Halogen inrush current

jbwiden

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I have a motorcycle and would like to use a switch in order to turn off the headlight(it is a dualsport and when offroad I would rather not have the headlight draw the extra power). So my question is how much inrush current do halogens(55w low /60w high) draw and for approximately how long? This is to make sure the switch I use can handle the current.


Thanks
Jbw
 

mdocod

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Hi jbw,

If you measure the cold resistance of the bulb, and divide that resistance into the supply voltage, you will have an estimate of the maximum possible peak cold-start current that would be reached in a situation where the flow of current to the bulb is practically unrestricted. In my experience, this testing would indicate that most bulbs could in theory draw upwards of 10X their normal running current on a cold-start in those first few milliseconds.

In reality, there are going to be many factors that reduce the effects of this maximum peak. While there is still a peak, it isn't going to be 10 fold the running current in most installations. Probably more like 3-5X the running current is more likely. Keep in mind, that even in normal installations with normal expected amounts of resistance, the warm-up time to normal running current occurs in a fraction of a second.

In choosing a switch, picking something that can handle 2-4X the normal running current of the bulb should be more than adequate for long term reliability.

Out of curiosity, is this a 6V or 12V system on board? Keep in mind that 6V systems are going to have double the current requirements and require significantly beefier wiring and switches to handle the current without major losses from resistance.

-Eric
 

jbwiden

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12v system. the 60watt bulb should steady state draw about 5 amps (just using nominal values). I bought a waterproof 25 amp switch which seems like it will be adequate.


Thanks for the help,
Josh
 

KiwiMark

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12v system. the 60watt bulb should steady state draw about 5 amps (just using nominal values). I bought a waterproof 25 amp switch which seems like it will be adequate.

It wouldn't matter if the current went above 25 amps - it would be so brief that it wouldn't have time to overheat and melt anything in the switch. If the switch can handle a constant 25A then it would have no problem handling 50A for less than a tenth of a second.

I can't see the switch being inadequate for the job - you should have no problems with it.
 

Ainsley

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When ever I have done any high current switching I have always just used a simple, inexpensive switch and then used a Power MOSFET as the relay. It is essentially the same set-up as with a relay but relays can get pricey in order to handle really high currents and mosfets are dirt cheap.
 

markmckelvin

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It wouldn't matter if the current went above 25 amps - it would be so brief that it wouldn't have time to overheat and melt anything in the switch. If the switch can handle a constant 25A then it would have no problem handling 50A for less than a tenth of a second.

I can't see the switch being inadequate for the job - you should have no problems with it.
A current twice the contact rating could cause the contacts to weld together. If it's rated 25 amps don't assume it can ever handle more than that, even for a split second, unless it is specifically states that it can.
 

alpg88

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I have a motorcycle and would like to use a switch in order to turn off the headlight(it is a dualsport and when offroad I would rather not have the headlight draw the extra power). So my question is how much inrush current do halogens(55w low /60w high) draw and for approximately how long? This is to make sure the switch I use can handle the current.


Thanks
Jbw
about 15 amps maxx on start up, assuming your wires are thick enough, if not, then even less amps. pretty much any automotive switch will handle it easy
 

AmplifierMark

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about 15 amps maxx on start up, assuming your wires are thick enough, if not, then even less amps. pretty much any automotive switch will handle it easy
Use an NTC Thermistor in series with the bulb and it will eliminate the surge current. amazon has a pack of 5 for less than $10: uxcell NTC Thermistor Resistors 8D-20 6A 8 Ohm Inrush Current Limiter - Pack of 5
 

vicv

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You guys are worrying too much. You don't have to worry about inrush current when choosing a switch. Think about a 15A switch in a house turning on and off a pump that draws 15A. A motor pulls 8x it's rated current at start up and it lasts much longer than a light bulb. It's for such a brief period of time that's it's a non-issue
 

lightfooted

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I'm with vicv, especially as I have done enough cobbling together 12V systems for lighting and what not that I can say I have never seen a switch rated for the current capacity ever fail due to inrush. Fail due to brief shorts? Absolutely, but they were seconds long, not milliseconds. Even those were more of a mechanical failure where the plastic immediately around the terminals melted and misaligned them than an absolute failure where the switch stopped working.
 

broadgage

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You guys are worrying too much. You don't have to worry about inrush current when choosing a switch. Think about a 15A switch in a house turning on and off a pump that draws 15A. A motor pulls 8x it's rated current at start up and it lasts much longer than a light bulb. It's for such a brief period of time that's it's a non-issue

I agree, if the switch was intended for general household use then it should be suited for controlling power tools, microwave ovens, air conditioners, large audio amplifiers, and other appliances with significant inrush currents.

If intended for automotive use, then it should be suitable for controlling headlight lamps, a very common application. A 12 volt 60 watt lamp will use about 5 amps. I would prefer to select a switch rated for at least 10 amps, not so much for inrush currents but to provide a margin for modern cheaply made switches.
 

Alaric Darconville

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I have a motorcycle and would like to use a switch in order to turn off the headlight(it is a dualsport and when offroad I would rather not have the headlight draw the extra power). So my question is how much inrush current do halogens(55w low /60w high) draw and for approximately how long? This is to make sure the switch I use can handle the current.

Use a switch to control an automotive relay so the switch handles just a tiny bit of current (~30 to ~40mA (milliamps)) and the relay does the heavy lifting.
 

broadgage

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Use a switch to control an automotive relay so the switch handles just a tiny bit of current (~30 to ~40mA (milliamps)) and the relay does the heavy lifting.
Certainly possible, and use of a relay to control headlights on modern cars is common practice. Less applicable to motorcycles though as the load is less (single headlight rather than two), wire runs are much shorter, and space to mount the relay is at a premium.
 
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