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From: John Larkin on 8 Jul 2010 15:12
On Thu, 8 Jul 2010 11:29:10 -0700 (PDT), whit3rd <whit3rd(a)gmail.com>
wrote:

>On Jul 8, 9:24�am, AM <thisthatandtheot...(a)beherenow.org> wrote:
>> On Thu, 08 Jul 2010 08:56:21 -0700, John Larkin
>
>> >> �That's what lasers are for. �Worst case, send them out to be custom
>> >>trimmed.
>
>> >We used to make planar manganin shunts using dies and a punch press,
>> >but that was expensive. We tried CO2 laser machining, but the laser
>> >mostly bounced off (manganin is mostly copper, an excellent IR
>> >reflector) and the edges were awful anyhow. Lately we have them
>> >photo-etched, which works beautifully. ...
>> >We don't trim our shunts, we just trim gain electronically downstream.
>
>> �Get a micro cleaver setup like the chemical/bio labs use to cleave off
>> pieces of frozen sample media for analysis.
>
>I've seen shunts that were trimmed by holding 'em up to a grinding
>wheel.
>The problem is, mechanical stress raises the resistance (until heat or
>age
>anneals the metal). Chemical etching leaves a raw-metal surface,
>which
>will in time oxidize (changing the resistance). Laser trimming is
>fine for SMALL resistors, removing serious metal from a 200A
>shunt is a frightening prospect.
>
>Make the shunt, anneal and passivate the surface, then use some
>other gain trim downstream.

We do anneal our shunts. Then dip the relevant parts in a little
dilute HCl to clean them up for soldering.

John

From: AM on 8 Jul 2010 15:19
On Thu, 8 Jul 2010 11:29:10 -0700 (PDT), whit3rd <whit3rd(a)gmail.com>
wrote:

>The problem is, mechanical stress raises the resistance (until heat or
>age
>anneals the metal).

The site I saw info on showed media where that would only be on the
surface molecules that were proximal to the grinding.

Also, one can grind carefully and slowly so as to reduce
thermo-mechanical stresses introduced.

They use a granular microstructure that also depletes and minimizes
such effects.

One can also stress relieve the part after the grinding and before the
next calibration reading is taken.
Or finish the trimming with a file.

Also, a punch might be a way to shear off pieces without introducing
time released stress modulus. Paper punch.
From: Spehro Pefhany on 9 Jul 2010 11:16
On Thu, 08 Jul 2010 12:12:35 -0700, John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Thu, 8 Jul 2010 11:29:10 -0700 (PDT), whit3rd <whit3rd(a)gmail.com>
>wrote:
>
>>On Jul 8, 9:24�am, AM <thisthatandtheot...(a)beherenow.org> wrote:
>>> On Thu, 08 Jul 2010 08:56:21 -0700, John Larkin
>>
>>> >> �That's what lasers are for. �Worst case, send them out to be custom
>>> >>trimmed.
>>
>>> >We used to make planar manganin shunts using dies and a punch press,
>>> >but that was expensive. We tried CO2 laser machining, but the laser
>>> >mostly bounced off (manganin is mostly copper, an excellent IR
>>> >reflector) and the edges were awful anyhow. Lately we have them
>>> >photo-etched, which works beautifully. ...
>>> >We don't trim our shunts, we just trim gain electronically downstream.
>>
>>> �Get a micro cleaver setup like the chemical/bio labs use to cleave off
>>> pieces of frozen sample media for analysis.
>>
>>I've seen shunts that were trimmed by holding 'em up to a grinding
>>wheel.
>>The problem is, mechanical stress raises the resistance (until heat or
>>age
>>anneals the metal). Chemical etching leaves a raw-metal surface,
>>which
>>will in time oxidize (changing the resistance). Laser trimming is
>>fine for SMALL resistors, removing serious metal from a 200A
>>shunt is a frightening prospect.
>>
>>Make the shunt, anneal and passivate the surface, then use some
>>other gain trim downstream.
>
>We do anneal our shunts. Then dip the relevant parts in a little
>dilute HCl to clean them up for soldering.
>
>John

I find it amusing that manganin is also used to make pressure
sensors... a nice linear piezoresistive response, and you can get a
percent or three change in resistance with (large) pressure. I imagine
you must have to consider the mounting method carefully to get
ppm-level stability, considering the heat generated and connections
required.

