Constant Current Driver

[john2k]

Hi guys i'm a newbie here needing some help and advice. I have a DC constant current LED driver that delivers 280mA. The input range is 12-24V DC and it gives a constant current output of 280mA. I've been told that the input range can handle the voltage fluctuations of a car which usually varies from 12v - 14v. However, when testing the driver, i realised that although the current output always delivers 280mA, the voltage output fluctates with the fluctuation of the car voltage. But because the current always remains at 280mA whether car is delivering 12v or 14v, the current seems to always be 280mA. I'm just wondering if the voltage fluctuation will in any way effect my LED's or reduce its lifespan? I was considering adding a Voltage regulator but someone recommended to use a 5W 12V Zener Diode instead which will block anything above 12v.

any help would be greatly appreciated.

thanks

[AnAppleSnail]

What load are you testing the driver with? If it drifts while not driving anything, you'll have to test it with a load of some kind. You might consider using an incandescent bulb with similar current draw. Watch to see if the voltage and current under load change.

If it drifts while pushing load, the zener will be helpful. BUT! A 5W Zener set to 12v will always be dumping voltage in your car. Healthy car electrical systems must reach 14v to charge the battery. A 12V Zener between the car and driver will be fighting your whole alternator output. This will probably toast the Zener. The Zener on the LED side is less likely to lead to exciting problems.

Say, this isn't for automotive safety lighting, is it (Headlights, turn signals, DRLs, brake lights, or other blinky things on the outside of the car? Sounds like an overly-bright interior light. Right?

[john2k]

AnAppleSnail, thanks for your reply.

The current does not really shift i've connected a multimeter inline to read the current draw and the current draw is always at 280 +/- 2. My LED unit that I am trying to power up contains 4 high power LED's wired up in parallel and the entire unit is rated at 12V 8W and I am basically trying to under-drive it to keep the brightness much lower. But my concern is whether the fluctuating voltage output will effect the LED's in any way which is why someone suggested adding a 5W 12v Zener. But then someone else said that if the current is regulated and constant, then the LED's will not get more than what they need and are well within their limits so should be fine. Just a little confused really.

I've tried wiring the LED unit up with this driver via a multimeter to measure the current draw and it's always 280 and also the brightness is definately reduced immensly as I wanted. I also wired up a halagon bulb to see if the driver restricts the current to the bulb, and it does it stops at the 280mA and the bulb doesnt power up. Whereas a sidelight 5W bulb powers up but very very dim.

From what I understand so far, since the 4 LED's are wired in parallel they will each get approx 70mA but will get a 12-14V voltage each, so my concern is whether that 12-14v is ok for the LED. I'm guessing the 70mA per LED is well within the limits because they are high power LED's and dont really warm up at all when I use this driver but warm up a little when I use the supplied power supply.

[AnAppleSnail]

LEDs don't run at 12v, so it's probably wired in series. No matter, with drivers you just need to match voltage operation range and current outputs. But if the driver runs them at constant current, then they can't get too much voltage. For a given current, there is a voltage that the LED will require (And vice versa). The relationship isn't linear like with lightbulbs.

[john2k]

Thanks for your reply AnAppleSnail very much appreciated,

they are definitely wired in parallel i've opened one unit up and checked it, there are 4 small high power led's inside the unit and the input wire has a label that says 12V 8W. The LED driver i'm using gives 280mA, I can regulate the voltage to 12V if needed using a voltage regulator. It all works fine and with less current i am getting the desired brightness but i'm just not sure if everything technically is ok with the voltage issue as i don't want to blow the led's

[alpg88]

there is no problem with voltage spikes, as long as they stay in leds forward voltage range.

as for 12v being ok for leds, for some it is, but for most, it isn't, most likely your leds have a resistor, or they would fry instantly from 12v.

