Assistance for understanding a sudden increase in current in power supply

[efelnavarro09]

Hello, Im working on a system that requires the following voltages 1.2 V, 3.3 V and ±5 V, all from a unregulated 7 to 9 V input source. Anyway, Here I attached my schematic so may some one can notice what is going on, because when I power one the board the current starts at few mAmps and suddenly, after some minutes, the system consumes about 1.5 Amp with any load. At this point I not feed any component, jus test the power stage with that result. The 3.3 V LDO regulator (LT1764) star heating a lot I can find out any short circuit. I will appreciate any look you can make.



Thanks in advance.

 

[mtwieg]

There's no good reason for that to happen. Did you check that all the bypass caps are still working?

 

[efelnavarro09]

mtwieg, thanks for your reply. Unfortunately I did check all capacitors and used the recommended ones by the manufacturers. Actually this is the first time I see that happend, that is why I'm asking for assistance.

Thanks again

View attachment powerSly.jpg

 

[Audioguru]

A low dropout capacitor oscillates and draws a high current when its output capacitor is bad.

Your schematic is a big as my neighbourhood so only small pieces of it fit on the screen of my monitor.

 

[DXNewcastle]

Also looking at the capacitors, I see there's some pointless variations in the voltage ratings. eg C4 is 50volts and C10 is 16volts, and C5 and C6 are both 6.3volts in series (why in series? they probably won't share the voltage equally!).
I'm also puzzled by the bridge rectifier - if its a DC input, why is there a rectifier? If its an AC input, is C2 unpolarised?

But my biggest concern is that some of the capacitor values are probably implemented with electrolytic caps (eg C5, C6, C8, C9, C10, C11, C14, C18, C20, C23 etc) but they are drawn as unpolarised. I strongly advise checking their polarisation. This also applies to C2, but you won't put an electrolytic there if the polarisation is either AC or indeterminate - and its also unecessary to put 10uF in there for an AC input.

Up-rate C5 and C6 to a single 1uF cap with 16volt or more rating. C16 doesn't have a voltage specified - check that its adequate.

I find the drawing very poorly laid out, with some psu elements drawn with neg at the top. There's some unpolarised caps in those positions, so check them again where the pos should be at the bottom of the drawing.

As a trouble shooting technique, simply disconnect the main parts to idenfify which part is consuming the current. Alternatively, insert a small value resistance between each part, and watch the voltage drop over the resistances to locate the current. I would certainly disconnect the negative voltage sub-circuit and try again - that would tell you whether the fault lies with the neg circuit or not.