Calculating Vf for MCE 4S- Mtbr.com

# Thread: Calculating Vf for MCE 4S

1. ## Calculating Vf for MCE 4S

Hi just wondering where you find Vf for an MCE wired 4S. Is it something you calculate or something you look up - if so where?

2. I have one that I have just checked 5 mins ago and it is 12.8 volts @ 700ma
It is not something you can calculate as they can vary a bit
but somewhere between 12.5 v and 13.5 v is normal

3. Cheers, now I can put it into the formulae on the maxflex sheet to see how much heat i'm going to make with the maxflex

4. The Vf is in the Cree MC-E datasheet.

5. Originally Posted by QsJK4Ht
The Vf is in the Cree MC-E datasheet.
The values in the datasheet are typical and maximum. They would be useful for Ants to determine approximately how much heat his maxFlex will develop. But, as Troutie's measured example shows, his 4S MC-E is running .8V less than the what the "typical" value from the datasheet would be. For many applications this would not be significant, but in the DIY bike light world, it makes a difference in choices of driver and battery configuration.

6. ants, what battery / pack/ voltage you planning to use?

7. Troutie's numbers are specific to his circuit, and his LED. They are 0.2v below Cree's typical (average) values per LED, but I don't know what equipment Troutie's actually using. The lower values may be indicative of Cree's quality control and process capability, but then they may not. They're also indicative of Troutie's quality control (his ability to measure accurately). Troutie doesn't say if he inserted an ammeter in his circuit, or what temp he measured at. It's probably best to use Cree's official numbers for your own designs, until you gain more experience with specifically known hardware.

I know that not everyone is an electrical engineer, but the technical information isn't that difficult to read. Here's what people should be using as a guide in their tinkering.

8. i'm looking at a sort of darkstar knock off - running two 4s MCE's in parallel off of an 11.1V Li-Ion battery through a maxflex

9. cheers QsJK4Ht..
ants, yep 4s led/3s li-ion battery/maxflex good combo ....
yo trout man, you bin insertin any dem ammeters lately?

10. Use a hipFlex and an 18.5V battery. Slightly more efficient circuit.

11. Originally Posted by Vancbiker
Use a hipFlex and an 18.5V battery. Slightly more efficient circuit.
+1
.......... but you must promise, never-to-crank-it-up-to-2.8A!!!!

12. ## No Inserting hammeters today

This is my measuring device .
as I am no electrical engineer , just an enthusiastic diyer.

so if this is a wrong way of measureing then I will happily be told the err of my ways
and shown the path to enlightenment

different MCE but same method only this one is wired 2s2p and the juice and current is there to see

temp is ambient about 15 c
and the block af ali has not yet warmed up much .
yes the vf does drop as the block warms

13. I think troutie gets the good stuff cutter keeps hidden under the counter.

Cree don't bin based on Vf (efficacy and flux at 350mA) so you never know what you might get.

For whatever reason I seem to get nice low Vf on my new XRE R2s but others still report 3.6V

Check out this discussion:
http://www.candlepowerforums.com/vb/...d.php?t=201190

14. Well I can tell you when I built the Darkstar I tried 11.1v batteries and it does work but...

Fresh of the charger you lose quite a lot of dimming capability as it goes to direct drive at lower currents so if you are not carefull you can burn the light as the dimming feature is not availalbe really for thermal protection.

I sell a 7.4V 6000MaH li-ion pack with the DS and it gets 3:15 runtime on high off the charger, before the maxflex cuts it at low voltage.

HTH

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