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Old 30th January 2006, 18:14   #1
DRC
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Default Why relay for high watt lamps

So I am asking this question.

If I want to go for 100W bulbs, why need a relay.
If I do not use it, what will get damaged? Pl be specific, dont say its electricals


Before you all get on to this.. Here are my thoughts...

A car's wiring system unlike the one in a bike, already has a realy.
A relay is a device whcih translate the light switch setting to the ones capable of handling more current.
a 60W buld takes 5A of current, i.e., 10 A switched through the relay (2 lights), for 110W, it is about 20A.. which I think is still fair being handled through the existing relay...

Now If we opt for the relay, how is the wiring, ... are you going to replace the existing relay by a new one.. may be one with the higher current rating??? Which I think is acceptable if the OEM one is in-adequate for high currents.. But most OEM relays are good enough for the high currents. ( I will edit the thread with accurate current rating later)

In my bike way back in college days I had experimented this one.. nonte the bike wirirng (Kawasaki Bajaj k100 rtz) did not have a relay. still used a 55/60 buld and screwed up the SWITCH. Yes, switch in the bike can not handle the make and break of the high currents, hence gor screwed.. more expensive than the bulb itself.,

But a car does not have this problem.. It already has relay..

Now the last thing.. the wirer it self. are the wires in adequate.. I think these ones are always over engineered for safety reasons.. may be good to use the thicker ones.. but are they obsolutely necessary??

Gurus, comments please...
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Old 30th January 2006, 23:40   #2
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Hi DRC,

You seem to have a good grasp of the system as is, but i will just start from square one again....

As DRC mentioned, the reasons why relays are needed in a car is so that the switch (in the light dipper/switch in this case) does not need to handle a very high current that is required for the headlamps.
Also, the fact that there is a lesser length of wire carrying a high current makes it safer in terms of short circuits (=fires).
Not to mention the fact that all conductors have a resistance, hence the longer the wire, the more the resistance. Therefore the shorter your current carrying wire is, the more current makes it through till the end. (= more power for your headlamps)

Now, as you mentioned, all cars already have a relay for headlamps! Why change it? ...it has to do with the ability to safely carry more current (or simply not trusting the OEM relays to handle the larger current your new bulbs will be drawing)

In terms of changing the existing wiring.
I WOULD reccomend it.
The larger wires minimize the voltage drop from your power source to your headlamps.
Intresting fact - a halogen bulb at 95% of its operating voltage puts out only 83% of its rated light output!!

Here is a phenomenal resource for automotive lighting -
http://www.danielsternlighting.com/home.html

cya
R

ps - let me know the ratings for some existing OEM relays and we will redo the math with some tricks

Last edited by Rehaan : 3rd February 2006 at 08:49.
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Old 30th November 2006, 14:12   #3
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Rehaan, what you say makes perfect sense. However, could we go further into detail on this.

1. What gauge wires are the norm for existing cars? That is, if such a norm exists.
2. How does one check their wiring to see if it is sufficient to take a certain load.
3. Similarly, what amps should your relay be able to handle to provide you with optimum performance from 100/90W bulbs?
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Old 30th November 2006, 14:18   #4
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Quote:
Originally Posted by Rtech View Post
1. What gauge wires are the norm for existing cars? That is, if such a norm exists.
2. How does one check their wiring to see if it is sufficient to take a certain load.
3. Similarly, what amps should your relay be able to handle to provide you with optimum performance from 100/90W bulbs?
1. To quote (and remember higher the gauge (ga), thinner the wire) >
Quote:
Typically we find 16 gauge wire (1.5 mm2) at best, more commonly 18 gauge (1.0 mm2) and in some cases even 20 gauge (0.5 mm2).
2. By using a voltmeter / multimeter at the bulb terminals and comparing that with the reading at the battery terminals you can check voltage drop.

