Why do Petzl Headlamps perform like this with NiMhs?

i am putting it here 'cos my question is related more to the electronics of...

So...here are my measurements on a Tactikka XP LuxIII modded with a SSC P4



and while the NiMh batts were fresh of the charger and in good condition, the output is lower than the new alkalines

My guess is that it has to do with how the electronics treat input voltage...Something like inetervening with a DD drive....?

but my knowledge stops here....your explanation is needed....

BTW...Petzl claims a 50& increase in boost (there is a thermistor on the back of the led that cuts boost at 20 sec lapsed)

Notice the huge difference in my measurements? Any chance that the LED is running DD (though the NiMh performance contradicts that). Most certainly a Vf issue? (if I remeber correctly the LuxII was a S-bin)

3xAAA to me means more or less DD in most of cases.

Since alkalines have more voltage at start, that would mean brighter. Once they are used, the voltage will decrease and the more stable NiMH will be better than alkalines.

The pros are simpler design is less prone to fail if the components have a minimun quality. I can see the point on using 3xAAA lights with brands like Petz, Mammut, etc unlike most 3xAAA ebay ones (If the price is right, of course)

I don't see anything surprising in the figures.

I'd assumed from the non-regulated nature of the lights that the Tikka XP
used either PWM with a series resistor, or switching different series resistors by multiple FETS, (or both), so lower battery voltage -> lower output and current draw.

I do have a Tikka XP (one I found buried under boulders and underwater in a cave streamway, which I managed to get working and upgrade to a P4), and that has multiple 16R and 1R5 resistors on the main board, so I'd guess the multiply-switched resistor idea is the likeliest.

If you draw a graph of current against battery voltage (presumably battery voltage off-load?), the points for the 'high' and 'medium' settings lie pretty much on a straight line hitting the voltage axis at about 2.8V, which is exactly what you'd expect for a resistor-limited circuit.

On the boost setting, the point don't lie on as 'good' a line, but that could easily be accounted for by voltage drop of the alkaline cells under load.

hmm let me see if i got that straight...

the electronics are in fact switching resistors as the voltage gets lower....so in effect the headlamp is a resistored drive.

I do have trouble visualizing what you say about the graph....

you measn that in both cases the peak is at 2.8Volts (is that under load?) and amp draw will not get any higher as voltage increases, because the extra voltage is consumed by the resitor that is selected each time?

VALI
e pros are simpler design is less prone to fail if the components have a minimun quality.


Does that mean it is the most rugged design, even more rugged than the malkoff electronics that have only 4 parts?
https://www.candlepowerforums.com/threads/237645

kosPap said:

hmm let me see if i got that straight...

the electronics are in fact switching resistors as the voltage gets lower....so in effect the headlamp is a resistored drive.

Click to expand...

I think it switches in different resistances for different power settings, but for a given power setting, there's only one fixed resistance.

kosPap said:

I do have trouble visualizing what you say about the graph....

Click to expand...

Basically I made a graph from your figures of current and battery voltage. For each battery voltage (full alkalines, used alkalines and NiMH cells), I plotted points for the boost, high and medium power levels, which gave three points for each of the power levels.

For the high power level, the thee points were pretty much on a straight line. Drawing a line through the points, and carrying it on, it crossed the voltage axis at around 2.8V
That's pretty much what would be expected for a fixed resistor drive. Since even the highest current (high power, full alkaline battery) was only 0.18A, the actual Vf of the LED at that current would have been very little different from the Vf at the lowest power level (0.12A), and would also not be massively different from the Vf of the LED when it was just barely turning on.
As a rough approximation, if we pretend that the Vf of your LED was close to constant, having a resistor in series with it would give a graph which started at Vf on the voltage axis, and then went in a straight line with increasing current with increasing battery voltage, with the slope depending on the resistance.

Things were very similar for the medium level, just with the crossing point being very slightly lower, though it's hard to be completely accurate with lower currents measured in 10mA steps

For the boost level, the three points weren't really in a straight line, but that might well have something to do with voltage measurements being made without a load. If the alkaline voltage levels had each been a little lower, a very similar picture would have emerged to that at the high and medium levels.

Doing some *very* rough calculations, for the high level the results are consistent with a total resistance of about 10 ohms, and for the medium level, about 19.
Bearing in mind the roughness of the calculations, that could be something like the high level using two 16R resistors in parallel (giving 8R) and the medium level using one (giving 16R), with possibly a little more resistance from elsewhere in the light (or one of the 1R5 resistors?), and/or just from error in the rough calculations and lack of precision in the numbers.

