Date: Tue Dec 23 12:25:01 2008

Author: chuck britton

Subject: Re: Pedagogical Resistors

Post:


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On Dec 23, 2008, at Dec 23(Tue) 11:59 , Bill Norwood wrote:

> Hi Taplers,
>
> We are in the throes of deciding how to best choose carbon resistors
> for replacing old ones on our 4-resistor boards (20, 30, 40 & 50 =20
> ohms, 5 watts).

My quick rough estimate sez you better not use much more =
than 6 =20
volts or you'll 'abuse' the parallel combo of these.
>
> These have the gold band for 5% tolerance, but some are off by 10%, =20=

> 15% or more.
>
> We know that these resistors have been overheated sometimes.
>
> These resistors measure 11/16 inch long and 5/16 inch in diameter
> (in case I am getting the wattage wrong).
>
> Questions:
> 1. Does overheating cause a resistor=92s value to increase =
permanently?

> 2. Does overheating cause a resistor=92s value to become erratic?

Certainly could for ANY semiconductor (which carbon is)

> 3. Does aging (apart from abuse) affect resistors?

Wouldn't think so.

> 4. Does the use of 1% or 2% tolerance resistors significantly =20
> reduce student confusion?

If they STAY low tolerance - it should help.

> 5. Are carbon resistors best for student lab use?

NO - why not use metal film???? The MODERN way to go. MUCH more =
stable.
I'm a bit surprised that you could FIND 1 or 2 % tolerance in =
Carbon =20
R's.

> 6. What is a good resistor source?

Can't beat DigiKey for selection and service.
You might beat their price at a surplus place but from DigiKey =
you =20
know what you're getting.
That's where the Pro's shop.=

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My quick rough estimate =
sez you better not use much more than 6 volts or you'll 'abuse' the =
parallel combo of these. Certainly could for ANY =
semiconductor (which carbon is) Wouldn't think =
so. If they STAY low =
tolerance - it should help. NO - why not use metal =
film???? =
The MODERN way to go. MUCH more stable. I'm a bit =
surprised that you could FIND 1 or 2 % tolerance in Carbon =
R's. Can't beat DigiKey for selection =
and service. You might beat their price at a =
surplus place but from DigiKey you know what you're =
getting. That's where the Pro's =
shop.=

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