DX 30mW Green - V(in) vs I(LD) & Board Pic

[Benm]

Update on the above... I opened by DX20, which contains and identical circuit board, so i'd say the only difference between 20 and 30 is KTP efficiency.

I didn't completely retrace the circuit, but i'm assuming its correct and found no errors so far. I did some experiments on the reference voltage at the 431 regulator. Stock, this voltage is 1.32 volts. I yanked it to the negative rail, and the unit stops producing any light as expected. Also, i connected it to the positive rail via a 10k resistor, and the reference voltage rises to 2.31 volts. The strange thing is: It does not make any difference in diode current!

There is obviously something else hindering proper regulation, and as of now i believe it to be a problem with the LA6358 opamp. This component is specced with a minimum supply voltage of 3.0 volts, and and output swing up to Vcc-1.5 volts.

Running of 2 AA cells loaded with 250 mA or so, these specifications are never met. The limitation in output swing could affect the ability of the 'upper' opamp to give sufficient base drive to the 617 transistor - it has to deliver 0.7 volts (Vbe) plus the voltage over the sense (say 0.3 volts) to the transistor.

One solution i see is to connect the 100 ohms base resistor of the 617 to Vcc instead of the output of the duff PD feedback opamp. The circuit might then be unable to reduce drive and blow up the diode, but perhaps this is worth the experiment?

Any insights welcome, and i'll post further analysis results if i do more work on it.

Also: if anyone can provide specifications for the pump diode used, that would be very welcome! With some specs i'd actually know what the diode requires, and i might be more able to hook something up!

 

[chimo]

Without much analysis, one risk with driving the 617 from Vcc is the potential loss of a soft start.

This one may just require a new driver board. Perhaps a nice LDO, although the lowest I have on hand right now is 1V.

Paul

Update on the above... I opened by DX20, which contains and identical circuit board, so i'd say the only difference between 20 and 30 is KTP efficiency.

I didn't completely retrace the circuit, but i'm assuming its correct and found no errors so far. I did some experiments on the reference voltage at the 431 regulator. Stock, this voltage is 1.32 volts. I yanked it to the negative rail, and the unit stops producing any light as expected. Also, i connected it to the positive rail via a 10k resistor, and the reference voltage rises to 2.31 volts. The strange thing is: It does not make any difference in diode current!

There is obviously something else hindering proper regulation, and as of now i believe it to be a problem with the LA6358 opamp. This component is specced with a minimum supply voltage of 3.0 volts, and and output swing up to Vcc-1.5 volts.

Running of 2 AA cells loaded with 250 mA or so, these specifications are never met. The limitation in output swing could affect the ability of the 'upper' opamp to give sufficient base drive to the 617 transistor - it has to deliver 0.7 volts (Vbe) plus the voltage over the sense (say 0.3 volts) to the transistor.

One solution i see is to connect the 100 ohms base resistor of the 617 to Vcc instead of the output of the duff PD feedback opamp. The circuit might then be unable to reduce drive and blow up the diode, but perhaps this is worth the experiment?

Any insights welcome, and i'll post further analysis results if i do more work on it.

Also: if anyone can provide specifications for the pump diode used, that would be very welcome! With some specs i'd actually know what the diode requires, and i might be more able to hook something up!

 

[Benm]

I guess it hasn't much of a soft start as it is, apart from the limiting effect of the 10 nF in the transistors base (from 100 ohms, that starts it in about a microsecond).

Do you still have that opened pionter laying around? Perhaps you could measure the voltage across the actual LD at a couple of input currents. This could give an bit of insight in how much room there is to work with in terms of regulation voltage.

I would prefer modding the existing driver as it mechanically fits the unit, though its probably feasilbe to fit a DIY circuit in there as a last resort.

 

[chimo]

Unfortunately it's all closed up. I did jot down some figures with a 2.4V supply voltage.

Voltage across the LD series elements:

LD: 1828mV
617: 330mV
1R0: 242mV

Cheers,

Paul

 

[Benm]

That's okay - that figure is useful, and comparable to mine really. Was the dot nice and bright at that reading?

