Post by thread:Re: [EE] Low resistance

I've got a PIC monitoring the on/off status of a small (2A 12V) motor and presently use a low-value resistor on the earthy side (the motor is switched in/out with a high-side relay). The voltage across this resistor is compared to a pre-set value in the PIC's comparator. Works well, no problems, the PIC has no trouble sensing when the motor is on/off But.......... When the customer had a particular motor, this resistor was a 0R47 0.25W carbon. Now, for advantageous economic and supply reasons, he's switched to a motor with a much lower winding R, and to make the circuit work the way it used to, the resistor has had to be lowered. What seems to work best are 0R03 6W resistors (from RadioSpares, basically because they were convenient) Unfortunately, being 6W W-W power resistors, they're much too big physically, certainly much bigger than a 1/4W, and I'd like to replace them with resistance wire What I have to hand is some Cuprothal (6.08 ohms/m) and Nichrome 80 (13.4 ohms/m). Work it out and you'll see that the length required for 0.03 ohms is fiddly (5mm and 2.24mm respectively) and I'd not be confident about my repeatability. Ideally I'd like something that was more around, say, 50mm. Which I guess makes it around 0.6 ohms/m Problem #1 - where to get a good selection of resistance wire in NZ Problem #2 - what exactly am I looking for or, alternatively, is there a "good" choice and a "bad" choice ? And would 0.6 ohms/m even be regarded as resistance wire ? Any short-cut help appreciated TIA ============================================== If you aren't part of the solution, you're part of the precipitate

Jinx, How critical is the resistance with temperature - could you use a copper wire or pcb track? And what is the actual power dissipation? RP On 18/01/06, Jinx <spam_OUTjoecolquittTakeThisOuTclear.net.nz> wrote: {Quote hidden}> I've got a PIC monitoring the on/off status of a small (2A 12V) > motor and presently use a low-value resistor on the earthy side > (the motor is switched in/out with a high-side relay). The voltage > across this resistor is compared to a pre-set value in the PIC's > comparator. Works well, no problems, the PIC has no trouble > sensing when the motor is on/off > > But.......... > > When the customer had a particular motor, this resistor was a > 0R47 0.25W carbon. Now, for advantageous economic and > supply reasons, he's switched to a motor with a much lower > winding R, and to make the circuit work the way it used to, > the resistor has had to be lowered. What seems to work best > are 0R03 6W resistors (from RadioSpares, basically because > they were convenient) > > Unfortunately, being 6W W-W power resistors, they're much > too big physically, certainly much bigger than a 1/4W, and I'd > like to replace them with resistance wire > > What I have to hand is some Cuprothal (6.08 ohms/m) and > Nichrome 80 (13.4 ohms/m). Work it out and you'll see that > the length required for 0.03 ohms is fiddly (5mm and 2.24mm > respectively) and I'd not be confident about my repeatability. > Ideally I'd like something that was more around, say, 50mm. > Which I guess makes it around 0.6 ohms/m > > Problem #1 - where to get a good selection of resistance wire > in NZ > > Problem #2 - what exactly am I looking for or, alternatively, is > there a "good" choice and a "bad" choice ? > > And would 0.6 ohms/m even be regarded as resistance wire ? > > Any short-cut help appreciated > > TIA > > ============================================== > If you aren't part of the solution, you're part of the precipitate > > -

> How critical is the resistance with temperature - could you use a > copper wire or pcb track? > And what is the actual power dissipation? Track was what I was thinking of, should be repeatable in theory. Or at least consistent for a given run of boards anyway Dissipation is low. The comparator is set at 60mV, and resistor (with one end at ground) voltage is 0V (motor off) or 90mV (motor on). Start current for motor is 2A, generally runs at 1.5A-ish for a very short while, no more than 15s. It's an adjustment arrangement with a driven screw thread. In that situation you can feel no warmth with a 0R47 1/4W, so a track or piece of wire would be fine Temperature ? As above, no significant heating apparent and ambient is never out of comfort zone

Hi Jinx. "Tinning" the edges with better conducting metal wont help to achieve 5mm of working length left you say ? :) WBR Dmitry. Jinx wrote: > What I have to hand is some Cuprothal (6.08 ohms/m) and > Nichrome 80 (13.4 ohms/m). Work it out and you'll see that > the length required for 0.03 ohms is fiddly (5mm and 2.24mm > respectively) and I'd not be confident about my repeatability. > Ideally I'd like something that was more around, say, 50mm. > Which I guess makes it around 0.6 ohms/m

