Shedding some light on the RPL auto shut-off

[Aseras]

Hi.. let me add this I think of Jack as a Honest car salesman. He's not the engineer who built the car and he's not pulling the slick shit and lying to make a sale like a certain other WLaser company. I'm glad he's so open and will actually talk about his products and stand by them. He's an A-1++ guy IMHO. You can't buy support as good as Jack has been giving here.

Here's some things that may get you going in the right direction. Jack any idea you can tell me what's in the big black chunk of heatshrink? Couldn't get a reading it it with the multimeter, and I just don't feel like cutting it off and soldering it all back together.

 

[Aseras]

Put one cell in the laser and run it until auto off happens(use diode current level 9 or a non-adjustable RPL so it will eventually shutdown). When auto off happens, shut the laser off and wait 5-10 seconds and turn it back on again and note how long it takes to auto off again. As soon as it auto offs, quickly swap the battery in the laser for brand new fully charged one and see how long it takes to auto off. This will show that it is mostly if not completely Dependant on the battery rather than the laser or diode heat.

The reason you can have auto off happen and then wait a few minutes and have it work again is that when the battery is under load it eventually hits the voltage level that causes the auto off. When the laser it turned off for a few minutes the battery recooperates itself somewhat. This too can be proven by making a test fixture that switches a 2 ohm (10 Watt) resistor across the battery and then measure the battery voltage under this load after 3 minutes. Then shut off the circuit and wait a few minutes or so and then switch the circuit on again and right away measure the voltage. What you measure will not be as low as it was at the end of the previous 3 minute run on the resistor.

As soon as mine starts "blinking" when the auto cut off happens I can flip the button off and on and presto full output again and it'll run for another few minutes just fine. Going down one power level I can run my laser nigh forever. It runs , and get pretty damn warm but it still runs nice and strong ~120mw. I let it go over an hour one night to freshen/discharge the battery, and the RPL was very warm, probably 120ish, but it was still going. I think the "protection" circuit has something to do with current but I cannot figure out how it works. it's very odd. Also the adjustable tailcap is weird too in that it's a resistor to limit maximum current. Setting 1 is usually ~40mw, but if the battery is low 1 doesn't work at all and 2, ~80mw works fine, but no 1.

 

[Deleted member 10578]

Hi Aseras,
The the heatshrinked item in your picture is a resistor, I don't remember the exact value, but it's value is pretty low, 1 ohm or less.

You can learn more about the battery protection circuit by downloading and reading the pdf file I linked to above.

The tail cap is just a BCD switch attached to a network of resistors. At current level 9 and using a zeroed out milliohm meter (zeroed to subtract out the resistance of the meter leads), the resistance is ~0.25ohms. When the current is set to level 1, the resistance is ~1.1 ohms. This .75ohm change means the difference between min and max current. This is because this is a very low impedance / high current device. That is why when you measure current you have to use a method that does not add more than ~0.03 ohms to the circuit, other wise output power will be reduced and the measurement won't be accurate.

Do not attempt to reduce the tail cap resistance by shorting it out or part of it out. It will destroy the laser in short order. The tail cap more than covers the optimum point and operating range.

I would also not be opening it up and taking it apart on what looks in the photo to be the top of wooden desk/table. The diode and mosfets are extremely ESD sensitive and an ESD protected workstation is needed to prevent damage. The damage can occur in one zap that you don't even notice and destroy it right away, or it can partially destroy it in that one ESD even does not out right ruin it, but instead waekens it and reduces it's operating life or sets it up for easier destruction by a 2nd ESD event.

I guess I'm trying to scare you into being very ESD safe and careful with these. The static discharge you would notice ie carpet or clothing zap is several thousand volts, the amount needed to destroy the diode or mosfets would be only need to be 10's of volts and you would not ever notice this small amount.

The reason the laser won't work on current level 1 when the battery is low is because the IR diode has a threshold current that must be reached before lasing can occur. With a low battery level 1 does not allow the amount of current needed to lase. If I remember correctly it requires ~450mA to turn on.

 

[Aseras]

It's a fancy desk/workbench, it has esd buttons for straps on the back edge... not that I ever use them though, I usually just ground out on the frame before I start. Haven't fried anything yet ( crosses fingers ) lol.

About the only time I bother with the ESD straps is for bare diodes and other loose electronics. Anything that has mass like this, I don't even bother. Mostly out of laziness and a general disregard for the static boogeymonster.

 

[pseudonomen137]

I played around some more, and indeed, I do get longer runtimes with slightly drained batteries.

Jack, I really don't understand why you insist its a circuit to protect the battery. I've yet to see a single remote shred of evidence to support that claim. On the other hand, so far everything seems to fit with the manufacturer's story of it being an odd way of determining when the diode is too warm. Again, if it were merely to protect the battery from discharging too far, the first auto-off would be the last - time to charge up a new battery.

Another point of intrigue, and this one I can't explain, is that I realized it doesn't truly shut off. It just drops the current to a fraction of regular operating level. :-?