 

[chimo]

I don't recall, I had it pointed away and was concentrated on taking the readings. I think that 3x 2/3AAA NiMH betteries would realy make this thing sing. The batteries are available but I have not ordered any yet.

Paul

That's okay - that figure is useful, and comparable to mine really. Was the dot nice and bright at that reading?

 

[Benm]

Probably.. i've tried yanking the base drive voltage up, but it really hardly has any effect on the light output. To get some good light out steadily it just requires more than the 2 AAA's will give it. At 3.4 volts input, the diode voltage is around 2.0 volts, and the light is good.

At this point i think the difference between supply voltage from 2 AAA's and the diode voltage is just too tight to make it reliable. The only way with only 400 mV of drop (in case of NiMHs) is to use only a series resistor... that would probably work, but then the laser would no longer work safely on alkalines.

I would do well of 3x 2/3 AAA's, or perhaps off a 3.6v 14500 lithium rechargeable (plus a dummy AAA to make that fit).

 

[chimo]

I would do well of 3x 2/3 AAA's, or perhaps off a 3.6v 14500 lithium rechargeable (plus a dummy AAA to make that fit).

I have done the 10440 Li-ion w/ a dummy spacer. It's pretty bright. I have not measured the voltage sag under load, however.

Paul

 

[dar303]

I took apart my new DX True 30mW and it seems that I have yet another version of the circuit!
I drilled a 14 mm hole in a piece of plywood and cut the piece in two and used that to hold the pointer in a vice and just wiggled the front piece loose, no heat used.
My version of the DX30 is a newwish, it's engraved in the brass. It has no spring for the + battery terminal insted uses pressfit into the outer housing for contact.
The brass assembly looks to be of good quality and the focus lens is adjustable like the Aixiz housing but made entirely of brass.
The potentiometer actually works and controls the power, it was set to max from the factory.
One red and one black wire runs from the pcb into the optical housing, anyone knows what they are used for?
The pointer sometimes dims in the usual DX manner and it's apparently not due to anything with the power, the current and volage is constant when powered from a good lab supply.
I think it is something with the crystal or cavity, the output improves in certain temeperature ranges propably due to something inside expanding or contracting.
If the pointer is cold I get weak output and if i warm it to much (to hand temperature) I get weak output, it's good only somewhere in between.
But hey, for $27, what can you expect!

I can post some pictures later if anybody wants to see.

 

[chimo]

Pics would be great. The wires are from an external photo diode/sensor for feedback control. I have experienced the same thing regarding temperature range. It seems to get worse over time/use.

 

[Benm]

I'd be interested to see pics of this too. I've heard about the separate sensor being found in older units - did you purchase this DX30 recently?

 

[dar303]

hmm, I cant post links!

 

[SenKat]

hmm, I cant post links!

You can post links once you hit 10 posts. Go ahead and post a partial link - leave off the www or the http:// and one of hte mods will go in and fix it for you.

 

[dar303]

I figured the wires were for some sort of sensor, it makes sense to have the feedback-loop after the crystal.
I bought the pointer two weeks ago.

Som really bad soldering in there...
Here are the links without h t t p : / /
http://farm3.static.flickr.com/2387/1587954912_30d2d6c119.jpg
http://farm3.static.flickr.com/2174/1587069357_bedc13f532.jpg
http://farm3.static.flickr.com/2376/1587069687_eb31a34ff0.jpg

*******Edit - fixed links, and added the pics as tags into the message.********

 

[Benm]

That's a totally different setup from the other DX30's... very strange how they are now more or less concurrently shipping 3 different lasers under the same name and product number.

I wonder if they found a box of old stock and are now shipping that, since i've only seen this setup in pointers from long ago.

 

[IgorT]

After reading through this thread, and observing how my DX200 has a current directly proportionall to the voltage, how are these "driver" circuits any better, than just using a resistor?

I mean, they have enough components on board, to regulate the power through the photo diode feedback, but simply don't use them, or what?