Post by thread:Pic based capacitance measurement

Dear PIClister's, Does anyone else out there have a drawer full of anonymous capacitors? I was wondering about the feasibility of modifying the Microchip app note that does A/D conversion using a resistor and capacitor and using it to measure the capacitor instead. Using the PIC as a timer it should be possible to get a semi-close measurement of the capacitance and you could even do auto-ranging and select different resistance values depending on the measurements you get. I really don't need too exact of a measurement, just something to tell me if a capacitor is 1uf or 1mf. So I think it should be possible to build something fairly low cost using a PIC. Ideas? Thoughts? Suggestions? Anyone done this? Adam Bryant (age 0x23) spam_OUTabryantTakeThisOuTpeaktech.com (work) .....adamdbKILLspam@spam@juno.com (home) Parker, CO, USA Robotics, RC Airplanes, anything using a PIC ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]

well, if its just between 1uF and 1mF (which im guessing its not :) then you can just build a simle audio oscilator out of a 555 and plug in the caps as timing caps. then you will have one tone for 1uF and another for 1mF ------------------------------------------------------------------------------ A member of the PI-100 Club: 3.1415926535897932384626433832795028841971693993751 058209749445923078164062862089986280348253421170679 On Mon, 1 Mar 1999, Adam Bryant wrote: {Quote hidden}> Dear PIClister's, > Does anyone else out there have a drawer full of anonymous capacitors? I > was wondering about the feasibility of modifying the Microchip app note > that does A/D conversion using a resistor and capacitor and using it to > measure the capacitor instead. Using the PIC as a timer it should be > possible to get a semi-close measurement of the capacitance and you could > even do auto-ranging and select different resistance values depending on > the measurements you get. I really don't need too exact of a > measurement, just something to tell me if a capacitor is 1uf or 1mf. So > I think it should be possible to build something fairly low cost using a > PIC. > > Ideas? Thoughts? Suggestions? Anyone done this? > > Adam Bryant (age 0x23) > abryantKILLspampeaktech.com (work) > .....adamdbKILLspam.....juno.com (home) > Parker, CO, USA > Robotics, RC Airplanes, anything using a PIC > > ___________________________________________________________________ > You don't need to buy Internet access to use free Internet e-mail. > Get completely free e-mail from Juno at http://www.juno.com/getjuno.html > or call Juno at (800) 654-JUNO [654-5866] >

I only used the 1uf to 1mf as an example. I want to measure a broad range of capacitors, but I only need to know the approximate value rather than the exact value down to the nth picofarad. I want to build a small device that measures the capacitor value and displays it on a small LCD. And being cheap I would rather build something with a PIC than purchase a multi-meter with capacitance measurement or even go to Tech America and buy a capacitance adapter for my multimeter for $13 bucks. :-) Adam Bryant (age 0x23) EraseMEabryantspam_OUTTakeThisOuTpeaktech.com Parker, CO, USA Robotics, RC Airplanes, anything using a PIC On Mon, 1 Mar 1999 14:48:47 -0800 Lynx {Glenn Jones} <jones_glspam_OUTEFN.ORG> writes: {Quote hidden}>well, if its just between 1uF and 1mF (which im guessing its not :) >then >you can just build a simle audio oscilator out of a 555 and plug in >the >caps as timing caps. then you will have one tone for 1uF and another >for >1mF > >------------------------------------------------------------------------------ >A member of the PI-100 Club: >3.1415926535897932384626433832795028841971693993751 >058209749445923078164062862089986280348253421170679 > >On Mon, 1 Mar 1999, Adam Bryant wrote: > >> Dear PIClister's, >> Does anyone else out there have a drawer full of anonymous >capacitors? I >> was wondering about the feasibility of modifying the Microchip app >note >> that does A/D conversion using a resistor and capacitor and using it >to >> measure the capacitor instead. Using the PIC as a timer it should >be >> possible to get a semi-close measurement of the capacitance and you >could >> even do auto-ranging and select different resistance values >depending on >> the measurements you get. I really don't need too exact of a >> measurement, just something to tell me if a capacitor is 1uf or 1mf. > So >> I think it should be possible to build something fairly low cost >using a >> PIC. >> >> Ideas? Thoughts? Suggestions? Anyone done this? >> >> Adam Bryant (age 0x23) >> @spam@abryantKILLspampeaktech.com (work) >> KILLspamadamdbKILLspamjuno.com (home) >> Parker, CO, USA >> Robotics, RC Airplanes, anything using a PIC >> >> ___________________________________________________________________ >> You don't need to buy Internet access to use free Internet e-mail. >> Get completely free e-mail from Juno at >http://www.juno.com/getjuno.html >> or call Juno at (800) 654-JUNO [654-5866] >> > ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]