3. P = VI (watts = volts x amps)
100+100 watts continuous on high beam. 100+100+90+90watts on flash.
Car battery puts out between 11 - 14volts.
Do the math

goodnight.
R
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Old 30th November 2006, 15:04   #5
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I want to change my indica's wiring(already have a relay) but could not find anybody in Delhi who can do it for me.
Delhi based tuners... help please!
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Old 30th November 2006, 16:25   #6
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As i understand , if the battery is 12v 9Ah ( ex bike battery, cars have more Ah batteries) , that means the battery is capable of 12X9 = 108 watts every hour . Is this correct ?
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Old 30th November 2006, 17:32   #7
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Quote:
As i understand , if the battery is 12v 9Ah ( ex bike battery, cars have more Ah batteries) , that means the battery is capable of 12X9 = 108 watts every hour . Is this correct ?
Wrong!

It means that the battery can supply 9 ampere of current for 1 hour or 1 ampere for 9 hours.
The formula you would be looking at is:
(X ampere) * (Y hour) = quoted charge capacity of battery in Ah.

Quoted voltage is the voltage across the +ve and ground when fully charged and when no current is being drawn.
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Old 30th November 2006, 18:16   #8
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very simple terms, the gauge of wire that come OE are not sufficient for 90/100 bulbs nor the relays equipped, for that matter the fuse rating also. please note that the fuse handles for each sides in most cars if iam not mistaken, so in effective they handle ur headlights plus tailights.

rest rehaan has given a detailed explanation. one good news is companies like PMP have ready made kits available with proper insulation and connectors along with relays which are very easy to install.
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Old 7th April 2009, 19:55   #9
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Quote:
But a car does not have this problem.. It already has relay..

Now the last thing.. the wirer it self. are the wires in adequate.. I think these ones are always over engineered for safety reasons.. may be good to use the thicker ones.. but are they obsolutely necessary??

Gurus, comments please...

Not all cars have relays . Maruti 800 ,ambassador , and some of the trucks do not have relays .

For example if you were to use a 100 w halogen bulb directly in a maruti 800 ,the bulb requires a minimum of 8.5 amps typically . So the wire and the light switch should be able to handle 8.5* 2 bulbs minimum . The O.E wiring is not really capable of handling 8.5 * 2 Bulbs =17 AMPS of current . Incase if you do try using higher capacity bulbs ,you might actually end up frying the combination switch assembly .


So that is the reason why relays are used for handling the higher current capacity .
Most of the cars come with 55/60W bulbs only . And for that current the O.E wiring is ok .

The cars which do have relays may not have the proper rated wiring or the fuses to carry huge currents .

Last edited by Rehaan : 8th April 2009 at 13:42. Reason: Quote fixed.
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Old 9th September 2010, 18:56   #10
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By using the relay assisted switching we have the main supply to the headlamp going thus: From battery, to fuse, to relay contacts, and to lamps.
I can cite two reasons for using a relay for the headlamps.
1. All above components are under the hood and the route length is shortest. Shorter wire means less transmission loss in wire. This enables the headlamps to burn brighter. Also, it is easier to choose thicker wire if it is kept shorter and does not have to be threaded all across the dash-board.

2. The dipper switch on your steering is more delicate and more expensive hence it is better to keep it on light load - just the load of the relay coil.

Good quality relays are built robust, and have contacts made of superior materials and large contact areas for longer life.
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Old 9th September 2010, 19:31   #11
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I had the same logic as the thread starter and plonked in 100/90's into my car. I just had one problem - the plastic bulb holders would melt (slightly) and lose the connection with the connectors on the bulb.

So upgrading the holders to ceramic ones is also important when going for 100/90.

Then, I went the whole nine yards and added the (second) head light relay with beefier wiring and ceramic holders - no problems so far.
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Old 9th September 2010, 22:34   #12
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Quote:
Originally Posted by Pankaj401 View Post
Wrong!

It means that the battery can supply 9 ampere of current for 1 hour or 1 ampere for 9 hours.
The formula you would be looking at is:
(X ampere) * (Y hour) = quoted charge capacity of battery in Ah.

Quoted voltage is the voltage across the +ve and ground when fully charged and when no current is being drawn.
Slight modification. As ratings go 9AH = 10 hours x 0.9A. For shorter duration the capacity decreases, for example it would be 1/2 hour x 9 A apprx. Refer to battery manufacturers charts for details.

In most cars you will notice that the lights are brighter with the engine on, as the 12V battery has 6*2.2 = 13.2V fully charged, but with alternator on it can go up to 14.5V+ hence brighter light. Thus with relay and thicker wire you get lower voltage drop giving brighter light!
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