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'[EE] Measuring line voltage'
2011\09\10@231833 by Forrest Christian

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I'm struggling with coming up with a circuit which I actually like for measuring AC line voltage and current.   The goal is to be able to read the voltage of AC line within a volt or two within a microchip PIC....   For obvious reasons I'd like something where in normal operation the AC line is isolated from the PIC side of the circuit.

The obvious brute force way is to use a transformer.  Unfortunately brute is pretty much the way to describe this - big and heavy, even at 1.5x1.5x1 inches.  Especially since I really need to monitor two sources at the same time. Although it definitely works.

I'd really like to do something more 'electronic' - I.E. smaller and lighter.   Unfortunately everything I am coming up with involves drawing at least 5-6mA from the AC line for things like driving the emitter on an optocoupler, plus circuitry for the optocoupler, including power supplies for such things as opamps or processors with ADC's  - which quickly equates to at least a half of a watt worth of current @ 120V, and often quite a bit more.   Which is a lot of heat for me to get rid of - I really don't want a heatsink, and I'd really like to be able to do this on the bottom of the board where it's hidden from wandering fingers, but unfortunately also where there isn't a lot of opportunity for heat to dissipate due to the confined space.

I keep hoping to come across a really teency isolation transformer or similar, and/or a simple, clean circuit that does what I need it to... But alas, after hours and hours of looking, I'm no closer.

I'm hoping someone can give me a hint as to what I might want to be looking for.

-forrest




2011\09\11@065419 by Isaac Marino Bavaresco

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You could make a circuit with a small PIC (8 pins) with A/D converter,
connected directly to the AC line, powered by a transformerless power
supply and using a resistive divider to measure the line voltage. Then
transmit the result of the measurement to the other side of an
opto-coupler to a circuit that implement the remaining functionalities
of your appliance.

This way you can read as many different signals as you wish.


Best regards,

Isaac



Em 11/9/2011 00:17, Forrest Christian escreveu:
{Quote hidden}

>

2011\09\11@082158 by M.L.

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On Sat, Sep 10, 2011 at 11:17 PM, Forrest Christian <spam_OUTforrestcTakeThisOuTspamimach.com> wrote:
> I keep hoping to come across a really teency isolation transformer or
> similar, and/or a simple, clean circuit that does what I need it to...
> But alas, after hours and hours of looking, I'm no closer.
>
> I'm hoping someone can give me a hint as to what I might want to be
> looking for.
>
> -forrest

Do you just need the peak voltage? The RMS? Or are you sampling at
some frequency?

-- Martin K

2011\09\12@062950 by cdb

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::I'd really like to do something more 'electronic' - I.E. smaller and
::lighter

Have a look at the LEM range of encapsulated 1 turn tx with hall effect sensor. They're not cheap though, I have an ancient LM25 (I think it is).

<www.lem.com/hq/en/component/option,com_catalog/task,displayserie/se
rie,LA%2025-200%20-P/output_type,/>

Digikey are supposed to stock them , but i can't find them, but here is an RS page

<http://australia.rs-online.com/web/c/?searchTerm=LEM&sra=oss>

Colin
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2011\09\12@063555 by cdb

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Should have pointed out it would be the LV-25-200 that you'd be looking for.

Data sheet - <http://www.lem.com/docs/products/lv%2025-200%20e.pdf>

Colin
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2011\09\12@093428 by Michael Watterson

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Forrest Christian wrote:
> I'm struggling with coming up with a circuit which I actually like for
> measuring AC line voltage and current.   The goal is to be able to read
> the voltage of AC line within a volt or two within a microchip PIC....  
> For obvious reasons I'd like something where in normal operation the AC
> line is isolated from the PIC side of the circuit.
>  
Neon and a photo transistor? Should work at about 200uA on  mains side?
Voltage varies the current thus brightness due to large series resistor. Better suited maybe to 230V AC than 110V AC.

Hall effect sensor to measure line current.

