Glue Gun Conundrum

[Diver300]

I checked the resistance of my glue gun as it was cooling down.

It was about 300 Ω, and as it cooled, the resistance increased. It is now virtually cold and it is about 850 Ω.

It seems odd, as I would have expected the resistance to decrease as it cooled.

I then measured the current. It took about 200 mA, falling as it heated up to about 125 mA. It would have gone lower but I didn't wait all that long.

The mains is 240 V and the glue gun is rated at 40 W. It takes about 300 W very briefly on turn-on, then wanders down to around 40 W

I can't work out why the resistance is falling as the glue gun heats up, or what stops it taking loads of current.

Has anyone got any ideas?

 

[crutschow]

Perhaps there's a PTC thermistor in series with the heating element to help control the temperature. As the element heats the thermistor resistance increases and reduces the current.

 

[Sceadwian]

Crut is probably right, hot glue guns aren't like soldering irons their temperature can't be allowed to float arbitrarily high or the glue would get burning hot, a PTC thermistor would fit the bill perfectly.

 

[Diver300]

I understand that a PTC would work as a heating element, but why does the resistance increase as it cools down?

 

[Sceadwian]

No, you don't understand, the PTC isn't the heating element, the PTC is physically in line with the heating element and coupled to it mechanically.

When it's cold it allows a lot of current to flow, your observed surge current. When it heats up it begins to limit thermally. The bulk resistance you're measuring as it cools under no power is a combination of the the element and the PTC, and possibly a thermocouple voltage, it is different than the true impedance under load.

 

[Diver300]

I did a bit of searching and found this:-

**broken link removed** (page 3)

It seems that the PTC thermistors used as heaters have a very fast rise in resistance at the operating point, but the resistance falls slightly as the temperature increases at lower temperatures. There is a temperature where the resistance is a minimum.

The glue gun runs above the minimum resistance temperature, but I was measuring below that temperature.

I had imagined that the thermistor characteristics would be monotonic.

 

[Sceadwian]

I can't think of any real world device that truly fits the description monotonic, it's a theoretical state, never in practice.

 

[Diver300]

Monotonic just means that the slope is always the same sign.

Most thermistors used for temperature sensing are monotonic. After all, they are called PTC (positive temperature coefficient) or NTC (negative temperature coefficient) which implies that the coefficients don't change from positive to negative.

I realise that thermistors are never entirely linear.

 

[crutschow]

I understand that a PTC would work as a heating element, but why does the resistance increase as it cools down?

That is a good question. I was thinking the other way around. A PTC would indeed cause a reduction in resistance at cold, not an increase.

So I don't have a good explanation for the increase in resistance at cold that you observed.

 

[Sceadwian]

NTC element, PTC current limiter coupled in series.

 

[pacovsplinter]

Maybe it has NTC thermistor in series with heater. That would explain it. When heater is cold you nedd to limit the current what is exactly what NTC does. PTC explanation doesnt have any sense. NTC when is cold have very high resistence.

 

[Diver300]

I has answered my own question (post no6, about 3 1/2 years ago!) and I've just updated the link as that had moved.

The resistance of heating thermistors rises very rapidly at the desired temperature. If they didn't do that, there would have to be a separate control mechanism.

What I found, and is confirmed by the EPCOS data sheet, is that the resistance falls slightly as the temperature increases from room temperature to just below the desired temperature. The glue gun cooled in the time it took me to unplug it and measure it, so it was well below the desired temperature, so the resistance had fallen to near the minimum. As it cooled further, the resistance increased slightly, but far less than the increase that must have occurred at the desired temperature.

It is possible that the heating thermistors are made with a combination of PTC and NTC parts, but they are sold and used as a single component.

 

[dr pepper]

This is an old thread.

Anyway I have two glue guns, the one I have took apart to replace the power lead had a little pcb inside, this would mess u p resistance readings.