R as a function of temp in NTC material / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

R as a function of temp in NTC material

previous 1 2 3 next

Move Thread

LAN_403

kell

Sat Oct 28 2006, 12:39AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

I found a mistake in the circuit. It makes of muck of current sensing because the mosfet turns off and no current goes through the sense resistor. In other words, the circuit would work only if allowed to operate in linear fashion -- without the feedback. But that's not an option. I need to think about this some more to see if there's some way I can make this circuit work.

Steve Conner

Sun Oct 29 2006, 09:11PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Kell: Sure thing, but remember I could be talking crap about using the pipe as the heating element, for the reason Sulaiman mentioned, amongst others.

kell

Mon Oct 30 2006, 03:43PM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

I contacted DBK to see about getting a decent liquid flow heater

they have one for twelve volts that they have apparently sold to people for the use I am considering. The guy I talked to said the regulating temperature of the PTC heating element in the heater is 180 C. Isn't that kind of high for something that's going to be heating the fluid medium to about 80 C? I could be wrong, it just sounds like the heater would have to be extremely inefficient to need that much heat. But probably people just don't insulate it, so the exposed backside of the aluminum-encased heating element just throws half the heat away.
These will be tubes with about 3/8" I.D. and probably less than a foot long, if DBK's are anything like the "Vegtherm" I already have. The heating element on the vegtherm is in a flat rectangular aluminum casing welded to the outside of the tube, so a lot of it is exposed.

Edit -- Here's the email I got from Steve Smith at DBK:
I found one in our inventory. I also found a specification sheet on it and I was mistaken
about the surface temperature it holds to around 165F with no oil. With an inlet water
temperature of 27C the heater produces (stabilized) 220 watts at 3l/hr flow rate. The
inrush current is about 25 amps. If the flow rate is higher or the ambient temperature is
lower, then the stabilized power will be at a higher level, the in-rush will be the same;
only the duration is lengthened at the higher flow rates and or colder ambient
temperatures.

By the way, that circuit ought to work with the addition of a capacitor from the drain to the negative input of the op-amp.

Steve Conner

Tue Oct 31 2006, 09:56AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I just realised, a NTC heater driven from a constant current source will have the same self-regulating behaviour as a PTC heater driven from a constant voltage source. So if you could make a switched-mode constant current driver, I guess you could use your original vegtherm heater fine.

benbradley

Tue Oct 31 2006, 04:35PM

Registered Member #312 Joined: Mon Mar 13 2006, 01:50AM
Location: Georgia, USA
Posts: 27

I'm reminded of a couple of science-related sayings...

So I think the fact that the hyperboloid fit so well is a coincidence of sorts.

"Correlation does not imply causation."

But it points to a weakness we humans have in looking for theories to fit the facts we have.
It was such a good fit, I thought it must mean something.

I read somewhere (this was back on the Fidonet science echo, but it's so memorable and elegant I suspect the person who posted it was quoting some semi-famous person rather than having though it up himself):
"There are two mistakes [related to scientific investigation] people make: seeing a pattern that does not actually exist, and missing a pattern that does exist." Has anyone heard that before/know who may have first said it?

But so much for philosophy. What I'm really wondering about is this "ceramic" that conducts electricity and is NTC. The ceramics I've known of are all pretty good electrical insulators, so I wonder what's different about this. Does it have a wire or other conductor embedded in it? Or was a conducting substance (such as carbon, which by itself, in at least one molecular form, has a NTC) mixed in with it when it was formed?
Could this be a large power resistor? I've got a few of these, about 1 inch diameter, 6 inches long hollow tube, made of ceramic with the resistance wire embedded within.

Bored Chemist

Tue Oct 31 2006, 05:55PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

"I just realised, a NTC heater driven from a constant current source will have the same self-regulating behaviour as a PTC heater driven from a constant voltage source."
I thought about a constant current source but I figured that it would have to deliver that current from the battery so it would waste a lot of power- I didn't think about switch mode (I must be getting old).

kell

Wed Nov 01 2006, 04:25PM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

I made a boo-boo and in the process learned something about measuring resistance.

I wanted to know the resistance of that thing at high temperature so I applied heat to it from an external source. To measure resistance I passed a constant 100mA through it and recorded the voltage. No matter how much heat I applied to the thing (I was using a propane torch), the resistance measurement just kept going down.

But as it turns out, a measurement such as I made does not reflect the reality of what happens when the element develops its own heat as a result of a current passing through it. The heating will element show a much higher resistance to high heating current than small measuring current at the same temperature.

The heating element does in fact have a point somewhere above 212 F that the curve changes and goes positive.

Steve Conner

Wed Nov 01 2006, 09:36PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, at least you had some fun pissing off the guys on the alternative fuel board.

benbradley

Thu Nov 02 2006, 07:55AM

Registered Member #312 Joined: Mon Mar 13 2006, 01:50AM
Location: Georgia, USA
Posts: 27

The heating will element show a much higher resistance to high heating current than small measuring current at the same temperature.

Then the heating element is nonlinear (and not just because of temperature changes)? A higher heating current will of course cause a larger voltage, but you're saying the voltage is disproportionately higher than a lower current at the same temperature? Presuming this is true, it makes things a bit more complicated.

I'd still like to know the composition of whatever carries the current through the heating element.

kell

Fri Nov 03 2006, 05:41PM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

Yesterday I took a steel spring and tested it at room temperature. Drove 100mA through it using a constant current source and read .06 volts.
Then I put it on a twelve volt power supply with an ammeter. Got 12 amps. The power supply didn't droop.
This increase from .6 to 1 ohm was not because of heating. If that had been the case, the steel spring would have conducted 20 amps at first and then the current would have fallen as it heated, but that's not what happened. The current went to 12 amps immediately and stayed there.
So resistance varies with current, in some materials. Maybe even this is true of most ordinary materials. The resistors in circuits are made of engineered materials. Maybe this is necessary primarily to achieve linearity at relatively high currents, as they do.

previous 1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob