EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: tigerstyle on August 08, 2021, 04:23:11 am
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I have a project where I want to install this LED into my car, flashing LED alarm deterrent.
And am new to LEDs, I've looked at lots of online advice but none seems to fit my question or thinking.
The spec sheet for this LED says 'Operating Voltage Max 12V' So is this safe to connect directly to the cars 12v system? No resistor required (I believe the resistor is actually built into the LED itself along with the flasher ciruit?)
https://www.jaycar.co.nz/medias/sys_master/images/images/9525069512734/ZD0240-dataSheetMain.pdf (https://www.jaycar.co.nz/medias/sys_master/images/images/9525069512734/ZD0240-dataSheetMain.pdf)
However the spec sheet later shows 2V as the max forward voltage, well where the LED draws 20mA of current and is brightest/safest, but again, is this taking into the account the built in resistor if there is one?
So basic, but I am so lost! Thanks :-+
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I don't see anything in that data sheet about it having its own resistor.
Based on that datasheet, I would use a resistor: (12-2) / 0.02 = 500 ohms. Using 560 ohms will keep the current to 18 or so ma.
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Given the operating voltage is specified as 12v and endurance test is at 10v, I would expect there to be either internal current limiting, or potentially just multiple die in series.
It would not make sense to specify either 10v or 12v in any other case, because of course your system voltage is irrelevant.
The charts will be for the bare die they have just copy-pasted into the datasheet.
However, remember your vehicle is **not** 12v, it's more like 14.4v when running.
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Its a flashing LED, which means it must contain two dies - the GaAsP/GaP LED itself and a Silicon controller chip for it, which when on probably provides constant current LED drive. The 12V abs. max. rating will be for the controller chip, and a series resistor on its own wont help as during the LED's off time it will draw very little current, so the voltage drop across the resistor will be very small.
Unfortunately automotive DC supplies are notoriously harsh electrical environments, with frequent positive and negative going spikes of up to several hundred volts and with sustained supply voltages significantly in excess of the nominal system voltage when the engine is running. For a nom. 12V vehicle, the running voltage will typically be in the 13.7V to 14.7V range and occasional sustained excursions above 15V may occur.
What happens if you overvoltage the LED by up to 25% is anyone's guess. It may last for years or it may die within minutes or even immediately. I'd bet on it giving trouble sooner rather than later.
The simplest option would be to add a series resistor and also put a 9.1V Zener (cathode positive) across the LED to prevent its off-state voltage rising higher. The datasheet is less than clear, but it looks like it probably draws 20mA in its on state. A 220 ohm resistor dropping from 14.7V to 9.1V will pass about 25mA. When the LED is off the Zener will dissipate about 230mW, so allowing for temperature extremes, a 500mW Zener would be advisable.
The fly in the ointment is the simple circuit more than doubles the current the LED draws - about 25mA all the time rather than just 20mA during the flashes only. Unfortunately 25mA is a significant drain on the battery, over double the self-discharge for most batteries for small and medium vehicles, so this simple solution is probably *NOT* advisable unless this vehicle is your daily driver and you disconnect the LED circuit if the vehicle is going to be unused for longer than a week or two.
Lastly if you don't want a vehicle fire, for %DEITY%'s sake fuse the feed to the LED! 100mA slow-blow should do the job.
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If the idea is to simulate an alarm system then you don't need to worry about the voltage when the engine is running, it will be on when the car is parked unattended. If you want it automatic you could use a normally closed relay with the coil powered by a circuit that receives power when the ignition is on.
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Its still going to get overvoltaged when you switch the ignition off and the relay connects it to a freshly charged battery. A 4.7V Zener in series with it (Zener cathode positive) to lop off a few volts would be a good idea.
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There was an app circuit in the old LM3909 single cell LED flasher datasheet which allowed it to run from high voltages (up to 100V depending on component values).
If you can find a NOS part, then this might be a more robust solution as the first component on the supply is a largish value series resistor. You might want to add a low current Zener to ground after the resistor but it probably isn't needed.
It would also give a more realistic short flash - and low power consumption. Most flashing LEDs tend to have a 50:50 duty cycle which is a it of a giveaway when compared with standard car alarms.
App circuit attached...
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The led will only be powered when the vehicle is off, so won't see 14.4v, but will see a charged battery so maximum will be just over 12v.
Is this what the LED is designed for?
Or start with the 500Ohm resistor and try there first? As this will be a safe starting point and won't blow the led. I have a bench top PSU I can test with before connecting to the car.
A different LED, but this PDF has better info than my first link: https://vcclite.com/wp-content/uploads/wpallimport/files/files/LTH5MM12VFR4100.pdf
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I'm surprised they make the absolute maximum voltage 12V, if that is indeed the absolute max it seems like it's just asking for people to connect it to a 12V battery and pop it. Given the low current involved, a 78L05 would be a really easy way to drop the voltage down, or even just a zener shunt regulator, for that you don't even need capacitors on the input and output.
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I'm surprised they make the absolute maximum voltage 12V, if that is indeed the absolute max it seems like it's just asking for people to connect it to a 12V battery
Yeah, so you're saying don't do that then?
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Yes, we all reckon you need something in-between to drop the voltage a bit to keep the flashing LED within its ratings when fed from a nom. 12V Lead Acid battery, and especially if said battery is connected to a vehicle alternator. As you can see from the above there's more way to drop the voltage than you can shake a stick at!
Note that James's suggestion of using a 78L05 regulator to drop the LED supply to 5V has its own input voltage limits that are likely to be transiently exceeded by spikes on the supply in a running vehicle, so should be combined with his previous suggestion of using a NC relay so its disconnected when the ignition switch is on.
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hi,
you gotta consider voltage between 8-9V and 15V and build a constant current supply for the led
how much current, it's up to you and led's datasheet (maybe you want 50% of his capabilities and he will live longer)
you want the led to blink, add this also, smaller the battery drain
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I don't believe modern cars actually have electrical systems which regularly produce nasty spikes. There are standards about load dump, but that only happens if the battery is disconnected, whislt the alternator is running.
Another thing to consider is how reliable does it need to be? If this were a control circuit for an airbag, then I would go to town on protection circuitary, but I doubt it's worth it in this case.
Here's my suggestion. It should be able to withstand continious voltages of up to 48V and handle short, high voltage spikes. It probably won't pass the strict standards, but should be reliable enough, for this application.
(https://www.eevblog.com/forum/beginners/help-with-a-basic-flashing-led-for-a-car-(is-this-led-12v-safe)/?action=dlattach;attach=1243714;image)
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Note that James's suggestion of using a 78L05 regulator to drop the LED supply to 5V has its own input voltage limits that are likely to be transiently exceeded by spikes on the supply in a running vehicle, so should be combined with his previous suggestion of using a NC relay so its disconnected when the ignition switch is on.
I once repaired the ECU (or maybe it was the ICU?) in a 80s VW and found it used a bog standard 7805 regulator. Incidentally the regulator itself was the fault so perhaps there is something to that, but I don't think they were generally known for failing.
The zener shunt is probably safer and will work just fine given the very low current involved. You could even use a transient suppression diode instead of a regular zener, they are very robust.