led driver selection for project

[Tha fios agaibh]

I am building a house number placard for the front of my house, that use 6 to 9 leds to illuminate each of the 4 numbers in my sign.

http://www.surrounding.com/Products/Luxello/Modern_LED_House_Number_5_Outdoor.asp

The customer support people say to run 12 to 16v ac or dc to supply it, but I have always heard it's better to run a constant current driver for leds. They can't seem to tell me the manufacture recommend constant current, or driver to supply it with. (They said 350ma constant current. That would surely fry it)
But I did get a breakdown of what each number draws at 12v; .033, .033, .020, .027
And @ 16v; ..050, .050, .030, .040
That totals 113 to 170ma or 1.4 to 2.7 watts.

I'm waiting to hear more from the vendor.
What are your thoughts?
Should I look for a cc driver of say, ...150ma? (Smallest I've seen)
Or just just go with a constant voltage driver of 12v?

Thanks, John

 

[Harald Kapp]

As the descriptions says these numbers can be driven from a standard doorbell transformer, they have probably (or should I say hopefully ) built-in current limiting, if only a few resistors.
Resistors would work wit a constant current source, too, they'd just drop unnecessary voltage. No problem as long as the LED driver's compliance voltage is high enough.

 

[AnalogKid]

Your question is not clear. Are you copying the Luxello product, or buying and using it? If you are buying the product, then follow the recommended power instructions. Many LEDs can handle over 300 mA, and CREE is a leader in high power LEDs for outdoor use. My guess is that there is a non-isolated, AC/DC input, constant-current regulator built into each digit.

ak

 

[Alec_t]

My guess is that there is a non-isolated, AC/DC input, constant-current regulator built into each digit.

The measured 20mA-50mA currents for 12V-15V input suggest otherwise. My guess is there's simply a series resistor.

 

[Tha fios agaibh]

Yes, I'm buying the product to use but their support people are not much help.

When asked what constant current driver to use, they said "350ma not to exceed 16v" I'm guessing they are confusing constant current, with constant voltage supply?

Barring getting some documentation, I don't really trust them.

 

[Tha fios agaibh]

As the descriptions says these numbers can be driven from a standard doorbell transformer, they have probably (or should I say hopefully ) built-in current limiting, if only a few resistors.
Resistors would work wit a constant current source, too, they'd just drop unnecessary voltage. No problem as long as the LED driver's compliance voltage is high enough.

Your probably right. If it didn't have some regulation, I'd think a crude supply like a doorbell xformer would be unstable to a dangerous level.

 

[Martaine2005]

Hi John,
My guess is it's safe to believe in the individual boards/numbers are regulated.
And probably quite nicely too hence,
Each number container 6 - 9 CREE LED. The LED panels are interchangeable and can be replaced at any time. The numbers are powered by a standard doorbell transformer or LED driver

Martin

 

[Tha fios agaibh]

Yes, I think your right. The leds would be burning out prematurely if there was no regulation.

I'm probably overthinking this one.

I will go with a constant 12v driver.

 

[Tha fios agaibh]

The measured 20mA-50mA currents for 12V-15V input suggest otherwise. My guess is there's simply a series resistor.

If it is regulated; I suppose when the supply voltage goes up, the current might also go up while the regulator circuit shunts the overcurrent on the load side of the circuit. In essence making the circuit look resistive from the line side?

 

[Tha fios agaibh]

Thanks Martin, I didn't catch that the panels are interchangeable.
This should allow me to inspect the board to see if there is a regulator chip, or just limiting resistors.
I guess I'll know in about a week.

 

[Tha fios agaibh]

Finally got my led numbers in the mail the other day. Packaging says 6 to 16volts ac or dc. I looked at one of the boards and found 10 leds with 2.7k resistors.
I did not see any regulator but there might be one on the other side of the board? I'd rather not take the board out and get too invasive.
Obviously, there must be a bridge to rectify an ac source.
Could leds function on such a wide range of voltages without a regulator?
I'm still wondered if I should just use a basic wall adapter, or add my own regulator circuit for stability and longevity of the lights?

 

[Alec_t]

I'd rather not take the board out and get too invasive.

Heck, it's only 2 more screws to remove .
My money says there's no regulator. Each LED has its own 2.7k current limiting resistor, so peak current per LED with a 16VAC supply would be about 7mA (assuming the LEDs and resistors are not all in one series string). Maybe the LEDs have some inbuilt protection against being reverse-biased, but I think you'd be wise to use a bridge rectifier with the AC source.

 

[Tha fios agaibh]

I'm thinking, if it can handle up to 16v and I use a 12vdc wall wort, any fluctuation would not harm the leds.
But again, I am surprised it will operate at such a wide voltage range.

 

[Alec_t]

Having seen the unit it's not surprising. At 12V DC each LED will draw about (12-3)/2.7 = 3.3mA, so the 10 LEDs together need 33mA and the whole unit will dissipate 12*33 = 396mW.
Cheap white LEDs are typically rated for up to 20mA, so a mere 3.3mA is nowhere near that.

 

[Tha fios agaibh]

Thanks Alec, the math works out pretty well. I hooked it up to 9vdc and got about 78ma Total. (Slightly high)
9-3/2.7=2.2ma X 34leds=75.5ma.

I plan to use a 12vdc ps and fuse it at .2A
I have a 125ma fuse, but I'm guessing that would be cutting it too close provided my 12v load draws 113ma?

John

 

[AnalogKid]

My bet is there there is no reverse voltage protection on the pc boards. The current limiting resistors are so large that even if the diodes go into reverse conduction the current is limited to a safe value.

ak

 

[Tha fios agaibh]

My bet is there there is no reverse voltage protection on the pc boards. The current limiting resistors are so large that even if the diodes go into reverse conduction the current is limited to a safe value.
ak

Thanks AK,
It must protect against reverse polarity with a bridge rectifier because It can accept an AC supply.
What I'm wondering is my .2A fuse sized correctly. Is there a rule of thumb formula for small non inductive loads like this? Say, 125%, or 150%?

 

[AnalogKid]

It must protect against reverse polarity with a bridge rectifier because It can accept an AC supply.

It might be a single series diode for half wave rectification. I'm having trouble with it being the only component on the bottom side of the board, and expensive manufacturing step. Any chance of a bottom-side view?

ak

 

[Tha fios agaibh]

It might be a single series diode for half wave rectification. I'm having trouble with it being the only component on the bottom side of the board, and expensive manufacturing step. Any chance of a bottom-side view?
ak

No, I didnt see the other side, but Its not a single series diode. I hooked up eight wires with no polarity marked and it worked with battery hooked up either way. At $60 per number it's not cheap.
NO, I have it mounted and silicone sealed now. I'll post a picture when I get it wired up.
Any insight on my fusing question?

 

[Tha fios agaibh]

Thanks for everyone's help.