a little help with capacitive power supply values

[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.

[Byte_Butcher]

Hmmm. 0.4 to 1.4 watts when idle sounds like quite a chunk to me.

Say, I've got a couple of Kill-A-Watt's lounging around here somewhere... I'll take one of these little transformers and put it to the 100 hour test like you did with the X-10 modules.

Edit: I just plugged one in and these little transformers, unloaded, show .02W and .09VA. I'll let it run for a few days and see how the watt-hours add up.

I kind of forget about the Kill-A-Watt's unless the grid power is down and I'm running on backup generator. Then I drag 'em out to keep track of voltage and frequency...

steve

[dhouston]

Hmmm. 0.4 to 1.4 watts when idle sounds like quite a chunk to me.

The 0.4 was the capacitive supply that really was idle; the 1.3 & 1.4 have superregenerative RF receivers which are never truly idle. The point was that capacitive supplies are not inherently wasteful.

Tle last 5 devices all use transformer PSUs as they have serial ports and require isolation.

[Pic_User]

More sites:
Step-down rectifier makes a simple dc power supply
[Electronic Design News]
http://www.edn.com/ednmag/archives/1...98/08df_06.htm
http://www.edn.com/ednmag/archives/1...98/08df_06.pdf

A Capacitor-Fed, Voltage-Step-Down, Single-Phase, Non-Isolated ...
http://www.grix.it/UserFiles/Powermo...L_acfvsdsp.pdf

+++++++++++++++++++ You may skip this part:

In my very best Rod Serling, Twilight Zone, imitation:

“Picture if you will; A man bent over his electronics’ workbench.
A man, with many decades of experience, working for a living, doing precisely what he is doing at this moment.
Analyzing an electronic circuit.

Not just any circuit, but a circuit made by X10.
A consumer circuit, brand new, just out of it’s pristine box, for the first time since it left the factory floor.
The device is powered through a two prong, AC, 120V (USA) power cord.

Only today, instead of analyzing the circuit to repair it, or analyzing it to modify the circuit, he is trying to learn something else.
He is attaching his multi thousand dollar oscilloscope to the X10 device, to find out how it works.
To learn the secrets of it’s communication wave form, if you will. (Rod Serling speak)
To SEE what is going on in this particular mysterious device.

He has done this countless times, to repair, to alter, to learn and to expand, on the knowledge gained by this analysis.

There is no schematic included.
The two wire cord lulls this “searcher of the truth”, into a false impression that the device MUST be isolated, MUST have a small power transformer, somewhere hidden from the casual observer.

Enter - The Twilight Zone:
He FAILS to notice, that there are NO outside connections needed, except the cord.
Just an all plastic case, with all plastic buttons. No jacks, no other cords, just a dead end for the power cord.
Cleverly designed by people who do this for a living.

Just a power cord attached to a plastic case with eight plastic switches.
The output for the device is really, signals going back down the power cord.
Signals riding on the 120V AC power line.
This is know but not thought about by the investigator at this time, overlooked.
He is not aware that this device is powered by a “transformerless” power supply.

With the standard, properly functioning oscilloscope, attached through a 100 times (100x) probe (for conservative safety), the examiner is ready to begin. He has carefully attached the probe, ground on the minus of the filter capacitor, probe on the plus of the low voltage supply.
He has done this, before he energizes the circuit, for obvious reasons.

He now, without the least hesitation, picks up the x10 power plug and plugs it directly into the 120Voutlet.

Also without the least hesitation, he watches half of the printed circuit traces disappear.
A loud bang, a bright flash, a lot of smoke are all that is left of that half of the circuit.

What does he do?
He doesn’t even examine the burnt x10. He knows what happened and why.
He unplugs the warm cord.
He wraps the cord around the unit.
He puts the burnt x10 switch back into its box.
He opens the second identical x10 unit box (an extra, back-up unit).
He opens the unit.
He places the oscilloscope probe on the identical place, only on the new x10 circuit board.
AND
He plugs the new x10 into 120V AC!

