Downboy and 'Low' mode

[akula88]

Downboy and \'Low\' mode

How does downboy circuit handle when tailcap is in 'low' mode ?

For the past two nights, I've been doing a 'runtime' test with my Aleph-3 TWOK/DB700 using 2xPila150s. Under high mode, the A3 lit up for 2hr 33min when the Pila protection sets in. Under low mode of 22-ohm tailcap, the A3 doubled its runtime to 5 hours, before complete shut down. While running under 22ohm 'low' mode, the output was contantly the same.

Is DB still regulating even while under resistored-mode?

The difference between the 2xPila vs 2xCR123 would be that the Pila would not go into 'moon' mode since cut-off voltage would be achieved before the threshold of DB is reached.

Any insights? Thanks.

 

[Billson]

Re: Downboy and \'Low\' mode

Hi Johnny,

I think the DB goes into direct drive if the vin goes below vf. The resistor does this to drop it out of regulation or the light would still be runnning at the set current of the board.

Am I making any sense? It's Monday morning and my brain still seems asleep.

 

[akula88]

Re: Downboy and \'Low\' mode

Monday blues /ubbthreads/images/graemlins/smile.gif

I'm really confused here. Since Vin (before a resistored-tailcap) would be 8.4volts, that would probably FRY the TWOK in a direct drive mode. The DB circuit is supposed to bucks down the voltage? (or current)...

Don, Darell, Tony (and all other electronics experts) -- please give us your take here...

 

[Flatscan]

Re: Downboy and \'Low\' mode

DB drops into DD if Vin < Vf. It has no boost circuitry, so it cannot regulate if it doesn't have the excess voltage to buck. With a resistor in series, Vbatt = Vr + Vdb (voltage of battery, resistor, and DownBoy, respectively). The low mode occurs because Vdb < Vf.

Were you metering the light during the runtime test? The output should have decreased, but it may not have been noticeable.

A little bit of trivia is that the boost boards (BB, NG, MM) continue to boost in low mode, but their provided voltage is too low to reach regulation (BB, NG) or just has less voltage to work with (MM).

 

[Billson]

Re: Downboy and \'Low\' mode

Double post

 

[Billson]

Re: Downboy and \'Low\' mode

DB is a buck converter so vin has to be higher than vf for the converter to work. It will only go into DD if the vin is below vf. The resistor drops the board out of regulation by dropping the voltage below what is required for the board to start regulating resulting in an under driven led and thus you get low mode. I hope I explained it correctly.

 

[Darell]

Re: Downboy and \'Low\' mode

Ha! Never assume that I'm any electronics expert! The good news is that the answers here are accurate... but that's about the extend of my knowledge! As long as the resistor brings the battery V down below the Vf of the emitter, you're in direct drive, and you are NOT regulated. Your final battery V after resistance will be different depending on the V that you start with (in other words, two rechargeables will be higher V than 2xCR123. If you start with hight enough battery V, then a resistor may not even bring the battery V low enough to enter low mode.

If you are truly in low mode (which it sounds like you are) then the output should not be totally flat. The the output of all Li chemistry is quite flat all on it's own, but it won't be as flat as regulation.

I've probably just repeated all the other posts *and* not made any sense. /ubbthreads/images/graemlins/icon3.gif

 

[McGizmo]

Re: Downboy and \'Low\' mode

Was the output constantly the same to your eyes or to a light meter? I would expect you to have seen or measured some drop as the voltage of the cells dropped. 22 ohms on 2x Pila must be a pretty darn bright low mode, yes?

 

[Chop]

Re: Downboy and \'Low\' mode

If that DB is feeding a Lux III, I would think that 60 ohms of resistance would be more appropriate, and you'd get a lot more runtime in low mode.

The li-ions do have a pretty flat discharge curve, as far as voltage goes. The dimming is ALMOST imperceptible.

 

[akula88]

Re: Downboy and \'Low\' mode

Everything was Eyeball Mk.1, so dimming is perceptive. I second what Chop has said, Pila does have a very flat discharge curve.

Darrel - It is in low mode. This was the A3 you built with Chop's 22-ohms tailcap.

Don/Chop - Yes, 22ohms is pretty bright with two Pila or 2xCR123 for that matter; but I like it that way.

Billson - Got it! But I'm no electronics experts, so the formulae are somewhere else. How do you compute for that? What is the expectant V(out) going into the Lux3 after the Batt-V (8.4v) is resistored with 22-ohms? /ubbthreads/images/graemlins/thinking.gif

Flatscan - Downboy in this case would have no chance in going into moon mode since Pila has a cut-off voltage of around 3.6, thus at 7.2v, it is still higher than the Vf (3.51-3.75) of the TWOK.

 

[PeterB]

Re: Downboy and \'Low\' mode

Only a small warning: You burn about 1W in your tailcap => might overload the resistors if you use it for long times.

Since VBatt is so much higher then VLed and the discharge curve is flat, you get a very flat low mode. Exactly the same es with resistorized lights: higher Vbat=> more constant brightness but reduced efficiency.

 

[Flatscan]

Re: Downboy and \'Low\' mode

[ QUOTE ]
akula88 said:
Billson - Got it! But I'm no electronics experts, so the formulae are somewhere else. How do you compute for that? What is the expectant V(out) going into the Lux3 after the Batt-V (8.4v) is resistored with 22-ohms? /ubbthreads/images/graemlins/thinking.gif

[/ QUOTE ]
You can't calculate it without knowing the Vf/current curve of your Lux. McGizmo's current/Vf/power/lux chart could help you get a rough estimate.

The easiest way to find your Vf on low is to meter it directly. Open the head from the top and measure voltage across the Lux while on. If the Lux turns off while you're doing this, get out of there, as you've shorted something. Luckily, the only likely component to burn is the 2-stage resistor board.

Vbatt = Vdb + Vr
Vdb = Vlux + Vd
Assume Vd = 0, this is the voltage dropped by the reverse-voltage protection diode, which should be pretty small.
Vr = i * R
R = 22Ohm
Vbatt - Vlux = Vr
i = (Vbatt - Vlux)/R
Vlux = 3.51V -> i = 222.3mA
Vlux = 3.0V -> i = 245.5mA
Vlux = 2.5V -> i = 268.2mA
Vlux = 0V -> i = 381.8mA (maximum current possible)

Is the Vf of your Lux going to affect its drive current and the power dissipated in the resistor board? Yes. Does it matter? Not really for the Lux, but as PeterB pointed out, you are burning a fair bit of power in the resistor board. If you really like the 22Ohm low, you should probably get another one made with more resistors to handle the power better.

What's "moon mode" on the DB? I don't remember ever reading about it.

 

[akula88]

Re: Downboy and \'Low\' mode

PeterB > Thanks for the warning. I never really thought about it. I now realize that the tailcap does get warmer compared with the head.

Flatscan > Very thorough explanation. Thanks. CPFers have been using the moon mode to denote a very low output condition. I think it also does apply, even if circuit is DB. I have been experiencing that kind of output when CR123s are well into their depletion stage.

/ubbthreads/images/graemlins/thanks.gif everyone. I need to digest Flatscan's post.