indicator lights

[v6a1a4]

A friend of mine asked me to wire up a portable hot water tank, easy-peasy. He also asked me to wire in indicator lights that would light when all was well with the heating elements and would go off when the elements burnt out. I want to stay away from sensing circuitry and am hoping for a very simple solution. Obviously a 120v lamp in parallel won't work, what about a small (12v) lamp in series? I'm thinking the current drawn by the heating element will blow the lamp. What about neon lamps? Any thoughts?
This smells of a very simple problem but it's just not coming to me.

Cheers, James Vancouver BC

[jimandy]

Will the elements be cycling on and off from a thermostat?

[Edd]

On the simplest side , how about a NE-2 lamp(s*) with its / their series current limiting resistor, both extended wires lead then safety covered with fiberglass woven “spaghetti” tubing with the leads terminated across the 2 screw terminals of the heater element.
(* The plurality being in reference to the duality of top and bottom heating elements on larger units, if being a small unit , only one element suffices)
The lamps side profile is then made visible thru a housing aperture.
Heater element OK:…….…no visible indication.
Heater element Open……...then the decreased temperature will have engaged the thermostat into its on state and the only thing in circuit will be the constantly on neon lamp; approximate thermal recovery time……...ad infinitum.

73's de Edd
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[v6a1a4]

yes, the elements cycle with the thermostat as with all h2o heaters.
p.s., having said that i'm sure there is some that do not:)

[rshayes]

I would consider using a current transformer with an indicator light. A pilot lamp, such as the #47, is rated at 6 volts and 150 milliamps. With a 1:100 ratio current transformer a primary current of 15 amps would light the bulb to full brilliance. The primary voltage drop would be about 60 millivolts.

A common method of construction for a current transformer is to wind the secondary turns on a toroid form, usually a tape wound core. A large hole is left in the center, and the primary consists of a single turn of wire passing through the center. This has the advantage that the insulation does not have to be removed from the primary wire.

The core does not have to be a toroid. An E-I core could also be used with one turn of wire looped around the center leg as the primary.

A current transformer should not be operated with an open circuit in the secondary circuit. The voltage can easily rise to levels sufficient to break down the transformer insulation. Since incandescent lamps tend to fail as an open circuit, I would place a pair of zener diodes in a back-to-back connection across the indicator lamp. This would limit the voltage if the lamp burns out.

<small>[ January 14, 2006, 08:37 AM: Message edited by: stephen ]</small>

[jimandy]

I might be missing something, but on Stephen's idea, I'm thinking you would not know if the element had gone bad if the t'stat was on but the element burned out (no current flow) because you would have the same indication (no current) when the thermostat was off when the water is at temperature. Edd's idea seems to have merit if, when the t'stat was on, the neon light was dark because the voltage drop across the element was not enough to fire the lamp. (At least I think that's the idea).

My question regarding the cycling on/off had to do with the notion that there may be some heaters that don't cycle but have a current varying circuit driven by a thermistor, although I don't know of such.

<small>[ January 14, 2006, 12:23 PM: Message edited by: jimandy ]</small>

[rshayes]

The current transformer would not indicate if the element is switched off by the thermostat. It basically indicates that heating is taking place, but does not cover the case where no heating is being commanded by the thermostat.

It has the advantage that the secondary is isolated from the power lines and can be grounded.

[Chris Smith]

I would expand a little on Ed's neon light post to include two or even three neons.

One can be for a power indicator to show there is power to the general area, the second one shows when the power is cycled on and off after the thermostat, and the third as already suggested, shows when the element burns out.

For a buck or less for all three, neon’s are cheap and reliable.

[jimandy]

I have an electric hot water heater at my weekend getaway shack and I too have been musing over the same problem.

The way I see it, you need to sense both the status of the thermostat (on/off) AND the current flow (flow/no-flow). If you analyze the problem logically then the possibilities are...
1) off, no-flow (status of element is unknown)
2) off, flow (can't happen)
3) on, no-flow (element burned out)
4) on, flow (all's well)

Condition 1) might suggest you would never know if the element is burned out but, in reality, this condition would not last long as the water temp will have to drop and the t'stat will come on at some point resulting in condition 3).

In real terms, I think the only "logical" way is to sense the status of the t'stat with a wire to it's "switched" side AND sense the current flow with the aforementioned current sensing transformer, translate these conditions to logic level signals connected to an appropriate logic gate to drive the indicator lamp. And, oh yes, I would have it drive a beeper so I wouldn't have to keep checking to see if the light was on.

[Gorgon]

I just read all the ideas and got one myself.
What if you made/got two small transformers, one transforming the on-voltage from the thermostat and the other transforming the current through the element. On the secondary side the current output could cancel the other in the normal condition. When the element broke this cancelation stoped and the output from the voltage transformer turned the lamp on. I'm not sure you could just use a neon directly on the output windings, due to the phase difference, but at least not many components, maybe a single capacitor?

TOK

[Chris Smith]

Transformers seem drastic and over engineered, compared to a simple neon tube.

One neon placed at the power line to ground indicates power is in the area.

Another neon placed after the thermostat, also to ground, indicates the cycles on and off.

And the third as Ed suggested, only lights up as the element is broken and the neon by passes the element to ground, and lights up.

One buck and no special wiring,....tops.

No fancy transformers, no formulaes, no wire sizing, just three indicator lights and all for less than a buck with no additional wiring or logic needed.

Just a simple panel to hold the bulbs and a sticker underneath each light to say what each bulb is for.

I think over engineering is not all that it is cracked up to be?

[jimandy]

And the third as Ed suggested, only lights up as the element is broken and the neon by passes the element to ground, and lights up..

If the 3rd neon lights up if and only if the element is burned out, why would you need the other two?

[frhrwa]

I was going to just add one neon to each leg to see if there was power.. not go so far as if an element was burned out.. I took care of all of that.. put in a gas water heater...

[Chris Smith]

CUZ,....all the reasons we install panel lights in the first place.

Reason #1 to tell if you have power present like all panel lights tell us.

Reason #2 to tell if the thermostat is working, on and off cycles say yes, with out having to pull it apart and meter it.

Reason #3 to tell if the heater element is burnt out with out the use of any meters, opening up the panel, and well, .....just for the normal reasons we have panel lights installed.
[were lazy]

AND, if the third neon is off, how do you know you have power?

Is it a power problem or is the element burnt?

#1 neon tells you this.

How do you know the thermostat isn’t the problem?

#2 tells you this.

You watch for a on and off signal, as you draw water out of the heater, and make the water inside cold.

And if the #3 stays off "with or with out a broken element", #1 and #2 could be the cause and you would never know. Its called “logic”.

No power = #3 stays off, same as a good element.

#1] solves this.

#2 shows the themostat works with out a screwdriver or tools.

They are cheap and they save us time.

Or like Wayne says.......
I was going to just add one neon to each leg to see if there was power.. not go so far as if an element was burned out.. I took care of all of that.. put in a gas water heater...

<small>[ January 14, 2006, 06:32 PM: Message edited by: Chris Smith ]</small>

[jimandy]

Yes, but...

Reason #3 to tell if the heater element is burnt out with out the use of any meters, opening up the panel, and well, .....just for the normal reasons we have panel lights installed.

...Isn't that the only reason for which the problem was stated? Sounds like the other two are sort of "over engineering" to me.