Led Lighting Problem

[JohnW2]

However, what is the "9.5 W at 350 mA" all about.

That's what the light(s) is/are. As rocky said, that's what they get supplied with those Osram 700mA drivers, but I imagine they would work with a 350mA driver, or anything in between.

Well, it depends upon what the specification actually says, and what it actually means. If it's talking about one and the same light, which can tolerate a current of 700 mA (in which case power is 18 W), but that if one reduces the current to 350 mA, the power then reduces to 9.5 W, then fair enough. As you say, it could have indicated what the power would be at any current below 700 mA.

However, if, in the case I questioned, it was talking about a light for which the 'normal' operating current was 350 mA (9.5 W), then connecting it to the (then) pictured 700 mA constant-current source could well result in 18 W being dissipated for a very brief period of time, followed by a puff of smoke.

Kind Regards, John

 

[JohnW2]

Oops my mistake john, my confusion ... on that driver it does only state, as you say 700ma, i never noticed that, the pics of the net with the same model no as mine, yet on mine it says 700ma but also says it does 500ma with 18kohm resistor in the control port, hence, two currents, 500ma and 700ma (Not 350ma and 700ma as i misread) ... This is the actual ones i have, i also note the output states 0-700ma just to confuse me more, i assume thats to do with the dimming side

Well, yes, I suppose that is the dimming. It seems that what you have is a 'variable current' constant-current supply - by applying a voltage (0-10V) to the control inputs, or by putting a resistor across those inputs, one can alter the "constant-current current", seemingly across the range 0-700 mA - and that would obvious be a method of making the led dimmer (than it would be at a full 700 mA).

Then in the fitting instructions is where my confusion came from, it says two wattages. 9.5 watt at 350ma as well as 18 watt at 700ma At first glance i took that to be the two outputs as stated on the driver, but on further reading the instructions, the two wattages are actually two different fitting models, but using same fitting instructions and possibly do have different drivers. So the maths was right, but relating to two seperate drivers/fittings, sorry

Possibly, although, as I've just written to BAS, there is another possible (and simpler) explanation. Maybe they are talking all about the same fitting and ('dimmable') driver, for which the full ('normal') operating current is 700 mA (at which current it will produce 18W), but if one reduces the current (with the dimming facility), the power will obviously reduce (LED will "dim") and they are merely indicating that, as an example, if current is reduced to 350 mA, power will then reduce to about 9.5W.

Kind Regards, John

 

[333rocky333]

Get what your saying but in this case the instructions say
model cnse 500, 9.5w, driver 350ma 8w
model cnse 1000, 18w, driver 700ma 18w
I suppose still may be a chance there using the same 700ma 18 watt lamp in both models, would that make sense to do

 

[JohnW2]

Get what your saying but in this case the instructions say
model cnse 500, 9.5w, driver 350ma 8w
model cnse 1000, 18w, driver 700ma 18w

Fair enough. In that case, I was probably right to question it in the first place1

I suppose still may be a chance there using the same 700ma 18 watt lamp in both models, would that make sense to do

Yes, I suppose that's possible - the 9.5 W light must simply be an 18 W lamp run at half current (by using a 350 mA driver). The extent to which that "makes sense" depends on how efficient the drivers are at lower voltage outputs. In general, if one wanted, say a 10 W lamp it would usually be better (more efficient) to have a 10W lamp than to have a 20 W one dimmed to half brightness!

Kind Regards, John

 

[bernardgreen]

Input Voltage 180 → 254 V
Output Voltage 2 → 25V
Operation Constant Current
Output Current 0.7A

Implies that this driver when not dimmed will "drive" 700 mA through a number of LED elements that are connected in series.... The number of LED elements is irrelevent provided the sum of forward voltages across the string of LED elements is greater that 5 volts and less than 25 volts.

Current through an LED element creates a voltage across the element. This is the forward voltage and it is reasonably constant irrespective of how much current is flowing through the element

When the driver is dimmed a current lower than 700 mA will be driven through the string of LEDs. At very low currents it may be that if there are only 2 white LED elements the voltage across the string may drop below 5 volts and the driver may then become un-stable. LED elements other than white have lower forward voltages so more would be required to ensure the minimum 5 volts was met.

 

[JohnW2]

When the driver is dimmed a current lower than 700 mA will be driven through the string of LEDs. At very low currents it may be that if there are only 2 white LED elements the voltage across the string may drop below 5 volts and the driver may then become un-stable. LED elements other than white have lower forward voltages so more would be required to ensure the minimum 5 volts was met.

I'm not sure where all this talk of 5V comes from. As you correctly quoted, Rocky's driver specifies an output voltage of 2V-15V - and 2V is sufficiently low that it could probably just about cope with a single non-white LED element, certainly two of them in series.

Kind Regards, John

 

[bernardgreen]

I'm not sure where all this talk of 5V comes from

Me and my error. I was about to correct tmy post to change 5 to 2 and change the minimum number of elements required.

 

[JohnW2]

I'm not sure where all this talk of 5V comes from

Me and my error. I was about to correct tmy post to change 5 to 2 and change the minimum number of elements required.

Fair enough - it's always reassuring to see that I'm not the only person who makes mistakes What I forget to add it relation to ..

When the driver is dimmed a current lower than 700 mA will be driven through the string of LEDs. At very low currents it may be that if there are only 2 white LED elements the voltage across the string may drop below 5 volts and the driver may then become un-stable.

... is that, if it is a well-designed driver, then that should not be allowed to happen - i.e. it should not allow the current to be turned down so low that the voltage output (with the load in question) is less than the minimum stable output voltage it can produce.

Kind Regards, John

 

[333rocky333]

thats the bit confusing me, with say a 700ma driver, how can it be a CONSTANT current driver if it lets you lower the current.
When you try to alter the current, why dont it try to self adjust back to 700ma

 

[JohnW2]

thats the bit confusing me, with say a 700ma driver, how can it be a CONSTANT current driver if it lets you lower the current. When you try to alter the current, why dont it try to self adjust back to 700ma

It's "constant-current" for any particular input to the control input, but by varying that control input alters the value of the 'constant current'.

In other words, for any control input it will put the same current through any applied load (within its range of voltage capabilities). If one alters the control input, that will still be the case, but the "same current it will put through any applied load" will be different. Hence, but altering the control input, one can make it a "700 mA constant current source", a "350 mA constant current source", a "123 mA constant current source" or whatever takes one's fancy!

As I said, one of my bench power supplies allow me to do just that. It may be easier for you to think of a 'constant-voltage" (i.e. regulated) bench power supply which (as they often/usually do) has a knob one can twiddle to vary that regulated voltage. In other words, it is a "constant-voltage" (but adjustable/selectable constant-voltage) power supply. It's exactly the same with a 'adjustable' constant-current supply.

Kind Regards, John

 

[333rocky333]

Thanks yet again, its obvious now, you are good at explaining things without being over technical

 

[JohnW2]

Thanks yet again, its obvious now, you are good at explaining things without being over technical

You're welcome. Other analogies would be many thermostatically-controlled things, or 'cruise control' or anything like that. You could call them "constant temperature" or "constant speed" providers - but there is often/usually 'a knob to twiddle' to alter the constant temperature, or constant speed, or whatever, that one wants to be maintained. Same with 'adjustable' constant-current sources.

Kind Regards, John