NCP1271 power supply not working under load
Good day folks. Bit of a dilemma here and I'd like some help so as not to spend too much time and resources on something that may be either really easy to fix or a rabbit hole.
I pulled out an old Bosch analog PTZ CCTV camera from one of our sites. The camera was dead - no power and no signal at all. I took it apart and after some tinkering I found it's not getting any power to its "brains". It takes a 24Vac input which is then rectified and dropped down to 12v (I think) by a small power supply based around an NCP1271. When I first got it, there was absolutely no power on the output of this internal PSU at all. I managed to get it going to a degree, but it's still not enough: initially, I found a short on the VCC pin of the IC (D359 - sorry I don't have a pic of the whole board and it's pretty messy on there). Based on how it was installed (anode to GND and cathode to VCC) I figured it's a protection diode and after removing it, I tried to fire it up without it, since I don't have a similar one. I now get 17v on the VCC pin of the IC and 12v on the output caps, which is fine, but when I connect the head of the camera, it shuts down and power cycles every 5s or so. I though the head itself may have some bugs of its own, like a short, so I tried a small 12v fan on that 12v output, but got the same result - the output shuts down and begins to power cycle. It's worth noting the the VCC pin of the IC ALSO drops down to almost nothing with each cycle. It then SLOWLY goes back up again and once it reaches around 12v, the output goes high again, before immediately shutting down. This makes perfect sense, since the datasheet clearly tells us UVLO is 12.6v.
Now I'm pretty stumped, since I already replaced the two caps on that VCC line, but still no change - the lightest load shuts it down. When unloaded, it's perfectly stable at 12v, but what good is that ? Can the chip itself be faulty in some way ? How come it runs normally when unloaded ? :|
It's not so much about the camera since I could just scrap it and tell them to get a replacement, or just wire a power adapter straight to the output of the internal supply and that would keep it going for a few more years. It's mostly about fixing stuff and enjoying the fruits of your labor. Cheers
EDIT: I now realize that diode may've been a zener diode...whether or not that affects the functionality, I can't tell....
I pulled out an old Bosch analog PTZ CCTV camera from one of our sites. The camera was dead - no power and no signal at all. I took it apart and after some tinkering I found it's not getting any power to its "brains". It takes a 24Vac input which is then rectified and dropped down to 12v (I think) by a small power supply based around an NCP1271. When I first got it, there was absolutely no power on the output of this internal PSU at all. I managed to get it going to a degree, but it's still not enough: initially, I found a short on the VCC pin of the IC (D359 - sorry I don't have a pic of the whole board and it's pretty messy on there). Based on how it was installed (anode to GND and cathode to VCC) I figured it's a protection diode and after removing it, I tried to fire it up without it, since I don't have a similar one. I now get 17v on the VCC pin of the IC and 12v on the output caps, which is fine, but when I connect the head of the camera, it shuts down and power cycles every 5s or so. I though the head itself may have some bugs of its own, like a short, so I tried a small 12v fan on that 12v output, but got the same result - the output shuts down and begins to power cycle. It's worth noting the the VCC pin of the IC ALSO drops down to almost nothing with each cycle. It then SLOWLY goes back up again and once it reaches around 12v, the output goes high again, before immediately shutting down. This makes perfect sense, since the datasheet clearly tells us UVLO is 12.6v.
Now I'm pretty stumped, since I already replaced the two caps on that VCC line, but still no change - the lightest load shuts it down. When unloaded, it's perfectly stable at 12v, but what good is that ? Can the chip itself be faulty in some way ? How come it runs normally when unloaded ? :|
It's not so much about the camera since I could just scrap it and tell them to get a replacement, or just wire a power adapter straight to the output of the internal supply and that would keep it going for a few more years. It's mostly about fixing stuff and enjoying the fruits of your labor. Cheers
EDIT: I now realize that diode may've been a zener diode...whether or not that affects the functionality, I can't tell....
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