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'[EE]: circuit analysis'
2002\02\15@142328 by

part 1 1294 bytes content-type:text/plain; (decoded 7bit)

Dear all,

I wonder if anyone has any ideas on how this circuit will act?
I'm sure it won't do what I want to do but I'm curious as to what anyone
thinks it will do!!

I've using a 2.5v reference, using an op-amp voltage doubler to bump up the
current output and double the voltage.. I feed this through a potential
divider with the bottom resistor being of variable resistance (actually a
sensor). The sensor is usually between 0 and 200 ohms so I've used a 200 ohm
resistor for the top resistor..... This gives between 0 and 2.5v from the
middle of the pot divider....
However, if it gets really cold the resistance will go >200 and the voltage
goes >2.5v. The resistance could easily go much higher and the PIC ADC will
see more than Vref+0.3v (from the 16F877 datasheet)....
So I drew up this circuit. Theoretically if the middle of the second pot
divider is 2.5v, any voltage on the first greater than this will cause the
diode to conduct... But where will the "volts" go???
My gut instinct on this circuit is that it will become an oscillator very
quickly... Any ideas??
How else does anyone limit the voltage... Low value zener diode? I know
recently these have been discussed and no-one was impressed.....

Thanks for any input,

Ben

part 2 2426 bytes content-type:image/gif; (decode)

part 3 105 bytes
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Hi,

Lets look at the limiting case.  Set the bottom resistor open.  Label
the junction of the two 2K resistors and the cathode of the diode "N".
You imply 5V at the top of the string and we will assume 0.6V across the
forward biased diode.  The current law states that (steady state) the
current into a node sums to zero.  The equation is:

(N/2000)+((N-5)/2000)+(((N+0.6)-5)/200)=0

This simplifies to:

N = 49/12 = 4.0833V

Of course if the bottom resistor is shorted the remainder of the circuit
is irrelevant and the output voltage is zero.

Regards,
Dave

Benjamin Bromilow wrote:

> However, if it gets really cold the resistance will go >200 and the voltage
> goes >2.5v. The resistance could easily go much higher and the PIC ADC will
> see more than Vref+0.3v (from the 16F877 datasheet)....
> So I drew up this circuit. Theoretically if the middle of the second pot
> divider is 2.5v, any voltage on the first greater than this will cause the
> diode to conduct... But where will the "volts" go???
> My gut instinct on this circuit is that it will become an oscillator very
> quickly... Any ideas??

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Hi,

Lets look at the limiting case.  Set the bottom resistor open.  Label
the junction of the two 2K resistors and the cathode of the diode "N".
You imply 5V at the top of the string and we will assume 0.6V across the
forward biased diode.  The current law states that (steady state) the
current into a node sums to zero.  The equation is:

(N/2000)+((N-5)/2000)+(((N+0.6)-5)/200)=0

This simplifies to:

N = 49/12 = 4.0833V

Of course if the bottom resistor is shorted the remainder of the circuit
is irrelevant and the output voltage is zero.

Regards,
Dave

P.S. All of this assumes no ADC input load and of course, the output to
the ADC will be 0.6V higher than N (4.6833V).

Benjamin Bromilow wrote:

> However, if it gets really cold the resistance will go >200 and the voltage
> goes >2.5v. The resistance could easily go much higher and the PIC ADC will
> see more than Vref+0.3v (from the 16F877 datasheet)....
> So I drew up this circuit. Theoretically if the middle of the second pot
> divider is 2.5v, any voltage on the first greater than this will cause the
> diode to conduct... But where will the "volts" go???
> My gut instinct on this circuit is that it will become an oscillator very
> quickly... Any ideas??

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At 07:23 PM 2/15/02 +0000, you wrote:

>However, if it gets really cold the resistance will go >200 and the voltage
>goes >2.5v. The resistance could easily go much higher and the PIC ADC will
>see more than Vref+0.3v (from the 16F877 datasheet)....

What happens under these conditions? The A/D converter "should" read 0x3FF if
Vin >= Vref. What does it actually do?  Are there "side affects" on other
channels or some such thing? Anyone know? I don't see anything in the midrange
reference manual, but the 0.3V > Vref thing IS on the data sheet.

>So I drew up this circuit. Theoretically if the middle of the second pot
>divider is 2.5v, any voltage on the first greater than this will cause the
>diode to conduct... But where will the "volts" go???
>My gut instinct on this circuit is that it will become an oscillator very
>quickly... Any ideas??

No, but the diode won't limit it to 0.3V either. The current will go into
the Vref divider so Vin will be > Vref by no more than about 0.6 or 0.7V.

>How else does anyone limit the voltage... Low value zener diode? I know
>recently these have been discussed and no-one was impressed.....

Zeners are not very good- you can make an almost perfect op-amp clamp.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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2002\02\16@080259 by
> Zeners are not very good- you can make an almost perfect op-amp clamp.
>
> Best regards,
>
> Spehro Pefhany --"it's the network..."            "The

Thanks for the Spehro, I suspected an op-amp circuit would be the way to go
but strangely my op-amp circuit book didn't include anything suitable and I
couldn't work out one that would work :) I just found a load on google

Ben

--

Benjamin, your circuit is designed for diferential output, basicaly it's a
bridge. The schematic as you design it hasn't too much sense because:
-- the input signal in adc is using only half bridge so the sensitivity
will be one half than the case of a full bridge usage
-- limiting the adc input voltage can be done using a resistor and a 0.5W
glass package 5v6 zenner at the output of an operational amplifier which
inputs are connected at middle of the bridge network.
-- the bridge operating modes are described in any measuring tehniques
book.

best regards, Vasile

On Fri, 15 Feb 2002, Benjamin Bromilow wrote:

{Quote hidden}

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