Hi, For sensing both ways with a lot of complication you might try high side= current sensing. A chip like MAX471 or MAX472 or similar gives you the current as a= voltage across a sense resistor and there is also a sign output that determines the= direction. *********** REPLY SEPARATOR *********** On 23/06/2003 at 16:53 Dominic Stratten wrote: >I was going to make a similar type of unit for measuring Car battery >charge/discharge. > >It looks pretty easy until you try it !!!! > >Measuring current flow one way is easy, use one side of the shunt as= ground >and the other to a Pic A2D for the current sensing, by adding the current= / >time you can sense the charge. > >Problem is when the current flows the other way round, you get a negative >voltage which the pic cant handle. > >One idea I had was to amplify the voltage across the shunt using a single >rail opamp. > >Lets say there is a 10mv drop across the shunt per Amp drawn. Using an >opamp >to amplify this by 100 would give 1v/Amp output. Feed this into an A2D >input >on the pic and you've got say a charge current. For sensing the discharge >current, use an opamp grounded on the other side of the shunt with the= same >amplification, you'll get a slight offset of the voltages but in theory, I >cant see why it wouldnt work. > >As I said, its easy to sense one way but to try to sense both ways appears >to be a bit of a pain. If you come across any easier solution (i.e.= running >the pic from a split rail supply) then let me know :-) > >Regards > >Dom >----- Original Message ----- >From: "John Nall" >To: >Sent: Monday, June 23, 2003 4:39 PM >Subject: [PIC]: Counter for how many amps a battery is down > > >> My sailboat has (or at least had) a battery monitor on it. You start >with >> a full battery, and this monitor displayed a number which represented= how >> many amp hours had been used. (Usually a negative number, but you could >> overcharge the battery and have a positive number). Alas, my monitor >seems >> to have been murdered. :-( >> >> The boat took a lightning hit while in the slip, and apparently some of >the >> electronics in the monitor got zapped. Although I could buy another= one, >a >> large part of the cost is the shunt, which was not harmed. They won't >sell >> the unit without the shunt, so I figure I should be able to use a PIC to >> build another monitor. The shunt is just a resistor, and is described= as >> follows: >> "When current flows through the shunt, a small voltage is developed >across >> the shunt which is proportional to the current flow. The battery monitor >> accurately measures this very low voltage and converts it to the "amps" >> reading on the meter. The resistance, which is the ratio between the >> voltage across the shunt and the current flowing through it, is a >constant >> for any particular shunt" >> So the PIC would measure the voltage coming from the shunt and use that >to >> calculate the corresponding current, and use a timer to convert to amp >> hours. The shunt itself is a dual MKB-500/50 (the dead monitor was a >Link, >> made by Heart Interface). >> >> NOW, please don't flame me for not having a specific, detailed question >to >> ask!! I merely want to know if someone on the Piclist might have done >> something like this, or have information that would be helpful to me in >> designing and building such a critter. If so, would appreciate such= info >> -- offline or online, as you choose. :-) >> >> Thanks, >> John >> >> -- >> http://www.piclist.com hint: The PICList is archived three different >> ways. See http://www.piclist.com/#archives for details. > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. ~~~~~~~~~~~oOo~~~~~~~~~~~ Ofer Safeman, M. Sc. Senior Design Engineer Paragon Communications 6 Pal-Yam Haifa, 33095 ISRAEL Phone: +972-4-8642775 Fax: +972-4-8642737 E-Mail: ofer@paragoncommtech.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.