Emitter Forward voltage - question

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Artiet59
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Emitter Forward voltage - question

I am wondering how forward voltage of an emitter effects .. well a flashlight’s driver, I guess?

For instance, if a flashlight has a driver appropriate for a led with a forward voltage (typical) of 3.3 volts, but I reflow in a led that has a forward voltage (typical) of 2.75 Volts, does this effect anything like how the driver operates or if the emitter will be surged with too much current? Or are typical current regulated drivers not so specific to account for one emitters forward voltage?

Scenario: I want to buy a Noctigon K1 with CULPM1 and reflow an sst20 into it (I would like to use the CULPM1 as my base light because I could use that emitter somewhere else). The forward voltage (I believe from reading the data sheet) of the CULPM1 is typical 3.3v, and the forward voltage typical for sst20 (I believe) is 2.73 volts.

Does this matter, per se? Will the light function fine with the sst20 in it? or is there some long term or short term repercussion from doing this emitter swap?

Thanks in advance.

merlot
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The rated voltage is a quite a conservative one, and different companies vary on how conservative they are. Instead, you can compare the voltage and current in the output tests found on this forum.

A quick peruse turns up Maukka’s output test for the sst20 here:

I can’t find an output test for the CULPM1, but the spec sheet has a graph of the voltage going above 3.4V, which is in the same range as the sst20. However it also shows the current going up to 8A which I think is a bit much for the sst20. If anyone else knows more about the CULPM1 I’m sure they could help you

Artiet59
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Thank you for psoting this Merlot. According to the CULPM1 data sheet, the typical forward voltage is 3.3 @ 6 amps. This is copied from the data sheet, whcih can be downloaded here: https://www.osram.com/ecat/OSLON%C2%AE%20Boost%20HX%20KW%20CULPM1.TG/com...
(english version)

Forward Voltage 5)
IF = 6000 mA
VF min. 3.00 V
typ. 3.35 V
max. 3.75 V

So I don’t know what that means in comparison to the sst20, except on the chart you shared the sst20 @ 6 amps = 3.725

Does this mean they would be fine using the same driver? I guess my concern is does it even matter? does a driver run differently based on the forward voltage of the led that it is powering? (these questions are for anyone)

Tom E
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About the driver dependent on the vF, depends on the driver design. Not sure exactly what Hank does in his K1 driver, but his Boost HX driver configuration is probably amp limited, not dependent on the vF - I'm only guessing here. I received a K1 with a W1 LED and swapped to the green W1. Since the green can handle more amps, I asked and he told me what resistor to change to result in increased amps, so I did swap the resistor and got more amps - been very pleased.

So, does the Boost HX K1 come with this same driver with a different resistor value? No idea. Certainly the SBT90.2 K1 has an open FET capability, so that driver must be different in some respect, maybe just firmware, or different design, not sure.

For me as a modder, Id' just swap the LED's, check the amps, and go from there. My best guess is the SST-20 would run anywhere between 6 and 8 amps which is probably fine. Hank has been good with answering tech question for me, so I'd start there.

EasyB
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I think that K1 driver is a constant current linear driver. Those differences in LED voltage won’t make a big difference in the driver function. In this case the higher sst20 voltage means it will fall out of regulation a bit sooner than the lower voltage WF LED.