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Re: Please help can't figure this out, been trying for ages

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In some lab I worked they used 10 turn trimpots (Bourns) for everything.
You will possibly also need that for opamp offset compensation etc.

Re: Please help can't figure this out, been trying for ages
On Friday, June 15, 2018 at 12:19:35 AM UTC+10, Tom Del Rosso wrote:
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He ought to. Resistor tolerance has a direct effect on price, and 0.1% resistors, while widely available, are more expensive than their 1% counterparts, and tend to be available in a wider range of values (typically the  
E96 range, where 1% parts tend to be offered in E24 values).

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He probably does. Most people do.

--  
Bill Sloman, Sydney

Re: Please help can't figure this out, been trying for ages
On Thursday, 14 June 2018 15:19:35 UTC+1, Tom Del Rosso  wrote:
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how much accuracy can one get by selecting resistors? When young I used to select from 5%ers to improve accuracy, and while ppms of drift limit what can be done it did get an improvement.

If your resistor divider pairs are thermally coupled, both drift similarly and the division ratio doesn't change as much.  


NT

Re: Please help can't figure this out, been trying for ages
On Friday, June 15, 2018 at 4:02:11 PM UTC+10, snipped-for-privacy@gmail.com wrote:
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It's a mugs game. For some resistors the manufacturers did it, and what you got shipped as 2% resistors didn't include any that could be shipped as 1% resistors, and a surprisingly higher proportion of parts that were right up against the 2% limits.
  
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If you buy thin film resistor arrays, you can get 0.1% resistance tolerance, and tighter - guaranteed - matching on resistance ratio versus temperature. I first took advantage of that in 1979.

--  
Bill Sloman, Sydney

Re: Please help can't figure this out, been trying for ages
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AFAIK, that might've happened back in the day, with carbon comps that were  
terrible to manufacture any more than vague values.

Modern processes laser etch parts to spec individually, so error is  
proportional to production time (more or less), making precision parts  
expensive and loose parts cheap.  This is still mostly swamped by overall  
production costs and especially distribution, but the difference is  
noticeable in quantities of millions.

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Please help can't figure this out, been trying for ages
On Friday, 15 June 2018 08:55:56 UTC+1, Tim Williams  wrote:
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I assume basic carbon films aren't laser trimmed, just rebinned.


NT

Re: Please help can't figure this out, been trying for ages
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True, they might be hand-filed by Chinese child labor... :^)

I haven't seen any studies that showed other than a windowed normal  
distribution for even the cheapest axials and chips.

Even the smallest Chinese factory can afford laser trimming machines,  
they're not at all the industrial enterprise equipment they used to be.  You  
can churn out many thousands of resistors per hour, if you don't mind the  
poor tolerances you get from the limited settling time.

Diode lasers for cutting and engraving are very cheap, as commercial tools  
are concerned.  You can buy one yourself for low kilobux.  Great for rapid  
prototyping in plywood or plastic.

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Please help can't figure this out, been trying for ages
General response here : Maybe it would be best if the thing was adjustable.
 Either ten turn pots or maybe just good design to set the range of the adj
ustments, like not making something that wants 130 volts adjustable to 300  
volts. Design, that's the word.  

But then to the pots drift ? I remember some things, like the HV in a TV be
ing set by cutting resistors, maybe something like that with the proper val
ues.  

The circuit is easy, it hits the voltage and up goes the output, then the n
ext threshold slams it down. If regular BJTs had a really stable Vbe, you c
ould do it with them. In fact the overvoltage sensing could be done that wa
y I think. If the comparator has an open collector output, you just add ano
ther open collector output.

Re: Please help can't figure this out, been trying for ages
On Saturday, June 16, 2018 at 9:32:45 AM UTC+10, snipped-for-privacy@gmail.com wrote:
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e. Either ten turn pots or maybe just good design to set the range of the a
djustments, like not making something that wants 130 volts adjustable to 30
0 volts. Design, that's the word.  
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being set by cutting resistors, maybe something like that with the proper v
alues.  

Pots get twiddled by people who don't know what they are doing, sometimes i
n final test.

