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Transformerless PSU using a zener/transil to drop voltage

Hi all,

For a hobby home automation system I need to make a very small, non isolated, direct-mains power supply unit regulated via a 7812 IC or such. Thus it has to be transformer-less, and since I need to draw something like 100mA, even a capacitor (of the kind that do not explode on mains!) would be too big. I have two ideas so far to lower the voltage to a range acceptable by the 7812:

1) a rectifier in serie with a zener (even better a transil), the latter used to drop the voltage so that only the peaks of the AC top waveform come through, and lessed by ~300 volts. Care must be taken because if the (230VAC case) RMS voltage can be anywhere from 210V to 250V, then it means that the DC coming out of this circuit will have the ~same offset range (eventually a fuse will be blown or, better, I'll use a polyswitch). Another thing that worries me is power dissipation: if I draw 100mA, then of course those 100mA will have to be multiplied by the zener/transil drop voltage, which (again, 230VAC case) will be, say, 300V: 30W! But anyway this will only happen for a fraction of the 50Hz waveform, when the diode conducts (probably much, charging a low voltage capacitor), so the average power dissipation of this diode hopefully should still be acceptable.

2) if the above is not feasible, I was thinking about working on the low part of the AC waveform rather than on the peaks: maybe a high voltage MOSFET which conducts only when the serie voltage is under e.g. 20V? For the rest of the AC waveform the MOSFET would be turned off. I should use a 20V zener (in a high impedance way, something zeners really don't like) and a P-MOSFET, intuitively, or a depletion-mode N-MOSFET.

Any suggestions/comments please?

Thanks! Andrea

Reply to
andrea
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snipped-for-privacy@NOSPAMPLEASE.com a écrit :

No, it still will be 30W (average current in = average current out).

I think it's Linear that has a circuit doing exactly this but I can't find the part# now.

You can also adapt this:

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Reply to
Fred Bartoli

Why non isolated, direct-mains?

Reply to
Homer J Simpson

Hello Andrea,

When it is a hobby project, and even a professional one, I would drop both options. 100mA at 12 volt is 1.2W and that is not low power in my opinion.

Many designs that does not use a transformer, use a capacitor directly connected to mains. The capacitor serves as a dissipation less current limiter. The capacitor feeds a 2 diode rectifier with storage capacitor and zener (regulator) diode.

The rectifier must be loaded, otherwise the voltage rises to several

100 V, therefore the regulator diode is added. Note that when your circuit doesn't consume the designed current, all power is dissipated by the regulator diode (in your case about 17V to feed the 7812).

A resistor is in series with the capacitor to absorb transients. Mostly a varistor across the mains is added to absorb large transients. The design of such a capacitor/rectifier circuit is not easy if you want that you circuit will survive over voltage category 3 or 4 transients.

When it comes to space saving, the capacitor will be in the 2..4 uF range and therefore will have a large size. I would consider a transformer for that power level. Even professional energy meters do use a small mains transformer rather then the transformerless approach

Hope this will help you a bit.

Best regards. Wim

Reply to
Wimpie

Use a comparitor to switch the MOSFET off if the output voltage exceeds 12V or the drop on the MOSFET exceeds, lets say, 6V.

The mains spends only a smallish time inthe 12V to 18V span so the MOSFET needs to be able to conduct way more than 100mA when on.

Reply to
MooseFET

could also use a lightdimmer !! the TRIAC or SCR could be a solution as it do not dissipate much power . Maybe it can be a problem to filter the voltage from the TRIAC/lightdimmer as the consumption is

100 mA but I guess it will be the same if a FET is on only until the voltage are over say 20 volt i
Reply to
Alex

I think that the Origonal Poster should consider using a transformer since the need for the larger capacitor is a hint that it isn't really a small supply.

In the idea of switching the power: About 15 years ago, I used an app note from Linear Technology to create a very high efficiency transformerless power supply (it was going into a well insulated case that plugged into the utility meter socket on industrial buildings).

The first stage of the supply was a simple and dirty switching supply. Basically (there were more parts involved) a capacitor with a transistor to the voltage input that turns on whenever the capacitor is < x volts. As soon as the capacitor is charged, the transistor shuts off. As the load draws power, the capacitor drops voltage slightly and the transistor drops more energy in from the supply. The hysterisis in the control of the transistor becomes ripple in the supply. In the system they were pushing in the app note, the output of the cheap and dirty switching supply went into a low dropout linear regulator that they had recently come out with.

Reply to
B Fuhrmann

I'd advise you not to try this. Too dangerous unless you now what you are doing. Wall warts are as big as they are for a reason.

If you are aware of the risks consider buying an AC-DC converter module perhaps..

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Reply to
CWatters

Exactly. So if the OP can afford the real estate to heat sink 30W, a transformer shouldn't be an issue.

A switched mode supply shouldn't be too demanding at this power level and will be lighter and smaller than one with a transformer. But if the OP is challenged by the exploding capacitor problem, I'd suggest a quick trip to the local electronics shop to buy a ready made wall wart.

100 mA @ 12V should be a cinch and will eliminate the safety risk. The stuff on the 12V side will be safe and easier to tinker with.
Reply to
Paul Hovnanian P.E.

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