burned out element detection

How automated does it have to be?

- One CT* and one LED per element (each LED should light whenever the related controller is calling for power).

- One CT and one LED with 6 wires running through the core (3 in one "direction" and 3 in the other) .. if the controller is calling for power and any one element is burned out then the LED lights. ;-)

Obviously you could make a more automated detector by using LEDs in optocouplers rather than visible LEDs to detect the currents and using optocouplers to detect the AC voltage. A little logic or a microcontroller would do the rest. You can't detect a bad heater until the controller calls for heat with this approach.

Another approach is to pass a small current through the heaters in the 'off' state, but it's hard to detect bad heaters when others are in parallel.

• CT = Current Transformer

Not "automated" at all. A light that comes on to indicate when an element is burned out is enough.

And there lies the problem. If this were a DC circuit, it would be a LOT easier because I could isolate each element with a diode and do a simple current/no current measurement.

Thanks for the reply.

If you need remote sensing.

Add a neon lamp across each element and add a photo-diode to each neon lamp.

A PIC can measure the current in the photo-diode and give a message about its status.

don

Put a current sensing transformer on each line to each element. Then build you human and or control interface.

Paul P.

This is a difficult problem to do right if you approach it wrong.

For example, suppose it has 1.2kW then this gives 10A of current and a resistance of 12Ohm. Adding 1Ohm resistor for "current" measurement would dissipate ~= 90W. One could drop down to 0.1Ohm for 10W but the voltage drop across this resistor is 1V. This is not too bad but requires some other discrete components such as a comparator or adc which requires a smaller supply.

You could try an active resistive method such as using a mosfet on the low side that is generally fully on(similarly how the ss relay is used to turn the element on/off) but for short periods of time you could increase the resistance and check for a voltage. Because the mosfet can have subohm resistance it will generally have a very low impact on the total power dissipation(

Don't miss Spehro's point, he's suggesting putting a current transformer in the circuit. When ac current flows it develops an ac voltage on the output, you can use that to drive an LED.

"- One CT* and one LED per element (each LED should light whenever the related controller is calling for power). "

I like this idea, (3 in one"direction" and 3 in the other), but I need to think out the phase situation. But it is a neat idea.

" One CT and one LED with 6 wires running through the core (3 in one "direction" and 3 in the other) .. if the controller is calling for power and any one element is burned out then the LED lights. ;-) )

• current transformer-- look it up.

Mike

This won't have any effect. The line voltage is supposedly stable, regardless of the load. The bulb will be lit all the time.

The obvious answer is to use current transformers.

"Use a PIC" posts are not allowed here without a full schematic and source code included. ;-)

Cheers! Rich

OK, then. A current transformer, detector, inverter, and indicator.

How about a light indicating "operating"? That could be as simple as a light bulb (incandescent or LED - maybe even neon, with enough turns in the CT) that will be on when current is flowing. An incandescent can serve as the burden resistor, - nah, if the bulb blows out, the volts will hit the moon. Maybe a parallel burden resistor...

Have Fun! Rich

On a sunny day (Tue, 3 Nov 2009 11:12:15 -0800 (PST)) it happened " snipped-for-privacy@gmail.com" wrote in :

So use 2 diodes in anti-parallel! And a PIC of course ;-)

I see his point, and agree. This is probably the easiest and most straightforward way to do this. The pieces are in the range of $10 each though.

to

This might help keep costs down for the 6-element heater, *if* it will work this way...

It tells me if AN element in the 6-element assembly is burned out, but not *which* element. Still, it saves parts/cost.

Thanks for the help everyone. I didn't think this would be an easy problem.

Add solid state relays to drive each of the elements that are now in parallel. Drive them with the line that now controls the group. Then you can use your neon lamps with some photo transistors and simple logic to sound an alarm.

How hot does it run? You might be able to find a thermochromic paint that would show when a heater fails if automation isn't needed.

Otherwise inductive current measuring clamps around the leads is probably the least invasive way to measure if current is flowing.

Regards, Martin Brown

Another low cost way to do this might be to put a very low ohm resistor in series with each element, then use the voltage sensed by this to turn on a transistor with an led in the collector circuit. If you put a resistor to base, the transistor turns on on half cycles and you only need to drop 0.6-0.8 volt or so to drive it.

I've seen circuits like this where the element itself is tapped to sense the voltage, but that might be too non standard...

Regards,

Chris

Ya, I agree CTs can be expensive, What is the current your heating elements use?

Mike

I had the same situation. CTs, as others have mentioned (with a burden resistor, a current limiting resistor and a LED for each CT) were the correct solution. A glance tells you instantly if all elements are heating, or, if not, which element(s) is/are not heating, whether due to an open element, blown fuse, failed contactor, whatever. Mine lit the LEDs when working properly, so it required no power source - the CT's powered the LEDs. Yours will require power to run the inverter circuits, if you must have led on = failure. Simpler to have led on = working.

Ed

I'm sorry Martin, I giggled when I read this, I was thinking of the liquid crystal-type indicatoes/paints. The bottom end of the reactor runs at between 600 and 700 degrees C. Thermochromic paint might won't work, but some of the other stuff used in ceramics might. We had a prof in the ceramics dept make some spacer plates for us and she wasn't too concerned about the max temps.

Thanks for the suggestion.

The smallest heating tape runs about 10.45A, the largest about 14A; the cartridges (set of 6) run about 5A each. You'd think the controller manufacturer would build in (or offer as an option) element burnout detection. They DO sense thermocouple breaks.

I think I could make the current transformers fairly easily, but it's a question ov whether it's cheaper to buy or pay my salary to make them (build vs buy). A long time ago (about 20 years), I used a circuit that had a minimal amount of parts to make a low voltage/ current from 120 VAC. It didn't use a transformer, but used (I think) a cap (or tow), and a diode (or two). Mostly used as a battery charging circuit. I was thinking I could use something like this, a current transformer, and a simple transistor NAND circuit to drive an LED when the element burns out. Gotta do some digging through my old notes (and my old mind...) and see if I can come up with that circuit.

Thanks again everyone. You've helped point me in the right direction!

I hate to re-post to replies, but I think I have an idea that might work. I can use a current transformer to drive a low-power relay, possibly a reed relay. A neon lamp in parallel to the heating element would go through the NC relay. When current flows through the element, current/voltage is produced in the current transformer, activating the relay, and keeping the light from coming on. If the element burns out, the relay stays closed and the neon lamp will light. Any problems w/this?