Indicator Lamp in Series With a Switch

You can buy toggle switches with a neon indicator built-in to the handle. It glows just based on being excited by the potential across the switch, no common necessary. But it glows when the switch is OFF (open), not when it is On, because the switch essentially shorts across the lamp. You would need your operator to know "If you don't see the light, the fan is on".

You could try "You cannot leave this area until the light comes on telling you it's OK", he/she will figure it out pretty quick.

If you need it to glow when the switch is on, you must have the common for the lamp, LED, Neon or otherwise.

If you are controlling an air handler, you have a ground.

Trucks run on 24V electrics - and there are plenty with warning lights built into their switches.

"Trucks run on 24V electrics - and there are plenty with warning lights built into their switches."

That's because they can use the chassis as a common. I forgot this was 24VDC though, so I'm not sure if that's a possibility on a building. I'm not fluent in Low Voltage HVAC control wiring codes. In general though, using ground as a current carrying common conductor is frowned upon.

Almost every power supply I use, and this includes AC and DC supplies, ties the chassis of the power supply to the chassis of the cabinet it is mounted in. If you don't, you have a floating ground for the supply. There are instances of this, but I doubt an air handler circuit is such an example. Of those, my indicators are grounded through the bulb housing, or have a ground wire going back to the neg side of the supply (essentially chassis ground). Some are in-line single wire AC or DC bulbs (i.e., neon, led, filament, etc), and those are grounded to the equipment chassis. No ripple and minimally measurable DC current is returned through the chassis of the air handler. I don't see the issue.

This would not be a code compliant installation in Canada. No matter what the voltage is, the use of a bonding conductor or bonded chassis for the return of current back to its source, is not permitted. I am quite surprised to hear it is there. This practice is compliant with NFPA 70?

Sounds like your also going to need a common at the switch area as well in order to run a lamp. They are only breaking the hot lead to the coil. Thus they only use the two wires. One in one out of switch. Neon may show an indication of power flow, but amp draw is so low it may not. Other wise run the common and use LED.

Please be aware that the contactor essentially has at least one normal open and one common close auxiliary contacts readily built in by the manufacturer. You just have to use these auxiliary contacts, selection of lamp and voltage source is up to your convenience.

I think he should explore the option of the auxiliary contacts like you suggested. That way the lamp will come on when the coil pulls in and not when the switch changes it's state, and please correct me if I'm wrong but it also removes the chance of an open circuit if the lamp ever fails/burns out while it's in series with the coil.

Why simply indicate? What if your operator forgets to look at the lamp?

Timer.

Can you use a timer switch similar to this?

If all else fails, interconnect the air handler switch to a magnetic door lock within the operators' work area, air handler on, no bye bye......problem solved.

You will still need to provide a return path for this, as the obvious was pointed out earlier in this thread. Sounds like someone is trying to over-engineer what is a simple solution.

With just the two wires, you're right - you need a series-connected indicator if you want it ON when the switch is on.

With 24V - forget neons.

Do you know what the load is? Can you give us details? If you could afford to lose a volt or two you could use an LED, but you'd need to put it across a series resistor sized to give the right voltage for the LED with the current drawn by the load.

May be able to find a piezo sounder which can run with a couple of volts (again assuming you can spare it) set up to beep every few seconds. Or a sounder/LED indicator powered by a separate source (battery?) but triggered by current flow. Need to know the current and max. allowable voltage drop, whichever you use.

Then again (he said, thinking "outside the box" ^{_{God I hate that phrase}}) - if you used an independent supply for an annunciator, you could rig it to be triggered by the absence of voltage across the switch. No voltage drop when ON, but possibly a teeny bit of current leakage when it's OFF.

Before putting something in series with the contactor coil, I'd poke around with a voltmeter. I'd be willing to bet that you can find a ground that would be 24V away from the lead going to the switch. An LED and a resistor, and you'd have a light.

Or run a wire back from the contactor?

I just don't like the idea of a series connection: indicator opens, blower doesn't work.

"... series connection: indicator opens, blower doesn't work ..." doesn't hold if the majority of the current is being passed by a series resistor, and the indicator is in parallel with the resistor.

True.

Thanks for the numbers, that is what I need - facts I can engineer with.

It would be best to have two lights, one red for not running and one green for running, but it is obvious from the thread that another wire will have to be run for a common.

In this case that is not all that difficult, but it is another job that will take time and materials.

Install a 15 minute wind up timer in place of the switch. There is no guarantee if you install 50 lights the operator will still turn it off. With the timer no matter what it will not run more than 15 minutes. Problem solved and no light to ever change.

Have you tried just the LED in series with the switch? Voltage drop depends on the type of LED used when forward biased - .6V for Silicon; .2V for Germanium; (?) for whatever the Zener voltage happens to be. I would also need to know how much current needs to go through the switch, which can be measured with an multimeter. The neon bulb has some resistance, and a forward-biased diode should present a short-circuit. I hope this helps.

"Voltage drop depends on the type of LED used when forward biased - .6V for Silicon; .2V for Germanium;[1] (?) for whatever the Zener voltage happens to be[2]. I would also need to know how much current needs to go through the switch, which can be measured with an multimeter[3]. The neon bulb has some resistance[4], and a forward-biased diode should present a short-circuit[2]."

[1] LED forward voltage is higher than either of your examples (which are about right for small-signal diodes). LEDs have a voltage drop of between about 1 and 5 volts, depending on the type (colour).

[2] Zener voltage does not come into it. LEDs are used forward biased (anode to +ve, cathode to -ve). Zener breakdown occurs the other way round.

[3] Enough current to pull in a contactor would destroy most indicator LEDs.

[4] Even so-called low voltage neons need about 70V to strike, and only pass a few milliamps. The control circuit here is 24V.

Sorry, but I felt this had to be said. I have marked this off topic.

Hi, not sure what voltage you control circuit is operating on, but I would suggest that you have an indicator wired across the switch the glows when the blower is off, when the blower is on the lamp goes off.

IE instead of having indication that the blower is operating, have indication that the blower is off.

Cheers

Joe

Guest #16 added note: The voltage drop over an actual LED is usually lower depending on the color. You can also try an R/S Latch with the LED to indicate when the system is on or off. I don't work for Radio Shack, but you might want to buy the Electronics Learning Lab they sell for the Truth Table and circuits.

Coldspot has not told us if the 24 volt is DC or AC. If it is AC, a current transformer could supply a lamp.....

OK here is a sugestion that I have used with sucess.

Assuming that your 24V operates a relay that activates the air handler.

The control relay will probably require ~20 mA to activate (close the contact to the air handler) Also, this is probably 24 V AC. So what I have done is put a couple of 10mA LEDs back to back in series with the 24V control switch. The back to back LEDs will conduct on both halves of the AC and prevent the relay from chattering. I also put a low value resistor in parallel with the LEDs. The resistor (if used, will provide just enough current to close the relay in the event of an LED failure. This will cause the LEDs to light when the switch is closed. The circuit will cause ~ 3 Volt drop in voltage but not enough to cause the relay to malfunction. You may have to adjust the resistor value depending on the current for the LEDs that you select. Also, you may want to put a capacitor (.01 uF) across the switch if the control wires are long. Long wires will have enough inductance to cause a voltage spike, when the switch is activated, which could cause damage to the LEDs.