2011\09\12@100858 by Isaac Marino Bavaresco

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Em 12/9/2011 10:34, Michael Watterson escreveu:
> Forrest Christian wrote:
>> I'm struggling with coming up with a circuit which I actually like for
>> measuring AC line voltage and current.   The goal is to be able to read
>> the voltage of AC line within a volt or two within a microchip PIC....  
>> For obvious reasons I'd like something where in normal operation the AC
>> line is isolated from the PIC side of the circuit.
>>  
> Neon and a photo transistor? Should work at about 200uA on  mains side?
> Voltage varies the current thus brightness due to large series resistor.
> Better suited maybe to 230V AC than 110V AC.
>
> Hall effect sensor to measure line current.


It seems that my suggestion got lost in the list:

"You could make a circuit with a small PIC (8 pins) with A/D converter,
connected directly to the AC line, powered by a transformerless power
supply and using a resistive divider to measure the line voltage. Then
transmit the result of the measurement to the other side of an
opto-coupler to a circuit that implement the remaining functionalities
of your appliance. This way you can read as many different signals as
you wish."


Isaac

2011\09\12@103527 by Michael Watterson

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Isaac Marino Bavaresco wrote:
{Quote hidden}

However ADI has isolated RS422/RS485/Can transeivers.  But I  don't know what  PSU draw is.

A PIC  (100uA to 200uA  via  capacitive dropper?)  feeding  serial data via small cheap pulse transformer or opto-coupler.

I once used a Quad DIL pulse transformer for isolated Analogue I/O
1) 500KHz for powering CPU and Opamps etc.
2) Data clock
3) Data in
4) Data out
CPU was a NEC 78HC11 with internal shift register parallel load/unload.
So host read "last response" while clocking in new command.

2011\09\12@115444 by Herbert Graf

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On Sat, 2011-09-10 at 21:17 -0600, Forrest Christian wrote:
> I'm struggling with coming up with a circuit which I actually like for
> measuring AC line voltage and current.   The goal is to be able to read
> the voltage of AC line within a volt or two within a microchip PIC....  
> For obvious reasons I'd like something where in normal operation the AC
> line is isolated from the PIC side of the circuit.

Hall effect sensor? Not the cheapest solution, but the total thing can
be VERY small.

TTYL

2011\09\12@123427 by Isaac Marino Bavaresco

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Em 12/9/2011 11:35, Michael Watterson escreveu:
{Quote hidden}

It seems that the OP is worried about power dissipation (heat), not
exactly current consumption.

The capacitive power supply is mostly reactive, not resistive, so the
power dissipation is small even for moderate currents.

If he uses a power resistor to drop the line voltage, then a few mA
generate a lot of heat, but with the capacitive power supply the power
dissipation is very small.


Isaac

2011\09\12@162443 by Richard Prosser

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On 13 September 2011 04:34, Isaac Marino Bavaresco
<.....isaacbavarescoKILLspamspam.....yahoo.com.br> wrote:
{Quote hidden}

Re the voltage Measurement
How about capacitive coupling? Especially if you're not looking at
particularly high accuracy.

The sensor could be a few 10/s of turns of wire wrapped around the
phase wire. The two layers of insulation will provide the required
level of isolation, and with "enough" turns you should be able to get
to 100pF or so coupling capacitance. Use this as the input cap on a
capacitive divider (to reduce frequency variations and the effect of
nose spikes) and feed to a very high impedance op amp. If the amp has
unity gain, you can feed the voltage back to a guard shield over the
wound wire to reduce external stray capacitance effects. This stage
may be followed by an appropriate gain/LPF/level shift stage which
feeds the ADC.  I've used a similar arrangement to detect if a cable
is live or not, this just takes it to the next level of actually
making a measurement.

If you can get 100pF coupling capacitance, the reactance is 26.5Mohm
at 60Hz. But the voltage is 340V pp for 120V input. So a voltage
divider ratio cap of about 70 is required to reduce it to 5Vpp. This
implies an earth leg  capacitor of  7nf (6.8nf ?) and reduces the
source reactance to about 3.7Megohms. So if you can arrange a bias
network and an opamp that will give you ~30Megohm input impedance and
a 2.5V offset you should be good to go.

