Hi All,

I saw some informed posts on this forum and decided to register to ask a question.  I have been using the 365 nm emission line in a MV lamp to start a chemical reaction (university setting).  The arc lamp is hot and I thought I would try a 365 nm LED instead.

I was looking at this one (UV LED) , which is rated at 700 mA and 4.1V. 

I understand how to set up a circuit with a dry battery and a resistor but for convenience I was thinking of using a mains voltage LED driver.  The specs seem more complicated than those of a simple circuit. 

For example, looking at this (Driver) , it is rated from 2.5 – 6 V and up to 700 mA.  That looks suitable but the specs speak about constant current and constant voltage modes, which is the bit I don't understand.  Can someone tell me if this is the right driver and how it can have different modes depending on the power?

The LED specs give a range of voltage and currents centered on the 4.1V 700 mA ideal.  Within its range, how does the driver determine the actual voltage to be delivered to the LED?

Thanks for your advice, Mike.

PS You need eye and skin protection to work with these LEDs, which will give you a tan you weren't expecting if you mess about with them.

Only way I could think of is the driver senses the Non linear curve and finds the right spot.

The LED ballast (=driver) is a source of a constant current power system, so a system, where the current is dictated by the power source (the LED ballast) and the actual voltage by the load (the LED). So the ballast always feeds whatever load (with the voltage drop within the rated limits) with the same 700mA into any load with voltage drop within the 2.5 and 6V limits. If you want your load to get more power, for a constant current system it means the load has to drop higher voltage. And if you want to supply more loads from a single source, you have to connect them in series. Then the sum of the voltages has to be within the power source's limits.
As the LED has about 4.1V drop (so well within the 2.5 to 6V ballast limits) and is rated to be fed by up to 700mA (the current the ballast feeds), it should be perfectly OK.

Compare to that, e.g. the mains is a constant voltage power system, so the voltage is dictated by the mains (the 120 or 230V or so; it will remain at that level with any load), while the current flowing into the load is dictated by the load. So a hikgher power load has to draw higher current. As well here are the limits, with the mains it is just the maximum (the circuit breaker rating), with some "electronic halogen transformers" there may be the minimum load limit as well (below that the electronic does not work correctly). And if you want more loads there, you have to connect them parallel. And make sure, the sum of the currents does not exceed the source's limits (the circuit breaker,...).

Normally which system to use depends on the natural load characteristic. If the load keeps the voltage about the same with varying current (e.g. all the discharges and/or LED's), in order to have the power transfer stable, you have to use the constant current system (as these loads have the tendency to dictate the voltage, what is exactly what the load should do in a constant current system).

With batteries the electrochemical potential and the chemical reaction rate steered by the current makes these a natural constant voltage sources, so are used like that as well.
For an universal power distribution the constant voltage system allows easy connection and disconnection of additional loads, so it is used for that task.
In fact the ease of varying the connected loads made the constant voltage system the most common.
The constant current systems are used only for special purpose tasks and because of the rather limited power range, virtually only for a fixed topology system.

Of course, sometimes a constant current source is needed, while only constant voltage supply is available (and sometimes the vice versa, but that is rather rare). For that we have what is called "ballasts". It may be just a single series resistor,  to make a constant current source (e.g. for a LED) out of the available constant voltage source (e.g. the battery). But that is rather inefficient and does not provide that constant current, so the output varies a lot.
Therefore there are the dedicated ballasts, which are first way more efficient while providing way better controlled output (all the fluorescent and/or discharge chokes and ballasts, or here this small box for the LED).