700mA LED

[Mecca]

Hi everyone,
I have a 350mA 18W supply going to a wine cabinet in my house. I also have a couple of 3W 700mA LED fixtures I would like to fit. Would I be able to run them in series on that driver at half brightness or is that not recommended? 1.5W per lamp of light would be fine. Or should I even run them in parallel? I'm not sure. Alternatively I'll need to get a 700mA driver or 350mA LEDs but would be nice to avoid that if possible.

 

[JohnD]

I have a 350mA 18W supply

You mean it's at 51 volts?

I don't understand.

 

[JohnW2]

You mean it's at 51 volts? I don't understand.

I imagine that he's talking about a constant-current driver - in which case, yes, 51V maximum.

Kind Regards, John

 

[JohnW2]

I have a 350mA 18W supply going to a wine cabinet in my house. I also have a couple of 3W 700mA LED fixtures I would like to fit. Would I be able to run them in series on that driver at half brightness or is that not recommended?

Is it a 'constant-current' 350mA power supply, and are your 700mA fixtures designed for a constant-current' supply?

If the answer to both questions is 'yes', the you could run one of the 700mA fixtures (or the two of them in series) from the 350mA constant-current source, but they would certainly run at 'half brightness' at best, quite possibly appreciably less than that.

If the answer to either of my questions is 'no', then the same is the answer to yours

Kind Regards, John

 

[aptsys]

but they would certainly run at 'half brightness' at best, quite possibly appreciably less than that.

Actually, they'll be greater than half brightness. Remember the Luminous Flux/Forward current curve is not a straight line and the highest gradient is in the first 3/4 of the maximum forward current.

 

[Mecca]

Thanks for your replies. Yes, constant current driver and LEDs. I'll hook them up and see if it's bright enough. I looked up the curve graph, quite interesting.

 

[JohnW2]

Actually, they'll be greater than half brightness. Remember the Luminous Flux/Forward current curve is not a straight line and the highest gradient is in the first 3/4 of the maximum forward current.

Thanks.

I must confess that, as will be apparent from what I wrote, that's not quite what I 'remembered'! However, having had a quick look around, it seems that it's pretty close to a straight line relationship in most cases - such that luminous flux at 'half current' is only fractionally more than half that at 'full current'. Whether or not a human being would perceive 'half the light output' as 'half the brightness' is, of course, a different question. This generic curve is fairly typical of what I found ....

Kind Regards, John

 

[JohnW2]

Thanks for your replies. Yes, constant current driver and LEDs. I'll hook them up and see if it's bright enough. I looked up the curve graph, quite interesting.

What did your curve look like - as I just wrote, the ones I found during a quick look around all seemed pretty linear?

Kind Regards, John

 

[Mecca]

What did your curve look like - as I just wrote, the ones I found during a quick look around all seemed pretty linear?

Kind Regards, John

No, you're right, I was looking at voltage v current curves which are not relevant here.

 

[JohnW2]

No, you're right, I was looking at voltage v current curves which are not relevant here.

Fair enough.

In any event, I think it is now clear that (contrary to what I initially implied) at 'half current' the light output will be no less than half of the output at 'full current'. However, as I've also said, whether a human being will perceive 'half light output' as 'half brightness' is a much more complicated question, to which I don't know the answer - but once 'you have done the experiment' you will probably know!

Kind Regards, John