capacitor explosion??

[MrDEB]

Just applied 24v ac to this circuit and the capacitor C5 went BANG!
C5 is/was 100uf 10v, C5 is 10uf 25v
The 7805 should have output 5vdc but why the BANG!!
Only thing I could think of the 7805 didn't do its job??

 

[Diver300]

Did you mean that C4 is 10 uF, 25 V?

24 V AC when rectified will give about 35 V.

That is about the maximum input voltage of the 7805, and well over the voltage of C4, if I have guessed correctly.

Did you connect C5 the right way round?

The input capacitor, C4 should be a lot larger than 10 uF. You need a capacitor that will keep the supply working between mains cycles. 10 uF, even with a 10 V drop, will only support about 10 mA. You would get excessive ripple current in C4, so you should be aiming for 10,000 uF or so, but you could use a much smaller transformer voltage.

 

[MrDEB]

The 24v ac is connected to the 4 diodes configured as a full wave bridge rectifier so it should only output 12v. Where you get 35V??

 

[Nigel Goodwin]

The 24v ac is connected to the 4 diodes configured as a full wave bridge rectifier so it should only output 12v. Where you get 35V??

24V in will give 35V odd on the capacitor, how did you imagine 12V?.

I'm presuming the diodes aren't 1N4148's either?

 

[shortbus=]

Input voltage * 1.414 = out put voltage of full wave bridge rectifier

 

[audioguru]

Mr. DEB,
1) Why is your schematic a NEGATIVE image with a BLACK background?
2) Why is it backwards? The input should be on the left side and the output should be .....
3) Didn't you know that the peak voltage of 24VAC is 34V? If you use a 24V center-tapped transformer (with the center-tap connected to 0V) and two rectifier diodes then the output voltage is about +16V.

 

[MrDEB]

I had a brain freeze, thats my story and I'm sticking to it.
The transform does not have a center tap.
the diodes are 1n4004.
I simulated using LTSPICE and it came up with 12v output?
Will do futher posts using a positive image.

 

[MrDEB]

simple revision

Already have mounted on pcboard. a simple solution. NOTE I need the 24v ac for the iron heater still.
Off to RS when they open. Don't have and zener diodes but have 3W 300 ohm resistors.

 

[Nigel Goodwin]

I suggest you work out how big your zener needs to be before you dash off to buy one - but it looks a VERY, VERY clumsy solution.

If the 5V supply only needs such a tiny amount of current, why not just use a 5.1V zener and dump the 7805?.

Assuming you want to continue using the 7805, then dump the zener - a resistor is all you need.

 

[MrDEB]

would not the zener provide a better stable voltage to the 7805?
Why does it look clumsy? Need a clean quick solution. Reason for the 7805= a stabile voltage for the PIC
will post entire circuit schematic. This is a PIC controlled soldering iron.

 

[MrDEB]

complete schematic

here is my complete schematic. A very basic circuit

 

[MrDEB]

power supply revision

Went with a simple voltage divider.
Changed the cap values
Only get 7v on 7805 output with a 20ma draw. Probably not enough. Need to recheck data sheet for min current draw for regulation.

 

[MrDEB]

Changed 7805 and tested with a 300 hm resistor across V=/grd
5v+ instead of 7V .
Reg must have been damaged during cap explosion!!BAM!!
Tested all voltage points.
Going to insert PIC and solder the LCD on tomorrow.

 

[audioguru]

The output of an LM7805 is supposed to be from 4.75V to 5.25V, not 7V. It might be oscillating because it doesn't have a little high frequency 0.22uF or 0.33uF capacitor to ground on its input as shown on its datasheet. It is spec'd with a minimum load current of 5mA.

 

[MrDEB]

It works as planed now after I changed out . The one that was in the circuit is the same one that I exploded the cap with.
I have seen data sheets with 1000uf on input as well as .22uf. Have also on the output .1 as well as 10uf.
All depends on how far from source of input as I interpreted the different data sheets.
I am going to add a .1 to output and possibly an additional one ? on input but with a 300 ohm resistor across therails I get 5v