From: John Larkin on 9 Jul 2010 11:33
On Fri, 09 Jul 2010 11:16:39 -0400, Spehro Pefhany
<speffSNIP(a)interlogDOTyou.knowwhat> wrote:

>On Thu, 08 Jul 2010 12:12:35 -0700, John Larkin
><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Thu, 8 Jul 2010 11:29:10 -0700 (PDT), whit3rd <whit3rd(a)gmail.com>
>>wrote:
>>
>>>On Jul 8, 9:24�am, AM <thisthatandtheot...(a)beherenow.org> wrote:
>>>> On Thu, 08 Jul 2010 08:56:21 -0700, John Larkin
>>>
>>>> >> �That's what lasers are for. �Worst case, send them out to be custom
>>>> >>trimmed.
>>>
>>>> >We used to make planar manganin shunts using dies and a punch press,
>>>> >but that was expensive. We tried CO2 laser machining, but the laser
>>>> >mostly bounced off (manganin is mostly copper, an excellent IR
>>>> >reflector) and the edges were awful anyhow. Lately we have them
>>>> >photo-etched, which works beautifully. ...
>>>> >We don't trim our shunts, we just trim gain electronically downstream.
>>>
>>>> �Get a micro cleaver setup like the chemical/bio labs use to cleave off
>>>> pieces of frozen sample media for analysis.
>>>
>>>I've seen shunts that were trimmed by holding 'em up to a grinding
>>>wheel.
>>>The problem is, mechanical stress raises the resistance (until heat or
>>>age
>>>anneals the metal). Chemical etching leaves a raw-metal surface,
>>>which
>>>will in time oxidize (changing the resistance). Laser trimming is
>>>fine for SMALL resistors, removing serious metal from a 200A
>>>shunt is a frightening prospect.
>>>
>>>Make the shunt, anneal and passivate the surface, then use some
>>>other gain trim downstream.
>>
>>We do anneal our shunts. Then dip the relevant parts in a little
>>dilute HCl to clean them up for soldering.
>>
>>John
>
>I find it amusing that manganin is also used to make pressure
>sensors... a nice linear piezoresistive response, and you can get a
>percent or three change in resistance with (large) pressure. I imagine
>you must have to consider the mounting method carefully to get
>ppm-level stability, considering the heat generated and connections
>required.

We glue the planar shunts to anodized aluminum blocks with thermal
epoxy. A surface-mount pcb (amps, dacs) gets stuck to the block too,
and a Minco or similar heater, and we stabilize the whole mess at 60C.
It doesn't appear to me that the TCE differential between the block
and the manganin is affecting the overall TC much.

The big problem turns out to be the eddy currents induced into the
block, messing up the transient response of the shunt. You can see a
tenth or two of a per cent voltage overshoot for a constant-current
pulse, and it decays in a most ugly diffusion-type manner, out into
milliseconds. We learned a lot of wasn't-obvious-to-us stuff here,
some by accident.

You can see awful eddy effects in most heatsunk-type planar resistors,
like some of the Vishay and Caddock parts.

John


From: Dave Platt on 9 Jul 2010 14:16
In article <lkfe36hr1od5sgs7npfig7tbs9894ir2ql(a)4ax.com>,
John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>We glue the planar shunts to anodized aluminum blocks with thermal
>epoxy. A surface-mount pcb (amps, dacs) gets stuck to the block too,
>and a Minco or similar heater, and we stabilize the whole mess at 60C.
>It doesn't appear to me that the TCE differential between the block
>and the manganin is affecting the overall TC much.
>
>The big problem turns out to be the eddy currents induced into the
>block, messing up the transient response of the shunt.

Could you avoid that by using a ceramic block of some sort (perhaps
natural marble) instead of aluminum? The thermal conductivity would
be somewhat lower, but might be sufficient?

--
Dave Platt <dplatt(a)radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
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