[john2k]

there is no problem with voltage spikes, as long as they stay in leds forward voltage range.

as for 12v being ok for leds, for some it is, but for most, it isn't, most likely your leds have a resistor, or they would fry instantly from 12v.

this is what i am having trouble understanding. exactly what you say is what i understand so far, but i opened up one of the units and the leds inside are just high power led's soldered to a heatsink kind of material and wired up in parallel. there is absolutely no resistors or any other components. The power supply that originally powers the LED unit has a label saying 8W 12V same as the label on the LED unit itself. So by me hooking up my 12V 280mA driver i am still giving the unit the exact same 12V that the original power supply was giving it but less current. So the bit I remain confused about is the fact that in parallel the voltage is not shared so every led is potentially receiving 12V each and from what people are saying a high power led cannot handle 12V forward current. So i am left confused as to why and how comes this is working? I have been using this 280mA driver for few months now with a 12V regulator just to be on the safe side and the led's all work fine.

any suggestions?

[alpg88]

wait.
led driver is cc so it will adjust output voltage, depending on how many leds are connected in SERIES, if your leds are in parallel, the driver will see it as 1 led, and supply correct voltage (3-4v) to the load. led cc drivers almost never specify output voltage, it isn't constant.

[john2k]

wait.
led driver is cc so it will adjust output voltage, depending on how many leds are connected in SERIES, if your leds are in parallel, the driver will see it as 1 led, and supply correct voltage (3-4v) to the load. led cc drivers almost never specify output voltage, it isn't constant.

ah now that makes sense, your a genius so i think that explains why my led driver says output voltage is 3-18v DC and output current is 280mA, i guess like you are saying if the led's are in parallel then the output voltage will only be for one led whereas for series it will be total for all the led's. so does that not technically mean that I do not need to worry about regulating or limiting the voltage in any way on the constant current driver because the voltage output will all be provided by the driver?

[wquiles]

so does that not technically mean that I do not need to worry about regulating or limiting the voltage in any way on the constant current driver because the voltage output will all be provided by the driver?

You still need to worry about feeding the constant current LED driver the appropriate voltage - all LED drivers specify what operational voltage range it can support, to guarantee constant current in the output.

Whether you connect the LED's in parallel or in series affects what type of LED driver you need. So the sum of the vf of LED's in series affects the topology of the LED driver. If the sum of the vf's for the LED's wired in series is greater than the input voltage to the LED driver, then you need a boost LED driver. If the sum is less than the input voltage to the LED driver, then you need a buck LED driver. You can read about some very good drivers (and a comparison table with input voltage ranges supported) from www.taskled.com

In this thread, post #3, I cover some of the basics on driving LED's:
http://www.candlepowerforums.com/vb/...lashlights-...


Will

[john2k]

Will, many thanks for your reply. Now i am confused once again LOL

Please can you or someone advise me on my particular setup if you dont mind.

My LED Unit is a sealed DRL unit which has a label saying 8W 12V. I've opened up a spare unit to see the wiring inside. Inside the unit there are 4 high power LED's which look like Luxeons and they are attached to a heatsink and definately wired in parallel.

The constant current driver that i've been using for a while to reduce the brightness and under-drive them has a label on it that says the input voltage is DC 7V-24V and output voltage is DC 3V-21V. and constant current output is 280mA. So from what I am understanding so far is that since my 4 LED's are wired in parallel the driver should ouput the required 280mA but yet only provide 3V of voltage whereas if the 4 led's were wired in series the driver would deliver 12V. Is this correct?

If so then doesnt this mean that I do not need to worry about fluctuating input voltage supplied from the car which fluctuates from 12V-14.4V? because the car's fluctuating voltage is within the input range parameter of the driver and the driver will only output the voltage needed for the LED's?

Many thanks once again

[alpg88]

The constant current driver that i've been using for a while to reduce the brightness and under-drive them has a label on it that says the input voltage is DC 7V-24V and output voltage is DC 3V-21V. and constant current output is 280mA. So from what I am understanding so far is that since my 4 LED's are wired in parallel the driver should ouput the required 280mA but yet only provide 3V of voltage whereas if the 4 led's were wired in series the driver would deliver 12V. Is this correct?

If so then doesnt this mean that I do not need to worry about fluctuating input voltage supplied from the car which fluctuates from 12V-14.4V? because the car's fluctuating voltage is within the input range parameter of the driver and the driver will only output the voltage needed for the LED's?

Many thanks once again

yes

[AnAppleSnail]

I think DRLs are regulated automotive equipment. Are yours OEM?

[john2k]

I think DRLs are regulated automotive equipment. Are yours OEM?

Yes they are OEM, but there is no other components other than the LED's themselves inside the unit. there is only the 4 led's themselves inside the unit

[wquiles]

Will, many thanks for your reply. Now i am confused once again LOL

Please can you or someone advise me on my particular setup if you dont mind.

My LED Unit is a sealed DRL unit which has a label saying 8W 12V. I've opened up a spare unit to see the wiring inside. Inside the unit there are 4 high power LED's which look like Luxeons and they are attached to a heatsink and definately wired in parallel.

When you connect LED's in parallel, the LED's share the total current being fed to them. So if the total output current is indeed 280mA, and you have 4x LED's in parallel, that 280mA is shared - so each one gets approximately 280/4, or about 70mA (I say approximately since it is highly unlikely that all 4x LED's have the same identical vf value).

So from what I am understanding so far is that since my 4 LED's are wired in parallel the driver should ouput the required 280mA but yet only provide 3V of voltage whereas if the 4 led's were wired in series the driver would deliver 12V. Is this correct?

Yes. The LED driver will increase its output voltage until it reaches the desired output current (in this case 280mA). So the output voltage is variable, and adjusts to meet that 280mA of output current.

If so then doesnt this mean that I do not need to worry about fluctuating input voltage supplied from the car which fluctuates from 12V-14.4V? because the car's fluctuating voltage is within the input range parameter of the driver and the driver will only output the voltage needed for the LED's?

Assuming that the designer of the LED module/driver did his/her job, yes, you should not have to worry. The driver "should" be able to automatically maintain its output current when the input voltage is within that range.


Will

[wquiles]

Yes they are OEM, but there is no other components other than the LED's themselves inside the unit. there is only the 4 led's themselves inside the unit

So there is no LED module/driver/something visible besides the 4x LED's? Nothing?

I think we need to see photos of this setup.

Will

[john2k]

The LED driver will increase its output voltage until it reaches the desired output current (in this case 280mA). So the output voltage is variable, and adjusts to meet that 280mA of output current.

Thanks for your reply Will. So since the output voltage is variable and adjusts to meet the output current, is there a chance that the output voltage might end up going high and blowing the LED's? is there a way with a multimeter perhaps to measure exactly how much voltage is being drawn by the unit? I've figured out how to measure the current drawn by adding the multimeter inline with the positive cable.

Do you think maybe it might be worthwhile maybe limiting the output voltage of the driver to 3.3V by adding a 3.3v regulator or something to prevent the output voltage going more than 3.3v?

I'm away from home for a few weeks on business so will try and get pics when i get back.

[wquiles]

So since the output voltage is variable and adjusts to meet the output current, is there a chance that the output voltage might end up going high and blowing the LED's?

That is exactly the job of the LED driver. To adjust the output voltage to maintain a constant current. If the output voltage ends up too high and kills the LED(s), it means that the LED driver failed to do its job, either because its operating range/conditions were exceeded, a poor design, a component/piece/part of the driver failed, or any of the above.

Again, I am assuming that this module/kit/package you are using actually "does" have some sort of LED driver - not just LED's corrected to the car's 12 supply. This is why need would like to see pictures of this setup, instead of guessing.

is there a way with a multimeter perhaps to measure exactly how much voltage is being drawn by the unit? I've figured out how to measure the current drawn by adding the multimeter inline with the positive cable.

