want it something around 12v.
can someone please tell me why is it showing 36v and how do i overcome it.
thanx in advance.

garuda

 

[IRstuff]

try measuring with some sort of real load TTFN

 

[BrianG]

There are several reasons why you can get more volts than you expected.
1)Measure the actual a.c. volts at the secondary - I've had a wrongly marked transformer in the past!.

2) As Irstuff says, the volts may be be higher if this is without a load. Don't forget that most small transformers try to compensate for copper and iron losses by "overwinding" the turns ratio so that it gives the right sort of output voltage at the rated load current. This may account for as much as 20% excess voltage at no load.

3) The previous explanantion is not likely to explain 300% overvoltage. One thing that might is if you have inadvertently connected the bridge to a centre-tapped 12V secondary, i.e. instead of being 0-12V connection to the bridge it's a 12-0-12V winding intended for a full-wave two diode circuit. This would give your bridge 24V input instead of 12V, and remembering that, off load, your capacitor will charge up to 1.414 x 24V (ignoring diode drops and allowing for overwinding) this gives about 36 volts!

4) You might want to check your transformer connections to make sure that you have not connected 220V to a 110V primary tapping, as this would also give you twice as many volts at the secondary.

Sounds obvious I know - but I speak from experience!

 

[6349342]

You may be reading the dc voltages on the ac range of your multimeter .

 

[dydt]

hi...
you are measuring the charge on the DC filter capacitor...try to put a resistor or light bulb with the
right voltage across and measure again..

dydt

 

[BrianR]

With a 12V transformer you cannot have 36V on the filter capacitor. The capacitor voltage will be 1.414 x 12V, ie. charging to the peak value of the AC voltage. Under load this will drop due to diode forward voltage drop (approx 1.4V) and transformer resistance. If you are reading 36V then either you do not have a 12V transformer or you do not have a serviceable meter set to read ACV. :-D

 

[Lewish]

Hi BrianR, actually you could get 36V from a 12V transformer, assuming that it is a Center-tapped transformer for Full-Wave rectification, as is often called out in the datasheets. You have 12V either side of the center-tap. Now if you use a 4 diode bridge, instead of the 2 diode full-wave rectifier scheme, you have 24V * 1.414 + a little more at no load = 36V. I have seen novices make this mistake before.

 

[BrianR]

Lewish,
I concede that point. It was a bit obvious but would certainly explain the result. (Of course if you ground the centre tap and use a bridge you get +/- 18V out).

 

[Lewish]

BrianR, that would be true at no load. But, if you want to get really tricky, I can ground one end of the 4 diode bridge and get +12V and +24V under load, by using the center tap for the +12V.

 

[BrianR]

Lewish,
You've been designing power supplies too long. That's cunning.

 

[Lewish]

But BrianR, I don't admit to designing power supplies. I am an ANALOG Engineer. You get that power supply stuff on you and its hard to get off.

 

[ulix]

- plase measure A.C. voltage on two wires conducted to your full wave rect, bridge.
- there would be 24 V A.C., as first described BrianG in his answer, part 3)

There is lot of solutions. Take, for instance integrated stabilizer type 7812, don't forget add two capacitors cca. 1 microfarad to in and out pins against middle pin which is conducted to common, and minus lead from your capacitor.
A heat sink must be mounted on it, depends of current.

Another simple way (if you don't need stabilized voltage) is to change your transformer with one 9 Volts output voltage. It will produce around 12 V rectified. Use same diodes bridge (Graetz) and capacitor. a rolling stone gathers no moss

 

[ulix]

P.S. Applying solution with an voltage stabilizer is not so good idea because of maximum input voltage, it is very close to this value, typicaly such stabilizers have 35V max. DC in.

Another reason is simple: 36 - 12 = 24 volts, and 2/3 of total used power would be dissipated into heat...
Much better result will be with input voltage of about 18 volts (if there is central point on transformer). a rolling stone gathers no moss

 

[garuda]

hi everyone,
i am grateful for all of ur valuable suggestions. but this time i tried something else. i grounded one end of 4 diode and the other end to the capcitor which i grounded again.
also, i grounded centre-tap. this resulted in the burning of the grounded centre-tap wire.
need ur help once more. plz tell me why this has happened.

thanQ,

garuda

 

[Lewish]

garuda, now that you have told us that your transformer is a center-tap design, how about you searching on the web for a full-wave rectifier design based on a center-tap transformer. You might get a clue that you only need 2 diodes when you ground the center tap!

 

[BrianR]

garuda,

Your "modification" to the circuit shorted out 2 of the diodes, resulting in one half the transformer being shorted on one half cycle and the other half on the other half cycle. That is until the diodes overheated and failed short circuit, (which is what they do), and then the transformer would have been shorted all the time.

The bridge rectifier is probably fried as is the transformer. Draw the circuit and work it out.