26-06-2024, 06:16 PM
The two PSUs were in different applications and the issue was failure, not leakage to humans.
1) The 5V 500mA USB charger had 0V connected to slider of a 250K pot across -96V and the 5V powering an LED voltmeter and Vsense connected to 0V. It worked and then the 5V charger (out of case) started smoking. It was rated 110V to 240V so I'd powered it off +150V DC. Input is a bridge rectifier. The tiny meter is still OK. The main +150V and -96 from a transformer with 4KV (minimum) split bobbin. No earth anywhere, no mains test gear connected.
2) The +12V 2.5A PSU (originally two core mains flex and no earth) was powering the transistor side of an opto-isolator driving a buffer. Later I was going to add a FET switch. The main PSU is 19.1V laptop PSU that has the 0V connected to mains Earth. A 3 core clover plug. Mains Earth (as with most of Ireland) is bonded to the neutral and a 1.2m (approx) earth spike near the meter box. You never ever get a "tingle" off earthed equipment here. Some 2 core gear does, but only if it has RFI cap(s) from mains to the output or case. Quite rare, but seen on one Sky box.. The +12V may have been connected to laptop PSU +19V, as that is the cathode and heater + on the tester. The heater - normally connects to an IRF1404Z connected to 0V (which is also earth on all the higher power laptop PSUs). Anyway, something in the +12V SMPSU failed (smoke!) and the main PSU was cutting out. The +12V SMPSU was replaced by a tiny 8V linear PSU (split bobbing transformer, just a bridge and 1000 μF cap) and everything was fine.
I took case off the +12V 2.5A SMPSU and wondered why the PCB terminals were labelled L and N rather than both ~. Also the track space between the mains wires doesn't meet UL spec for 110V to 120V mains, though still enough in reality for 240V as long as dry!
I don't know why either PSU died, but the transformer in the USB charger is the size of a sugar lump. The 12V 2.5A SMPSU mysteriously has two small transformers and seems very complicated.
I will investigate more and do leakage tests first. It may be a subtle problem. The 5V USB charger had never been used because it was only 500mA and was "europlug" styled. It came with some gadget. Maybe it was going to die anyway!
The +12V 2.5A SMPSU had a two core mains flex, but not double PVC. That's legal in USA, but not here or UK now (though it used to be common). Perhaps I had it connected wrong? Putting 19V across the output would not make it happy, though I often design PSUs so they don't die if +30V is across them. Only shunt regulated PSUs can sink external current.
The commonest cause of fires in the home now apart from cooking and smoking is SMPSUs used to charge phones, ecigs, scooters, bikes and power stuff like decorative LED lights or gadgets.
It's a mystery. However with the linear PSUs you can see the transformer construction and insulation. A SMPSU can be a box of mysteries. I think I first designed one over 50 years ago and unless a cheap easy to replace part had obviously exploded, I had no joy repairing them. Now they are scrapped, though I might remove any easily removable and testable part!
1) The 5V 500mA USB charger had 0V connected to slider of a 250K pot across -96V and the 5V powering an LED voltmeter and Vsense connected to 0V. It worked and then the 5V charger (out of case) started smoking. It was rated 110V to 240V so I'd powered it off +150V DC. Input is a bridge rectifier. The tiny meter is still OK. The main +150V and -96 from a transformer with 4KV (minimum) split bobbin. No earth anywhere, no mains test gear connected.
2) The +12V 2.5A PSU (originally two core mains flex and no earth) was powering the transistor side of an opto-isolator driving a buffer. Later I was going to add a FET switch. The main PSU is 19.1V laptop PSU that has the 0V connected to mains Earth. A 3 core clover plug. Mains Earth (as with most of Ireland) is bonded to the neutral and a 1.2m (approx) earth spike near the meter box. You never ever get a "tingle" off earthed equipment here. Some 2 core gear does, but only if it has RFI cap(s) from mains to the output or case. Quite rare, but seen on one Sky box.. The +12V may have been connected to laptop PSU +19V, as that is the cathode and heater + on the tester. The heater - normally connects to an IRF1404Z connected to 0V (which is also earth on all the higher power laptop PSUs). Anyway, something in the +12V SMPSU failed (smoke!) and the main PSU was cutting out. The +12V SMPSU was replaced by a tiny 8V linear PSU (split bobbing transformer, just a bridge and 1000 μF cap) and everything was fine.
I took case off the +12V 2.5A SMPSU and wondered why the PCB terminals were labelled L and N rather than both ~. Also the track space between the mains wires doesn't meet UL spec for 110V to 120V mains, though still enough in reality for 240V as long as dry!
I don't know why either PSU died, but the transformer in the USB charger is the size of a sugar lump. The 12V 2.5A SMPSU mysteriously has two small transformers and seems very complicated.
I will investigate more and do leakage tests first. It may be a subtle problem. The 5V USB charger had never been used because it was only 500mA and was "europlug" styled. It came with some gadget. Maybe it was going to die anyway!
The +12V 2.5A SMPSU had a two core mains flex, but not double PVC. That's legal in USA, but not here or UK now (though it used to be common). Perhaps I had it connected wrong? Putting 19V across the output would not make it happy, though I often design PSUs so they don't die if +30V is across them. Only shunt regulated PSUs can sink external current.
The commonest cause of fires in the home now apart from cooking and smoking is SMPSUs used to charge phones, ecigs, scooters, bikes and power stuff like decorative LED lights or gadgets.
It's a mystery. However with the linear PSUs you can see the transformer construction and insulation. A SMPSU can be a box of mysteries. I think I first designed one over 50 years ago and unless a cheap easy to replace part had obviously exploded, I had no joy repairing them. Now they are scrapped, though I might remove any easily removable and testable part!