RP

2011\09\12@191816 by M.L.

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On Sun, Sep 11, 2011 at 8:21 AM, M.L. <EraseMEmspam_OUTspamTakeThisOuTlkeng.net> wrote:
> On Sat, Sep 10, 2011 at 11:17 PM, Forrest Christian <forrestcspamspam_OUTimach.com> wrote:
>> I keep hoping to come across a really teency isolation transformer or
>> similar, and/or a simple, clean circuit that does what I need it to...
>> But alas, after hours and hours of looking, I'm no closer.
>>
>> I'm hoping someone can give me a hint as to what I might want to be
>> looking for.
>>
>> -forrest
>
> Do you just need the peak voltage? The RMS? Or are you sampling at
> some frequency?
>
> --
> Martin K.
>


Do you need the peak voltage or the RMS or what?

-- Martin K

2011\09\13@063549 by Forrest Christian

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I'm not picky.  Peak would be fine, as would be RMS, since I can basically do math to adjust either.  I don't need a fancy true-rms reading or anything like that.

-forrest

On 9/12/2011 5:17 PM, M.L. wrote:
{Quote hidden}

>

2011\09\13@065750 by Forrest Christian

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Ok, to clarify/address a few things....

The circuit as currently designed uses an 240VAC primary transformer and a lightly loaded secondary.   The transformer and associated parts (diodes, caps, etc on the secondary) cost about $2-$3, but is biggish and heavyish.   I.E. about 1.5 inches square by 1 inch tall, and probably doubles the weight of the product.  Which is a lot of space to consume - since I really have two of these on board.

The advantage of the transformer is that any inefficiencies will dissipate out that big heavy device pretty efficiently - no heat sink required.   Plus, due to the lead arrangement, I can generally keep the AC on the bottom of the board - which is important since this is an industrial type product designed to be in a cabinet where people will be doing work from time to time and I'd like to limit finger-contact options.

As I mentioned in my original email - the problem I'm running into with *any* circuit which uses an optocoupler is that the LED in the opto, plus a bit for the rest of the circuit usually adds up to a few mA, which is in the watts category when you use a resistive power supply off of the line.   Especially when you want it to operate over a wide range of voltages - I need to think internationally here to some extent (I have a few places where 230V is the norm).    If I put the circuitry on the bottom of the board, I run into a heat entrapment issue.   On the top of the board, I run into the 'dangerous voltages for fingers' problem.   Or I run into having to somehow insulate that circuitry.   All with a limited budget of say $5 or less for this portion of the circuit, including any insulation.

I was looking at the capacitive power supplies, but it looks like the required capacitors for those are rather large, and probably not surface mount.   Of course, I may have missed some schematic somewhere which uses smaller sized/valued caps for this... but the general one needs big ugly capacitors.  Plus, it seems these are very line voltage sensitive, which means I get to burn off some more heat in a voltage regulator (aka resistor and/or zener, or a linear regulator).   Or I can spend money on a switcher which isn't in the budget.

In all, I'm back to the $3 transformer - big, ugly, but apparently elegant solution.

I'm intrigued by the idea of using some sort of hall effect sensor.

I'm also intrigued by the idea of using something more like an audio isolation transformer...  with some circuitry to prevent overloading/flames..

Or I'm interested in someone pointing out something I missed.

-forres

2011\09\13@071720 by Isaac Marino Bavaresco

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By your first post, I understood that your problem is the heating, right?


Did you analyze the capacitive power supply idea? If you use a simple
divider with power resistors, the heating will be great, even for very
small currents, but with the capacitive power supply you may get some
tens of mA without noticeable heating.

You could drop the voltage with a capacitor and a Zener to say, 9V, and
then use a 78L05 to regulate to 5V for the small PIC with A/D. It is
perfectly possible to use an opto-coupler spending 5mA, plus perhaps 1mA
for the PIC.

Using SMD and creative board layout it should be possible to accommodate
all this in 1/2 square inch.