DE DE DE DE – DE DE DE DE (that’s Twilight Zone theme music, look it up)

NOT into the same normal AC receptacle, but INTO an isolation transformer. His bench-top isolation transformer.

He would have done this with the first x10 but he wasn’t paying attention.”

That inattentive investigator was... ME. This is a true story.
I had plugged the first (now ruined) x10 into the normal AC receptacle with the neutral side and the line side flipped.

This is why they make and we use line isolation transformers.
++++++++++++++++++++++

You should never service a transformerless supply without one.

-Adam-

[mackrackit]

Adam,

I am glad it was not a
Tales from the Crypt
rendition.

[dhouston]

To learn the secrets of it’s communication wave form, if you will.

Simpler and safer is to use a stepdown transformer connected to the powerline and view the communication wave form through it. You don't even have to open the X-10 modules. A high pass filter passes 120kHz and blocks the 60Hz.

[Dennis]

Hi All

This is turning out to be a very interesting discussion..(well at least for me anyway!)
If you look right the way back to the start of this thread I as asking for a little help regarding the calculations mentioned in the Microchip appnote 954.Plenty have been referenced and suggested since then, and thanks to all who have contributed!

To tie things up I wonder if we could start from scratch again and take a close look at (and discuss) the circuit as we build the circuit up in 3 stages.
Hopefully we could get to a circuit which is able to supply a PIC in the correct way and be as safe as an UNSAFE and DANGEROUS circuit could be.

And the stages are :
1. The mains supply stage up to where it becomes DC
2. The circuit-load stage (where we use the 'DC' portion to power the circuit
(a possible zero crossing detector here and LED and pushbutton)
3. The output-load stage (where a load is switched either by means of a relay or a triac.

To start the ball rolling I would like to draw your attention to the attached diagram which depicts the basic types of transformer-less AC to DC power supply.

Each has its own unique chartacteristics (see AC circuit theory study notes or Google them). Understand that there may be phase shifting to consider as well as the zero crossing (these two will not be discussed yet)

SOMETIMES you may need to power a low voltage circuit such as a

microcontroller with HV AC line current.

The basic steps would be to :

1. Use a reactance to limit the current,

2. Rectify the voltage with a diode (half-wave rectification or bridge
rectifier for full-wave rectification),

3. Regulate the voltage with a zener,

4. Add a large electrolytic capacitor to filter out the ripples.

Transformerless supplies don't offer isolation from the HIGH VOLTAGE

line and present a MAJOR SAFETY ISSUE. I will put all of the possible safety tips together in point form and outline the reasons why each one is used or practiced.

I would like to be using a transformer-less CAPACITIVE circuit with common NEUTRAL (although we still have not discussed the effect on the PIC pin and the triac when there is no load!!)

I have not drawn a fuse in the diagrams yet although it is generally agreed that it is a best practice to place the fuse on the LIVE line not NEUTRAL and also not have a second fuse for example one fuse on LIVE and one on NEUTRAL!

With this in mind please could you confirm the following calculations for 220VAC and 50Hz, max current would be +-30mA.
(check the appnote and posts #14 and #16 for the calculations(Thanks Amgen and Melanie))

Calculations as per Microchip Application Note 954 :
(Please note .. Microchip are using a COMMON LIVE example - I am still not 100% sure why)

NOTE - VOLTAGE is either 230V and 240V (RMS) see appnote for 120V


VRMS 230 240
VZ 5 5.1
C1 0.00000047
R1 100
Freq 49.5 50.1

Equation 4
IIN-Min 0.018482937 Amps 18.48 ma
IIN-Max 0.029218819 Amps 29.22 ma

Equation 6
PR1 0.199702245 Watts 0.399404491 Double Watt

Equation 7
PD1 0.217302793 Watts 0.434605585 Double Watt

Equation 8
PD2 0.020453173 Watts 0.040906347 Double Watt

Please feel free to check my values and comment back.

So at this stage all I would like is some consensus on the values calculated and the different types of circuit.

Can we all agree on this so far ?

Kind regards
Dennis