Re: Reading a High Voltage
From: Dr Engelbert Buxbaum (engelbert_buxbaum_at_hotmail.com)
Date: 08/04/04
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Date: Wed, 04 Aug 2004 10:11:55 +0200
CFoley1064 wrote:
> Hi, Jas. Mr. Popelish has this exactly right. I'd like to add one small piece
> of advice, if I could. If it's possible your 0-500V input voltage is going to
> change quickly, you also have to take into account the capacitive divider, too.
> All resistors have an inherent capacitance in parallel with their resistance.
> It isn't much (low pF level), but in the event of rapidly changing input
> voltage, it could result in a nanosecond excursion at the input pin of a couple
> hundred volts.
>
> If I had to use a VDVR to measure high voltage with a PIC, I'd use a 470 pF cap
> to GND in parallel with the 10K resistor, which should swamp out the capacitive
> VDVR. It will also slow down your response time, but will give you some
> improvement in input noise.
>
> High Voltage VDVR
> Voltage Input PIC Input
> ___
> o---|___|--o----o--------o
> R1 | |
> .-. |
> | | ---
> R2| | ---
> '-' | C1
> | |
> === ===
> GND GND
And possibly replace R1 with several smaller resistors in series, to
prevent high voltage from leaking into the PIC. I would additionally
place protection diodes from the PIC input to ground and positive
supply, just in case.
o +Ub
|
_|__
/\ \ ZD5V6
/ \
----
|
___ ____ ____ ____ |
o---|___|--|____|--|____|--|____|--o----o-----o--o PIC input
| | |
4 x 1.275 M .-. | _|__
| | --- /\
R2| | --- / \ DUS
'-' | C1 ----
| | |
=== === ===
GND GND
This should limit the input voltage of the PIC to -0.7 to +5.6 V,
probably survivable.
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