a little help with capacitive power supply values

[Melanie]

Back in Post #2 of this Thread you were directed to posts which had schematics. The discussion also continued as to what I thought of Microchips App Notes! The product had a production run of several thousand and many hundreds were installed in Hospitals (this I thought was a great idea, since if anything did go wrong, the punter was already in the right place to get assisstance). To my knowledge, in the intervening years, nobody's died and niether has the product, since we never did get any back for service. Looking at the schematic of that product, one end of the TRIAC was tied to NEUTRAL, the other went through the LOAD through a second FUSE (the first Fuse was on the Capacitive Supply to the PIC) to LIVE (you'll note the board had two Fuses). My Gate Resistor from the PIC to the TRIAC was 1K8 - but yours will depend on what your TRIAC's Datasheet tells you.

So how do we find news ways of doing things if we are stuck designing only in the confines of a transformer-based supply.

You go to school and learn the theory of what components can and can't do. You learn the math behind calculating component values. Then, and only then, you will have armed yourself with the knowledge that should stop you from doing something stupid. I've said this before - Darwin had a great theory of "Natural Selection", unfortunately every now and again, good people are also lost along the way.

3. is that I can go extremely small if I use SMD components not so ?

Prime example of lack of knowledge. SMD parts seldom come in high voltage ratings. By reducing the size you reduce the separation between adjacent connections... as a rough guide you need 1mm per 100v. Go pull the Datasheet on an 0805 series Resistor and tell me what the maximum voltage rating is!

[Byte_Butcher]

I was just curious if you could even GET a 0.47uF 500V cap in surface mount, so I did a Digikey search.
The answer is yes, but the only ones I found are ceramic and at $10.45USD each it makes those little transformers start looking pretty cheap.

In through-hole they're available in poly film for about $1 but they ain't particularly small.

I know the answer to Melanies question about 0805 resistor voltage rating. The Vishay parts I use are rated at 150 volts max...

Since you're working on a PIR project, have you seen these?
http://www.zilog.com/docs/devtools/PS0284.pdf
Serial interface to your MCU and can even tell whether something is moving left to right or right to left relative to the sensor. I bought a handful of 'em a couple months ago and am just starting to have fun with them.
I've got parts coming this week to do some serious play with them. Hopefully the UPS man should be bringing me my boards and parts on thursday. Woohoo!


steve

[Dennis]

Hi all

Melanie..

Regarding SMD, I have seen some designs showing SMD components placed after the rectification section section, is it a bad thing to consider SMD in a transformer-less design ?

The voltage ratings are 0805 = 150V and for 1206 = 200V it's quite a whack but not enough for the rectifying stage here.

Would you suggest I utilize that PSU degin you posted mentioned in #2 ?
Just curious as to why the MOV is placed where it is ?
EDIT **** OK I saw it in the forum post ..It's to accomodate for a weakspot at the the zener ****
Did you ever try replacing the 5V regulator with a 5V1 or 5V6 zener instead ?
Have you had any strange anomalies in the PSU's use ?
Have you had any blow?
Is there any ripple or noise encountered on the outputs ? The reason I ask is because I was wondering if it would be ok to add a wireless module to the circuit which the supply would power.

Steve.. the prices certainly are expensive and even for an X2 cap no surface mount !

Those transformers you found certainly are well priced !

Regarding the PIR sensors ...WOW and they do what they do at such a low voltage too ...awesome!
You could easily pack something like that into a matchbox or smaller and have it as a bed-side alarm !
Problem is that it would trigger every time either you or your partner moved in your sleep :-)

Will definitely be looking into those too !

Keep well

Kind regards
Dennis


-

[Melanie]

> Would you suggest I utilize that PSU degin you posted mentioned in #2 ?

I make no recommendations... use at your own risk after taking your own requirements and circumstances into account.

> Did you ever try replacing the 5V regulator with a 5V1 or 5V6 zener instead ?

No, because I was using the PICs ADC in my design, and wanted a closer repeatable tolerance and higher current than a Zener could provide.

> Have you had any strange anomalies in the PSU's use ?

None

> Have you had any blow?

No, the design is short-circuit proof and failsafe.

> Is there any ripple or noise encountered on the outputs ?

None beyond the design specification of the 78L05.

[Byte_Butcher]

Steve.. the prices certainly are expensive and even for an X2 cap no surface mount !

Those transformers you found certainly are well priced !

Yes, they're pretty reasonable... and now I notice that they come in an even smaller size!

http://ww2.pulseeng.com/products/dat...2007_02-04.pdf

13 mA @6V, with the same 22mm x 23 mm footprint as the others, but only 12mm high! With a SMD rectifier, regulator, and caps, it's probably not much bigger than a xformerless supply with it's big cap, MOV, and all the
rest.
I'm sure it's still a little more expensive. But maybe not much...

Regarding the PIR sensors ...WOW and they do what they do at such a low voltage too ...awesome!

Those are cute, aren't they? And tiny! I'm starting to use them around the house to turn LED "night lights" on and off. When I stumble into the bathroom at night there's enough lumens to find my target without having to fumble for a light switch.


steve

[Melanie]

The efficiency of a Transformer at 50/60Hz is dependent on the amount of iron in it's core... once you drop below about 1.5VA (which is just over an inch cube with an EI30 core) the amount of iron is so small that half the power consumption is used just to heat up the core and windings the efficiency plummets to only 50% or less.

[boroko]

Melanie, you never cease to amaze me with your practical grasp of such a wide range of issues.

You never know, keep it up and you might just have a future in this "electronics" thing.

Bo

[Byte_Butcher]

The efficiency of a Transformer at 50/60Hz is dependent on the amount of iron in it's core... once you drop below about 1.5VA (which is just over an inch cube with an EI30 core) the amount of iron is so small that half the power consumption is used just to heat up the core and windings the efficiency plummets to only 50% or less.

Well, I'm sure those little transformers aren't the greatest for efficiency, but I don't think the medium small ones (6v@83mA) I've been using are all that bad.
I've got one sitting here on the bench for days with a light (10-15mA?) load on it, and it's only barely warmer than it's surroundings. It can't be dissipating very much energy. Maybe one of these days when I'm feeling less lazy I'll attempt to measure the efficiency on one.
It certainly doesn't heat up like cheap chinese wall wart transformers do.

But how efficient is a xformerless supply if it's not "precisely loaded"? Input current is always the same regardless of load! And unless that fancy X2 cap is sized exactly to the needs of the circuit, it looks like that zener is gonna be dissipating a good dose of heat.

If your load draws a constant current and the cap is sized correctly I can see how it could be very efficient. If the load varies over a wide range as the PIC switches stuff on and off, but the power supply draws the maximum current all the time, then I don't see how that can be very efficient...



steve

[dhouston]

But how efficient is a xformerless supply if it's not "precisely loaded"? Input current is always the same regardless of load! And unless that fancy X2 cap is sized exactly to the needs of the circuit, it looks like that zener is gonna be dissipating a good dose of heat.

I have measured several X10 modules, when idle, to see how much power they waste. It's not much. http://davehouston.org/x10-power.htm

Some of the highest use transformer supplies. SMPS tend to have the best efficiencies.