Transformerless Power Suppply

[ERIK]

Hi there,

Despite the obvious safety issues etc, I am very interested in building a transformerless power supply for my PICs. I am in New Zealand where the power is 240VAC @ 50Hz so most of the App Notes/Tech briefs on the web aren't quite appropriate for these conditions. I've been using rectifier diodes and or bridge rectifiers, a Zener, and some heavy duty resistors and caps. I've asked my friend who is an electrical engineer to have a think about it and he begrudgingly agreed so I'll see what he reckons. I was just wondering if anybody else out there has built a good/simple transformerless power supply to run on 240VAC @ 50Hz?

[muddy0409]

Silicon Chip had a project about a light dimmer. That had a method of scourcing power DIRECTLY off the mains. I'll have a look for the mag and post the details if I can find it.
From memory, this small module was fitted inside a lamp with no transformer or anything else, other than direct connection to the Active mains power for its supply.

[Melanie]

Why no transformer?

A transformer is there for good reason (whether it's a conventional iron cored item or a high-frequency switcher), apart from the obvious in dropping the incoming mains supply down to something safe to play with, it also provides ISOLATION from the supply mains which is a very important feature. Without that ISOLATION your circuit is LIVE regardless what voltage you play with.

[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.

[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.


---------

[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.

[NavMicroSystems]

I'm not using any transformerless PSU's at all.

Some time ago I could get hold of a large number of well designed switching PSU-units at about $0,50 each (stock/production overhead)

They provide well regulated 7,5V @ 600mA
and are very small in size.
(Actually not more in PCB-Space than a good transformerless design would require since they are fully SMD)

I could not even design and build any transformerless PSU at comparable cost.

Why develop/build something that is already available on the market at lowest cost?

OK,
for someone selling 10thousands of units per year it is a different story,
but if you are selling "only" several hundreds of units I still feel it is a good option to search the web for those stock overheads.

The only rule is:
get a sample before your bulk order and test it carefully.
If the sample is OK (meets or exceeds the requirements):
order a quantity that lasts at least to the planned production/life-cycle of your product.

That way I have saved a lot of time & money.

[NavMicroSystems]

... 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.

Mel, I doubt you would risk your reputation by marketing crap just to make money.
(please correct me if I'm wrong)

[Melanie]

You just have to walk down any High Street in any Town, anywhere in the world to see what I mean...

[NavMicroSystems]

You just have to walk down any High Street in any Town, anywhere in the world to see what I mean...

I'm almost sure I know what you mean and hope you haven't taken my comment too serious ;-)

[nomada]

Hi,

Be carefull, be very carefull, when using a power supply without transformer is like walking in the wire without the net underneath...

Atached you can see the circuit I use to put some power ... in PICs and other stuff. I found this circuit some years ago when I bought a kit (K6713)from Velleman, its a IR receiver to switch on/off a load connected to mains trough a relay.

One more advise, if by any chance you are planning to use this circuit in a touch sensor or similar do not forget to use two resistors in series for the sensor, the contour voltage for a regular 1/4W is less than 200V, unless you like to get a curly hair...

regards

[Melanie]

If you're still looking for components, attached PIC I've taken gives you an idea of what to look for... The 11V VDR is pretty small (VDR2 in this pic) - and also laying alongside with a view of a typical Capacitor you can use. If it's an X2 type you're OK, otherwise you'll have to fit one with a 400vDC rating. It's a real good idea to absolutely and positively mark the PCB that it's LIVE... even if it's just for yourself. Some time in the future you'll forget and it'll bite you.

[ERIK]

Thanks Mel, I have finally got some 275V X2 caps and some VDR's (I had to buy 100 of them but thats OK). So I'll sort out a PCB (well marked, and well housed) and let you know how it goes...

[sayzer]

Hi Melanie,

Going back to this post,

Would that be too much to ask for your complete fan controller schematic?
and, if possible, for the code also?

I am having a similar inquiry and would like to see your approach.
Or if there is anyone else who had a similar design.

Regards

[Melanie]

Hi Sayzer

This product is still in volume production (about 1000/month), and I'm treading on thin ice just posting the PCB view (a couple of posts above) and some snippets out of the PSU schematic - the remainder if you're clever you can figure from most of the components in use and eliminating those I've already revealed... a licence for the complete Software and Schematics would cost... I envisage around $5K.

Melanie

[sayzer]

May be, I can support that thin ice, so it will not crack.

Since you already have it in production, I may need not to work on it;

This is of course if you are able to sell it to other customers. Are you?

Should I PM to you?

[Melanie]

Sure, continue this by email, as I can then send you the product Datasheet so you can see if it fits your requirements.

[sayzer]

Upon the discussion with Melanie, attached is the capacitor value table for the current (mA) that your circuit draws.

Table is quite simple to figure out.

Table is for 110V@60Hz and 220V@50Hz.

Example: If your circuit draws 12mA, and your power line is 110V 60Hz, then, you use 0.289uF (290nF) for C2 in the circuit that Melanie posted here.

Of course you will have to adjust D2 and VDR accordingly.

