24 Dec
2011
24 Dec
'11
12:51 p.m.
If it's lower than the mains it's running in
shutdown mode where it
tries to start
and then shuts down. As a result it sounds like a very slow click.
With some SMPSUs you get a burst of oscillations driving the chopper (at
the normal switching freqeuncy) for each click.
The likely reason for that is NO LOAD, most older switchers have to be
loaded
to about 10% of total load to start and run correctly. The other fault
could be
overload where the load is a short of other fault. If those things are
In many cases, the reason why the PSU shuts down with no load is that you
get a spike on the output which causes the crowbar to fire, effectively
short-circuitng the supply. It then shuts down and tries again.
_Never_ disable the crowbar or overcurrent protection circuits in any PSU
unless you really know what you are doing. The results caan be exxpensive
an unpleasat. I rememrbr disabling the overcurrent protection in a DEC
H754 -15V regulator brick (which is a switching regualtor, of course),
only for find that the crowbar was firing. WIthout the overcurrent
proteciton, I ended up with most of the transsitors dead (several of them
blown off the board), resistors burnt out, the 723 useless, and so on.
Yes, I did get it going again...
-tony