One fertile source of modification requests at Cambridge Instruments was fr
om production, who'd want a wider range pot because they couldn't get the o
utput they wanted.

Most of the time this represented them working out their own calibration pr
ocedure, which was quicker than the official routine, but set up up the pot
s in the wrong order.

Once the machine is out in the field pots get twiddled by people who really
 don't know what they are doing - graduate students are a particularly dang
erous group.

Any pot you can design out represents design time well spent.

--  
Bill Sloman, Sydney

Re: Please help can't figure this out, been trying for ages
snipped-for-privacy@gmail.com wrote:

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The idea with trimpots (10 turns) is to make an accurate voltage divider to get close with say 1% resistors,
and then move the output voltage around a bit with the trimpot.
For example:

 +
 |  
[ ] 10
 |
 |----------------->
 |      |
 |     [ ] >100   +
[ ] 5   |         |
 |       ------> [ ] 10
 |                |
///              ///  

Values are relative resistor values, the '100' from the trimpot reduces its effect,
and also its drift effect.


Re: Please help can't figure this out, been trying for ages
On Friday, 15 June 2018 19:25:55 UTC+1, Tim Williams  wrote:
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not adjusting their values at all would make a lot more sense. Whatever value they are they're saleable as is.


NT

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Re: Please help can't figure this out, been trying for ages
Neon John wrote:
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I saw a youtube review of one of the cheap ones.  Plug it in and it  
burns a hole through the table.  The laser comes on at full power and  
doesn't turn off until the CPU boots.

Even I can design a driver that would prevent that, but it made me  
wonder how a truly reliable one would be designed so that one or more  
components could fail and it would never do that.




Re: Please help can't figure this out, been trying for ages
Neon John wrote:
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Maybe the reviewed unit was focused the same way, but it still shouldn't  
power on with the laser turned on.  Of course I didn't mean that yours  
does that.  Only that you have to beware of the cheap ones.




Re: Please help can't figure this out, been trying for ages
Neon John wrote:
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For a cutting laser I don't think they should rely on the CPU at all.  
What if it locks up with the LASER on?  A one-shot could turn it on so  
the CPU would have to trigger it repeatedly to keep it on.  Hardware POR  
could ensure it starts off.




Re: Please help can't figure this out, been trying for ages
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What if it locks up with the LASER on?  A one-shot could turn it on so
the CPU would have to trigger it repeatedly to keep it on.  Hardware POR
could ensure it starts off."

I think I would want an old fashioned frikken toggle switch. Automatic schmautomatic. I got a friend who will not buy a car with an automatic transmission. I like that.  

Re: Please help can't figure this out, been trying for ages
On Sunday, June 17, 2018 at 9:59:00 PM UTC-4, snipped-for-privacy@gmail.com wrote:
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The car I just put a deposit on doesn't have an automatic transmission.  :)  

Rick C.  

Re: Please help can't figure this out, been trying for ages
On Monday, 18 June 2018 05:17:43 UTC+1, snipped-for-privacy@gmail.com  wrote:
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Ditto. I wouldn't get an automatic, I don't know why mericans seem to love them.


NT

Re: Please help can't figure this out, been trying for ages
snipped-for-privacy@gmail.com wrote:
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I have driven both, prefer automatic any time.
I once had a Peugot 404 station,
it had a hand crack in case your battery was empty,
4 sure something to have for the real drivers ;-)

And with electric do we still need gears?
  
But auto-pilot? No.

Re: Please help can't figure this out, been trying for ages
On 06/18/18 07:06, snipped-for-privacy@nospam.org wrote:
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I've always had stick shift cars.  My wife has had about half and half.  
The major reason they're so popular around here is city traffic and many  
hills.

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Yes.  The maximum torque from an electric motor is proportional to its  
volume, so unless you want to always be effectively starting off in top  
gear, you still need variable gear ratios (either a normal transmission  
or a CVT).

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I'll probably want one when I'm 85 and can't drive myself anymore, but  
not sooner.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Please help can't figure this out, been trying for ages
mandag den 18. juni 2018 kl. 15.45.38 UTC+2 skrev Phil Hobbs:
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The tesla only have one gear, there's also some of the teams in formula E that only have one gear  




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