Sagging power supply

[jerryd]

Electro Tech,

I'm using a 120VAC to 9VAC 1A wall transformer, a full wave
bridge and a 7805 regulator for my 5 volt supply. There's
a 1000uf cap on the input of the regulator and a 220uf
on the output. This yields 4.96 to 5 volts.

As soon as I start to add devices to my board the 5 volts
starts to sag. With just one LMC555N the voltage drops
to 4.4 or 4.6 volts.

I think I have ample grounds and there's a 1uf cap on
the supply pin of the 555. The only other thing I have
on the board right now is an led for a power indicator.

I'm measuring the voltage with a Radio Shack digital
multimeter.

Am I missing something here?

jerryd

 

[MikeMl]

When the supply is unloaded, what is the voltage at the 1000uF capacitor?

What is it when the supply is loaded?

 

[colin55]

Something is faulty with the 7805. Or some diodes may be faulty or not connected or something is wired incorrectly. Try another multimeter.

 

[audioguru]

The small size of the filter capacitor prevents the 7805 from regulating when the output current is about 200mA.
If it does not have a heatsink then its heat will cause the output voltage to drop when it is loaded.

 

[colin55]

The small size of the filter capacitor prevents the 7805 from regulating when the output current is about 200mA.

Totally untrue.

1,000u allows about 1 amp and 10mV ripple. The 220u on the output will reduce this to about 1mV.

 

[ronv]

Go back to post #2 and check the input voltage. This will tell if it is a circuit problem or???

 

[jerryd]

Electro Tech,

The voltage at the 1000uf cap is 12.52 VDC.

There is no heat sink but there is almost no load.

When unloaded the ouput of the 7805 varries between 5.05 and 4.93
every time I read it. Maybe it's the wonderful Radio Shack meter.

Thanks for the replies so far.
jerryd

 

[ronv]

Should work. Is it possibly backwards?

 

[atferrari]

You could do something else

I'm using

I think I have

The only other thing I have

Am I missing something here?

Yes, to post the circuit to avoid the previous gentlemen to discuss about something that they do not actually know how it is.

 

[gerty]

And we need to know how much current your loads require.

 

[wuchy143]

Totally untrue.

1,000u allows about 1 amp and 10mV ripple. The 220u on the output will reduce this to about 1mV.

Colin,

What equations are you using for your calculation?

 

[audioguru]

My graph shows that the ripple voltage from a full-wave rectifier bridge and a 1000uf filter capacitor with a 1A load is off the graph at maybe 6V p-p.
With a 0.5A load the ripple is about 3V p-p. If the LM317 does not dropout then it will reduce 6V p-p of ripple to about 3mV p-p.