2K smt tempco part number

rosch · Joined: Oct 03, 2009 Posts: 164 Location: germany

hi guys.
i've been searching the forum for some info that i know is somewhere around here, but couldn't find where it was.

i've read that some of you used smt tempcos 2K 3500ppm instead of the through hole types.
i'm looking for the mouser or farnell parts numbers of SMT tempcos that work.
any help / tip would be highly appreciated, whether it's a part nbr or maybe you know where the infos were posted here on EM
thanks!

LFLab · Posted: Mon Dec 05, 2011 5:42 am Post subject:

Don't think they go upto 2k, at least not at Mouser. I'm using to 1k ones in series.

Mouser link

Checked, and they go up to 1.8k (and they are 3300ppm).

MadScientist · Joined: Nov 28, 2011 Posts: 63 Location: Denmark

If you have an account with them, then Farnell have them in both +3000 and +3900 ppm. They are:

#1174312: 2K0, +/-5%, 0805 SMD, +3000ppm (PTC).
#1174317: 2K0, +/-5%, 0805 SMD, +3900ppm (PTC).

rosch · Joined: Oct 03, 2009 Posts: 164 Location: germany

thank you!

would this farnell type work too?
it has 2K and a tempco of 3000 ppm/C.

http://de.farnell.com/te-connectivity/lt7330002k0jtg/widerstand-temperatursensor-2k/dp/1174312

is that a huge difference to 3500? or is it just important when i intend to use a VCO with a 8 octaves keyboard and want it absolute stable?

rosch · Joined: Oct 03, 2009 Posts: 164 Location: germany

thanks, mad scientist.
if these are close enough i'll try the farnell smt's.
i guess the 3900 will be better then, as they're closer.

MadScientist · Joined: Nov 28, 2011 Posts: 63 Location: Denmark

Depends on the exact VCO you are building, I suspect. Some designers specify +3300 ppm, and some ask for +3500ppm. In reality I wonder whether it may not matter much which one you select. The tolerances on the PTC coefficients is quite large, above +/- 10% for those thermistors I checked.

I would be interested in hearing about other people's experiences in this area.

Frank.

MadScientist · Joined: Nov 28, 2011 Posts: 63 Location: Denmark

Re-reading the ElectroNotes Supplement #19, it seems clear the correct beta value to aim for is +3300ppm, not 3500.

However, one way of getting around not having the exact value, would be to use another thermistor with a higher beta, and then just run a standard metal film resistor in parallel with it.

Say we want a 2.0K/+3500ppm combination. My pocket calculator claims you can get this *exactly* - or at least to within component tolerances - by using a 2.2k/+4100 ppm thermistor in parallel with a standard 22K metal film resistor. Thermistor goes on the transistor pair as usual, the 22K resistor is placed on the main PCB or where-ever.

Farnell also carries an appropriate part for that: Catalog #1761131. Vishay 1% thermistor, 2.2K, +4100 ppm. A bit more expensive, but that is probably what I will go for.

rosch · Joined: Oct 03, 2009 Posts: 164 Location: germany

that sounds good. and farnell is a much better place for me to order than US based, as long as they have stock in EU.
thank you for your efforts, i'll read through the article also.
i know i've read an explanation by Ian Fritz somewhere here, how to combine different ppm types to obtain certain levels, but i can't find it anymore, must be hidden well in some thread.
next time i'll bookmark such information, or copy and paste it in some text file... thanks again

LFLab · Posted: Tue Dec 06, 2011 10:42 am Post subject:

http://home.comcast.net/~ijfritz/sy_cir9.htm

LektroiD · Posted: Tue Dec 06, 2011 11:01 pm Post subject: