H
Hobbyist
My son bought a cheap Chinese light display unit which worked
fine at first but refused to turn on after it had been turned off
for some time. I did notice that the AC plug was a little loose
in the socket. I opened the separate 5V 1A switched-mode power
adapter and sure enough, the inrush limiting resistor was open.
It's a 1-ohm TH unit and looks like a 0.25W carbon type. I had a
0.5W piece handy and used that. Before putting the PCB back into
the adapter housing, I connected it to an AC outlet (not the same
one) and turned it on, and it worked. However, after assembling
the adapter, I carelessly plugged it into a live socket, i.e.,
one that was alreasy switched on (without connecting it to the
display unit. The same resistor went phffft! and the adapter went
dead again.
My question to the experts is: Is it normal for the adapter to be
so intolerant of the contact bounces it must have encountered? Is
it likely that there's some other defect? The resistor goes to a
bridge rectifier and the main DC filter is a 2.2uF 400V electro.
I thought that it wouldn't have enough inrush current to keep
blowing the resistor even though I *was* rather careless with the
plugging-in. Please note that I'm asking this to gain a better
understanding of the technical reasons rather than a desire to
save a few bucks by repairing the adapter.
fine at first but refused to turn on after it had been turned off
for some time. I did notice that the AC plug was a little loose
in the socket. I opened the separate 5V 1A switched-mode power
adapter and sure enough, the inrush limiting resistor was open.
It's a 1-ohm TH unit and looks like a 0.25W carbon type. I had a
0.5W piece handy and used that. Before putting the PCB back into
the adapter housing, I connected it to an AC outlet (not the same
one) and turned it on, and it worked. However, after assembling
the adapter, I carelessly plugged it into a live socket, i.e.,
one that was alreasy switched on (without connecting it to the
display unit. The same resistor went phffft! and the adapter went
dead again.
My question to the experts is: Is it normal for the adapter to be
so intolerant of the contact bounces it must have encountered? Is
it likely that there's some other defect? The resistor goes to a
bridge rectifier and the main DC filter is a 2.2uF 400V electro.
I thought that it wouldn't have enough inrush current to keep
blowing the resistor even though I *was* rather careless with the
plugging-in. Please note that I'm asking this to gain a better
understanding of the technical reasons rather than a desire to
save a few bucks by repairing the adapter.
