good morning
i am working with old circuit board . with 7812 voltage regulator .so i changed with new voltage regulator l7812cv . but it is over heated within 20 sec and output is also not proper . i need output of 12v . i am in confused state.
24ac---rectifier----dc24----7812----- 12v----555ic --
same sequence is carried out what may be cause... whether i have to use high rated 7812 ic

 

Check and recheck the pinout on the regulator. They are NOT all the same.

 

Get a 16VAC transformer. The 24VAC transformer provides too much DC input voltage to the VReg

 

An unloaded 24 VAC transformer will put out 27 VAC. If you use a fullwave bridge, you’ll be putting in 38 volts DC to that regulator. The maximum input for that part is only 35 volts.

Just using a single diode for halfwave will get it down to about 12 volts.

Ideally, you want the input voltage to be about 3 volts above the regulation voltage to produce the least amount of heat. If you’re hardly drawing any current, you may even want to go as low as a 6.3 VAC transformer.

 

I wasn't real clear on my post, but Ernie clarified it for me.

Get a 16VAC transformer. The 24VAC transformer provides too much DC input voltage to the VReg

What I should have said was...

The 24VAC transformer provides too much DC input voltage after full wave rectification to the VReg. The transformer itself does not turn AC to DC but rather the diode array that follows it (Full Wave Rectification). So as Ernie said, you are supplying the VReg with roughly 38VDC in which the max input voltage of it is rated only for 35V.