Transformerless Power Suppply

[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.

[sayzer]

On Microchip's WEB site, there is a Transformerless Power Supply concept similar to Melanie's.

The file name is TB008.pdf and attached here.


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[Melanie]

I have mentioned before that some of Microchips Application Notes are downright dangerous... and that is one of them.

Connecting Neutral and Earth together will trip most ELCB's a small fact that the author of that AN seems to have overlooked. Not fusing the Live is plain stupidity (as is any power distribution system that allows users to plug appliances in either way around)!

[NavMicroSystems]

I have mentioned before that some of Microchips Application Notes are downright dangerous... and that is one of them.

Connecting Neutral and Earth together will trip most ELCB's a small fact that the author of that AN seems to have overlooked. Not fusing the Live is plain stupidity (as is any power distribution system that allows users to plug appliances in either way around)!

I absolutely agree !

Unfortunately DO most european systems allow users to plug in appliances in either way around.

Hey Mel, you must be lucky living on an island with some better system!

There is one thing in addition I would like to mention:

Guess what happens if your mains is only "kind of" 50Hz sine?

Do you see where I'm going?

I have had several portable devices on my bench for repair.
Some of them had poor designed low-cost transformerless PSUs.

i.e. coffee (or espresso) makers a.o.

Users had thought it would be nice to use that coffee maker in their caravan
and hooked it up to one of those low-cost power inverters.
The output of most of the low-cost inverters is not even kind of sine, it is actually square, so all the transformerless PSU did was saying goodby by generating smoke.

You'll wonder what strange ideas users come up with...

[Melanie]

It's a 3-pin ONLY system in the UK. Yes you can get continental 2-pin shaver adaptors, but I'm told you men like to live dangerously in the bathroom.

Let's just look at TB008 for a moment... and just consider...

Neutral is grounded at the Power Station, and at the substation, and probably at the Building incommer... Here we have a fuse between Neutral and Earth... why? It's completely USELESS!!! If the fuse wasn't there (or it blows for whatever reason), you STILL have a FULL conductive path between LIVE and EARTH. Actually he's (I'm assuming 'he' as a girl I'd expect to know better *smiles*) even using EARTH as a return path of his circuit. Plug that circuit into most properly wired homes and the first thing that happens is the ELCB breaker trips! The author of TB008 should be prosecuted for the most dangerous suggestion you could possibly think of!!!

I'm well aware of the square wave inverters... still... the great unwashed (general public) hasn't got a clue... which is why you can market complete crap and they'll still pay good money for it.

[dhouston]

I have mentioned before that some of Microchips Application Notes are downright dangerous... and that is one of them.

Connecting Neutral and Earth together will trip most ELCB's a small fact that the author of that AN seems to have overlooked. Not fusing the Live is plain stupidity (as is any power distribution system that allows users to plug appliances in either way around)!

Melanie,

This may be accurate when talking about UK or European wiring but your advice is both inaccurate and dangerous when it comes to North American wiring. The fuse between neutral and ground shown in FIGURE 3 and FIGURE 4 of TB008 is a safety feature as the authors of the app note state.

Here's a URL which briefly explains the 3-wire system used here for many, many years.

• http://hyperphysics.phy-astr.gsu.edu...ic/bregnd.html

Circuits which might be plugged in either way around will not have the ground wire and thus will not have the fuse and will also require double-insulated, non-conductive enclosures. In fact, the only way to plug a North American two-wire appliance in either way around is to file down the wider neutral prong of the polarised plug.

You might also read...

• http://en.wikipedia.org/wiki/Double_insulated

[Melanie]

You are kidding me!

Neutral and Ground are basically one and the same. In many buildings all over the world (including in the US) the NEUTRAL incommer is tied to the Building GROUND (just as it is at the generating plant).

Go remove that fuse from the NEUTRAL line in figure 3 of TB008. Then stick your finger in the HOT terminal with one hand (or the 1uF Capacitor), and go grip a Water pipe in the other and tell me if you feel anything! They don't label that terminal LIVE for fun.

Oh, btw... I've a whole heap of US plugs and sockets that have the Neutral and Live EXACTLY the same size. So the Neutral terminal being the larger must be a recent innovation for the US.

The Transformerless Power Supply is not designed for portable equipment that can be connected either way around, but more suitable for fixed plant where the installer wires LIVE to LIVE.