If you open the current path from the output of the LED driver to the LED's and use a voltmeter/multimeter that has current measurement capabilities, you should be able to measure the output current going to the LED's.

Do you think maybe it might be worthwhile maybe limiting the output voltage of the driver to 3.3V by adding a 3.3v regulator or something to prevent the output voltage going more than 3.3v?

No. That would completely negate having a constant current LED driver in the first place. Plus, a voltage regulator does not work will LED's, since as LED's heat up, their vf lowers (becomes smaller), so the current through the LED increases (since the voltage regulator maintains a constant voltage, NOT a constant current), the LED gets even warmer with more current, the vf keeps getting smaller, which drivers a higher current, the LED keeps warming up, etc. until something melts/blows up ...


Will

[mattheww50]

It is unlikely that the output voltage would go up high enough to damage the LED. LED's are non-linear devices, the current rises very rapidly with very small changes in voltage once the LED actually lights up. As long as the current to the LED is well below the device limits, the voltage will remain well below device limits.

So as long as the LED's in the device are all working, there is zero probability of exceeding the voltage rating at the current your driver is providing. This makes further protection such as from a 3 terminal
3.3v regulator a waste of time and effort.

[john2k]

That is exactly the job of the LED driver. To adjust the output voltage to maintain a constant current. If the output voltage ends up too high and kills the LED(s), it means that the LED driver failed to do its job, either because its operating range/conditions were exceeded, a poor design, a component/piece/part of the driver failed, or any of the above.

Again, I am assuming that this module/kit/package you are using actually "does" have some sort of LED driver - not just LED's corrected to the car's 12 supply. This is why need would like to see pictures of this setup, instead of guessing.

The LED lamp unit does have a driver unit that connects to the LED unit itself. So there are basically 2 parts, one is the LED unit itself which consists of the 4 LED's wired in parallel and the other part is the driver to power it which has a label saying 8W 12 on it. But what i've been trying to do is under-drive the LED's because the original driver that comes with the LED unit makes the LED's too bright and they do warm up a little. I want them much dimmer. So thats why I bought a 280mA constant current driver.

How does the driver detect if it's parallel or series and give it either 3V or 12V? Just wondering my led's are in parallel and the driver should i'm assuming give no more than 4v max whereas if it was in series it should give 12v.

[wquiles]

The LED lamp unit does have a driver unit that connects to the LED unit itself. So there are basically 2 parts, one is the LED unit itself which consists of the 4 LED's wired in parallel and the other part is the driver to power it which has a label saying 8W 12 on it. But what i've been trying to do is under-drive the LED's because the original driver that comes with the LED unit makes the LED's too bright and they do warm up a little. I want them much dimmer. So thats why I bought a 280mA constant current driver.

So you are basically replacing the circuit that came with this kit with a different circuit? Do you know what current was being fed to the LED's with the original circuit?

How does the driver detect if it's parallel or series and give it either 3V or 12V? Just wondering my led's are in parallel and the driver should i'm assuming give no more than 4v max whereas if it was in series it should give 12v.

Usually there is a small-value, current sense resistor, either inside the driver IC, or external to the driver IC. Each driver does this a little bit different, but they have a feedback loop in which they:
- measure current on that sense resistor - basically a voltage is generated as current goes through a known, high precision resistor (most of the time with a value less than 1ohm, like 0.1 to 0.01 ohms).
- if the current is still too low, it increases the voltage
- measure current again, keep adjusting the output voltage
- if the current is too high, lower the output voltage (there should be some hysteresis as well)
- etc.

Some drivers also have provisions for open circuit on the output, short circuit on the output, thermal overload, etc..

Will

[john2k]

So you are basically replacing the circuit that came with this kit with a different circuit? Do you know what current was being fed to the LED's with the original circuit?

Yeap that's exactly what i am doing. all i know is that the original circuit has a label that says output 12V 8W nothing else.

they have a feedback loop in which they:
- measure current on that sense resistor - basically a voltage is generated as current goes through a known, high precision resistor (most of the time with a value less than 1ohm, like 0.1 to 0.01 ohms).
- if the current is still too low, it increases the voltage
- measure current again, keep adjusting the output voltage
- if the current is too high, lower the output voltage (there should be some hysteresis as well)
- etc.

So from what you explain above it seems like the as long as the constant current output on this new driver does not exceed the max current the led's are rated for (in this case 8W which will be over 600mA) then technically it should be fine because the driver will slowly raise the voltage until the desired current is met in this case the desired current for the LED driver is 280mA. But can 280mA not be delivered in many different voltages? like for example lets say the current reaches it 280mA and the voltage gets to 3V, and still continues to climb up but the current stays at 280mA, will the LED technically speaking still be ok?

[john2k]

I have just come across an article that says the following:

If you are planning on using the driver to power an individual LED, or to run numerous LED's in series, you would need to ensure that you get a constant current driver.

If however you are planning to run multiple LED's in parallel, you would require a constant voltage driver.

Does this mean I am using the wrong type of driver all this time ?

[alpg88]

that is wrong, to drive leds you need cc driver, period, burn that article.

[wquiles]

Yeap that's exactly what i am doing. all i know is that the original circuit has a label that says output 12V 8W nothing else.

Do you know if the new LED driver is rated/designed to work in auto/car environments? Voltage spikes, temperature, etc.?

So from what you explain above it seems like the as long as the constant current output on this new driver does not exceed the max current the led's are rated for (in this case 8W which will be over 600mA) then technically it should be fine because the driver will slowly raise the voltage until the desired current is met in this case the desired current for the LED driver is 280mA.

The constant current LED driver does not care nor knows what is the current rating on the LED's. It only knows to try to regulate current by varying the output voltage, until the current is about 280mA, or until an internal limit is hit, or the device over-heats, etc.. You can actually use that LED driver, connect it to a 5mm LED (which a rated current of about 20mA), and pretty much kill it instantly. OR, you can use that driver with an SST-90 (current rating of about 9Amps), and the LED driver will simply under-drive the LED and provide about 280mA - even if the LED is rated to 9Amps.

But can 280mA not be delivered in many different voltages? like for example lets say the current reaches it 280mA and the voltage gets to 3V, and still continues to climb up but the current stays at 280mA, will the LED technically speaking still be ok?

No, that would be violating Ohms Law:

Voltage = Current * Resistance

In the case of the LED, the LED represents a variable resistor (as its vf curve varies with the current and temperature), but the law still holds true.

I have just come across an article that says the following:
---Quote---
If you are planning on using the driver to power an individual LED, or to run numerous LED's in series, you would need to ensure that you get a constant current driver.

If however you are planning to run multiple LED's in parallel, you would require a constant voltage driver.
---End Quote---
Does this mean I am using the wrong type of driver all this time ?

In general, and taking those sentences out of context, without knowing more about the article and other information, I would say the article is wrong. LED's need a current regulated supply, regardless of how they are wired. Period.

But the real problem is really driving LED's in parallel: It is generally not a good idea, as the vf of the LED's is usually not the same, so the current flowing through each LED will be different, even as the voltage is held constant across all LED's. The way around this problem is to test and match the vf's of the LED's as best as possible, so that they draw about the same current (which in the case of 4x LED's, each LED would draw about 1/4 of the current).

Will

[alpg88]

what i,m confused about, is why your leds are in parallel, your driver can drive them just fine in series, and they'll be brighter too.
unless they are soldered to a heatsink\star by heatpad and negative(or positive, doesn't matter really) common terminal, but in that case why it is marked 12v280ma??? you will burn them if you put 12v to them.
was the light new in the box when you got it? or it was used, and someone might rewire or put different leds???

[wquiles]

what i,m confused about, is why your leds are in parallel, your driver can drive them just fine in series, and they'll be brighter too.
unless they are soldered to a heatsink\star by heatpad and negative(or positive, doesn't matter really) terminals, but in that case why it is marked 12v280ma??? you will burn them if you put 12v to them.
was the light new in the box when you got it? or it was used, and someone might rewire or put different leds???

