Help on Schematic please....

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- 29th June 2010, 11:48
financecatalyst

Help on Schematic please....

Hi, I have a circuit here which uses 105K capacitor (250V) for isolation of mains and resistance networked to finally provide 12V to 7805.
12V is maintained by using a Zener diode & 1Watt resistance of 6.8Ohm.

Problem:
The unit has to be installed where mains AC voltage can be from 150V AC - 260V AC @ 50Hz. The system hangs when it has to operate 4 relays at around 150V. It works perfectly with all 4 relays at 230V & above.

Because of limited knowledge in electronics, I am unable to decide what should I do or change in the circuit. All relays take their drive from 12V via 9013(npn) transistor with base connected to pic via 10K, and don't leave enough current for the PIC to operate at LOW voltage so the whole system hangs. It works fine when only 1 or 2 relays are operating at 150V.
Coil current for each relay is around 60mA.

I tried to increase the capacitor value from 105K to 155K, but my zener diode burned on OFF LOAD (when no relay is ON). I tried putting 3 1Watt 12V zener in parallel and they were still getting hot.

I am aware that there could be some complex calculations over current required for the circuit, based on that the capacitor value to choose & other specific components, hence I am here hoping to get an answer or an existing schematic with this kind of circuit. Any help in the right direction is much appreciated.
- 29th June 2010, 15:27
ronsimpson

1 Attachment(s)

Attached is how I would do it. I moved the resistor to the input of the diode bridge. I think it helps protect the diodes from lightning. I used a low wattage zener and a power transistor. The transistor will get hot but you can attach a heat sink to it. Some people use only two diodes. In this case I would use the 4-diode bridge to get more power. Set C1 size so that at low voltage with all relays on you just make 12 (11) volts. Then at high voltage the transistor will have to burn up the extra power.
- 29th June 2010, 20:48
financecatalyst

Quote:

Originally Posted by ronsimpson

Attached is how I would do it. I moved the resistor to the input of the diode bridge. I think it helps protect the diodes from lightning. I used a low wattage zener and a power transistor. The transistor will get hot but you can attach a heat sink to it. Some people use only two diodes. In this case I would use the 4-diode bridge to get more power. Set C1 size so that at low voltage with all relays on you just make 12 (11) volts. Then at high voltage the transistor will have to burn up the extra power.

Thanks for the heads up. I am very short of space, heat sink is not a possibility in that space.
- 30th June 2010, 05:34
ronsimpson

Quote:

Originally Posted by financecatalyst

Thanks for the heads up. I am very short of space, heat sink is not a possibility in that space.

A capacitor power supply creates heat at ligh line voltage. Burning this heat in a zener or a transistorit is still heat.

Can your micro measure the differente between 110 and 220? There may be a way!

220 60hz is 20% hotter than 220 50hz.