this is interesting... I would also be interested in such a device... >I only used the 1uf to 1mf as an example. I want to measure a broad >range of capacitors, but I only need to know the approximate value rather >than the exact value down to the nth picofarad. I want to build a small >device that measures the capacitor value and displays it on a small LCD. >And being cheap I would rather build something with a PIC than purchase a >multi-meter with capacitance measurement or even go to Tech America and >buy a capacitance adapter for my multimeter for $13 bucks. :-) > >Adam Bryant (age 0x23) >RemoveMEabryantTakeThisOuTpeaktech.com >Parker, CO, USA >Robotics, RC Airplanes, anything using a PIC > >On Mon, 1 Mar 1999 14:48:47 -0800 Lynx {Glenn Jones} <spamBeGonejones_glspamBeGoneEFN.ORG> >writes: >>well, if its just between 1uF and 1mF (which im guessing its not :) >>then >>you can just build a simle audio oscilator out of a 555 and plug in >>the >>caps as timing caps. then you will have one tone for 1uF and another >>for >>1mF >> >>---------------------------------------------------------- -------------------- >>A member of the PI-100 Club: >>3.1415926535897932384626433832795028841971693993751 >>058209749445923078164062862089986280348253421170679 >> >>On Mon, 1 Mar 1999, Adam Bryant wrote: >> >>> Dear PIClister's, >>> Does anyone else out there have a drawer full of anonymous >>capacitors? I >>> was wondering about the feasibility of modifying the Microchip app >>note >>> that does A/D conversion using a resistor and capacitor and using it >>to >>> measure the capacitor instead. Using the PIC as a timer it should >>be >>> possible to get a semi-close measurement of the capacitance and you >>could >>> even do auto-ranging and select different resistance values >>depending on >>> the measurements you get. I really don't need too exact of a >>> measurement, just something to tell me if a capacitor is 1uf or 1mf. >> So >>> I think it should be possible to build something fairly low cost {Quote hidden}>>using a >>> PIC. >>> >>> Ideas? Thoughts? Suggestions? Anyone done this? >>> >>> Adam Bryant (age 0x23) >>> TakeThisOuTabryantEraseMEspam_OUTpeaktech.com (work) >>> RemoveMEadamdbTakeThisOuTjuno.com (home) >>> Parker, CO, USA >>> Robotics, RC Airplanes, anything using a PIC >>> >>> ____________________________________________________________ _______ >>> You don't need to buy Internet access to use free Internet e-mail. >>> Get completely free e-mail from Juno at >>http://www.juno.com/getjuno.html >>> or call Juno at (800) 654-JUNO [654-5866] >>> >> > >___________________________________________________________ ________ >You don't need to buy Internet access to use free Internet e-mail. >Get completely free e-mail from Juno at http://www.juno.com/getjuno.html >or call Juno at (800) 654-JUNO [654-5866] >

Hi, On 1 Mar 99 at 13:27, Adam Bryant wrote: > Does anyone else out there have a drawer full of anonymous capacitors? I > was wondering about the feasibility of modifying the Microchip app note > that does A/D conversion using a resistor and capacitor and using it to > measure the capacitor instead. Using the PIC as a timer it should be Sure - it's possible. Use 1 or more known value resistors to charge the unknown cap and measure the time it takes to trip the PIC input threshold. If you want a reference you can charge a known cap through the same circuit but for just a rough measurement you don't need one. E.g. R1 known resistor from RB0 to CAP+ (lets say 100K 1%, choose to give a reasonable time constant, you can use more than one) RB1 to CAP+ Known cap CAP+ to RB2 (Value in the middle of your measurement range, best precision you can get, you can use more than one ref. cap) i.e. the (-) of the cap connects to RB2 and the (+) to RB1 and resistor. Measured cap CAP+ to RB3 Measurement cycle: 1) Tristate RB1, RB3. 2) Set RB0 to 0. 3) Set RB2 to output 0. 4) Set RB0 to 1. 5) Measure time till RB1 reads 1. (reference capacitance) 6) Set RB0 to 0. 7) Wait measured time + some to discharge ref cap. 8) Tristate RB2. 9) Set RB3 to 0 10) Set RB0 to 1. 11) Measure time till RB1 reads 1. (measured cap) 12) Set RB0 to 0. 13) Wait for discharge. tref = reference time, tmes = measured cap time (in seconds) Cref = reference cap value (In Farads), Cmes = measured cap value tref = a*R*Cref (a is a constant) tmes = a*R*Cmes tref/tmes = Cref/Cmes Cmes = tmes*Cref/tref Check my math and pay attention to the units... You should probably protect the input pin (RB1) with a resistor (1K?). For extra protection put two more resistors, one from Cref(-) to RB2 and the other one from Cmes(-) to RB3. You can use more the one charging resistors to measure across a bigger range of caps quickly. You can also use more than one reference cap. Also, consider the case when the cap is already charged when connected to the PIC (with it's maximal voltage rating...). Have fun... Hope this helps, Guy - mlsirtonEraseME.....inter.net.il

>I only used the 1uf to 1mf as an example. I want to measure a broad >range of capacitors, but I only need to know the approximate value >rather than the exact value down to the nth picofarad. Why not check out Electronics Now issure June 1996. It has a simple PIC based LC meter that has the virtue of being 'self calibrating'. The oscillator is based upon a LM311 comparitor - it looks simple and reliable. The PIC reads the frequency (or period - I can't remember), scales it, and sends it to an LCD display. It sounds like a great start for you if you still want to roll your own version. dwayne Dwayne Reid <EraseMEdwaynerplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (403) 489-3199 voice (403) 487-6397 fax Celebrating 15 years of Engineering Innovation (1984 - 1999) * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Do NOT send unsolicited commercial email to this email address. My posting messages to Usenet neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email.

