question about dual power supply (+15V -15V) using db102

[spacekid71]

Hi everyone,

I am building a dual power supply (+15V -15V) circuit using center-tapped transformer (18V-0-18V).

The CT transformer gives me about 21.5VAC without load in isolation and when using 4x 1N4004 bridge rectification I was getting about +28.5VDC -28.5VDC. This is quite high as the input voltages for my LM7815/LM7915 combo and was making them run quite hot.

Looking for a solution this morning I remembered that I had a db102 rectifier chip laying around and I decided to see what would happen if I replaced the 1N4004s with the chip. Instead of giving me about +28.5VDC and -28.5VDC, the rectifier chip was giving me +19.3VDC and -19.3VDC. I haven't tried to plug it into the 7815/7915 but it would seem that they would run much less hot.

I am by no means an expert in power supply circuits so I thought I would ask the experts on this forum. Any help would be greatly appreciated, thanks in advance.

[EATyourGuitar]

Rectified AC does not give pure DC. It gives rectified AC which is AC.
And DC mixed. If you add a cap it is filtered. But it will have a significant AC component until it goes through a regulator. Transformer selection also depends on cap selection and max load requirements. As the cap gets bigger the average voltage from input to out of the regulator is increased so the heat goes up. Are you measuring the minimum DC voltage or the peak DC voltage or the average DC voltage? Did you already add the cap? Unloaded, your transformer will be slightly different in AC voltage. I'm not sure how you measured DC voltages. That would help me figure what you have.

[antonis]

Can't be 11VDC drop for DB102..!!!

What is your load current..??

[spacekid71]

I will make a 1N4004 schematic and a db102 schematic to show what I am doing. Might take a couple of days though, please bare with me. 

[PRR]

> a db102 rectifier chip

That's a plain Si bridge. Works the same as four 1N400x.

There's several things here and you need to keep them straight.

A "18V" transformer is likely to un-sag 20% at no-load, making 21.6V AC, as you report.

When you connect the rectifier to a capacitor it "Peak Catches". The peak of a 21.6V Sine wave is 1.414 or 30.5VDC, ideally. Your 28.5V DC would be reasonable real-world values. 

Your 19.3V values are most likely without any large capacitor. (The Average factor is 0.9, which makes 19.44V.) This is not smooth DC but big pulsations, dropping to zero 100 times a second. If you feed your regulators it may look near-right on a meter, but buzz very badly.

[Rob Strand]

Pic for PRR's words.  (Minor detail/difference is the rectifier in your case is two full-waves (+ and -) but this pic shows a bridge.)

[antonis]

More close to your circuit..