Best regards,

Isaac



Em 13/9/2011 07:34, Forrest Christian escreveu:
{Quote hidden}

>>

2011\09\13@073837 by Isaac Marino Bavaresco

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Em 13/9/2011 07:56, Forrest Christian escreveu:
> I was looking at the capacitive power supplies, but it looks like the
> required capacitors for those are rather large, and probably not surface
> mount.   Of course, I may have missed some schematic somewhere which
> uses smaller sized/valued caps for this... but the general one needs big
> ugly capacitors.  Plus, it seems these are very line voltage sensitive,
> which means I get to burn off some more heat in a voltage regulator (aka
> resistor and/or zener, or a linear regulator).   Or I can spend money on
> a switcher which isn't in the budget.


For 10mA @ 220V @ 60Hz (Brazil), you would need a 120nF x 400V
capacitor, not that bad. Double the capacitance because probably you
will use half-cycle rectification to be able to have a "ground"
reference (neutral).

For 50Hz it is slightly larger, but perhaps you won't need 10mA.

The capacitive power supply is not critical, and you will probably use a
Zener as a primary "coarse" voltage regulator followed by a 78Lxx.

A 150nF x 450V SMD capacitor is available in 1812 package (4.5mm x 3.2mm
x 2.0mm).


Best regards,

Isaac

2011\09\13@080028 by M.L.

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On Tue, Sep 13, 2011 at 7:38 AM, Isaac Marino Bavaresco
<TakeThisOuTisaacbavarescoEraseMEspamspam_OUTyahoo.com.br> wrote:
> A 150nF x 450V SMD capacitor is available in 1812 package (4.5mm x 3.2mm
> x 2.0mm).
>

Are safety-rated caps (X1/Y1) required for this type of supply?

-- Martin K

2011\09\13@092142 by Isaac Marino Bavaresco

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part 1 914 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable)

Em 13/9/2011 08:59, M.L. escreveu:
> On Tue, Sep 13, 2011 at 7:38 AM, Isaac Marino Bavaresco
> <RemoveMEisaacbavarescospamTakeThisOuTyahoo.com.br> wrote:
>> A 150nF x 450V SMD capacitor is available in 1812 package (4.5mm x 3.2mm
>> x 2.0mm).
>>
> Are safety-rated caps (X1/Y1) required for this type of supply?
>


Perhaps not, because no one will use the capacitor alone, usually it is
used together with a small resistor (around 100 ohm) in series to avoid
a huge inrush current when connecting the circuit to the AC line.

The OP could use a thru-hole resistor for this role, to obey the IEC
norm that specifies 8mm of clearance between conductors before a current
limiting device (the resistor itself). I don't remember what norm is
that, and I am sure it was superseded already by another one.

See the attached circuit.



Best regards,

Isaac


part 2 17724 bytes content-type:image/jpeg; name="Capacitive.JPG" (decode)


part 3 181 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2011\09\13@125351 by Electron

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Nice circuit. Perhaps unnecessary to say, but a fuse probably wouldn't be wasted
in this application, you never know the capacitor shorts..

A question: if I'm 100% sure that I'm getting my neutral/line wiring right, is
there any safety risk/concern in touching the neutral wire? I mean, could there
ever happen anything at the power station that makes me get an electrical shock
if I touch the neutral wire? I don't think they'll ever swap neutral and line,
it would even be quite hard to do technically for them (as, as far as I know,
neutral wires are all grounded).

Another question.. if I use an isolation transformer, neither touching either of
the secondary wires will provoke an electrical shock (of course not touch both
wires together :D ), right?
But what is that makes me get a little shock when I touch the line wire, the
resistive path to earth or the capacitive coupling with the earth (thus it would
give me a shock even if I was flying and not touching the ground)?

Cheers,
Mario



At 15.21 2011.09.13, you wrote:
{Quote hidden}

>

2011\09\13@134805 by Isaac Marino Bavaresco

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Em 13/9/2011 13:51, Electron escreveu:
> Nice circuit.
Thanks!


> Perhaps unnecessary to say, but a fuse probably wouldn't be wasted
> in this application, you never know the capacitor shorts..

