Dead heating element.

[rayband]

I have a 36 VDC heated stage that is dead. The input power checks OK, but the heating element itself shows dead short, 0 ohms resistance. An open circuit I could understand, but not a dead short. The leads & connections look OK, no obvious damage. What's going on here?

RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM

 

[Skogsgurra]

Is it 0.000 ohms? Or just close to zero ohms?

A heating element with 1000 W would have 0.77 ohms when hot and around 0.6 ohms when cold.

So, is it really a dead short or just close to zero?

Gunnar Englund

http://www.gke.org

 

[IRstuff]

Note that the probe contact resistance can be on the order of 0.5 ohm.

TTFN

 

[VEBill]

"36 VDC heated stage"

Part of a microscope? If so then it wouldn't be 1000 watts.

As far as how it could be shorted? There's always a rational explanation but it might be difficult to determine via the Internet.

 

[analogkid2digitalman]

VE1BLL:

Actually the other day I came across a heated stage accessory for a microscope that would heat samples up to 1500C (yes C)to study phase transitions and crystal structure changes. That probably consumes 100's of W to kW, I wish I could find the link again because it caught my eye- learn something new everyday.

rayband:
Have you checked if the any connections or the element is shorted to ground and thus possibly shunting power away from the element?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
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[itsmoked]

analogkid2digitalman; The question is why would you want to look through a microscope at something at 1500C?!?! Ouch.

rayband; More info? Microscope stage? Do you know what wattage your stage is supposed to be?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[VEBill]

I assumed that "36VDC" was a clue that it probably isn't very high power (xx watts max). If it required 480VAC 3-phase then I'd assume the opposite.

There are DVOMs around that use "0" to indicate Open Circuit and 0.0 to indicate zero ohms. Ya never know.

 

[rayband]

Thanks to all. The plate/element is removed from the unit, which is a Genisys 3D Printer used in foundry modelling, is removed from the unit, so is not connected to ground. The Omega 882 Digital Multimeter I use has a lowest scale of 2 k-ohms and reads .0005-6 when the leads are directly shorted and the same when the element is measured. However, after reading your replys and digging harder, I finally found the markings which were 36 VDC and 250 watts, so the resistance would most likely be below the meter's sensitivity (I haven't calculated it yet).

This is an old unit we (metallurgical engineers, not EE's) are trying to rebuild. It works by 'printing' a layer of plastic (sort of like hot melt glue) in the x-y plane, then indexing up the z plane and repeating until a solid model has been 'printed'. The heated platen's purpose is simply to keep the model under 'construction' stuck to the platen during the process.

Again, thanks to all.

Ray A.



RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM

 

[itsmoked]

P = V*V/R
R = V*V/P

R = 36*36/250

Hence R = 5.18 ohms

Your meter should be able to differentiate that over shorting the probes together.

You have an open heater element.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[IRstuff]

Unless it's shorted to the plate

TTFN

 

[VEBill]

Perhaps the wires are shorted together where the outside leads are connected to the element. For some reason, the designers of such things always like to bring the two wires ~very~ close together at that point (to help ensure future failure).

This assumes that the leads are normal wire. Safe bet that the heater element is something like nichrome or similar.

In other words, perhaps the element itself is fine. If you can get inside then it might be easy to repair.

Maybe...

Maybe not...

 

[Iamjonscranium]

itsmoked says you have an open heating element. you said you removed the heating element. put it back and it will probably close the circuit.

unless im not understanding this correctly...

 

[itsmoked]

Iamjonscranium; I got it backwards. I meant he should be able to differentiate between 0 ohms and 5.18 ohms with just about any ohm meter. 0 would be shorted. Which you rarely ever see in a heater failure.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rayband]

Yes, a shorted element is common, but an open element is what puzzled me and why I posted it here. Per skogsgurra's post, I rechecked and it does read about 5.6 ohms and, as stated, the power supply of ~36 V checks OK. But nothing happens when they are connected. The element seems to be like a perforated red rubber mat. There may be nichrome wires embedded, but I can't see them and don't want to probe. Anyway, the element seems OK (not shorted) as does the power supply, but nothing happens when connected. There is a small device, not a resistor, on one of the leads; I don't know what it is.

RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM

 

[VEBill]

The challenge is always the slow leak of important details...

Perhaps the little device is a thermal overload ('too hot') protection device ? If it is in series and open circuit (the device), then perhaps you've found the problem.

 

[IRstuff]

What happens when you connect the power supply to the same place you measure 5.6 ohms?

TTFN

 

[rayband]

Well, we are now back to the 'little device' or the power supply, as I found an old, unused variable DC power supply that still worked in the back of the corrosion lab. I could only get about 10.5 VDC out of it but when connected it drew about 1.5 A. and the element put out heat. So the element is probably OK. Getting closer. Stay tuned.

RAYBAND
Rock Island Arsenal
AMSTA-RIA-SEM

 

[OperaHouse]

Does it look like a metal tube and the size of a 3A diode? These are a chemical thermal fuse/protector commonly used in hair dryers and coffee pots. They are crimped onto the heating element lead to sense when things are going wrong. They can often go bad after years of use for no reason. Radio Shack has always carried them for some reason and are about $2. There is no current spec, but come three temperatures. Should be a three digit number followed by a C for th temperature. Search radioshack.com for 270-1320, 270-1321, and 270-1320 for picture. Be careful how you replace it. A crimp is best, soldering will destroy it. Effective temperature can be increased slightly by increasing lead length.

 

[itsmoked]

http://www.nteinc.com/specs/8000to8999/pdf/TCOs.pdf

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

As you have probably guessed, the last number should be 1322. Any time saved using a word processor is consumed doing corrections.

You just can't beat going down to Radio Shack and getting a part. I designed a jet engine test cell for the Air Force that was PC based. They had tried forcing me to use some old Data Generalmicro. Later in an AF presentation they called it Radio Shack Compatability. I saved that page! I also tried to save thm $51K, that got me in so much trouble. You choose your battles with the military very carefully.