 

[Deleted member 10578]

I played around some more, and indeed, I do get longer runtimes with slightly drained batteries.

Jack, I really don't understand why you insist its a circuit to protect the battery. I've yet to see a single remote shred of evidence to support that claim. On the other hand, so far everything seems to fit with the manufacturer's story of it being an odd way of determining when the diode is too warm. Again, if it were merely to protect the battery from discharging too far, the first auto-off would be the last - time to charge up a new battery.

Another point of intrigue, and this one I can't explain, is that I realized it doesn't truly shut off. It just drops the current to a fraction of regular operating level. :-?

Hi Carter,
I'll explain why I insist it's a circuit to protect the battery.
Download this spec.
ftp://ftp.sii.co.jp/pub/ic/spd_dtst/dt_sht_e/lithium/S8211C_E.pdf

This is the same device used in the RPL.
Here are the reasons I insist it's a circuit to protect the battery.
The description of the S-8211C on page one says "Battery protection IC for 1-cell Pack"
Now that doesn't mean it is used in that way or for that purpose.

If you look on page 25 (figure 14), there is a connection example diagram shown, it is an exact copy of protection circuit board in the RPL. Wired in exactly the same manner you will see if you make a schematic of the traces and components on the protection board. The FET1 and FET2 are not discrete FETS, but are a dual FET IC with the drains tied together internally. It's an NEC PA1870 or it's equivelent part. I've seen both the NEC part and a generic equivelent used. In fact the circuit even has values of R1, C1 and R2 that are within the recommended range for the example.

What you describe as working better after some discharge is very possible, but not the norm as I've seen. What it could be is that you will notice the protection IC comes in many configurations which have the thresholds hard wired internally. I've seen several different ones used and the models made before 1/2007 have thresholds which are easier to trigger. There is a threshold called "overcharge detection voltage" and it could be that the charger is getting the battery right up to this threshold so that the FETs are partially shutting off/ reducing the current. This would make the beam output lower until the cell is dicharged enough so the FETs are fully turned back on. This is just a guess as to why some may work better with a partially discharged cell.

Take care Carter, I gotta go tuck in the kids and read the younger one a story.

Jack

 

[pseudonomen137]

Well, I understand that that circuit may be originally used for battery protection. However, that's not how its being used in the laser. Not only would it be nonsensical to put one in like that, and the manufacturer says otherwise, but in use it doesn't operate that way. Again, if it were to protect the battery from going out, it should only activate when the battery is weak. It activates readliy on a fresh battery, and the battery works just fine after a little cool down time for the laser.

I'm guessing that the circuit may be in there with the diode, using the overdischarge feature to detect when the diode is being overpowered (IE, not being used for the batt, but the diode instead).

And then again, it could just be there for the battery as you say, but if that's the case it very clearly is not contributing to the auto-off in that case.

I'm still interested in why the auto shut off keeps running a little bit of current through the diode... that may lead to some more insight into the workings here.

 

[jayrob]

For longer run times, if a guy had an RPL 300 with an adj. tail cap, does anybody know aprox. what kind of mw the laser output would be on level 9, 8, 7, 6 etc...?
Jay

- Edit -
Nevermind I understand now....set to optimum level and not higher. Unless maybe the battery starts to get low...then maybe turn it up a click or two.....

 

[iLikeLight]

Recently there's been a bit of confusion about the RPL's auto shut off and what triggers it, so I tried to do some investigation. At first it was said to be to protect the laser from overheating, then Jack mentioned it having more to do with the batteries. Finally, someone (I think it was Aseras) mentioned getting longer runtimes with slightly used batteries. Well, it looks like they are all partially correct. Here's the situation as I understand it from the broken English:

The RPL auto shut-off is ultimately to combat the laser overheating, but it doesn't do this by a temperature sensor. As a laser diode heats up, it becomes less efficient and so the RPL circuit makes it draw more power to sustain output. Eventually it starts trying to draw more current than it was spec'd to, and that's when the auto cutoff triggers to shut down the laser temporarily.

How the actual driver circuit works on the RPL puzzles me, but I did learn that a fully charged battery results in both higher input and output powers for the diode/laser. Therefore, using a fully charged battery, or a higher capacity battery will cause the diode current to already be close to auto off levels, whereas letting the battery drain a little before use may allow unlimited runtimes.

I hope that helps all you RPL users.



PS: If you are considering an RPL, send me a PM as I may have some useful info for you.


PPS: I haven't had time to work on my site recently (Will be called A1 lasers and used for selling the random stuff that passes through my laser collection), but eventually I will get it online and be able to sell additional tailcaps to you RPL owners who would like to have both (I bought my RPL 300 with the adjustable endcap, and my RPL Blue 17mW with the non-adjustable endcap. I found I much prefer the non adjustable one, but I only have one of those endcaps for two lasers)

I think it would be great to have an RPL, but how soon will there be an actual green diode available for cheap? I've heard that they exist but they're currently very expensive.

I would be disappointed if the new diode technology became available 1 year after I bought an RPL. :'(