.... then the resistor limits the current to around 10A and blows almost
immediately.

> A question: if I'm 100% sure that I'm getting my neutral/line wiring right, is
> there any safety risk/concern in touching the neutral wire?


It should be safe, unless there is some problem in the wiring.


>  I mean, could there
> ever happen anything at the power station that makes me get an electrical shock
> if I touch the neutral wire? I don't think they'll ever swap neutral and line,
> it would even be quite hard to do technically for them (as, as far as I know,
> neutral wires are all grounded).

It is not at the power station, it is at you utility pole or at the fuse
box in your house. A bad electrician can do this to you.


> Another question.. if I use an isolation transformer, neither touching either of
> the secondary wires will provoke an electrical shock (of course not touch both
> wires together :D ), right?


Yes.


> But what is that makes me get a little shock when I touch the line wire, the
> resistive path to earth or the capacitive coupling with the earth (thus it would
> give me a shock even if I was flying and not touching the ground)?


The resistive path is the most probable. Under certain conditions even
the atmosphere can conduct enough current to give a shock.


With my circuit you don't need to worry about shocks, because you are
supposed to use an optoisolator (most of them isolate up to 7500V) to
transmit the data to the rest of your circuit. The part of the circuit
that is connected to the AC line is completely isolated from the rest,
you may even swap line and neutral that it will work correctly and safely.


Best regards,

Isaac

2011\09\13@163702 by John Coppens

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On Tue, 13 Sep 2011 14:48:01 -0300
Isaac Marino Bavaresco <EraseMEisaacbavarescospamyahoo.com.br> wrote:

> > Another question.. if I use an isolation transformer, neither touching either of
> > the secondary wires will provoke an electrical shock (of course not touch both
> > wires together :D ), right?
>
>
> Yes.
>
Don't trust an isolation transformer unless there is an electrostatic
shield between the primary and secondary, and it is connected to a
ground.
Joh

2011\09\13@211541 by Richard Prosser

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On 14 September 2011 04:51, Electron <RemoveMEelectron2k4EraseMEspamEraseMEinfinito.it> wrote:
{Quote hidden}

Mario,

In NZ, the regulations state to treat the neutral as being live. The
problem is that although you may have the neutral & phase correct, you
can't guarantee that the supply will have it correct. It's less likely
in a large industrail situation that it will be wrong, but there's
alway a chance.

R

2011\09\14@035045 by cdb

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Not a criticism, just a statement.

Some people would prefer to split the MAINS side resistors into two for safety reasons.

The resistors data sheet should be checked to make sure they are rated for 230-250v, many aren't.

Some people would bung in a bleed resistor to discharge the caps if the circuit being powered suddenly opened.

Colin
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2011\09\14@042317 by Electron

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Very interesting. What is the minimum current to actually feel the line?

And the current to risk fibrillation?

Cheers (not if I'm touching the line wire though),
Mario


At 19.48 2011.09.13, you wrote:
{Quote hidden}

>

2011\09\14@203157 by Oli Glaser

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On 14/09/2011 09:19, Electron wrote:
> Very interesting. What is the minimum current to actually feel the line?

About 0.5mA at 60Hz.

>
> And the current to risk fibrillation?

I think for 60Hz severe pain and difficulty breathing will start around 20mA, and fibrillation is possible at or above this - usual figures are  >50mA.
Useful link here:
http://www.allaboutcircuits.com/vol_1/chpt_3/4.html

2011\09\15@055932 by Electron

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At 02.30 2011.09.15, Oli Glaser wrote:
>On 14/09/2011 09:19, Electron wrote:
>> Very interesting. What is the minimum current to actually feel the line?
>
>About 0.5mA at 60Hz.

Well, then 230V RMS is 650V peak to peak it means 1.3 Mohm, but why does one
gets a shock even when he uses shoes.. if I measure my resistance to earth in
such a situation, it will be in the gigaohm side.. does it mean that e.g. wood
shoes have a low breakdown voltage? Only that can explain it, I think.

