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'[EE] Low resistance - resistance wire etc'
2006\01\18@041103 by

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 <joecolquittclear.net.nz> wrote:
{Quote hidden}

> -
> 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-bouncesmit.edu [piclist-bouncesmit.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

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> 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.

>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

> But, several physically small resistors in parallel would work well.
> eg 3 x Philips SFR16 0R1 would be small and close enough

I had a look a couple of days ago through Active's stock of Philips
everything and haven't found anything under 1R0 yet ;-(

Hadn't dismissed using paralleled resistors - if I can find them that is

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

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

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 <joecolquittclear.net.nz> wrote:
{Quote hidden}

> --

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

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