how important is a driver

[Axkiker]

Im working on my lighting project which will be used in the auto industry. How important is a driver?

Say I were to use an led which required a voltage drop of 3.65V. If I used 4 of those in series that would require a source voltage of 14.6V.

So a typical car alternator pumps out 13.5 - 14.5 V when charging and of course when the auto is off its a straight 12v. With thise setup even with the alt charging I am still within the acceptable range for the leds voltage requirements. Yeah when its not charging the leds will be producing slightly less but im not really concerned with that.

Now from my understanding leds last much longer when powered by a driver. Im just curious how much of a difference there is.

If we are talking about going from a 40,000 hr life to a 20,000 hr life then I dont see a need for a driver. However if it cuts the life down dramatically then I see the need to incorporate a driver.

Being that I have not been able to experiment I thought I would ask the led gurus here

thanks

 

[tebore]

Im working on my lighting project which will be used in the auto industry. How important is a driver?

Say I were to use an led which required a voltage drop of 3.65V. If I used 4 of those in series that would require a source voltage of 14.6V.

So a typical car alternator pumps out 13.5 - 14.5 V when charging and of course when the auto is off its a straight 12v. With thise setup even with the alt charging I am still within the acceptable range for the leds voltage requirements. Yeah when its not charging the leds will be producing slightly less but im not really concerned with that.

Now from my understanding leds last much longer when powered by a driver. Im just curious how much of a difference there is.

If we are talking about going from a 40,000 hr life to a 20,000 hr life then I dont see a need for a driver. However if it cuts the life down dramatically then I see the need to incorporate a driver.

Being that I have not been able to experiment I thought I would ask the led gurus here

thanks

In an automotive environment a ROBUST driver will mean the difference between an LED's full life (50,000 hours) or 1 - 2 starts of the engine.

 

[Axkiker]

In an automotive environment a ROBUST driver will mean the difference between an LED's full life (50,000 hours) or 1 - 2 starts of the engine.

Could you explain more. I would not have thought that starting of the engine would have caused such a deastic loss in life. I just asumed as long as the max ratings were not exceeded that all would be good.

however the heck if i know lol

 

[Illum]

when starting the engine, do not expect the DC bus will read just 13.8V [or whatever a full starter battery will read]

LEDs fear power surge, and engine starting will create alot of it, for a few millisecond in the least which is more than sufficient to fry something on the microscopic scale.

 

[Alan B]

LEDs don't behave like light bulbs or resistors. A small change in voltage or modest change in temperature changes the current (and light output) substantially. You must manage the current in LEDs, not the voltage. That is what the driver does.

The automotive environment is very difficult for electronics. There are occasional high voltage spikes that can "punch through" the junctions in the LEDs and ruin them in a microsecond. The driver must protect the LEDs from transients.

If you are working on something that needs constant illumination, like a headlight, then the light output changes with temperature must also be compensated for. The driver can do that also.

The driver is the hard part, the LEDs are simple. So the real engineering goes into the driver, and possibly the optics. There is substantial complexity to getting it right.

 

[Axkiker]

good info... A driver it is


thanks

 

[tebore]

As stated above
spikes = bad

starting the engine is when most spikes happen and when the power in a car is dirtiest.

That's the reason for my drastic statement. Also that's why some electronics that plug in the cigarette lighter say to disconnect prior to starting the engine. Some car makers know people leave stuff plugged in when starting their car and have a safety. The car cuts power to all non-essential or high or possible high current devices/electronics in a key position just before going to the start position. Some older cars don't have this.

 

[blasterman]

You must manage the current in LEDs, not the voltage

Pardon my basic knowledge of electronics, but I have to ask:

If the voltage in a circuit never exceeds +V, and the circuit is designed so that +V can never hurt the LEDs passively, then why does current management matter? Just asking...

The reason we have to manage current is because voltage is bouncing all over the place, right? Not because there's some fundamental law of LEDs that demand we need to regulate current, correct?

Another question: LEDs, at least according to marketing, are robust when it comes to constantly turning them on/off. How does a LED tell the difference then between being turned on/off jillions of times -vs- being on circuit with erratic power?

So the real engineering goes into the driver

'Should' or 'does'. Clarify please

 

[Alan B]

Pardon my basic knowledge of electronics, but I have to ask:

If the voltage in a circuit never exceeds +V, and the circuit is designed so that +V can never hurt the LEDs passively, then why does current management matter? Just asking...

The reason we have to manage current is because voltage is bouncing all over the place, right? Not because there's some fundamental law of LEDs that demand we need to regulate current, correct?

Another question: LEDs, at least according to marketing, are robust when it comes to constantly turning them on/off. How does a LED tell the difference then between being turned on/off jillions of times -vs- being on circuit with erratic power?

'Should' or 'does'. Clarify please

If you design for max +V and the voltage never exceeds +V then fine. There is always a max voltage in the design. But the transient voltage can be 60V or more. Also the max voltage in an automotive setting is much higher than the average. So unregulated LEDs may vary considerably in their light output. If you design for a maxV of 16V, and the average is 12.6, and use a resistor and 3 LEDs in series the light output might be down by half or more.

