Post by thread:[EE]: circuit analysis

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 -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

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?? -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

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?? -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu

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" .....speffKILLspam.....interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

> 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 though... Thanks for the lead, Ben -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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}> 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 > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.