ld drv

Benm said:

I would also recommend using a 1k or so resistor across the diode, this improves turn-on behaviour as you crack the current up from zero using the pot.

Click to expand...

Sorry to go a bit off topic, but why does that help?

Benm said:

[quote author=phenol link=1193741215/0#12 date=1193782823]
True that. I managed to kill one IR LD because of such a current slug.
For shorter settling times high speed op amps should be used.

Click to expand...

The capacitor is usually a good idea. Regardless of slew rate, while powering on the opamp briefly works off a voltage too low for reliable operation. What actually happens depends on the opamp, but usually there is a little current spike with this sort of setup. 10 uF is sufficient to surpress that to below LD threshold voltage.

I would also recommend using a 1k or so resistor across the diode, this improves turn-on behaviour as you crack the current up from zero using the pot. [/quote]

Benm said:

[quote author=phenol link=1193741215/0#12 date=1193782823]
True that. I managed to kill one IR LD because of such a current slug.
For shorter settling times high speed op amps should be used.

Click to expand...

The capacitor is usually a good idea. Regardless of slew rate, while powering on the opamp briefly works off a voltage too low for reliable operation. What actually happens depends on the opamp, but usually there is a little current spike with this sort of setup. 10 uF is sufficient to surpress that to below LD threshold voltage.

I would also recommend using a 1k or so resistor across the diode, this improves turn-on behaviour as you crack the current up from zero using the pot. [/quote]

Trans1 is a simulation of this circuit's behaviour during a power-on event occurring at t=500us with the following assumptions:
-the resistance of the closed switch is 0.01ohms
-the rise time of the switch is 10ns
-the current is ~100mA
-LD is actually 3 1N4151 diodes in series

Trans2 is the same thing with a 1uF capacitor across LD

a_pyro_is said:

[quote author=Benm link=1193741215/15#15 date=1193838017]I would also recommend using a 1k or so resistor across the diode, this improves turn-on behaviour as you crack the current up from zero using the pot.

Click to expand...

Sorry to go a bit off topic, but why does that help?[/quote]

It helps because otherwise there is a chance of odd behaviour starting from zero current. When you configure the current for zero, the circuit is actually out of regulation and the output of the opamp is near the positive rail. As you increase curent setting, it quickly has to move to about 2 volts below positive which gives a sort of flickering effect in the laser diode - a resistor catches that flickering a bit a provides a more stable load to the regulator at very low currents.

The simulation is nice btw, i've tried driving LED's without the capactor and you can actually see the power-up spike in real applications when set for zero current. For a LED this spike is no problem, but LD's can fail even in those few microseconds due to COD.

Good to know a 1uF is enough, a lot of people are using 47uF Caps and while I'm sure that gives a softer start, it makes for a larger circuit.

I've cut down to 10uf 16 volt capacitors for battery power, and I use 47uf 35 volt for my labby. The nice thing about the 10uf 16v is it easily fits in the standard aixiz soldered directly across the diode, and this is the best place for the capacitor to be.

a_pyro_is said:

Good to know a 1uF is enough, a lot of people are using 47uF Caps and while I'm sure that gives a softer start, it makes for a larger circuit.

Click to expand...

I would add a ceramic cap /10-100n,../ in parallel with the electrolyte one since they are not particularly good at absorbing nanosecond-long pulses and at blocking HF for that matter...

A bit off topic, yet interesting. These are pictures of the emitting side of a chip-type LD/ found in cheap pointers/ magnified 500x. lase1 shows the LD pumped with ~1mA. It is not lasing yet.The pink surfce is the cleaved mirror facet with a scratch and speckles of contaminants. lase2 is the same thing with ambient lighting switched off. in lase3 is the LD is at about threshold current. Higher than that the webcamera was saturated.

lase2...

and lase3.

Amazing pics! and from a webcam??? How in the world did you do that?

a_pyro_is said:

Amazing pics! and from a webcam??? How in the world did you do that?

Click to expand...

I aligned its objective with the eyepiece of the microscope.

lol @ me. I should have seen that. Still a great set of pics!

phenol said:

[quote author=a_pyro_is link=1193741215/15#26 date=1194039480]Amazing pics! and from a webcam??? How in the world did you do that?

Click to expand...

I aligned its objective with the eyepiece of the microscope.
[/quote]

Very nice close up's. Thanks for sharing them..

another driver

A brief description...
This is yet another constant current source that benefits from having LD's cathode connected to ground. The regulating element is a P-type FET. This broadens the compliance range of the regulator. Its main advantage is that the output current does not depend on the power supply voltage inasmuch as the current is low enough for the circuit to regulate it. The reference voltage is obtained from a shunt regulator LM336-2.5 /2.5V/. U1B is a summing amplifier and does the following: Uout(U1B)=Udd-(Udd-ILD*Rsense)=ILD*Rsense. Thus, the output current is calculated as follows: I=Vreference/Rsense. For low currents, Rsense should be increased. This would reduce the loop gain and improve stability. MCP6022 can be replaced with another similar dual op amp capable of swinging its output within supply rails and having common-mode input range also within supply rails, such as LMC6482.
For higher currents the max power dissipation of the FET must be taken into account. A small heatsink should be gued on it or the drain pad be made bigger with ground copper pour running under its case with some thermoconductive grease for better heat transfer. Again, as a lethal current spike could appear upon power-up, the LD should be clamped by a capacitor. I used 2x1uF ceramic caps in case one fails.....
I extracted a red LD from a 20x LG-H55N. At ~175mA it develops 2.37V across the junction. This LD is an open-case one, looks exactly like those Lite-On ones, but the latter seem to have higher Vf compared to the LG one. The 20x Lite-ON LD I have has 2.51V across it @ 150mA.