One thing i wonder aboujt with these capacitance mesure-ers is the following: As i understand it, you hook up the capacitor like so Vcc | Res | | Pic Pin | ------------| | Cap | GND now then you hold the pic pin low, then release it untill the voltage trigers the input. However, if you do it again, hold the pic pin low, it would seem that quite a lot more than 20ma is going to go through the pic pin... am i mistaken? ------------------------------------------------------------------------------ A member of the PI-100 Club: 3.1415926535897932384626433832795028841971693993751 058209749445923078164062862089986280348253421170679

Lynx {Glenn Jones} wrote: > As i understand it, you hook up the capacitor like so > Vcc > | > Res > Pic Pin | > ------------| > | > Cap > | > GND > it would seem that quite a lot more than 20ma is going to go through > the pic pin... am i mistaken? Two options; 5V/ 20mA is conservatively, 270 ohms. Put that in series with the pin (will undoubtedly be much less than Res). Put Res on the PIC pin side so there's no voltage divider effect. Alternately, use Res istelf to limit the current. Hold it low for *much* longer than the measuring interval, then alternate between holding it high for a few cycles and sampling it as an input. The standard "POT" function ( http://www.dontronics.com/see.html#pot ) in PIC basic variants works this way. -- Cheers, Paul B.

Glenn, True, you should worry about the current going in to the PIC pin. The actual wiring looks like the following (and I apologize in advance for my ASCII art). Pic RA0 ----------------| | ResA | Pic RA1 -----ResB-------| | Cap | | GND To discharge the cap set RA0 and RA1 to outputs and drive them low. To do the measurement RA0 is set as output and driven high. RA1 is set as input. As soon as the Capacitor has charged to the threshold voltage of RA1, RA1 will go high. Resistor A is chosen to give an RC time constant measurable by the PIC (I am currently using 1K) and Resistor B will limit the current into RA1 and can be a fairly high value (100K is what I am using) to provide high input impedance on RA1. My initial tests with this setup last night were very encouraging. My program basically runs in a loop and does the discharge/measurement over and over, and after the first couple of cycles the measured value settled down into a very consistent value. I still have some work to do to tweak the code a little and to better interpret the results, but for the small amount of time I have spent on it I am very pleased with the results. Adam Bryant (age 0x23) RemoveMEabryantEraseMEEraseMEpeaktech.com (work) RemoveMEadamdbspam_OUTKILLspamjuno.com (home) Parker, CO, USA Robotics, RC Airplanes, anything using a PIC On Tue, 2 Mar 1999 11:12:02 -0800 Lynx {Glenn Jones} <RemoveMEjones_glTakeThisOuTspamEFN.ORG> writes: {Quote hidden}>One thing i wonder aboujt with these capacitance mesure-ers is the >following: As i understand it, you hook up the capacitor like so > > Vcc > | > Res > | > | >Pic Pin | >------------| > | > Cap > | > GND > >now then you hold the pic pin low, then release it untill the voltage >trigers the input. However, if you do it again, hold the pic pin low, >it >would seem that quite a lot more than 20ma is going to go through the >pic >pin... am i mistaken? > >------------------------------------------------------------------------------ >A member of the PI-100 Club: >3.1415926535897932384626433832795028841971693993751 >058209749445923078164062862089986280348253421170679 > ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866]

Dwayne Reid wrote: > > >I only used the 1uf to 1mf as an example. I want to measure a broad > >range of capacitors, but I only need to know the approximate value >rather > than the exact value down to the nth picofarad. > > Why not check out Electronics Now issure June 1996. It has a simple PIC > based LC meter that has the virtue of being 'self calibrating'. The > oscillator is based upon a LM311 comparitor - it looks simple and reliable. > The PIC reads the frequency (or period - I can't remember), scales it, and > sends it to an LCD display. It sounds like a great start for you if you > still want to roll your own version. > > dwayne ===================== Is there a way of getting a copy of that article? Harley L. Miller EraseMEhmillerspamspamBeGonesound.net WB0ROQ

>Why not check out Electronics Now issure June 1996. It has a simple PIC >based LC meter that has the virtue of being 'self calibrating'. The >oscillator is based upon a LM311 comparitor - it looks simple and reliable. >The PIC reads the frequency (or period - I can't remember), scales it, and >sends it to an LCD display. It sounds like a great start for you if you >still want to roll your own version. > >dwayne This project is fantastic. I built one and it's still working great. /"\ Friendly Regards \ / X ASCII RIBBON CAMPAIGN Octavio Nogueira / \ AGAINST HTML MAIL =================================================== RemoveMEnogueiraKILLspampropic2.com ICQ# 19841898 >From the creator of ProPic,ProPic 2 now much better New ProPic 2 homepage: http://www.propic2.com PIC Programmer for Windows with down to earth price ===================================================