transformer and rectifier...

Thread Starter

hazim

Joined Jan 3, 2008
419
I connected a 220 to 6V transformer with a fullwave rectifier, and I put two 330μF capacitors.. I got about 8 to 9V DC at output... What does I have to do in order to have an output of 6V or if possible 5V without using voltage regulators such as LM7805/6 and LM7905/6...??? Thanks
 

davebee

Joined Oct 22, 2008
539
The trouble is that the AC will charge the capacitors to their peak voltage with no load on them, but there is only a tiny part of the AC cycle supplying power at the peak of the cycle.

So while with no load, you'll get a peak voltage, a tiny load will immediately drop that voltage by a large amount, then as the load increases, more and more of the AC cycle is providing power, so the voltage seems to stabilize at a lower value.

One way to get a more stable voltage without a regulator is to add a dummy load (a resistor) to pull the voltage down to where it is more stable. You'll waste some power in the dummy load, but then adding additional load won't vary the voltage nearly as much.
 

Bernard

Joined Aug 7, 2008
5,227
What is your average load current on DC side. Possibilities : add a choke between rectifier and capacitors, replace choke with resistor of proper ohmage & wattage, or add a resistor in parallel with load to bring output to desired level, or a combination of preceding.
 

S_lannan

Joined Jun 20, 2007
247
you could use series diodes. each dropping .6 or so of a volt until you have the voltage you need. Regulation is not good but it works. A zener diode shunt regulator will do the trick too.
Having said that a standard regulator is the best way to go. You could use a switch mode regulator but it would be uneconomical.
 

rvh002@gmail.com

Joined May 15, 2009
119
No chance, you must use a regulator. Remember ac out when rectified produces 1.4+ times the ac voltage. This is fairly basic.
 

Thread Starter

hazim

Joined Jan 3, 2008
419
I have tried to put a 2W resistor, 220ohm I think, voltage dropped a little.. and the output isn't stable... I can't use this voltage supply since it's not regulated as circuits I'm using require... I may use voltage regulators...
 

Audioguru

Joined Dec 20, 2007
11,251
A 6VAC transformer has an output of 6VAC only when its input voltage is correct and when its load current is what it is rated for. It might have an output that is 10VAC when the load current is low.
the 10VAC has a peak voltage that is 14.14VDC which is reduced to 12.7V by the full-wave bridge rectifier. Then the output voltage drops when the load current increases.

If you want a regulated output voltage then of course you must use a voltage regulator.
 

Thread Starter

hazim

Joined Jan 3, 2008
419
What do you want to power with 6V?
My senior project... there is many circuits and some 6v dc motors...etc. there is no constant current but I think the current will not exceeds 1A.
 

Thread Starter

hazim

Joined Jan 3, 2008
419
for the capacitors between -6V and ground, the negative terminal of the capacitor is connected to -6 and the positive to 0, right?
 

leftyretro

Joined Nov 25, 2008
394
for the capacitors between -6V and ground, the negative terminal of the capacitor is connected to -6 and the positive to 0, right?
Correct, the negative lead of the cap goes wires to the most negative voltage.

Lefty
 
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