Tempco Resistor Question

[delayed]

Hi,
Will this part below work to replace the PT094 2K tempco in the MFOS VCO or other projects that require a tempco resistor?

This is a 1K so would two work out?

Thanks,


1k PTC from mouser LT1/4LT52R102J3900


http://www.mouser.com/ProductDetail/KOA-Speer/LT1-4LT52R102J3900/?qs=bQqEcw2qv2QjD1HR3vMEzw%3d%3d

Mouser Part #: 660-LT1/4LT52R102J39
Manufacturer Part #: LT1/4LT52R102J3900
Manufacturer: KOA Speer
Description: Thermistors - PTC 1K OHM 5%

[magman]

A pair of these resistors in series would be better than a 2K Metal Film resistor, the only note being that these appear to be 3900ppm, rather than the 3500ppm specified.

An alternate source of 2k 3500ppm tempco's though is Magic Smoke. They have them listed on their website here:

http://www.magsmoke.com/tempco-resistors.asp

But there was also a recent thread in the Thomas Henry Designs sub forum where Tim Servo advised that they are now available at about half these prices:

http://electro-music.com/forum/topic-19122-125.html (scroll towards the bottom of these posts)

Good hunting.

Regards

Magman

[delayed]

Thank you for the reply and the link.

What does the "3900ppm, rather than the 3500ppm specified" number mean? Or how would it change the circuit? I have not ever used one of these and don't know the difference.

Thanks again,

[magman]

I think the ppm is parts per million and is the degree of temperature change per degree Celsius.

In other words, for a 3500ppm part, the resistance will change by 0.35% per degree C change in temperature.

With normal resistors you want a small temperature drift (normally less than 200ppm), but the tempco is used to adjust a value to keep an oscillators frequency constant as the temperature changes (such as on stage or even as the temperature changes in your house), so you want a much bigger change in resistance per degree C change.

Here's a useful link from Elby Designs which explains how they adjust a 3500ppm tempco to fit a 3300ppm requirement for an ASM1/2 oscillator:

http://www.elby-designs.com/datasheets/tempco.pdf

The 3900ppm part you originally linked to would probably have to have a similar adjustment process to work in the MFOS application as designed.

Hope that helps.

Magman

[delayed]

That helps a lot. Thank you for explaining that.

[Tim Servo]

We have those in stock, and yes, we have them from a new supplier at a lower price.
1 for $1.80, 2 to 5 for $1.50 ea, 6 or more $1.20 ea
shipping = $1.50 US/Canada, $3.50 EU/UK/Aus

Tim (shipping stuff this weekend) Servo

[inlifeindeath]

what is your company name and how can i buy these through mailorder? Thanks

[Scott Stites]

http://www.magsmoke.com/magsmoke.asp

[Tim Servo]

Whoops, thanks for helping with that Scott. I made the mistake of assuming that inlife already knew about Magic Smoke. My bad.

You can go to www.magsmoke.com and look around, or just send us an email to magsmoke@gmail.com and I'll send you a copy of the latest price list. For payment, we take PayPal to magsmoke@gmail.com

Thanks!

Tim (packing and shipping) Servo

[RobotDad]

So looking around on mouser and a few other places I don't see ppm clearly specified. What are the other common ratings that ppm can be derived from? For example, this one:
http://www.mouser.com/ProductDetail/Honeywell/135-202FAG-J01/?qs=sGAEpiMZZMv1TUPJeFpwbuXgoFDGQ8gkuYNqvRcfgJk%3d

I see a rating in the datasheet for a beta of 3468. Is that some alternate description of the ppm rating?

[frijitz]

[RobotDad]

Ah, thanks. Looking up the right type of part seems key.

[frijitz]

[darkomen]

Going off of the link that magman posted, I just want to run this by to see if I did the math correctly.

If I take the 1k ohm 3900 ppm/C resistor and run it in series with a 340R resistor that's 100 ppm/C (both at 25 degress C), that would give me 3347.8 ppm/C, correct?
Also, if I wanted a 2K ohm tempco resistor and took 2 of the 1K in series would the 340 ohm resistor still be sufficient or do I need to recalculate?

[frijitz]

[darkomen]

Thanks for checking this for me (I realize that I actually calculated it for the 2k not the 1k, thus getting the 340 and not the 170 in my post). And thanks for the info, guess I'm better off just buying tempcos at 3500 ppm/C instead of fudging it.
Also you mentioned that you had some tempcos that you were trying to compensate for, how did you go about testing them to find the ideal resistor for compensating the tempco. Did you heat them for a little and check the drift on the resistance?

[frijitz]

[adambee7]

Thanks Ian for the article. that has taught me more about temps than a book ever could. Would a heating pad used to heat tanks for reptiles be good enough?

[frijitz]

[adambee7]

[Starspawn]

Looking for 1K 3500ppm, whats my best bet?