>-----Original Message----- >From: .....piclist-bouncesKILLspam@spam@mit.edu [piclist-bouncesKILLspammit.edu] >Sent: 18 January 2006 09:11 >To: pic microcontroller discussion list >Subject: [EE] Low resistance - resistance wire etc > > >What I have to hand is some Cuprothal (6.08 ohms/m) and >Nichrome 80 (13.4 ohms/m). Work it out and you'll see that the >length required for 0.03 ohms is fiddly (5mm and 2.24mm >respectively) and I'd not be confident about my repeatability. >Ideally I'd like something that was more around, say, 50mm. >Which I guess makes it around 0.6 ohms/m No problem, just use 20 lengths of 50mm in parallel ;) Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. =======================================================================

> No problem, just use 20 lengths of 50mm in parallel ;) See, now that's why I come here - lateral thinking (smiley noted) It might not be out of the question. Maybe not 20 ....... This little snippet I picked up puts a number on a track. I'd rather it be wider than 10 thou, so if the following is correct I'd have to do a little scaling. Many sources give track the resistance as (p * L)/A. Before I can work that out I'd have to find out what the board copper thickness is http://www.analog.com/library/analogDialogue/Anniversary/10.html "Q. Surely the resistance of a short length of copper in small- signal circuits is unimportant? A. Consider a 16-bit a/d converter with 5-kohms input impedance Suppose that the signal conductor to its input consists of 10 cm of typical PC track-0.25 mm (0.010") wide and 0.038 mm (0.0015") thick. This will have a resistance of approximately 0.18 ohms at room temperature, which is slightly less than 2 x 2-16 of 5 kohms; this introduces a gain error of 2 LSB of full scale"

Dissipation is low. The comparator is set at 60mV, and resistor (with one end at ground) voltage is 0V (motor off) or 90mV (motor on). Start current for motor is 2A, generally runs at 1.5A-ish for a very short while, no more than 15s. I presume those currents are for the old motor. >It's an adjustment arrangement with a driven screw thread. >In that situation you can feel no warmth with a 0R47 1/4W, >so a track or piece of wire would be fine I take it that the idea for measuring the current is to limit power amp dissipation, and detect limit stops. As such you probably do not need to be too accurate with your resistor tolerance. What gauge nichrome wire are you using? I seem to remember my dad producing some 0R47 resistors from heater nichrome wire, which is about 0.5mm dia, and that still required a reasonable length that had to be coiled up. He used some silver solder to make spots at the correct distance apart that I could solder to.

> ... Nichrome ... ? A C Nielsons. Dick Smith Jaycar South Island Component Centre. But, several physically small resistors in parallel would work well. eg 3 x Philips SFR16 0R1 would be small and close enough. Any number of SMT resistors in parallel ... 0805s will mount between 2 veroboard tracks so putting N in parallel is relatively easy. The nice people at Surface Mount Devices Ltd in Pakuranga http://www.smd.net.nz/ will sell you small quantities at OK prices. But, whoops, they seem to start at 1R0 :-(. You could wind something with copper wire. You could use multiple strands of Nichrome etc in parallel. You could build an electronic device that synthesised low resistance - could be as compact as desired with SMT. You could ..... RM

> I take it that the idea for measuring the current is to limit power > amp dissipation, and detect limit stops What happens is that the motor moves two screws. One is to adjust the machinery, the other is to activate cross-wired miro-switches. When one is tripped, it cuts the power, stopping movement in that direction. The PIC detects the loss of power and knows then that it can drive only in the opposite direction. A neat little arrangement between the PIC, two microswitches and a DPCO relay > As such you probably do not need to be too accurate with your > resistor tolerance That's true. Although because this resistor is the earthy connection of the motor, it has to be in a certain low range. Whatever is chosen, the comparator voltage can be set to accomodate it, so the absolute value isn't critical > What gauge nichrome wire are you using? I'm not, as yet. Mentioned it simply as an alternative to the bulk of an RS power resistor. The two types are DSE stuff I have to make some fractional-ohm (but >> 30mohms) for a PSU > I seem to remember my dad producing some 0R47 resistors > from heater nichrome wire, which is about 0.5mm dia, and > that still required a reasonable length that had to be coiled up I thought about using a bit of a bar heater element. That's about 70 ohms cold, 0.5mm diameter, probably a metre long ? 0.4mm to get 30mohms. Hmmm, maybe not

Why not use a low forward voltage-drop diode in series with the motor, when on, you'll get a small voltage-drop of say 0.2-0.4 volts, depending on diode, which is easilly detected, and you don't need to change it depending of which motor the customer uses, it will always be the same, make sure you rate it for the maximum power-diss and current. If AC, use two diodes in parallell. With best regards Tomas Larsson Sweden Verus Amicus Est Tamquam Alter Idem > {Original Message removed}

Jinx wrote: > What I have to hand is some Cuprothal (6.08 ohms/m) and Nichrome 80 (13.4 > ohms/m). Work it out and you'll see that the length required for 0.03 > ohms is fiddly (5mm and 2.24mm respectively) and I'd not be confident > about my repeatability. Why not solder the wire flat between two pads that are 5 mm apart? > Ideally I'd like something that was more around, say, 50mm. > Which I guess makes it around 0.6 ohms/m Thicker or more conductive (copper) wire :) I've done something like that with about 2 cm of 1.5 mm^2 copper wire. (Was for higher currents though... up to 40 A.) Gerhard

What about using longer wire lengths and paralleling them. You get the ease of length measurment and a repeatable process. Larry ---- Jinx <.....joecolquittKILLspam.....clear.net.nz> wrote: {Quote hidden}> I've got a PIC monitoring the on/off status of a small (2A 12V) > motor and presently use a low-value resistor on the earthy side > (the motor is switched in/out with a high-side relay). The voltage > across this resistor is compared to a pre-set value in the PIC's > comparator. Works well, no problems, the PIC has no trouble > sensing when the motor is on/off > > But.......... > > When the customer had a particular motor, this resistor was a > 0R47 0.25W carbon. Now, for advantageous economic and > supply reasons, he's switched to a motor with a much lower > winding R, and to make the circuit work the way it used to, > the resistor has had to be lowered. What seems to work best > are 0R03 6W resistors (from RadioSpares, basically because > they were convenient) > > Unfortunately, being 6W W-W power resistors, they're much > too big physically, certainly much bigger than a 1/4W, and I'd > like to replace them with resistance wire > > What I have to hand is some Cuprothal (6.08 ohms/m) and > Nichrome 80 (13.4 ohms/m). Work it out and you'll see that > the length required for 0.03 ohms is fiddly (5mm and 2.24mm > respectively) and I'd not be confident about my repeatability. > Ideally I'd like something that was more around, say, 50mm. > Which I guess makes it around 0.6 ohms/m > > Problem #1 - where to get a good selection of resistance wire > in NZ > > Problem #2 - what exactly am I looking for or, alternatively, is > there a "good" choice and a "bad" choice ? > > And would 0.6 ohms/m even be regarded as resistance wire ? > > Any short-cut help appreciated > > TIA > > ============================================== > If you aren't part of the solution, you're part of the precipitate > > --

On Wed, 18 Jan 2006, Jinx wrote: > Problem #1 - where to get a good selection of resistance wire > in NZ You already have it. Use several longer pieces in parallel (f.ex. wind several turns between 2 posts on the board). > And would 0.6 ohms/m even be regarded as resistance wire ? If you do something with low voltage lead acid batteries (like 2V) then it is 'high resistivity wire'. Peter

On Wed, 18 Jan 2006, Jinx wrote: >> How critical is the resistance with temperature - could you use a >> copper wire or pcb track? >> And what is the actual power dissipation? > > Track was what I was thinking of, should be repeatable in theory. > Or at least consistent for a given run of boards anyway > > Dissipation is low. The comparator is set at 60mV, and resistor > (with one end at ground) voltage is 0V (motor off) or 90mV (motor > on). Start current for motor is 2A, generally runs at 1.5A-ish for > a very short while, no more than 15s. It's an adjustment arrangement > with a driven screw thread. In that situation you can feel no warmth > with a 0R47 1/4W, so a track or piece of wire would be fine Don't you have ground noise messing up the measurement ? Normally you would aim for a 1%-of-the-power shunt so Pmotor=25W->Pshunt=0.25W which at 2A is 0.06 ohms. So 2 x 0.1 Ohms 0.25W in parallel should work? But this also requires >200 times noise suppression (motor electrical noise) to work. Peter