# ksd 301 thermal switch.



## Hippofeet (Nov 26, 2012)

I am trying to use a thermal switch as a ... well, as a thermal switch. I have three identical ksd 301 thermal switches (thermostat) that operate by temperature deformation of a bi-metal disc, with in turn opens a contact switch inside the ceramic body of the switch itself. I have a light in the circuit, to use as an indicator. With the thermal switch bypassed, the heating element heats up quickly, and reaches a temperature hot enough to burn paint, melt wiring heat shrink tubing, and discolor spade connectors. Its hot. With the thermal switches in the circuit, the heating elements never reach a temperature high enough to shut down the circuit, which they should. I should be shutting of the lights at 300 deg. F.

If these were resistance type thermistors, I would understand what is going on. But they aren't. 

Anyone have any idea why these thermal switches would behave like a resistance load in the circuit?

I have gutted one of the thermal switches, it IS definitely a normally closed, contact type thermal switch. This isn't a function of time. Without the thermal switches, the heating elements are getting out of control and popping a 20 amp breaker in about 9 minutes. With the thermal switches, 45 minutes, and no temperature change, but also no opening of the circuit, with accompanying light shut-off.

What the heck is goin' on 'round here?

Any ideas?


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## Hippofeet (Nov 26, 2012)

Is it possible the lights themselves (some kind of photography light, wattage unknown, but rated at 6 amps) are restricting the current in the circuit?


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## Hippofeet (Nov 26, 2012)

Ok, nevermind, I got it. I think. >.>

With the lights out of the circuit, the heating elements are working properly again. I should have used a normal bulb and ceramic socket, but I didn't have any on hand.

The paint is scorching because the housings for the heating elements (from china) are painted with regular spray paint, apparently, while the thermal switches allow 300 deg. F. at the housing. 

Had they used a high temp paint, there would have been less worry. I would like the heating elements to hover around the 300 deg mark (drop down to 250, heat back up to 300) without getting so hot as to shut down the circuit.

I guess I will let the elements heat up for 1/2 hour increments, and check the thermal switches with a meter. A light would have been nice, lol. Im lazy.


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## Hippofeet (Nov 26, 2012)

Well, for anyone following this thread, but not chiming in,

The aluminum block of the heating element melted, poured out of the housing, and ran across the table like lava.

The thermal switches never shut down the circuit. The 20 amp wall breaker never popped. 

Even if the Chinese heating elements are cast of zinc, instead of the aluminum I assume, that is still a melting point over the rated opening of the thermal switch.

Oddly enough, an identical thermal switch to the ones mounted on the heater housings, resting on the housing body itself, DID trip, and showed open on the meter, and then returned to a normally closed state after a short time cooling down.

I cant help but think im missing something here.

The thermal switches should be bi-directional, I mean, what does it care which way current flows?

If there is so much lag between the temperature inside the element, and the thermal switch on the housing, what does that say about Chinese design? Not much of a safety factor if it melts down to a liquid state before the circuit fails. o.0

Clearly, I am missing some vital understanding of how these things work. Time to hit the google.


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## Otaku (Dec 3, 2004)

What are you using to control the power going to the heater(s)? I use heater cartridges with Watlow 930 controllers, and without a closed-loop thermocouple feedback circuit, they'll just put out whatever power setting is programmed into them until the cartridge burns out. Without the TC, they don't know when to stop. Does your controller have PID capability to ramp the power, or is it simple on/off?


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## Hippofeet (Nov 26, 2012)

The controller is simple on/off. Ive got the thermal switches working as they should now (Im letting them heat up, and checking the switches with a multimeter to time the warm up to circuit shut off) and Im trying to time the cooling cycle to just hover under the thermal switches shut off of 180 deg C.

I could do a thermocouple feedback, but Im trying not to first...


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## Hippofeet (Nov 26, 2012)

Well. This turned out really, really, well. I had to play with the air gap from the heater to the thermal switch, to increase the temperature of the heater, and then time the cooling cycle, but its pretty much dialed in. It ran most of the day without a hiccup. Im stoked.


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## Dan The Welder (Jul 18, 2012)

Atleast you weren't talking to yourself. I thought that was a pretty interesting idea


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## Otaku (Dec 3, 2004)

Nice! But what happens when the ambient temp changes, will you need to tweak the air gap?
One thing I used to obtain a reliable and voltage-adjustable control temp when doing _in situ_ TC testing was a T-220 35W resistor. These guys can handle some pretty high temps and not burn out. You should be able to run one in series with your heater and have it act as an active, adjustable controller. Since they're a relatively large flat package you can use them for direct surface contact and they're consistent to Ts ± 2 °F. I tried the air gap solution but even the slightest air movement would jog the temp wildly, like a 5 °F drop in < 1 second. And if the lab was a little cooler that day I needed to change the gap distance.

Caveat: The TC I was monitoring was a 38 ga. Type T, pretty small and susceptible to temp swings if you just looked at it sideways.


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## Hippofeet (Nov 26, 2012)

The ambient air WILL affect the heater temp, you are correct. I will try the resistor, I can have direct surface contact easy. 

I had thought that the heaters, as long as they are under the shutoff temp, would pull more amps to maintain a fairly flat temp. I also am looking at some adjustable thermal switches, but im having trouble finding the disc type in the 300 to 400 deg Fahrenheit range. They seem to max out around 300, not quite what im looking for, but workable.

As it sits, bodged together on the bench, im running the heaters at 350-ish, with a 1/4 inch air gap on 180 deg (Celsius) thermal switches, dropped into a metal housing, insulated heater, but open air from heater block to the thermal switch.

I ran 3 hours without going over-temp, or under, so im thrilled. As you mentioned, I am thinking about the ambient air, 50 degrees would make a big difference in the timing of the cool down cycle, extending the heating cycle of one side by I don't know how much. Hmmm. Could I do a variable resistance? And then relate that to a marked dial, to offset the ambient air temp? Or will the 35w resistors, done correctly, do that themselves?

So if it dropped 30 degrees, I could turn up the shut-off temp accordingly?

Dan, I think more techy folks would jump in, if I spoke the language better. As it is, I am clearly barely semi-literate in electronics, and so it is hard for me to convey just what I mean. It takes a person who can speak a little idiotese.

Thanks, Otaku.


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## BIGANT (Jul 13, 2010)

To chime in about the light bulb thing, if it was a bulb with a filament then yes it will act a resistor. Its resistance will grow dramatically as the element gets hotter which probably explains why the thermal switches never worked right


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## Hippofeet (Nov 26, 2012)

Ah. I had hoped to use some lights in the circuit, but I ended just pulling leads, and checking across the thermal switch with a meter. It works, its just more work. I suppose I could use a relay somehow, maybe an audible alarm when the circuit shuts down. Anyway, Im getting an IR thermometer, which will help me out a lot.


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## BIGANT (Jul 13, 2010)

Hippofeet said:


> Ah. I had hoped to use some lights in the circuit, but I ended just pulling leads, and checking across the thermal switch with a meter. It works, its just more work. I suppose I could use a relay somehow, maybe an audible alarm when the circuit shuts down. Anyway, Im getting an IR thermometer, which will help me out a lot.


IR thermometers are great!


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