What does it mean when the voltage lower it self up?

[LegendBreath]

Hi,
What does it mean if I did a power supply, it's giving out 7v but, when I mesure it it's goes like 7-6-5-4-3-2-1-0... like really fast. Does it mean that my capacitor is too big or too small? I got a 100uF capacitor.

 

[Alun]

This could mean many things it depends on the type of power supply you have, without looking at it I'd automatically suggest using a bigger capacitor but there again some linear regualtors can oscillate if the load is a high-Q capacitance. Could you please post the schematic.

 

[LegendBreath]

This could mean many things it depends on the type of power supply you have, without looking at it I'd automatically suggest using a bigger capacitor but there again some linear regualtors can oscillate if the load is a high-Q capacitance. Could you please post the schematic.

Ok... ummm, well it's a power supply that i did my self pretty easy:


Transformer (5 pins - 2 for 7AC & 2 for 12AC) - Bridge Rectrifier (4 diodes) - Capacitor (4700uF - with 4700uF it's work more correctly - Tried 100uF - Tried 1uF (lol!) - Regulator KIA7806P (dunno the specs, need to find them only thing I know it's a 6V pos regulator)

And wth do you mean by "there again some linear regualtors can oscillate if the load is a high-Q capacitance"!?

 

[LegendBreath]

I'm guessing you don't have a feedback load resistor. 
Gain goes max, regulator shuts down ...

: ;D

ermm... A feedback load resistor?

 

[Decade]

How about a bad solder joint?
Or to old instrument that load the output of the psu  ;D

 

[LegendBreath]

How about a bad solder joint?
Or to old instrument that load the output of the psu  ;D

Lol don't think so, I got a pretty new Digital Meter and my solders are pretty good (not!, but its not the problem)

 

[audioguru2]

I did a power supply, it's giving out 7v

How can a 6V regulator IC have a 7V output? It's busted or you have it connected backwards.
Please attach your schematic.

I also don't know what is a "feedback load resistor", but a 7806 regulator should have a +6V output (within 5%) without any load. It should also be cold without a load. I bet yours is frying.

 

[LegendBreath]

How can a 6V regulator IC have a 7V output? It's busted or you have it connected backwards.
Please attach your schematic.

I also don't know what is a "feedback load resistor", but a 7806 regulator should have a +6V output (within 5%) without any load. It should also be cold without a load. I bet yours is frying.

Ah sry, that's without the regulator. And... I don't have any schematic...

 

[Alun]

And wth do you mean by "there again some linear regualtors can oscillate if the load is a high-Q capacitance"!?

I can't give and example but there is a linear regulator that oscillates when you place a large value low ESR capacitor on its output.

 

[Decade]

I would follow mr audiogurus advice, oh yes they could oscillate quite easily but 7 volts from 7806 make me suspichous.
LegendBreath I just wanted to put some positive feelings in the thread
not to offend but think you got me right

 

[Alun]

If it is a LM7806 then it shouldn't be a problem as this kind of regulator dosen't suffer from this. I agree with audioguru the regulator is probably broken, try replacing it.

 

[audioguru2]

A 7806 regulator needs at least 8.0V on its input to provide voltage regulation when it has a load. Look at its datasheet! The 7VDC input is much too low.

If you used 12VAC from the transformer, the rectified and filtered 15.5VDC would allow the regulator to work properly, but it might get hot with a load.
If you used 7VAC from the transformer, the rectified and filtered 8.5V is much higher than the 7VDC you measured. How did you lose 1.5VDC?

 

[Alun]

7VDC?
I made the assumption this was 7VAC from the transformer!

Go for 8V or alternatively use a low dropout regulator.

 

[LegendBreath]

Geez... I'm f.... up...  ??? ??? I can't understand a word you guys say  :-[  But I managed to fix the 6VDC thing up, but I got another problem, I'll explain it:
I got a transformer that takes 120VAC and put it to 12VAC and 7VAC, I use 2 bridge rectrifier to convert both of them to 12VDC and 7VDC. And then, I use 2 4700uF capacitors to stabilize those two voltages. And then I use 1 voltage regulator that's goes on 7VDC and transform it to 6VDC and for the 12VDC I don't use a voltage regulator.

At the end, it's gives me a 6VDC and a 11VDC (that's the current problem, the 11VDC)

 

[Alun]

That's very odd you should get about 15.6V if you put 12AC into a rectifier and capacitor, are you sure the bridge recitifier is working properly?

 

[LegendBreath]

That's very odd you should get about 15.6V if you put 12AC into a rectifier and capacitor, are you sure the bridge recitifier is working properly?

Oups... You just made me remarked that one diode is in the wrong way... I'll put it in the good way (after that the soldering iron is hot... )

EDIT: Nope I'm getting 9.3VDC. But, if I mesure before my capacitor, it's giving me a unstable 10.45VDC. Perhaps someone could help me on MSN or AIM?

 

[audioguru2]

I can't understand a word you guys say

Hee, hee. ;D We speak geek-talk! ;D

 

[LegendBreath]

Hee, hee. ;D We speak geek-talk! ;D

Lol I know Well I do understand some words I'm not a electonic geek (but want to become one ) I'm a PC geek!

But anyway that doesn't help me much... can someone help me with my 12VAC that doesn't gives out 15.6VDC problem?

 

[audioguru2]

If you can't post a simple schematic then how can we help you?
I use Microsoft Paint to draw all my schematics. With the Shift key depressed, the mouse makes perfectly straight lines. It can copy and paste stuff from other schematics.
I copied and modified this tiny schematic and your circuit should be wired the same:

View attachment 37855