With even rougher calculations, in the case of the boost level, that seems like a resistance of something like 2 ohms, which could be a 1.5R resistor and extra resistance in the circuit or LED, though at lower resistances, it's a lot harder to be sure since all kinds of things become relatively more important.

kosPap said:

you measn that in both cases the peak is at 2.8Volts (is that under load?) and amp draw will not get any higher as voltage increases, because the extra voltage is consumed by the resitor that is selected each time?

Click to expand...

Very roughly yes, at least at the medium and high settings.
At those relatively low currents, the Vf won't vary much, so as a first approximation, an LED and resistor could be visualised as a 'constant voltage sink' with the current flowing in the circuit varying linearly with the 'surplus' battery voltage above the level of Vf.

wow! what can i say...yuo have been very helpful with this and the some other times in the past....

guess at these current draw levels (excluding boost) Vrest and Vload will be close enough...

I really do not know waht to think of the Petzls at these era of regulated headlamps...maybe tehy are still the workhorses (and the "maglites") of the headlamp world.....

So....I ahve saved the thread to read better at home...
Thanks SO much for your help! Kostas

I was already vaguely interested in how the Tikka XP worked, but had never really got around to really thinking about it until you asked, and with the figures you measured and a peek inside, it was fairly easy to make an educated guess as to what was happening.

I'd assumed it might use PWM for at least some of the power control, but I hadn't even done a quick swipe test to see if it did or not.

Though the XP is a handy little light for a toolbox, and I use it round the workbench a fair bit (looking for things that have dropped on the floor/behind equipment, where the on/off UI makes it great for a grab light), I haven't really used it for anything important since I found it at the start of the year, since it rather falls in between the Zebralight I sometimes take as a backup light underground and the other larger and rather brighter LED headlights I use for most above-ground things.

In some ways I think the unregulated nature is maybe best for a 3xAAA light - you don't expect stellar performance from a 3xAAA, and having a nice slow fade does give you confidence you'll have *some* light.

My second-best larger headlight is a 3xAA Ring Cybalite Extreme.
They're really nicely-built lights, with O-rings on the headset and battery case, and are a complete doddle to modify from the original 1W side-emitting Luxeon to a Seoul P4, giving far more light and a much nicer beam. After getting one and modding it, I ended up buying/modding/selling about another 25 of them, since an outdoor store was shifting end-of-line stock, and they were being sold off ridiculously cheap.

However, they have a buck/boost circuit inside which doesn't have *any* kind of graceful tail-off - they run regulated right to the point where the cells are dead, and then just switch off, and even trying to turn them back on at a lower power rarely gives light for more than a few seconds.
They're fine if you have spare cells, but I do wish they had at least some kind of tail-off, at least for the last 10-20% of cell capacity.

indeed this might be the best selling point of the Petzls..Average for the average user....

I did not observe any PWM in the low mode

Also... here is a passage I prepared at home

Well at the quiteness of home one can really see into things...

First of all. I really understood what you were saying about that line going to 2.8Volts....

Originally I though it was an horizontal line, but once I got over that everything made sense...So I looked at the SSC P4 datasheet, and what do you know, at about that amp draw the Vf is 2.8Volts..guess you knew better.....

It seems that you also remember well your Petzl innards....Somehwere in your post you mentioned a 15 Ohm resistor....well looking at the pics by fellow meber Szemhazai in this thread ...what do you know again.....

2x 1 ohm and 3x 15 ohm resistors, aren't they?

So....I recharged the batts and took measurements again....

Vbatt = 4.35V SSC P4 U-bin

Mode...........Vin......Adraw...........Vf......Vdrop........Equiv Resistance
Boost......4.05 V…...0.62 A.........3.5 V.......0.55 V..........0.88 Ohm
High.......4.26 V......0.15 A.......2.95 V.......1.31 V............8.7Ohm
Med........4.29 V…...0.09 A.........2.8 V.......1.49 V...........16.5 Ohm
Low........4.33 V…..0.035 A.......2.75 V.....1.575 V.............45 Ohm

So...
Low: 3x 15ohm in series are 45 ohms
Medium: 15ohm
High: 2x 15ohm in parallel is 7.5 ohm
Boost: ?
Here I am lost 2x 1ohm resistors in parallel are 0.5 ohm right?

And a second "new alkaline / 4.53V" measurement gave me this
Mode...........Vin......Adraw...........Vf.....Vdrop......Equiv Resistance
Boost......4.09 V…...0.68 A.......3.6 V....0.49 V......0.72Ohm

Could one be safe to assume that all resitance above the assumed 0.5 Ohm is circuit resistance?