Alarm Led

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martiale

Copper - Posts: 52

Joined: February 01, 2006

Location: United States

Posted: February 01, 2006 at 9:32 AM / IP Logged

Hi all,

this is my first post in this forum and my question is regarding alarm led power consumtion, i did remove the 5.1K resistor as per this link: http://www.unofficialbmw.com/e36/interior/e36_euro_alarm_led.html it is blinking much better now, before you could barely see the blinking. I think as a result of removing the resistor, power consumption has increased, 1.is there a way to measure (i have a voltmeter) this? 2.Is it better to e new resistor in there like 3K or 2K? My alarm is BMW made by Alpine and i added this Euro Led because it matches the interior.

'99 BMW E36 M3 Coupe

ripley01

Member - Posts: 33

Joined: November 27, 2005

Location: Netherlands

Posted: February 02, 2006 at 12:03 PM / IP Logged

I wouldn't worry about power consumption. A typical LED will consume 15 mA. Of course, if the LED is blinking, this current will only run while it's lighted.

If you want to measure the current, you should but a DMM in between one lead and the alarm, and switch the alarm to a mode where the LED is permanently on, e.g. valet mode if you have a Clifford. Now guesstimate the duty cycle, i.e. the fraction of the time the LED is on when it's blinking. Usually a very short amount of time, like 10%. Multiply the two and you'll have a pretty good indication of average power consumption.

With 15 mA steady state and 10% duty cycle, average consumption will be 1.5 mA. Theoretically, it will take about a month to consume 1 Ah from the battery. In practice, the battery will discharge more than this on its own.

martiale

Copper - Posts: 52

Joined: February 01, 2006

Location: United States

Posted: February 02, 2006 at 12:15 PM / IP Logged

ripley01 wrote:

I wouldn't worry about power consumption. A typical LED will consume 15 mA. Of course, if the LED is blinking, this current will only run while it's lighted.

This info helps a lot. My question now is: 1.Why there was 5.1K resistor there? 2.And what else (in addition to brighter light) changed after the resistor have been removed? 3.Do i have to be concerned for anything by removing this 5.1K resistor?

Thanks again

'99 BMW E36 M3 Coupe

dualsport

Silver - Posts: 983

Joined: September 27, 2005

Location: United States

Posted: February 02, 2006 at 1:21 PM / IP Logged

The LED was likely intended for an application where it's driven directly by 12V power.

Your alarm output is probably doing the current limiting internally and dropping the voltage down; otherwise it would probably have blown the LED when you bypassed the resistor- don't think any LED would take a full 12V across it..

You might be overdriving the LED a bit, depending on what you have inside the alarm, but if it's still working, it's probably okay. Worst case is the LED life will be diminished if it's overdriven; you can check the current as mentioned. If you want to play it safe, put a resistor in parallel with the 5.1K instead of directly shorting it, try 1K to start and change values as necessary for your desired brightness.

martiale

Copper - Posts: 52

Joined: February 01, 2006

Location: United States

Posted: February 02, 2006 at 1:25 PM / IP Logged

Where can resistors be purchased? Is Radio Shack a good place?

'99 BMW E36 M3 Coupe

tex12gage

Copper - Posts: 83

Joined: September 28, 2005

Location: United States

Posted: February 02, 2006 at 1:39 PM / IP Logged

Radio shack is fine.

tex

ripley01

Member - Posts: 33

Joined: November 27, 2005

Location: Netherlands

Posted: February 02, 2006 at 1:42 PM / IP Logged

The 5k resistor limits the current through the LED to approx 2mA when connected to 12V. That's a pretty low current, so it might be a good idea to put a small resistor in there in case the alarm system drives a lot more current through the LED. Don't live in the US, but yeah, Radio Shack is a good place for buying resistors.

martiale

Copper - Posts: 52

Joined: February 01, 2006

Location: United States

Posted: February 02, 2006 at 1:48 PM / IP Logged

I'll give a try with 1K and monitor the brightness, it should be OK somewhere between 1-3K,.

Thank you all.

'99 BMW E36 M3 Coupe

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Alarm Led

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