Power Supply Filter Capacitors

[Alexsgarage]

I am building a power supply to give +12V, 0V, and -12V. I have a regular 24V transformer (no centre tap). I was using some large 470µF filter capacitors and I still get distortion on the output side. Also when I test the voltage on the output of the diodes the voltage is 24V DC but when I attach the filter capacitors the voltage changes to 38-40V DC can someone explain why this happens?

 

[Sceadwian]

If you want a good solid -12 0 and +12 you really need a center tapped transformer. Your meter may not be reading the ripple voltage straight off the diodes correctly, with the capacitors it will buffer the maximum voltage the rectifier is putting out, as soon as you apply a load to the output that 38-40 peak voltage will drop as the current draw increases. This is perfectly normal (and should be expected) from bridge rectified AC.

 

[Alexsgarage]

This is perfectly normal (and should be expected) from bridge rectified AC.

I am not using a bridge rectifier I am just using two IN4004 rectifier diodes to get the positive and negative voltages. The diodes are connected to one wire of the transformer and the 0V connection is the other wire. Also how should I eliminate this distortion on the output side.

 

[Sceadwian]

Let me rephrase that then, it's typical of any AC rectifier. You're doing two half wave rectifiers so it's actually worse, you're always going to have a lot of ripple. The only way to filter it is to use a good voltage regulator, and really big filter caps.

 

[Alexsgarage]

Ok I have a surplus of filter capacitors but I have no voltage regulator for this project, can I use a voltage divider?

 

[kchriste]

I am building a power supply to give +12V, 0V, and -12V. I have a regular 24V transformer (no centre tap). I was using some large 470µF filter capacitors and I still get distortion on the output side.

What do you mean by distortion?

Also when I test the voltage on the output of the diodes the voltage is 24V DC but when I attach the filter capacitors the voltage changes to 38-40V DC can someone explain why this happens?

The capacitors charge up to the Peak AC voltage of the 24Vrms.
Vpk = Vrms * 1.414
33.936 = 24 * 1.414
The above calc ignores the typical diode voltage drop of 0.7V. You are getting more than 33.9V because the transformer is putting out more than 24Vac when lightly loaded.

 

[Alexsgarage]

What do you mean by distortion?

On the oscilloscope there is the peaks of the sinewave input, somewhat smoothed with the filter capacitors.

 

[Alexsgarage]

What do you mean by distortion?

On the oscilloscope there is the peaks of the sinewave input, somewhat smoothed with the filter capacitors.

 

[Sceadwian]

No, you need a voltage regulator to properly regulate the voltage, or at the very least a ripple regulator. It basically has to be an active device, or you're going to get variation and ripple on the output under load.

 

[kchriste]

On the oscilloscope there is the peaks of the sinewave input, somewhat smoothed with the filter capacitors.

You can add more capacitors in parallel with the 470uF caps to reduce the ripple. Since you are making a ±12V supply, you will need regulator chips to bring the voltage down anyway. As Sceadwian has already pointed out, the regulators will eliminate most of the ripple. A 24Vac transformer is a bad choice for a ±12V supply because a lot of power will be wasted in heat. A 24Vac center tapped transformer would be perfect.

 

[Alexsgarage]

Does anyone know where I can get a 7912 and a 7812 regulator, I checked ebay but it will cost me 10$ for a 2 regulators!

 

[superfrog]

rs seems to have them for around 10p fr a to92 version and 40p for a to220 version, that is in UK.

I am pretty sure you should be able to get these kind of prices from digikey or any online component reseller. If you just need that, it might make sense to find a local shop selling this kind of products, even though , in my experience, they rarely stock fixed voltage regulators, and only have adjustable ones.

Hope this helps

 

[kchriste]

Does anyone know where I can get a 7912 and a 7812 regulator, I checked ebay but it will cost me 10$ for a 2 regulators!

Some other choices:
Voltage Regulator L7812CV - dipmicro electronics $0.77 each (7812 only)
Digi-Key - LM7812CTFS-ND (Manufacturer - LM7812CT) $0.58 each
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM7912CTFS-ND $0.58 each

You might also want to look at the LM317/LM337 combo if you want an adjustable supply. Also, the 7812/7912 only will handle apx 35V input and will not let you supply much current at that voltage due to excessive power dissipation. You need a center tap on that 24V transformer!

 

[bountyhunter]

What do you mean by distortion?

On the oscilloscope there is the peaks of the sinewave input, somewhat smoothed with the filter capacitors.

The peaks of the sine wave will always look flattened some when using a diode rectifier bridg and filter cap.