Role of output capacitor with 7805 regulator, An unusual finding.

[rizwanspirit]

I have to power up an embedded server, which takes 5v, ~500mA. I made a 7805 regulator circuit, by using 47uf input and output capacitors, server initialized well but after some time it reboots.
I was told to increase the value of out put capacitor. By using 470uf 25v I/p and O/p capacitors, performance increased but still issue of rebooting was there.
Now i am using two parallel 470uf capacitors at output, the unusual thing i cannot understand is that sometimes output voltage climbs up slowly and after some time it reaches maximum level, say 11v (with 12v input to yhe circuit) and then jumps back to 5v.
I am unable to understand this phenominon.
I would appreciate if someone explain this, and clarify role of output capacitor.

 

[jiripolivka]

I have to power up an embedded server, which takes 5v, ~500mA. I made a 7805 regulator circuit, by using 47uf input and output capacitors, server initialized well but after some time it reboots.
I was told to increase the value of out put capacitor. By using 470uf 25v I/p and O/p capacitors, performance increased but still issue of rebooting was there.
Now i am using two parallel 470uf capacitors at output, the unusual thing i cannot understand is that sometimes output voltage climbs up slowly and after some time it reaches maximum level, say 11v (with 12v input to yhe circuit) and then jumps back to 5v.
I am unable to understand this phenominon.
I would appreciate if someone explain this, and clarify role of output capacitor.

Rebooting a server requires a huge DC voltage variation. I do not think it has anything to do with the capacitors around your 7805 regulator.
If you suspect it, change the 7805. Check if the input DC voltage is always >+8VDC to make 7805 regulate. Check for external interference, connecting cables can bring some pulses from outside.

 

[rizwanspirit]

Rebooting a server requires a huge DC voltage variation. I do not think it has anything to do with the capacitors around your 7805 regulator.
If you suspect it, change the 7805. Check if the input DC voltage is always >+8VDC to make 7805 regulate. Check for external interference, connecting cables can bring some pulses from outside.

Input is 12v Dc, there is no external interference,
Large Capacitors were introduced to minimize effect of possible current surges (Which may cause rebooting embedded server), In my openion these current surges may be introduced at time when some peripherals are turned on.

 

[123jack]

12v into a 7805 suggests some heat. Do you have a good heatsink in place?
As suggested above - I doubt the caps are a problem but you could be suffering thermal shutdown.

 

[smijesh]

Input is 12v Dc, there is no external interference,
Large Capacitors were introduced to minimize effect of possible current surges (Which may cause rebooting embedded server), In my openion these current surges may be introduced at time when some peripherals are turned on.

Electrolytic capacitors are using for smoothing (Ripple reduction) the DC, not for nose filtering. Similar problem I faced few years back. I had found differential mode noise flowing in the circuit. I had used one inductor and two ceramic capacitor as a pi filter and solved the issue. You can try minimum 100nF ceramic capacitors on both input and output side of the 7805.

 

[mtwieg]

Sounds like a thermal issue, rather than an issue with the output caps. Try using a better heatsink or use a SMPS.

 

[iop95]

What type is 12V power supply? If there are connectors involved, check contacts, may be a contact resistance problem.

 

[david002]

Why you do not just implement a diode in the chip if it's such an obvious point of failure?Do it i am also waiting for your reply :lol: