Here's a link to a sketch of the circuit I am talking about http://members.socket.net/+AH4-llile/misc/Mosfet+ACU-20Pass.JPG Don't be afraid to learn new tricks about mosfets - they solve a lot of the problems that old BJT's had. Look for International Rectifier on the web - they will have some good tutorials about using mosfets. Call your IR rep, and ask for a databook, and pick his brain. Now, a PNP power transistor would work similarly to a P channel modfet, but you always have a minimum Vce drop across your transistor, which sets your minimum dropout voltage on your whole supply, and also sets the efficiency. If you are looking for low dropout characteristics, a Mosfet is your friend. Think of a mosfet as a voltage controller potentiometer. They can go down to well under an ohm when fully on, .06 ohms is a common spec. This means power dissipation is nil. A T0220 mosfet is often specced at 20 or 30 amps+ACE- When the Gate (think Base) voltage is about ten volts relative to the source (think Emitter) then the mosfet is turned on. In a P channel this means the gate is 10 volts negative. Mosfets have zero gate current, except for charging and discharging the gate capacitance, so worries about base drive power dissipation are gone. Mosfets have a high gate capacitance, since the gate is capacitively coupled to the rest of the device. So you have to drive them with a low impedance driver if you want to drive them fast. To soup up the schematic I sent, you could drive the mosfet with a PNP-NPN totem pole pair. A 2n2222 and a 2n2907 work quite well. But for your purposes, this probably isn't necessary. Size the capacitor in the feedback loop to knock out most all the high frequency component of the chopped signal you are putting in. Batteries don't change fast, so your cap could be plenty big. The 14.7 volt number came from my leaky brain, prolly wrong. --Lawrence ----- Original Message ----- From: +ACI-jb+ACI- +ADw-alphaone+AEA-ROF.NET+AD4- To: +ADw-PICLIST+AEA-MITVMA.MIT.EDU+AD4- Sent: Wednesday, January 23, 2002 7:38 PM Subject: Re: +AFs-ee+AF0-: solar site charging system My MOSFET knowledge is pretty limited, not sure how to drive that with a PIC running at 5V. Wouldn't a PNP power transistor do essentially the same thing since the CE drop in saturation is what, maybe .2-.4V? Use a 2n2222 to drive the base on the PNP hard with the PIC output, and use the A/D input as you said to detect hi/low thresholds? Perhaps the MOSFET idea is better, but absent much experience with them, I'm trying to stick with what I know. Also, you could be right on that 14.7V, but the data sheet on this battery claimed that in an application like mine where the battery will be drawn down frequently, it's okay to set the constant voltage charger to 15V. Of course, now that you've challenged that figure, I can't seem to locate that data sheet, but I'm pretty sure that's what it said. JB +AD4- -----Original Message----- +AD4- From: pic microcontroller discussion list +AD4- +AFs-mailto:PICLIST+AEA-MITVMA.MIT.EDU+AF0-On Behalf Of Lawrence Lile +AD4- Sent: Wednesday, January 23, 2002 4:04 PM +AD4- To: PICLIST+AEA-MITVMA.MIT.EDU +AD4- Subject: Re: +AFs-ee+AF0-: solar site charging system +AD4- +AD4- +AD4- I built a charge controller for a friend's solar system some time ago. +AD4- +AD4- I've built a linear charge controller out of a 2N3055 and +AD4- some other stuff +AD4- for this very application. After having lived with it, i'd +AD4- stay away from +AD4- any linear regulation of any kind. The LM317 or the 2n3055 +AD4- are power hogs, +AD4- won't work with low dropout voltages, and also have all the +AD4- same probs you +AD4- mentioned. +AD4- +AD4- Ditto for the zener regulators. Dump them with Nixon, 8 tracks, the +AD4- bicentennial and all that other 70's stuff. +AD4- +AD4- Take your PIC and drive a P-channel mosfet as a pass +AD4- transistor, filter the +AD4- output a little and run it into your A/D port. If the +AD4- voltage is too high, +AD4- turn off the mosfet, if it is too low, turn it on. You'll be +AD4- surprised how +AD4- easy this is. +ACQ-5 should build it, and a mosfet can handle a +AD4- lot more than +AD4- 1.5 amps no sweat. +AD4- +AD4- Check your output voltage - I think lead acid batteries are +AD4- fully charged at +AD4- around 14.7 volts but my memory leaks so look it up. I'd regulate the +AD4- output at the fully-charged voltage of the battery, and shut the solar +AD4- charging down when the battery gets full. +AD4- +AD4- +AD4- --Lawrence +AD4- +AD4- ----- Original Message ----- +AD4- From: +ACI-JB+ACI- +ADw-alphaone+AEA-ROF.NET+AD4- +AD4- To: +ADw-PICLIST+AEA-MITVMA.MIT.EDU+AD4- +AD4- Sent: Wednesday, January 23, 2002 12:37 PM +AD4- Subject: +AFs-ee+AF0-: solar site charging system +AD4- +AD4- +AD4- +AD4- I have a panel that delivers up to 21.5V in full sun with 3.5A +AD4- short-circuit +AD4- +AD4- current. This panel is used to charge a 17Ah 12V sealed +AD4- lead-acid battery +AD4- that +AD4- +AD4- can be floated at 15V for cyclic usage patterns, with a +AD4- maximum charge +AD4- rate of +AD4- +AD4- around 1.5A. Using a LM317-T set at 15V with no ballast +AD4- resistor (using +AD4- +AD4- internal battery resistance) this works fine in full sun, +AD4- but anything +AD4- short of +AD4- +AD4- about 17V at the panel results in zero charging. I would +AD4- like to utilize +AD4- the +AD4- +AD4- power available when the panel is at any point above the +AD4- battery voltage +AD4- so +AD4- +AD4- that I am not wasting that energy. +AD4- +AD4- +AD4- +AD4- A switching regulator with a wide input range, say 12-30V +AD4- to 15V out would +AD4- be +AD4- +AD4- perfect, but I have yet to find one. +AD4- +AD4- +AD4- +AD4- Should I drop the 317 and simply shunt the panel output at +AD4- 15V with a +AD4- Zener? I +AD4- +AD4- don't care about anything over the 15V point, as I don't want to +AD4- over-charge +AD4- +AD4- the battery. This of course assumes I can find a 15V/50W +AD4- zener to mount on +AD4- the +AD4- +AD4- panel. +AD4- +AD4- +AD4- +AD4- Are there better alternatives? There is a 16F873 in the project with +AD4- available +AD4- +AD4- A/D and TTL ports, so that may be an option with the right +AD4- components, but +AD4- I'm +AD4- +AD4- running out of experimentation time, so I'd rather not venture into +AD4- something +AD4- +AD4- that +ACI-might+ACI- work. +AD4- +AD4- +AD4- +AD4- I can justify up to about +ACQ-50 for the components. +AD4- +AD4- +AD4- +AD4- -- +AD4- +AD4- http://www.piclist.com hint: The list server can filter out +AD4- subtopics +AD4- +AD4- (like ads or off topics) for you. See http://www.piclist.com/+ACM-topics +AD4- +AD4- +AD4- +AD4- +AD4- +AD4- -- +AD4- http://www.piclist.com hint: The list server can filter out subtopics +AD4- (like ads or off topics) for you. See http://www.piclist.com/+ACM-topics +AD4- +AD4- +AD4- -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/+ACM-topics -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.