2011\09\15@055932 by Electron

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At 02.30 2011.09.15, Oli Glaser wrote:
>On 14/09/2011 09:19, Electron wrote:
>> Very interesting. What is the minimum current to actually feel the line?
>
>About 0.5mA at 60Hz.

Well, then 230V RMS is 650V peak to peak it means 1.3 Mohm, but why does one
gets a shock even when he uses shoes.. if I measure my resistance to earth in
such a situation, it will be in the gigaohm side.. does it mean that e.g. wood
shoes have a low breakdown voltage? Only that can explain it, I think.

2011\09\15@061425 by Isaac Marino Bavaresco

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Em 15/9/2011 06:59, Electron escreveu:
> At 02.30 2011.09.15, Oli Glaser wrote:
>> On 14/09/2011 09:19, Electron wrote:
>>> Very interesting. What is the minimum current to actually feel the line?
>> About 0.5mA at 60Hz.
> Well, then 230V RMS is 650V peak to peak


Nope, the peaks (positive or negative) happen at different times. The
maximum instantaneous voltage between line and neutral is half that.


>  it means 1.3 Mohm, but why does one
> gets a shock even when he uses shoes.. if I measure my resistance to earth in
> such a situation, it will be in the gigaohm side.. does it mean that e.g. wood
> shoes have a low breakdown voltage? Only that can explain it, I think.


Resistance of some materials is not linear with voltage. As you guessed,
breakdown voltage plays its role there. Perhaps not of the rubber
itself, but of impurities and contaminants on the surface.


Isaac

2011\09\15@071232 by Michael Watterson

face picon face
Electron wrote:
> At 02.30 2011.09.15, Oli Glaser wrote:
>  
>> On 14/09/2011 09:19, Electron wrote:
>>    
>>> Very interesting. What is the minimum current to actually feel the line?
>>>      
>> About 0.5mA at 60Hz.
>>    
>
> Well, then 230V RMS is 650V peak to peak it means 1.3 Mohm, but why does one
> gets a shock even when he uses shoes.. if I measure my resistance to earth in
> such a situation, it will be in the gigaohm side.. does it mean that e.g. wood
> shoes have a low breakdown voltage? Only that can explain it, I think.
>
>  
Capacitance?
The charging current?
You need a pressed down foil insert to measure shoe resistance to earth. a probe only touches on part and foot the entire area.

Maybe even an artificial foot.

2011\09\15@071754 by Michael Watterson

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Isaac Marino Bavaresco wrote:
{Quote hidden}

Yet on a "leakage tester" they will read "dead short".
http://vintagetvandradio.myfreeforum.org/sutra3243.php#3243

Since these are connecting usually between a Valve (tube) Anode and the next Grid (which might have 220K Ohm to 4.7M Ohm resistor and > 20M Ohms input impedance, the results are usually bad. Output valve overheats and often burns out transformer(s).

2011\09\19@114240 by David Harmon

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On Tue, 13 Sep 2011 18:51:48 +0200, Electron
<RemoveMEelectron2k4TakeThisOuTspamspaminfinito.it> wrote:

>A question: if I'm 100% sure that I'm getting my neutral/line wiring right, is
>there any safety risk/concern in touching the neutral wire?

Yes.  For just one example, suppose the wall socket is old and worn,
and the neutral blade stops making good contact.  Now you are
touching the hot wire, through whatever circuitry you have

2011\09\19@133140 by Electron

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face
At 17.42 2011.09.19, you wrote:
>On Tue, 13 Sep 2011 18:51:48 +0200, Electron
><EraseMEelectron2k4spamspamspamBeGoneinfinito.it> wrote:
>
>>A question: if I'm 100% sure that I'm getting my neutral/line wiring right, is
>>there any safety risk/concern in touching the neutral wire?
>
>Yes.  For just one example, suppose the wall socket is old and worn,
>and the neutral blade stops making good contact.  Now you are
>touching the hot wire, through whatever circuitry you have.

But if the PIC was getting 1 uA through that RC circuit, I will too. Not more than that, or not?

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