Hi

i read that if 5v stand by are "gone" one of the reason can be 7805. Is it usually checked on the board or it should be desoldered?

how is the test actually done ?

Thanks in advance

 

You can really only check a '7805 by applying 7V+ and checking the output voltage. You could do this with it in circuit, checking the input and output voltages (unless something connected to the output is short circuit).

You talk about '5V standby' and this term is often used in relation to PC and MAC power supplies an in this case they don't generally use a '7805 but a separate switch mode supply.

 

You can really only check a '7805 by applying 7V+ and checking the output voltage. You could do this with it in circuit, checking the input and output voltages (unless something connected to the output is short circuit).

You talk about '5V standby' and this term is often used in relation to PC and MAC power supplies an in this case they don't generally use a '7805 but a separate switch mode supply.

As above - but if there's any aluminium electrolytics on the input and output of the regulator, they can be to blame. Under certain fault conditions, the regulator can burst into oscillation and not produce the output expected.

Tantalum bead capacitors are a popular choice with these regulators - they can go very leaky or completely shorted. On the output; a short would activate the internal current protection, but I'd expect the regulator to get a bit warm. A shorted tantalum bead on the input would probably go off like a match head - the hermetically sealed cylindrical tantalums used by military & aerospace would probably just go short and cause the damage to happen upstream.

 

Now i am looking for "it" and cannot find...

https://flic.kr/s/aHskTapyLD

Please tell me where to look for it.

I think it should be near to 4 diodes at that picture / in the middle/

https://www.flickr.com/photos/142883181@N04/33283899032/in/album-72157679481399141/

but in the left of them i see 2 Bipollar transistors...

Thanks for your advices

 

Now i am looking for "it" and cannot find...

That's likely because there isn't one, as @AlbertHall warned. They're pretty obvious if they are there.

 

You talk about '5V standby' and this term is often used in relation to PC and MAC power supplies an in this case they don't generally use a '7805 but a separate switch mode supply.

the article i read is old, which is in my language.

About that "separate switch mode supply" where to search it - is it integrated in main IC of the PSU or is a basic element which i can see?

thanks

 

About that "separate switch mode supply" where to search it - is it integrated in main IC of the PSU or is a basic element which i can see?

There are three transformers in a line across the PCB, one big one and two small ones. The big one is for the main supply (or main supplies).
That line across the board separates the low voltage output circuits from the high voltage mains circuits and maintains the isolation between the two parts. There will be some feedback across that line to control the output voltage. This is commonly an optoisolator straddling that dividing line. I suspect that one of those small transformers will be supplying the '5V standby supply'.

This circuit is an example PC power supply. Here T1 is the main supply transformer. T2 supplies the base drive for the transistors feeding T1. T3 and Q6 provide the +5V standby supply. U1 is the optoisolator providing the feedback.

 

You talk about '5V standby' and this term is often used in relation to PC and MAC power supplies an in this case they don't generally use a '7805 but a separate switch mode supply.

So...

the article i read is old, which is in my language.

About that "separate switch mode supply" where to search it - is it integrated in main IC of the PSU or is a basic element which i can see?

thanks

Most of the ATX aux 5V circuits I've seen were discrete component designs - most of those use a BJT because the circuit is simpler and cheaper.

Every once in a while I see one of the "usual suspects" SMPSU chips driving a MOSFET.

 

hi again. i have reread the thread and have here one PSU that i want to fix for learning purposes.

It was :
shorted fuse on the input
gone 1piece 2sc2625
1 disconected/fired/ track
and 1 little higher than normal electrolytic capacitor, bulging.

Have done :
new fuse.
new transistor. about the second one of this type 99.9% it`s healthy.I should have measured it
have used a pin of electrolytic cap for shorting the track
replaced the cap with new one

After connect the main there is no even 5v stb. Above you wrote about the little transformer on the oposite side of the big one. how to test it actually? About the mosfets i will all of them which i see.

Please be here for me