If this table is any help to anyone!


Regards

[Melanie]

Nice table, but if you limited the Capacitor values to those dozen or so X2 values available (eg 10n, 22n, 47n, 100n, 220n etc, etc), you'll be able to pick the one closest to your current requirement. No point in having a 290n value when you can't buy one, and putting heaps in parallel negates the advantages (cost, size and weight) of a transformerless design to start with.

[sayzer]

Yes, of course.

Just as in your design, C2 is 470nF but in the table the nearest value is 477nF.

I am sure 330nF can also be used there although is not a near value.

Similar to this example, one can look at the table and pick a range.

I will try to make this table as an active WEB page so the user can enter the values of Frequency and Current then have an info for the capacitor range that "can" be used.

-----------------

[psdayama]

I have seen in many posts that Melanie is asking to use X2 capacitors.
For the requirement of circuit the value posted in tb008 is correctly just
470n 400V type. These are available at half price than X2 types because
X2 are required for special protection function and 'never short' across the
mains requirement.
Also the resistor across this capacitor can be 220k 1/4 W instead of 100K
1Watt. This resistor is for discharging capacitor when ckt is not in use and
not for providing resistive current. Those who are having 110V/60Hz should
double value and save money.
If one is not finding proper value of capacitor then it is not good that
one should have much higher value available. If used then low voltage regulators (zener or series type ) will dissipate much more power and may
burn out. Consider that this ckt is constant current generator and that when no load is there it will be dissipated in zener. It is better that shunt type
voltage regulators are designed with proper wattage rating so that there is
no problem later.
( I had been manufacturing X2 and normal plastic film capacitors for more than 12 years in India)

[Melanie]

There is a reason I specify X2... and you said it yourself... "they (are specified to) never short accross". If in a Transformerless Power Supply the Capacitor DID short accross it would be very bad news!!! If you don't fit X2's, then yes, your costs are very much reduced, but do you want to take the risk?

[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.

[dhouston]

I suggest you actually take the time to read the URLs I cited.

We've had polarized plugs and wall sockets from even before the time the 3-wire system was implemented. I live in a 60+ year old building that predates the 3-wire system and all of the wall outlets are polarized. My memory is a bit fuzzy but I believe the 3-wire system was required from about 1960.

If you have non-polarized plugs and sockets they are likely be for IEC Class II use which you might have grasped had you bothered to read the URLs I cited.

Transformerless power supplies have been used by X-10 in their plug-in modules (some 3-wire, some 2-wire, all polarized) and switches since 1976 or so.

[Melanie]

We'll have to disagree on this one. However I've not killed anybody yet (on the basis it's bad for business) and my designs are used internationally - many with UL certification, so either I'm lucky or I must be doing something right. *smiles*

[Luciano]

Hi,

Ground Fault Circuit Interrupters (GFCI)

A GFCI will trip under a shorted Neutral-Ground condition even
if no load is connected to the circuit -- that is, no load
other than the short between neutral and ground.

Here in Italy the GFCI will trip because of this fuse between
neutral and ground. (See figure 3 and 4 Microchip TB008).



Best regards,

Luciano

[dhouston]

The circuit shown is probably not intended to be plugged into a CFGI nor into any power grid other than in North America.

Here, ground and neutral are not necessarily at the same potential. Recent changes in the National Electric Code call for the ground lead to connect to the center-tap of the utility transformer and for neutral to connect to earth ground at the entrance panel.

I haven't said that fuse should be used in the UK or Europe nor have I, in any way, been critical of Melanie's design skills.

[Luciano]

Here, ground and neutral are not necessarily at the same potential.

The fuse will like that!

Best regards,

Luciano

[Melanie]

Actually Dave you quoted me as "both inaccurate and dangerous".

I challenge that.

I don't care what part of the world you're in, the Fuse in Figure 3 of TB008 is a complete waste of time, offering NO protection - let's repeat that in case anyone missed it first time around - offering NO protection to the User of that circuit. If that Fuse is in or out, the circuit will be LIVE. Microchip have dropped a bollock on this one - so let's direct the "inaccurate and dangerous" where it's needed - the author of TB008.

[Pic_User]

Hi All,

There seems to be a great deal of confusion among hobbyists and new professionals concerning ground and circuit ground.

I stopped calling the “common” connections in electrical / electronic circuits, “ground”, 30 years ago. Most of us, have worked out the dual meaning of ground. On one hand we actually mean earth ground, next we simply mean a common connection in our circuit.

This dual meaning is okay most of the time, but sometimes it causes misunderstandings and even dangerous situations. Transformerless power supplies are a common battle ground for the semantics of the term, “ground”. This is because, the misunderstandings lead to safety problems. Problems that cannot be ignored.

Melanie, as usual is the voice of reason and spot on. The TB008 application note, is right out of the twilight zone. The author, obviously has some confusion about “ground” and “common”.

I post this diagram to help illustrate what Melanie already pointed out.

[Luciano]

Hi,

What I have in my house.

(Click to enlarge).

Best regards,

Luciano