Steve, We've done a lot of voltage measurement in the aviation world, which is similar to your situation. Several points: a. I wouldn't worry about the 10K input Z for the ADC for such a low frequency signal (DC). We use 57.6K and 6.11K resistors (1% or better) and have been able to hold +/- 0.1 volt reading without problems. Yes, that's less than 10K but we've also done boards using Meg ohm resistors and it worked fine. Note that we use an external voltage reference since you can't count on Atmel's internal one being consistant from chip to chip. We've built a board that used a Atmel 169 picopower and a bare glass LCD. With the display on, it used an average of 25 uA (if my memory is correct) and displayed time and voltage. b. Isn't there some sort of a master switch that turns off all the power to this tiny little motor? (I could resist the dig, since we deal with jet engines.) Couldn't you put your voltage divider on the other side so it's shut off when the boat is shut off? BTW, even 50 mA is probably much less than the self discharge rate of the battery. c. I haven't seen any mention of surge protection etc. I'd assume that you'll get a nasty voltage spike with the motor is turned on/off. We always include a TVS, fuse and diode on the input. We've actually had the power pulses from avionics knock out the power supply on our boards. Watch your grounds. Regards, Tim Gilbert JEM Innovation Inc. 303-926-9053 (office) 303-437-4342 (cell) 720-890-8582 (fax) www.jeminnovation.com www.pdksolutions.com ----- Original Message ----- From: Steven Hodge To: AVR-Chat@yahoogroups.com Sent: Friday, December 14, 2007 9:23 AM Subject: RE: [AVR-Chat] Re: ADC Vref Ok, OP back again. I was away for most of yesterday. Yes, my application is (sort of) automotive. Specifically it is a boat environment, which is 12 VDC. My guess is that in some respects it's not quite as harsh as automotive, but in others, eg, high power RF transmissions and need to shield magnetic compasses from EM fields, it is far worse. I have many uses for uC's but the specific one that the post was about is that I have put in a "bow thruster", which is a huge 5.0 KW, 12 VDC motor that draws about 400 A (which is another story). This needs to be run from its own local 12 V battery (an Optima AGM starting type) because to run it from the main boat's bank in the aft part of the boat, the voltage drops even in large 4/0 cable would kill a lot of the motor's thrust. Even so, the local battery, if the thruster is run too long (like 5-7 minutes), can get drawn down to the point where its voltage - I'll call it Vb here -- gets so low the solenoids in its circuitry start to chatter and within a few seconds fuse together and bingo, motor stuck on and battery completely drained (ie, major problem). So my intent is to use a uC to monitor the thruster battery voltage, Vb, when under load and when it gets too low, shut everything down. One feature of this is that the uC power supply voltage (Vcc) will come from the boat's main bank but the voltage to be monitored is the separate local battery (in other words I'm not powering the uC from the voltage to be monitored). True I could do this simple task totally in hardware, with a voltage comparator, but there are other reasons which require using a uC so the easiest solution is to just have the uC do the voltage measurement. Obviously not a whole lot of accuracy is required. Regarding voltage divider current drains, my assumption is that from the data sheets the analog ADC input likes to see 10 Kohms maximum impedance. For a maximum input to the ADC of Vcc = 3V this implies a current drain through the divider, regardless of amount of dividing, of 0.3 mA. Yeah, heck, compared to 400 A it is quite negligible, but overall I have so many of these mA to 10 mA to 50 mA drains that they all add up to a significant 24/7 drain on the batteries, and so I have a general philosophy of trying to minimize these "background" drains if at all possible. I have used VP3203 enhancement mode P-channel mosfets (actually DMOS FET's, I just loosely call them mosfets) on PCB's in the past and I see no reason why one of these, with its gate driven directly by a uC output pin, could not be used to turn on/off the voltage to the divider. Their threshold voltage is only 1 V so a 3 V signal from the uC will do the trick. Leakage current when off should be less than 10 uA. Connect the gate with a pull-up to Vb, source to Vb and drain to the top of the divider. I always but TVS's and big input capacitors on the power input to any PCB I build and (so far, knock on wood) haven't had any problem with transients. In some other potential applications, not this particular one, I will have need to measure 2 or 3 voltages, all well above Vcc. In this case I would simply add one of these mosfets to each divider but drive them all with the same gate signal from the uC. Am I missing something here? Steve From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf Of Graham Davies Sent: Friday, December 14, 2007 5:43 AM To: AVR-Chat@yahoogroups.com Subject: [AVR-Chat] Re: ADC Vref --- In AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> , "Roy E. Burrage" <RBurrage@...> wrote: > Why not just stick with MOSFETs, Graham? Sure. An NPN transistor is just an alternative. > He might also want to use some sort > of gate/source protection scheme ... That would depend on the application, which we don't know. > Why would he not be able to expect > pretty fair accuracy from your scheme? He would. As you say, the on resistance of the MOSFET would be of no consequence, so it's just a matter of using the appropriate resistors in the divider and a good reference for the ADC. Some of the other suggestions, though, did not strive for accuracy. Again, it may not be necessary in this application. We don't know what the OP hasn't told us. Recall that his original post didn't mention the need to draw almost no power, it was just about what the data sheet said about the ADC reference input. Graham. __________ NOD32 2723 (20071214) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com [Non-text portions of this message have been removed] [Non-text portions of this message have been removed]
Message
Re: [AVR-Chat] Re: ADC Vref
2007-12-14 by Tim Gilbert