If you raise the voltage on an LED just a little bit, the current goes WAY up. Not like a resistor. The important ratings for LEDs are in terms of current. The voltage at the rated current will vary depending on the temperature and the particular LED. If you have a batch of LEDs the constant among them is the current. Each will have a slightly different voltage at that current.

Turning them on/off is not a problem if the current is not exceeded. Wandering voltage will make the current go all over. If the current is not limited it may exceed the ratings.

Remember these are DIODES not resistors or filaments. When the voltage or temperature changes the current can change drastically. The voltage-current relationship is nonlinear. So you control things by regulating the current and allowing the voltage to do what it will. The regulator takes up the extra voltage, spikes, etc. The parameters that have to be controlled for a diode are forward current, reverse voltage and adequate heatsinking.

There are many cases of poorly designed drivers causing rapid LED failure. One example is LED lighting for the RV industry. Some designers figure the voltage is 12.0 and use a resistor with three white LEDs in series, set for 12.0 volts to get the design current. These bulb replacement LEDs fail quickly when subjected to the constant output of the RV battery charging system which is always higher than 12.0 volts when the RV is plugged in. So in a few weeks these expensive LEDs are dark.

 

[blasterman]

So unregulated LEDs may vary considerably in their light output. If you design for a maxV of 16V, and the average is 12.6, and use a resistor and 3 LEDs in series the light output might be down by half or more.

Ah, I got it now. The poorly regulated voltage environment of an automotive electrical system would cause unregulated, LED based headlights (dashboards, etc) to dim up and down, etc. This would be very untolerable, if not dangerous at night.

The voltage at the rated current will vary depending on the temperature and the particular LED.

Heatsinking is a far more controllable variable and one that *shouldn't* be an issue (there's that word 'should' again) , but I get the point.

If you have a batch of LEDs the constant among them is the current. Each will have a slightly different voltage at that current.

But the over-all delta should even out though, correct? Obviously if you have multiple parallel/series of LEDs, and one string has a large cumulative Vf variance than the other strings (basically seen as a single LED to the driver), then it will cause havoc with the driver trying to regulate current, and then causes problems with the other strings that are fine (?)

So...shouldn't we solve this by simply keeping a better Q/C of the LEDs we are using? This is also why it seems there are more problems with 'shower-head' type arrays of small LEDs vs arrays of power LEDs? More chances for Vf to vary.

There are many cases of poorly designed drivers causing rapid LED failure.

It's a huge problem on the fixed lighting side. Drivers are causing more MTF failures than the initial power fluxuations they are supposed to fix. I'm using computer PSU's as LED drivers for such a reason.

I guess the only issue I have with this is I think it's a bit simplistic to say 'just use a driver/regulator' when it comes to using LEDs in/on a car. Obviously we need a driver/regulator that itself is reliable and designed for the specific electrical environment that it's going in for safety reasons. It's not like buying a LED lightbulb at Walmart that will die in a few months and you just replace it. Hope I'm making sense and not confusing the original poster

 

[Alan B]

Ah, I got it now. The poorly regulated voltage environment of an automotive electrical system would cause unregulated, LED based headlights (dashboards, etc) to dim up and down, etc. This would be very untolerable, if not dangerous at night.

Heatsinking is a far more controllable variable and one that *shouldn't* be an issue (there's that word 'should' again) , but I get the point.

But the over-all delta should even out though, correct? Obviously if you have multiple parallel/series of LEDs, and one string has a large cumulative Vf variance than the other strings (basically seen as a single LED to the driver), then it will cause havoc with the driver trying to regulate current, and then causes problems with the other strings that are fine (?)

So...shouldn't we solve this by simply keeping a better Q/C of the LEDs we are using? This is also why it seems there are more problems with 'shower-head' type arrays of small LEDs vs arrays of power LEDs? More chances for Vf to vary.

It's a huge problem on the fixed lighting side. Drivers are causing more MTF failures than the initial power fluxuations they are supposed to fix. I'm using computer PSU's as LED drivers for such a reason.

I guess the only issue I have with this is I think it's a bit simplistic to say 'just use a driver/regulator' when it comes to using LEDs in/on a car. Obviously we need a driver/regulator that itself is reliable and designed for the specific electrical environment that it's going in for safety reasons. It's not like buying a LED lightbulb at Walmart that will die in a few months and you just replace it. Hope I'm making sense and not confusing the original poster

Unregulated LED in an automotive enfironment will likely fail when the transients occur. For example if a simple resistor is used to set the current in a series string of LEDs designed for 15V, when a 60V transient comes along the LEDs will briefly see more than 10x the design current. Say the string of 3 LEDs has Vf of 3.5V each and the resistor is designed to see the remaining 15V-10.5V or 4.5V the resistor will see 60-11V = 49V and the current will spike up by a factor of more than 10x over normal. So using a resistor as a regulator causes fluctuations in the supply voltage to be exagerated in current.

LED QC is a problem. They make them, and then then sort them out. They cannot control them precisely enough.

Best arrangement is to use a separate current control for each series string.

