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'[PIC]: Input clamp current'
2001\02\12@061424 by Alan B. Pearce

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The problem I have with any direct connection between mains supplies and a PIC
pin is that the high value resistor required does not need much capacitance
across it to couple any mains borne noise straight into the PIC pin. My
preference would be to use a 2 stage voltage divider, even allowing that this
will push the price by needing 4 resistors.

I am afraid that I have to agree with Roman that relying on the internal diodes
in the PIC is really suicidal for a commercial product.

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2001\02\13@110006 by Dwayne Reid

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face
At 11:14 AM 2/12/01 +0000, Alan B. Pearce wrote:
>The problem I have with any direct connection between mains supplies and a PIC
>pin is that the high value resistor required does not need much capacitance
>across it to couple any mains borne noise straight into the PIC pin. My
>preference would be to use a 2 stage voltage divider, even allowing that this
>will push the price by needing 4 resistors.
>
>I am afraid that I have to agree with Roman that relying on the internal
>diodes
>in the PIC is really suicidal for a commercial product.

You know - I have NO problem with sampling 120 Vac directly into a CMOS
input pin having adequate internal protection.  I always use 2 identical
resistors in series - the ones I use are rated at 500V max (assume that to
be peak voltage) and have many thousands of units out in the field over the
past 15 years or so with no failures attributable to this technique.

I also do this with 12c508 but, since the input pin I use is GP3 which is
also /MCLR & Program, I have to clamp the voltage on the positive
side.  Because of board layout considerations, I used a 1n4148 clamping to
VDD instead of using a 4.7V zener to ground.  This is a fairly new product
and there are only a few hundred in the field but again, no failures so far.

Nor do I expect any failures.  The capacitance on the pin is much larger
than the capacitance of the series resistors - that capacitance coupled
with the large values of series resistor forms a low pass filter that stops
sudden transients cold.

The key to using this technique is to ensure that voltage breakdown on the
sense resistors cannot occur by using enough appropriately rated resistors
in series.

Others have mentioned using a voltage divider before feeding the PIC
pin.  Couple of things there:  you still have to be concerned with the peak
voltage across the input resistor (at least 2 resistors required).  The
other thing is that I am looking for zero crossing.  I need to see the AC
voltage cross zero - that means that I can't use a voltage divider if I
want any accuracy at all.

One final thought - this technique is NOT suitable if using a PIC with a/d
convertor on-board.  I have found that ANY substrate current in the 16c71 /
73 / 74 series parts introduces un-acceptable a/d error.  I have to assume
that all other PICs with a/d convertor suffer the same sensitivity.  In
that case, a zener clamp is best.

dwayne


Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2001\02\13@110925 by Spehro Pefhany

picon face
At 09:39 PM 2/12/01 -0700, you wrote:

>One final thought - this technique is NOT suitable if using a PIC with a/d
>convertor on-board.  I have found that ANY substrate current in the 16c71 /
>73 / 74 series parts introduces un-acceptable a/d error.  I have to assume
>that all other PICs with a/d convertor suffer the same sensitivity.  In
>that case, a zener clamp is best.

Agree with yr. comments incl. those snipped . Instead of a simple zener,
I'd suggest a zener biased from the power supply voltage, with a switching
diode (eg. 1N4148) to the signal. The "knee" on low voltage zeners is rather
soft for uA current levels, and this also can keep the current out of
the power supply rail if you choose the zener voltage wisely wrt the
guaranteed input threshold voltage. The zener 'sees' a substantial
and known minimum current, and only nA (typically) flow to the input when
it is less than the digital input threshold. A series resistor from the
clamp to the microprocessor pin is also a good idea.

Best regards,


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