# Constant current driver



## SirJMD (Jan 3, 2010)

Anyone know a good constant current driver, that delivers around 300mA?

Got a few requirements:
- Dont want to buy a driver
- Prefer leaded instead of SMD
- Input voltage @ 21VDC or 15VAC
- 300mA constant
- Good effeciency (80+)


Ive made a schematic with LM3407, that should be 90+ efficient. Downside is, that the LM3407 is a SMD - a quite small one actually (recommended footprint for the pins is only ~1x0.4mm!).

So what im looking for, is like "Have a look at the IC named xxx", or something like that 


Looking forward to see what you guys can come up with


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## zzonbi (Jan 3, 2010)

34063


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## SirJMD (Jan 3, 2010)

zzonbi said:


> 34063



Looks interesting, but from what i can see, you need to use its reference voltage to set the current. Seems to me, like it isnt that good for constant current source.


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## zzonbi (Jan 3, 2010)

The current may be set by Rsc, with a lower drop (I guess they all use some voltage as reference).

AMC7150 is even simpler, with only 5 terminals.


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## SirJMD (Jan 4, 2010)

zzonbi said:


> The current may be set by Rsc, with a lower drop (I guess they all use some voltage as reference).
> 
> AMC7150 is even simpler, with only 5 terminals.



Altho its an SMD, the footprint is actually quite okay. Gonna give it a look aswell.


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## SirJMD (Jan 5, 2010)

Updated the requirements a bit. Im gonna do some calculations - i might just go with a capacitor and an inductor. Should give an efficiency close to 99%.


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## Ekke (Jan 5, 2010)

SirJMD said:


> Should give an efficiency close to 99%.



With what and how? I don't think so..


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## SirJMD (Jan 5, 2010)

Ekke said:


> With what and how? I don't think so..



Well, the capacitor doesnt dissipate power and neither does the inductor - that just leaves the LEDs.

After a few ms, the inductor will act as a short - which just leaves the capacitor and the LEDs. I plan to use 7x1W. If we believe 99% is true, that would mean 70 mW dissipated in the capacitor - sounds a bit high to me.


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## Ekke (Jan 5, 2010)

SirJMD said:


> Well, the capacitor doesnt dissipate power and neither does the inductor - that just leaves the LEDs.
> 
> After a few ms, the inductor will act as a short - which just leaves the capacitor and the LEDs. I plan to use 7x1W. If we believe 99% is true, that would mean 70 mW dissipated in the capacitor - sounds a bit high to me.



Constant current driver? Huh? :thinking:


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## SirJMD (Jan 5, 2010)

Ekke said:


> Constant current driver? Huh? :thinking:



The capacitor will limit the current thru the circuit. However, if the applied voltage changed, so does the current. But if it does change, it will only be a tiny bit - it wont matter.

At a given frequency, the capacitor will have a reactance equal to 1/(2*pi*f*C), with f being the frequency and C being the capacitance in farad.


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## Th232 (Jan 5, 2010)

Could you post up a circuit diagram?

Right now the only similar circuit I can think of is where you put the cap in series between the power source and the LED. We use that trick in neurostimulators for controlling charge injection, but that's essentially an AC signal and has no inductor, so I can't see how it would work here.


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## zzonbi (Jan 5, 2010)

If the inductor acts as a short, the capacitor acts as an open circuit. So you wouldn't even need those...


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## SirJMD (Jan 6, 2010)

zzonbi said:


> If the inductor acts as a short, the capacitor acts as an open circuit. So you wouldn't even need those...



Do you know what a damped oscillation is?


Th232 --> Sure, here it is:






Normally you would use a resistor instead of the inductor. The reason why its even needed, is because if the surge-current you risk having thru the LEDs, if you turn on the circuit at the top of the sine wave.
The idea behind it all, is that the capacitor doesnt dissipate power - eventho it limits the current, just like a normal resistor. We like that  But we dont like the surge-resistor, since it will dissipate power.


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## r0g3r (Jan 6, 2010)

Im using Lt1676 to drive 3 LEDS in series.
Voltage range is 9 to 50 volt DC with around 87% efficiency.
To limit the current im using a 1.5 Ohm resistor and a pnp transistor.
When the current is 350mA the voltage drop over the 1.5 Ohm resistor
starts the pnp to conduct and the collector (connected to the feedback pin)regulates the voltage.
lovecpf


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## Th232 (Jan 6, 2010)

Thanks for the diagram, now it makes sense, especially the inductor bit. Would you believe that after mentioning an AC signal in my first post I then thought "But nah... why would he use AC?", completely missing the part of your OP where you mentioned 15 VAC?  Very interesting way of doing things.

