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| Cap dropper circuit. |
| If you have a circuit that needs only a few milliamps to power it, you can avoid the expense of a transformer by using a capacitor dropper cicuit. This has the advantage over a resistor dropper circuit by producing little heat. You do, however, need to regard the circuit and anything powered from it as live, so keep them well enclosed. |
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| This is a
basic circuit showing the idea. C1 will probably be in the range 0.47uF
to 1.0uF depending on the current you need (the higher the capacitance,
the higher the current capability) - one of the X2 class capacitors are
ideal here. R2 provides a discharge path for the capacitor when the power is disconnected - otherwise you would get a good belt from it! R1 limits the power-on surge when C1 briefly looks like a short circuit, so something in the 47R to 100R 2W-3W area is good here. D1 & D2 rectify the mains input and should be 1N4006 or 1N4007. Z1 should be chosen for your required output voltage. C2 will smooth out your DC voltage and is again dependent on your load; but 470uF is a good all-rounder here. |
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| An
improvement on the first circuit uses a standard 78xx voltage regulator
on the output. I have used this circuit for applications that are a bit sensitive to some of the muck that comes through the mains, like spikes and brown-outs. This one is particularly useful if your application is microcontroller driven where a brown-out can crash your control program into the twilight zone. C3 and C4 can be ceramic or polybox caps in the 10nF to 100nF range and C5 around 100uF. It's a good idea with either of these circuits to add a 0.1uF X2 across the mains input to help absorb some incoming nasties and also limit any interference your circuit may want to send back into the mains supply. Please remember that these circuits are LIVE and all the usual precautions should be taken. |
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