+1

Same thoughts/worries that I have. Something is not quite adding up

Will

[john2k]

what i,m confused about, is why your leds are in parallel, your driver can drive them just fine in series, and they'll be brighter too.
unless they are soldered to a heatsink\star by heatpad and negative(or positive, doesn't matter really) common terminal, but in that case why it is marked 12v280ma??? you will burn them if you put 12v to them.
was the light new in the box when you got it? or it was used, and someone might rewire or put different leds???

no they are brand new phillips brand, the driver that comes with the unit is not marked as 12v 280mA its marked as 12v 8W but i am trying to under-drive the unit with my 12v 280mA driver

[john2k]

Will, I think I am now understanding a bit better. I've got a few of those 280mA led drivers in my car and a multimeter so had some time to pop into a local electronics store nearby and bought myself a few 1W LEDs to do some testing with my multimeter and the driver. I carried out the following tests:

First I wired the 3 led's in series, connected them via my 280mA driver and added the multimeter in the current line to measure the current draw, it read exactly 0.28 (280mA). I then removed the multimeter so that while the 3 LED's were lit I can measure the voltage and the voltage read just above 9V so thats around 3V drop across the 3 LED's.

Then I decided to remove 1 LED from the series so that 2 were wired in series and then the voltage read 6V which means the driver is correctly adjusting the voltage. And because it's wired in series the voltage is adding up per LED.

I then decided to wire the 3 LED's in parallel and connect them via the driver. And first i tested the current draw and it was also exactly 0.28 (280mA) I then measured the voltage while the 3 LED's were lit in parallel and the voltage read 3.2V. So i think this test basically answers my questions.

So it seems that with this 280mA driver my parallel wired LED unit will get 280mA with a voltage of 3.2V because it's in parallel, but if the exact same unit was to be modified into series then the same driver would deliver 280mA with 9V.

So I am assuming that my concern about the fluctuating input voltage from the car power source will not be a problem because firstly it's within the drivers input range and secondly the driver will only output the voltage as tested above?

[wquiles]

Will, I think I am now understanding a bit better. I've got a few of those 280mA led drivers in my car and a multimeter so had some time to pop into a local electronics store nearby and bought myself a few 1W LEDs to do some testing with my multimeter and the driver. I carried out the following tests:

First I wired the 3 led's in series, connected them via my 280mA driver and added the multimeter in the current line to measure the current draw, it read exactly 0.28 (280mA). I then removed the multimeter so that while the 3 LED's were lit I can measure the voltage and the voltage read just above 9V so thats around 3V drop across the 3 LED's.

Then I decided to remove 1 LED from the series so that 2 were wired in series and then the voltage read 6V which means the driver is correctly adjusting the voltage. And because it's wired in series the voltage is adding up per LED.

I then decided to wire the 3 LED's in parallel and connect them via the driver. And first i tested the current draw and it was also exactly 0.28 (280mA) I then measured the voltage while the 3 LED's were lit in parallel and the voltage read 3.2V. So i think this test basically answers my questions.

So it seems that with this 280mA driver my parallel wired LED unit will get 280mA with a voltage of 3.2V because it's in parallel, but if the exact same unit was to be modified into series then the same driver would deliver 280mA with 9V.

So I am assuming that my concern about the fluctuating input voltage from the car power source will not be a problem because firstly it's within the drivers input range and secondly the driver will only output the voltage as tested above?

That surely behaves as a buck-type, constant current LED driver. That is good

As to whether it will survive in the harsh environment of a car? Typically when designing circuits, you design for a particular worst case scenario, in a known type of environment. For small electronics, the auto/car represents a tough case, specially with regards to temperature, inductive voltage spikes, etc., so who knows how well that LED driver will work/last in a car. That I can't answer.

Will