Question on PTCs

bugbrand

Its not so easy or cheap to find suitable PTC resistors for VCO temp compensation etc, so I wondered if anyone could help identify whether these may be useful:::????::::

1k PTC axial

And another similar one

fonik

ickystay · Joined: Nov 15, 2006 Posts: 142 Location: Oregon

Those look nice!

This guy uses Farnell smt tempcos and transistor pairs in his quad vco:

http://hem.bredband.net/bersyn/VCO/vco_quad.htm

(scroll down a bit).

I checked into ordering from the US, and it was going to be some freaking astronomical shipping for what amounted to an ounce of parts. Ditched that idea post haste.

etaoin · Joined: Jun 30, 2005 Posts: 761 Location: Utrecht, NL

bugbrand · Posted: Tue Feb 27, 2007 3:17 am Post subject:

Yeah, I think you can make up a trading name / company if you want to register. I don't use them too much, but there's some useful bits.. I find searching for anything pretty tricky 'cos they stock so many items - got the catalogues and they're meaty!

Lets discuss some more about these PTCs - if they're deemed to be useful then I'd be able to order a bunch next time I order from Farnell and then ship 'em out wherever for very little..

But first I want INFO! -- I'm sure there was a webpage somewhere that really explained about exchanging different ptc values -- for example, if I want to use them in the XRVCO should I series two of them or just use one and halve the input resistors?

Thanks guys

fonik · Posted: Tue Feb 27, 2007 4:03 am Post subject:

bugbrand · Posted: Tue Feb 27, 2007 4:07 am Post subject:

Ahha - yes he's got 49.9k input resistors and a 1k tempco in this VCO -- ahhhh, Ray's always the source!

fonik · Posted: Tue Feb 27, 2007 5:09 am Post subject:

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

One thing I don't get - from Renee Schmitz' site:

para · Joined: Oct 16, 2006 Posts: 276

if you don't mind me jumping in here Tom,

i've been meaning to ask if there is a difference between the $8 thermistor and the NTC which runs like $0.35, that i'm not seeing?

the $8 tempco's run at : -40°C to 125°C / which is : -104°F to 257°F (i think i did that right?)

and NTC's run at : 55°C to 125°C / which is : 131°F to 257°F (that too?)

obviously the tempco has a much bigger range but i can only image the NTC running hotter then room temp anyway, so i would think 131°F and up is completely reasonable, even expected.

if they are both meant to hold a steady resistance at excessive temperatures then why are they not interchangeable?

steven

Dan Lavin · Posted: Wed Feb 28, 2007 11:05 am Post subject:

para:

'tempcos' and 'NTC's" are 2 forms of thermistors = thermally sensitive resistors. They don't 'run' at temperatures (unless you get a switching type PTC thermistor); they vary their resistance with temperature.

First, there is really no such thing as a 'tempco'. It is really a PTC thermistor (positive temperature coefficient) thermistor. If you use the term "tempco' outside of the synth DIY circle, engineers and techs will look at you like you've got a third eye. 'Tempco' came from the original SSM music IC application data sheets where the schematics appreviated a device with a 3300ppm/degreeC positive temperature coefficient device as a "+3300ppm/C tempco" device. We've been living with that term ever since.

Next, the temp ranges mentions are the operating ambient ranges for those devices. I think the NTC should be minus 55C (-55C) not +55C.

NTC's can be used in place of PTC's but you have to take into account that NTC's have a logarithmic R-T curve (resistance vs. Temperature) and the 'tempco' has a linear R-T curve. Also, NTC's typically change -30,000 to -50,000ppm vs 3000 to 3500ppm for 'tempcos'. Generally you can linearize and knock down the R-T slope with a series and parallel fixed resistor. And of course the circuit location will need to be changed. For instance, if you had a voltage divider with a PTC on top and a fixed resistor on the bottom, you would need to replace the PTC with a fixed resistor and the original fixed resistor with the NTC network described above.

Well I hope this doesn't add to the confusion

Dan Lavin · Posted: Wed Feb 28, 2007 11:13 am Post subject:

para · Joined: Oct 16, 2006 Posts: 276

actually that makes it more complicated then i was expecting. i was under the impression that its job was to keep a steady resistance under varying temps, but you say that its job is to intentionally change a specified amount based the temp it is "told" to run at or however you want to say it ????????

hmmm

and the datasheet for the NTC i read didn't indicate - minus 55C ?? it doesn’t matter i’m not going to be able to figure out how to get it to react properly in things like the XRVCO's i have Bugs printing me or anything else anytime soon. its a bit over my head to honest, but i'll keep beating my head against it and one day it will make sense, just like everything else.

thanks ant!

steven

Dan Lavin · Posted: Wed Feb 28, 2007 1:16 pm Post subject:

Fernando

Fonik, I'd like to build some Rene's 4069 VCOs too (oops, assuming you meant this one!)

It seems the 4069 gets hot. Maybe a heatsink glued to the chip will help?

I may add some simple extras to it:
Saw phase shifter
http://home.swipnet.se/cfmd/synths/schematics/
Serge waveshaper
http://members.tripod.com/urekarm/synth/sergews.pdf

and a sync input:
http://www.aleph.co.jp/%7Etakeda/radio/farmVCO4069E.html
http://www.aleph.co.jp/~takeda/radio/img/4069VCO-scheme.gif
(sync input in upper right corner)

fonik

mosc · Posted: Fri Mar 02, 2007 4:02 pm Post subject:

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Sorry to quote myself, but isn't there a problem here: