24 VAC

24VAC is used a lot on equipment that has no electronics. Just relays. Cuts cost. Transformer is less expensive then a DC power supply.

If the engineers just specify 24V shouldn't AC or DC both work? They didn't specify one or the other.

Well we're (I and my fellow workers) the "engineers" and we want the 24VDC version. We just are stupid and forget to mark it correctly. Don't know why the AC version doesn't work on DC. You would think that wouldn't make a difference to a resistive load, would you?

I got one here right now - I can try test this.

Relays only - makes sense.

There is no reason of which I am aware as to why a standard Halogen globe will not work on AC or DC. There is some argument that DC causes a bit more migration of the tungsten, but this would not stop it from working at all as seems to be your problem.

The only concern associated with DC operation is that, should the filament break, then the globe may arc between the internal posts, which can draw quite high currents, so adequate fusing is advisable.

I assume that there is no soft start circuitry involved here so is it possible that the globe incorporates half wave rectification (don't ask me why - maybe for lamp life (not sure this would work as the globe glass may not then get hot enough to recycle the tungsten) or maybe it's for colour rendition?), but this is the only reason that I can see why the globe would not work on DC. Try reversing the connections to the globe and see what happens, alternatively meter the terminals of the globe for continuity.

I'm dropping back to some of my fading chemistry/physics schooling memories but the only thing different I would expect a halogen light would care about AC or DC supply would possibly be filament vaporization. Thus the lifetime of a lamp maybe attenuated if using the wrong type. The inductance impedance current limiting of a filament would not make much difference in the current draw.

As for 24 VAC as a control voltage, I've only seen it done with some very simple process control systems that required just a few relays and switches for use. With the drop in price of switching supplies today a simple 24 VAC transformer can be more expensive.

I was afraid in reality it was a 12V bulb with internal circuitry in the housing - nope. It works fine on 24VDC. Problem we have now is we have customers that particular they will insist on a label on the fixture that says 24VDC.

For information on 24VAC controls ask an electrician who is at least 55 years old. If he worked on controls 30 years ago, 24VAC is what was almost always used. There were very few solid state controls at that time and the systems entailed mostly relays and step-down transformers with a whole lot of push button switches. The machine transformers were usually very big due to the many taps both on the primary and secondary. They supplies a lot of current so that was a big weight and size contributor.

If you ever come across one of those transformers they can easily be made part of a high current battery charger (lead-acid), a bench top low voltage high current linear power supply, door weights or row boat anchors. Just don't drop it on your foot!

Good Luck, Old Salt

And be careful not to short one out. They don't burn very well, but the oil used to cool them makes a LOT of really smelly smoke!

Ask me how I know!

OK, I've done a little searching to find the difference in driving a halogen lamp with AC or DC, and so far all I've found is hearsay. Does the electric field influence where the tungsten bromide disassociates and deposits tungsten back on the filament? Or what is the difference?

See response to post before you - only difference is the model number and posted voltage on the housing. As I stated to Redfred - we have customers that will find the label and insist on changing it.

Sometimes in marine or dockside locations, 24VAC controls are used to minimize shock hazards from pendant stations and the like. A small control transformer (say 480/24 or 240/24) provides the lower voltage.

Pretty much all of the home and business Central AC and Heating systems installed in the US use 24VAC.

Yes. And do they have need for a tank light in these systems?

Your question was "who uses 24vac control voltage?".

Without regard to the availability of 24VAC lights, I seem to remember that one reason automotive bulbs (All running on DC) have a problem with filament notching that causes them to require a more robust filament (meaning one that provides less actual light at a specified voltage, since the filament is thicker, and designed not to get AS hot). If you run that bulb on AC (same voltage), it will provide more light for longer without replacement.

But my memory may be faulty. I can't right now remember why it might develop filament notches, and thus weak spots in the filament.

Anyone else remember hearing this, or know why it might happen?

Yeah - you're right. the cheap thermostat in my house runs on 24VAC - forgot about that.

how about the door bell?

Yes, door bells are 24VAC, but when do we use a tank light in a house? I am trying to figure out what industry needs a tank light and still uses 24VAC instead of 24VDC.

There appears to be an obsession here with the tank light needing to be DC.

If I were designing the system, then I would most likely use AC if the light was the over-riding consideration and provided it fitted with the electrical requirements of the other equipment. Does the rest of your componentry particularly require DC?

Contrary to what has been said here, lots of electronic circuitry, both basic and complex, uses AC input and control, I personally prefer it when using SCRs as it provides a convenient switch off method on the negative half cycle. For example, quite a few commercial switch mode power supplies are designed around an AC input for this very reason of easy control, they just won't work with a DC input.

I am a process skid controls engineer and heavily into analog instrumentation. AC is not a good thing with 4-20ma control. I can't even run it in the same wireway and have to segregate in the panel. Nearly everything we build has a PLC involved somewhere, if not on the immediate piece of equipment we are building, definitely somewhere nearby and our equipment will be connected to it, and ALWAYS some 4-20ma instrumentation on it. The tank in question with the 24VAC light has a temperature transmitter, a level transmitter and a pressure transmitter running on 4-20ma loop power and a point level with 24VDC signal. This is very common in the pharmaceutical and bio-tech industries.I sure don't want to run another conduit up the side of the tank and install a stepdown transformer in my panel for one piece of equipment.

The question here was who still uses 24VAC control, as I don't ever see it in the above mentioned disciplines, nor in the occasional food processing industry skids we get either. I haven't seen any analog instruments running AC lately, and all the discrete instruments we use are generally 24VDC. I can see where these could be 24VAC, but I never see them.

Material Handling Equipment. Example trash compactors, balers and elevating docks use 24VAC where it may be a hazard to the operator to run higher voltage. Like a remote switch out in the weather. A lot of time these panels are the same so the design is for one panel to fit all applications.

Thank you. To that I'll reply as to several before you, "where is there a need for a tank light in those applications?" This is most definitely a sanitary design for use with tank sight glasses. I still don't see any uses listed that would require a sanitary tank light, and 24VAC wiring. Perhaps the elevating dock people can use it? Think it would be a tad bit weak on lumens for this operation.

Still looking for where there is an industry using 24VAC and requiring sanitary tank lighting.

Stepping down the 220V or 110V AC to minimal magnitude for safety and significant value just for activation gases of light bulbs is reasonable. Putting up a rectifier and filter on the supply adds up inefficiency or losses of electrical energy, when all your goal is to activate the gas filled bulb to produce photons. Operating at lower AC voltage is material wise and cost wise also, wouldn't it. It depends on its use. 24 AC is not commonly used to industrial process instruments so to speak.

I would think that the manufacturer would rate their lights 24 V AC/DC.

Now, there is no difference in using AC or DC. DC MIGHT cause the lamps to last longer. Certain sensors can pick up the intensity variations of AC. Low voltage lamps tend to me more vibration resistant. I used a lot of ELH 120 V Halogens on AC and DC and EYC low voltage Halogens on DC for doing measurements on sensors.

What COULD be different is the switch and switch protection. Not all switches can switch DC.

Ask them what the difference is. Take on apart and compare the switch and switch protection components.