I made a 0.025 ohm resistor in the shop over lunch hour. It ended up being 14" of #24AWG tinned solid copper wire at 250C. I measured it with a recently calibrated $3000 HP 4 wire ohmmeter just to make sure it was what I needed. For anyone else trying to build a sense resistor, if you don't have a $3000 ohmmeter handy, this is a good recipe. Wound around a 1/4" dowel, it ends up about 1" long. FYI: Yes, I am in the process of designing a battery charger for NiMH batteries. This is step 3 of 5000 steps. The sense resistor and low-side amplifier are going to feed current info into a PIC. To answer your question, David: 1. Using the smallest sense resistor I can get away with is a way to get more efficiency out of my system. I don't want to waste power anywhere. Besides, I've got motors to run, don't want to reduce the voltage to them with an oversized sense resistor. 2. Right now I have started designing a linear current regulator, and already have realized it is only 50% efficient. Very bad! I will probably end up with a switching regulator before I am done, to get the efficiency high. Never having designed one before, this should be fun! 3. I don't want to use a cheap linear regulator (LM7805), because it is hard to find a 2 amp regulator, souping them up for higher current is almost as hard as building a regulator from scratch, and I need to also be able to turn OFF the regulator, and linear regulators are going to be ~~50% to 75% efficient. Bad! All these functions(feedback, battery voltage sensing, battery temperature sensing, shutting down the current regulator, trickle charging) are eventually going into a PIC. 4. A copper resistor is going to change resistance as it warms up. It is also inductive, being an air core wirewound resistor. Copper coefficient of thermal resistance is 3930 ppm/Degree C. My resistor is dissipating 0.1 watts, and it's as big as a 3 watt resistor, so I don't imagine I'll have to worry about self-heating too much. Ambient temperature changes are going to make a bigger difference. If it's a big deal, I'll compensate for them later. Good Point, David! Aha! Nichrome wire (the stuff they use in toasters!) is only 150 ppm/0C Mucho Better! I'll wind my next resistor out of nichrome. 150ppm/0C X 120C is only .0018 ohms, a change of only 7% between 250C and 370C (75F to 100F). Not too bad. 5. Yes, this is part of the constant current regulator for an NiMH battery charger, eventually part of a mowbot. I'm sort of designing this thing online, and people keep coming in in the middle of the discussion and asking me "What the hell are you doing? Why don't you just design a battery charger?" It's getting kind of funny. I'll keep posting more design details as I go. Hopefully this will benefit somebody else doing the same thing!Pictures are on the way! --Lawrence Lile ----- Original Message ----- From: "David Cary" To: Sent: Friday, May 18, 2001 11:47 AM Subject: Re: [EE]: NiMH batteries > Dear Lawrence Lile, > > Lawrence Lile on 2001-05-18 09:22:56 AM wrote: > > I needed a 0.025 ohm resistor as a sense resistor. This could be built up > > from 29 Cm of 24 AWG solid copper wire, if such a resistor can't be found. > > Onward to the constant current circuit! > > Why don't you a big resistor (10 Ohms ?) as a sense resistor ? > > I imagine you're going to use a cheap linear regulator -- would the > constant-current circuit in the LM317 data sheet work ? I think I downloaded my > datasheet from > Philips http://www-us.philips.com/ > > . > > Given a constant voltage input, a battery voltage output, and a linear > constant-current regulators, you're forced to waste power *somewhere*. I think > it's simpler and cheaper to waste it in the sense resistor than in your > constant-current regulator transistor. Is there a way to do something clever > like Olin Lathrop and make the same resistor do triple-duty as a current sense, > a voltage sense, and part of the constant-current regulator ? > > Um... some resistors change their resistance significantly when they warm up. > > -- > David Cary > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics