Quick newbie question. I am still getting 6.72 volts out of a 5 V regulator.

Hi,


I am getting 6.5 volts out of a 5v ragulator output leg. The total
input voltage is via a 6 VDC volts AC adapter. it goes into a bridge
regulator, a 100 UF cap, then into the 5 volts regulator. At its
input jack - I am reading 10.72 voltage ( WTF, the adapter its rated
as 6 vdc output, ? ) , then at output side of the voltage regulator, I
am getting 6.72 volts.


I dont quite understand it as why?



The voltage is indeed dropping to 6.72 from 10.74. I thought the
voltage regulator should drop it to at least 5 volts?

I have tried two voltage regulator, and both reads the same output.

I am wondering if my volt meter is defective, or if both regulators
are defective.


I cannot see wiring the regulator wrong. It has three legs. the
middle is ground. Checking the inbound first leg, it reads the full
10.72, the outbound *( last leg ) reads at 6.72


What could be going wrong here guys,?


Sorry if this question is very basic, I am just a newbie playing
around with basic tools., and a bread board. Trying to learn

Thanks to everyone in advance.

 

Are you using a 7905 maybe? Or somthing else?

Harrie

 

Two possibilities:

You have in and out reversed.
You have not added the correct bypassing capacitors. (Oscillations)


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Mebbe your meter needs a new battery ??

 

Hi,

You really need to give more information if you expect a useful answer. If you
have a TO-220 LM7805/LM340T-5.0 or an LM317, these are the pinouts (view in
fixed font as Courier or in M$ Notepad):



.---------.
| |
| o |
| |
|---------|
| |
| |
| |
| |
| |
'---------'
| | |
| | |
| | |

1 2 3

(Pinouts with plastic front of TO-220 facing viewer)

LM317
1. ADJ.
2. V(out)
3. V(in)

LM7805/LM340T-5.9
1. V(in)
2. COM
3. V(out)

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

When checking a breadboard circuit that doesn't work, always double check the
wiring and pinouts. If that isn't it, check 'em again. Then look at other
possibilities for three minutes. Then check your wiring and pinouts a third
time. This simple procedure solves the problem about 80 to 90% of the time,
and 5% of the time, the problem still ends up being the wiring and pinouts.

(By the way, you almost certainly need a bigger input filter cap, too).

Good luck.
Chris

 

follow up.



Once again many thanks to everyone!!!!!




It was a darn battery on a digital meter. I am a newbie, and It was an
honest mistake. Never thought the battery being the problem.

I apologize for such a dumb msitake :*(

 

Check your bridge with an ohm meter, sounds like you have alot of
ripple, you didn't indicate if it was full or half wave, in any case
increase the capacitor to 470uf, also replace the cap, because the ESR
may be too high, and it could be dried out.

if it's 7805 1 amp regulator, you need to have at least 2 volts rms
higher than the output, if it is less than the regulator won't function
properly. In your case it should be 8.6 volts RMS, and not 6, you have
to account for the diode drops, you can get the information out any good
electronics text book. it's a good idea to place a 0.1uf cap on both
input and output to get rid of any noise that adapter may spill into the
line voltage.

if all fails, replace the regulator, but only after you scope the cap
and bridge network, to make sure you getting rectified DC from the
input.

nells
Electronics Eng.