Small question though (mainly since I haven't played around with this), what'll the current ripple be like? My initial impression is that you'll have the LEDs changing brightness by some amount whatever freq the AC signal is at,but by how much I'm not really sure.


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## SirJMD (Jan 6, 2010)

r0g3r said:


> Im using Lt1676 to drive 3 LEDS in series.
> Voltage range is 9 to 50 volt DC with around 87% efficiency.
> To limit the current im using a 1.5 Ohm resistor and a pnp transistor.
> When the current is 350mA the voltage drop over the 1.5 Ohm resistor
> ...



Throw in a schematic  always fun to see how others have made their own stuff 





Th232 said:


> Thanks for the diagram, now it makes sense, especially the inductor bit. Would you believe that after mentioning an AC signal in my first post I then thought "But nah... why would he use AC?", completely missing the part of your OP where you mentioned 15 VAC?  Very interesting way of doing things.
> 
> Small question though (mainly since I haven't played around with this), what'll the current ripple be like? My initial impression is that you'll have the LEDs changing brightness by some amount whatever freq the AC signal is at,but by how much I'm not really sure.



Yea i can imagine  This forum's focus is more on DC, since most people in here only deal with flashlights.

Youre correct - the LEDs will flash with ~50 Hz. Ive done some testing, and not really noticeable. Its a small sacrifice to make, in order to get a higher efficiency - which is my primary focus, since im going for highest lumen/watt as possible.

Ill update the thread when i get rest of the calculations done


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## zzonbi (Jan 6, 2010)

"Do you know what a damped oscillation is?"

An oscillation consumed through an element able to dissipate energy, such as a resistor or those leds?

So you count on the coil too to limit the inrush current. So in exchange for using more leds and flicker you save efficiency and a constant current driver.


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## SirJMD (Jan 6, 2010)

zzonbi said:


> "Do you know what a damped oscillation is?"
> 
> An oscillation consumed through an element able to dissipate energy, such as a resistor or those leds?
> 
> So you count on the coil too to limit the inrush current. So in exchange for using more leds and flicker you save efficiency and a constant current driver.



A damped oscillation could look like this:





What i want here, is the amplitude not getting too high - i dont want a big surge-current.


An inductor will try to prevent any sudden change in current, where a capacitor will try to prevent any sudden change in voltage. The idea here, is to use the inductor as a surge-resistor. As time goes (were talking ms here), the inductor will conduct more and more - at the same time, the capacitor will limit the current more and more, going towards the designed value.


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## r0g3r (Jan 6, 2010)

> Throw in a schematic  always fun to see how others have made their own stuff


Ok here it is.
The red line shows the led current.


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## space (Jan 6, 2010)

This is for SirJMD:
I think you should start to simulate this circut. I don't belive this can be called constant current. LED's will go to 0 current on each cycle. What frequency is the supply? The inductor could probably be dropped.

space


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## zzonbi (Jan 6, 2010)

The leds work half the time, but at higher current peaks, to get the same flux. What you save in driver losses could well go in leds losses.

It's nice and simple I agree, but you are putting the driver in the backseat...


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## SirJMD (Jan 7, 2010)

r0g3r --> Look interesting - what housing does that LT1676 have?. Ive been working a bit with the LM3407, and its eMSOP - extremly small :/

space --> Ill simulate it when i have calculated the values of the two components. Just need to figure out a model for it. Im quite sure it will work - so does my teacher at the university (electrical engineering).
The LEDs are supplied with 15VAC @ 50 Hz. You cannot drop the inductor, and the reason why is simple. If you turn on the circuit at the top of a sine wave, you will have a massive current surging thru the LEDs, since the capacitor isnt charged!


zzonbi --> Since its AC, yes. When youre working with AC, there's a peak current and a RMS current. Both can be calculated. I will of course make sure the peak current doesnt damage the LEDs. But as far as i see it, it should be a problem here at all.

Hopefully i get some time today, to do some calculations.


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## space (Jan 7, 2010)

SirJMD: 
As I said you really should start to simulate this circut. 
The output impedance of your transformer will very likely be so high that the inductor is not needed. (Your 15VAC transformer is also an inductor. A rather large one.)

space


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## SirJMD (Jan 9, 2010)

space said:


> SirJMD:
> As I said you really should start to simulate this circut.
> The output impedance of your transformer will very likely be so high that the inductor is not needed. (Your 15VAC transformer is also an inductor. A rather large one.)
> 
> space



Indeed, it is an inductor - problem is tho, that its a salvaged transformer, so i have no idea about its specs (input and output). Maby i should give it a run in a LCR meter.. not a bad idea !


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