Transformerless Power Suppply

[muddy0409]

Ahh....there we are....

SILICON CHIP October 2005. You can get a reprint from their website www.siliconchip.com.au for a small reasonable price...

Hmmmmm.............I was wrong, it's a fan controller, not a dimmer.

Yes Mel, you are correct ( as always ) but in this case, built into a plastic box with no easy access, once installed in the roof, there shouldn't be a problem. I may even try this for a couple of projects at sometime in the future.
Anyway, read and see what you think.]

[Melanie]

If you must... here's part of a circuit I designed (coincidentally for a fan controller for Hospitals)... total disclaimer if you die whilst playing with it - it is LIVE - you have been warned! No big dropper Resistors (R1 is a standard 0.25W), no heat disspation problems...

The only real justification for using a circuit like this is space... The total physical real-estate a circuit like this requires is a fraction of that taken by an iron cored transformer or a switcher.

[ERIK]

Thanks for those. Yes space is exactly why I don't want a tranny. I'll try your's Mel it looks good. And I'll make sure it is well housed. I was getting some heat disspation problems as you mentioned by using all the ones with the big dropper resistors. Nice looking fan controller by the way.

[ERIK]

Hey Mel, sorry about going back to this dreaded topic again but I was wondering a couple things about your transformerless power supply circuit.

1 - What sort of 470nF 400V cap did you use? I can't seem to find a suitable one down here in my funny little country.

2 - Is the voltage dependant resistor an integral part of the circuit or a somewhat optional component?

No more questions on this subject after this I promise.

[Melanie]

Ask as many questions as you want... that's what the forum is here for.

Only C1 (10nF X2 type) is surplus. This PSU was part of a Fan Controller which used a Triac controlling the Fan. C1 was there for noise surpression and in your case can be removed.

C2 is either an X2 grade capacitor, or, any 400vDC rated Capacitor. Look for metalised polypropylene or metallised polyester. Typical manufacturers are Dachs-Kondel, Arcotronics, BC Components (Vishay), LCR and TruCap.

VDR1 (11v) is a safety component. My original design was going into Hospitals... if Zener D2 failed and there is negligable power consumption by the target circuit, the voltage at the junction of D3 and C3 would rise rapidly towards supply mains level destroying the Regulator and every low-voltage component (ie the PIC) downstream. The Zener is the weak spot... if it fails, basically everything is toast. That VDR is sufficient to keep the supply from rising much above 18v, still well within the parameters (30v) for the Regulator. My PIC had additional circuitry (two Resistors as a potential divider to an ADC pin) that detected that the Input DC supply to the Regulator Q1 had risen above 14v and flashed an LED to tell maintenance engineers that the unit is broke. Omit VDR1 at your risk.

PM me off-list if you have trouble finding suitable parts and I'll see what I can do.

[sayzer]

Hi ERIK,

Is your current (I) always the same or it changes? In terms of mA, what is the current of the entire PIC circuit at its max and min ?

Also, how is the stability of the power lines in your city? Is it always 240V or increases on holidays and decreases on working days? If not stable, how much is the change?

I may come up with a different solution!

[Melanie]

I'm always interested in alternatives and to see how other engineers approach problems, as I'm sure others on this forum are... try this spec... Input supply 220-250vAC 50/60Hz. Output 5vDC (regulated), operating current range 0-30mA (total), short-circuit proof. Which is roughly the specification of the circuit I've submitted. The important features are (a) that there must be minimal heat dissipation... so high wattage dropper Resistors or high voltage Regulators like the VB409 which require heatsinks are out... and (b) minimal size both physically and in PCB real-estate usage.