a little help with capacitive power supply values

[Dennis]

Hi all

Dave .. thanks once again for the extra info and tips :-) they always provide good reads and info :-)

Melanie,Steve and anoymouse...

While re-reading the Microchip appnotes, this one in particular

I noticed some interesting points...
Firstly this one regarding the current draw,

The PIC12/16/17 microcontrollers draw a maximum of 10 mA, even at the highest frequency and voltage of operation, therefore low current availability is not an issue. AC line voltage isolation can be addressed by using MOVs or transient suppressors on the PIC12/16/17.

and then something strange about the FUSE on the NEUTRAL LINE ???

In most applications, the output voltage should be reg-
ulated. Figure3 shows a diagram for a practical circuit
where a +/- 5V regulated output is generated. Note that
the neutral is connected to ground through a fuse. This
+5V
would guard against improper AC wiring.

Any comments on these statements?

Kind regards
Dennis

[mackrackit]

Where did you get the second quote? The one about the neutral.

Have you checked your local electrical codes about the proper way to wire something for the system you have? Might clear up the neutral thing for you. Here the ground and neutral are "bonded in the box" not at the device. I can not see yours being any different, I am sure you have a transformer with a center tap providing power to you.

BTW, you never said why you want to do this or what you think the benefit will be.

[Dennis]

Hi Dave

Thanks for tip, I was going to get eyes in on the local sub-station but will do so tomorrow ;-) and update as soon as possible :-)

The quotation is actually from a Microchip appnote TB008 , the paragraph is just above figure 3. in the appnote. [HTML]http://ww1.microchip.com/downloads/en/AppNotes/91008C.pdf[/HTML]

Ok the project is for the PIR-sensor based security lights, I need to tap into the PIR and replace the existing LDR and sensor circuit which are also transformer-less and triac controlled. Each sensor light PIC transmit back to a central PIC.

Given the space I have I cannot fit a transformer based supply into the housing or wall point (someone did a good job installing them) and there is no way I can mount the circuit externally.
So transformer-less it is !
I have seen circuits aplenty on the web yet it seems most are common live and also mainly for 120VAC it seems.
Some even with dual fuses (one on LIVE and one on NEUTRAL!)

So it seems there is quite a lot of confusion in this arena and for good reason I suppose, fear of death being the ultimate reason people shy away and of course once bitten twice shy(being shocked into submission certainly shys people away).

So my quest was to research the topic of transformer-less supplies before I dive into anything like building one only to encounter serious injury and the possibility of my circuit frying !

Nothing I have seen out on the web quite explains how one should wire things up on the pic if you're using a transformer-less PSU.
So appnotes are created and design note etc ... but the very simple steps seem to be missing, the questions (FAQ's) and their solutions.

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

In the early years I remember learning about the effects of capacitive, inductive and resistive circuits.
For example :

With a CAPACITIVE circuit, A current will flow through the circuit, first in one direction, then in the other.
No current actually flows through the capacitor. Electrons build up on the one plate and are drained off from the other plate in very rapid succession, giving the impression that the current flows through the insulator separating the plates.
However, a capacitor in an AC circuit does offer resistance to the overall current flow. We define a quantity called the CAPACITIVE REACTANCE, Xc,

In the designs I have seen, people place and swop around the components as the desire almost as though it's a little slap-dash design which may or may not work and your mileage may vary.

For example, I changed my original circuit design around to make it safer (which was a copy and make-up of the Microchip appnotes (or close to it ) , why would they suggest such an unsafe circuit I wonder ?
Then I keep getting told to use the Microchip designs ... yes I would love to but are they the safest ? And most if not all of them are for 120VAC without worked examples for 220V or 230V.
You may have already noted that there COMMON HOT(LIVE) and COMMON NEUTRAL transformer-less supplies.
So after having changed the initial part of the circuit to make it a common neutral supply , this raised more questions
what then? , what else needs to be changed ?
Are the diodes and other components correctly setup and placed the correct way around ?

And as regards the benefits... well
1. it is space saving,
2. is cost (the tiny transformers are extremely expensive here) and
3. is that I can go extremely small if I use SMD components not so ?

I have to put the same circuit in about 15 lights.
If you do manage to see a mistake please let me know.

Anyways that it from me.. I hope that exaplins it so far :-

Kind regards

Dennis

[Dennis]

Check out this power supply ..take particular note of the input side..
[HTML]http://www.redcircuits.com/Page134.htm[/HTML]

Dave Houston this is the 135KHz injector I was referring to [HTML]http://www.redcircuits.com/Page56.htm[/HTML]

Enjoy

Dennis

[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


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