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I wish to use some 12 V, 5 W xenon light bulbs under separate shelves of a cabinet. I'd like them to power-on using a magnetic reed switch attached to the cabinet door.

The magnetic reed switch I have has a current rating of 0.5 A 20 V DC: 10 W max.

My question: Since the lights are 12 V and NOT 20 V, would stepping down to 12 V change the ratings at all in regards to wattage? Or in other words, could the switch handle 3 of these 12V 5W bulbs (15 watts) since the voltage is less? Not sure if that changes the max specs or not.

The lighting company also offers 3 watt bulbs but I'm trying to get the maximum brightness possible.

I'll have to get a DC transformer as well to match specs.

Any and all advice is very much appreciated!

clabacchio
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BobH
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3 Answers3

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My question: Since the lights are 12v and NOT 20v, would stepping down to 12v change the ratings at all in regards to wattage?

There are are few things to think about:

  • The maximum current will be a fixed amount. This is because the contacts and conductors will get hotter the more current that goes through. In terms of current you get less power ('wattage') for the same current as the voltage goes down, so you get fewer watts at 12V than you do at 20.
  • When switches open you sometimes get sparks, this depends on 2 things: The voltage and the inductance of the load. Sparks/arcing can damage the contacts so it's bad news. The maximum rating may well assume no inductance, and with a xenon bulb there might be quite a lot of inductance.

The easiest way to get a lot of light without a lot of current and inductance is to use LEDs

Will
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The rating of switch or relay contacts is generally based on current - in other words, there's a maximum current the switch can handle no matter what the voltage is. Because different types of load have different characteristics when switched - for example, incandescent lamps draw a large current when switched on because the cold filament has a lower resistance - switches often have different ratings for different load types (example).

Separately, the switch will have a maximum voltage rating based on the insulation, separation between conductors etc.

In your application, if you don't want to change to a different type of bulb or LED, the best approach would be to use the reed switch contacts to operate a relay, or switch on a transistor, that then switches the power to the bulbs.

nekomatic
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The reeds are nominally within their limits once the lamp is running but, depending on which sort of Xenon lamp it is, inrush current could give them a nasty fright. For long lifetime I'd add some sort of buffer - see below.

You say "Xenon light bulbs". This might mean

  • Xenon filled filament maps

  • Xenon HID arc lamps

Which it is affects the answer.
A Xenon filled filament lamp will have substantial inrush current which means a different solution is probably needed.
A Xenon HID lamp will have a controller. Some of these offer zero inrush current. Others may have an inrush type effecyt, although the mechanaism is differnt to that which applies in a filament lamp.

Maximum brightness possible will probably be achieved by using LED bulbs.
The best of these offer much more light than even the best halogen-xenon filament lamps.
HID Xenon lamps are closer to LEDs in efficiency but a good LED lamp is still better.

Load current:

Current for a 12V at 5 Watt load is notionally given by
I = Power / Voltage
= 5W / 12V ~= 0.42A
so a 0.5A switch is notionally just within its rating.
In practice it will probably have a short life.

DC switch current ratings are usually related to the ability to not suffer arc damage when opened under load. Speed of opening, contact material and quite a lot more may ply a part.
Running an eg 20V switch at say 12V may slightly improve the effective rating and will usually do no harm or lower ratings.
At very low voltages and currents some switches may have contact oxidation problems but that can be disregarded here here.

Even with a resistive load I'd be wary at operating a switch at 80% plus of its rated limit. It might last well, but may not - you are dependant on the manufacturer 'knowing their stuff'. Many do - but many don't.

A filament lamp has a low cold resistance and initial current may be 10 to 20 times the run current. Even at 10x the reed contacts will be overrated by a factor of about 10x. They MAY survive this, but no guarantees. Inrush current can be reduced by "soft starting" with eg PWM or something which switches out a series resistor after a short while.

A Xenon HID lamp CAN have no inrush current with a well designed controller. Here's a semi randomly chosen HID kit claimed to have no inrush current

An LED bulb or strip of LEDs can, with proper choice, provide somewhat more light per Watt than an HID Xenon system. It need have no inrush current. The best LEDs have > 150 l/W ((lumen per Watt) efficiency ratings. It will take care to achieve this and good commercial finished bulbs may be not vastly better than HID. Bad commercial LED bulbs, of which there are many, have lower l/W than HID.

Current buffer:

If you have free access to all wiring points and can run an extra wire then a controlled switch can take the load off the reed contacts. I'd usually use a MOSFET but a bipolar transistor or relay or commercial solid state relay could do the job.

If you have freedom of design choice then a Hall sensor plus current buffer would last indefinitely.

Tell us more and we can advise further.

Russell McMahon
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  • FWIW I downvoted your answer but not the question because it was pretty much a total non-answer in original form which can be seen from the edit history. I'll retract but must say I'm not keen of placeholder "answers" in an attempt to keep a question open and always vote on a post at the point I see it. You have enough rep to see it has no close votes and a single downvote doesn't mean much. – PeterJ Sep 10 '14 at 11:59
  • @PeterJ THanks for the input. I'm meant to be doing tax - what fun. If I think I'll get back later I sometimes find a question has died aborning. I did not want to risk that and the downvote did not augur well. I see that the question downvote has now gone too, so all is well. | I'm happy with your reason for downvoting. I don't like them when they may mislead people as to what is a worthwhile answer. Effect on rep is obviously not an issue. Input always welcome. Thanks - R – Russell McMahon Sep 10 '14 at 14:09