Heatsinking is a major issue. If you are trying to make a high power LED run reliably you have to heatsink for the highest operating temperature. On a hot day in a hot climate that can be quite hot. So cooling the LED becomes problematic. So if that is not done right the LED headlight will fail when turned on in the desert. You need a LOT more heatsink for hot conditions.

If you regulate the supply voltage (such as your computer ps) then a simple resistor per LED string can be an adequate current regulation unless the Vf shifts a lot due to environmental temperatures, or the light output needs to be regulated.

An LED string with series resistor is a current regulator. The quality of the regulation is a function of how much voltage across / power you waste in the resistor. More power wasted equals better regulation. Fewer LEDs in series makes for more resistors and more voltage across those resistors which makes better current regulation. It is cheap to design and build but wastes power.

Paralleling LEDs is bad unless they are matched in Vf and thermal properties, and are at the same temperature. IE paralleling dies in a P7 works because they are matched and in the same thermal environment. If not, the Vf's won't match and the currents won't divide equally. That is why LEDs are series connected.

 

[ledstein]

Im working on my lighting project which will be used in the auto industry. How important is a driver?


thanks

I didnt see any decent or above led product that didnt use a driver for high power leds. A LED driver is paramount for LED life. If you want to build a quality product you must use a driver.

 

[Axkiker]

So while we are on this subject maybe you can answer a few more questions.

My project involves using 6 leds running at 700ma a piece. As you know this is for an automotive application so 12V is all that we have to work with.

I had planned on running them all in one housing. Most likely would have 2 sets of 3 in series then run those 2 sets in parallel.

So here comes the problem. That has me running at 1.4 amps which is fine but I cant find a driver capable of putting out that much juice.

So my next thought was to just run each set of 3 off a seperate driver.

Can anyone else think of another way or a driver which will put out my 1.4A.

I havent really gotten to researching drivers yet. The place I found that seems to have a descent amount to choose from is LED supply. I figured I could use a couple of their buck pucks.

any ideas

 

[HarryN]

Hi - taskled.com (cpf r georges80) makes various drivers that will work for your application. At least some of them are designed specifically for automotive applications. The boost driver cchipo can for sure do this all in series and the maxflex might be able to do them in series as well with 1 driver. In a buck format, the hipcc should work.

The taskled.com web site has a link to an FAQ area where you can ask more detailed questions. George usually responds within 1 -2 days. His personality is a bit "direct to the point" if you know what I mean, but the products are solid and he is great to work with. He helped me with the design of the electronics of my "breeze" light project.

I have some of his drivers - they are pretty robust.

 

[Alan B]

So while we are on this subject maybe you can answer a few more questions.

My project involves using 6 leds running at 700ma a piece. As you know this is for an automotive application so 12V is all that we have to work with.

I had planned on running them all in one housing. Most likely would have 2 sets of 3 in series then run those 2 sets in parallel.

So here comes the problem. That has me running at 1.4 amps which is fine but I cant find a driver capable of putting out that much juice.

So my next thought was to just run each set of 3 off a seperate driver.

Can anyone else think of another way or a driver which will put out my 1.4A.

I havent really gotten to researching drivers yet. The place I found that seems to have a descent amount to choose from is LED supply. I figured I could use a couple of their buck pucks.

any ideas

A driver per series string is best. However, if you are using a driver you can series them all up and use just one driver at 700mA. Just use a boost driver.

 

[Axkiker]

thanks

all great info guys

 

[Steve K]

For reference, you may want to check SAE (Society of Automotive Engineers) document J1113-11. "Immunity to Conducted Transients on Power Leads"

SAE J1113-11 describes the nasty voltage transients that your device will need to survive. These transients can include negative voltages as large as -100v (for "pulse 1") or -150v (for "pulse 3A"), or as high +100v (for "pulse 3B").

At a minimum, you'll want a series diode to block negative input voltages. A power zener diode is useful to clamp some of the high voltage pulses, and a good input filter to block incoming noise (as well as reduce noise generated by your driver circuit).

There are a lot of aspects to a successful design. Better to learn from the mistakes of others than to make the mistakes yourself.

good luck,
Steve K.

 

[Axkiker]

For reference, you may want to check SAE (Society of Automotive Engineers) document J1113-11. "Immunity to Conducted Transients on Power Leads"

SAE J1113-11 describes the nasty voltage transients that your device will need to survive. These transients can include negative voltages as large as -100v (for "pulse 1") or -150v (for "pulse 3A"), or as high +100v (for "pulse 3B").

At a minimum, you'll want a series diode to block negative input voltages. A power zener diode is useful to clamp some of the high voltage pulses, and a good input filter to block incoming noise (as well as reduce noise generated by your driver circuit).

There are a lot of aspects to a successful design. Better to learn from the mistakes of others than to make the mistakes yourself.

good luck,
Steve K.

I agree I would rather get it right the first time.

I emailed the guy at task led today and am hoping that he can help me out. It looks like he has done some automotive led conversions so im hoping some of his drivers already incorporate these components.

I wish I had an engineer friend who could design out something for me however I dont. So I have to run with prefab items