pics and info on prototyped 2amp ld current source

[Rick]

Hi guys,

I found a couple of high current pnp darlington transistors (a TIP125 from some
late 1970s circuit board and a new NTE264) and went ahead and built up what I
have so far in a modulated current source for a high power laser diode. Upon
realizing that the only rail to rail op-amp I had in my parts box could only
source/sink about 4mA, I added a 2907 to drive the base of the darlington. The
actual parts that I used are illustrated on this schematic:

http://www.skyko.com/isource/ldcursrc.JPG


I first tested the unit with the NTE264 and a Vin of ~4 volts (for a 2amp
current):

http://www.skyko.com/isource/CONST2AMP.JPG

I then modulated Vin with a 4V 10Khz square wave from a function generator
(channel 1) and measured the current across a ~1.9 ohm resistor as the load
(channel 2):

http://www.skyko.com/isource/NTE264.JPG

I then swapped the NTE264 pnp darlington for the dusty TIP125 and did the same
measurement:

http://www.skyko.com/isource/TIP125.JPG

Here is a pic of Vin modulated with 4V 10Khz (channel 1) with the 1.9 ohm
replaced by 3 chunky diodes in series (to simulate a laser diode voltage drop of
~2.2V). Channel 2 is measuring across all 3 diodes:

http://www.skyko.com/isource/DIODEVOLT.JPG

Here is a pic of the breadboarded circuit (note the output transistor and sense
resistor are soldered to a piece of circuit board to help save the breadboard


http://www.skyko.com/isource/CIRCUIT.JPG


All in all, a pretty good start. If I had some small gate capacitance nfets I
would try replacing Q2 (and possibly Q3) with them. By the way, thanks to Win
Hill and Tony Williams for the hints, tips and circuits.

Rick

 

[GPG]

Hi guys,

I found a couple of high current pnp darlington transistors (a TIP125 from some
late 1970s circuit board and a new NTE264) and went ahead and built up what I
have so far in a modulated current source for a high power laser diode. Upon
realizing that the only rail to rail op-amp I had in my parts box could only
source/sink about 4mA, I added a 2907 to drive the base of the darlington. The
actual parts that I used are illustrated on this schematic:

http://www.skyko.com/isource/ldcursrc.JPG


I first tested the unit with the NTE264 and a Vin of ~4 volts (for a 2amp
current):

http://www.skyko.com/isource/CONST2AMP.JPG

I then modulated Vin with a 4V 10Khz square wave from a function generator
(channel 1) and measured the current across a ~1.9 ohm resistor as the load
(channel 2):

http://www.skyko.com/isource/NTE264.JPG

I then swapped the NTE264 pnp darlington for the dusty TIP125 and did the same
measurement:

http://www.skyko.com/isource/TIP125.JPG

Here is a pic of Vin modulated with 4V 10Khz (channel 1) with the 1.9 ohm
replaced by 3 chunky diodes in series (to simulate a laser diode voltage drop of
~2.2V). Channel 2 is measuring across all 3 diodes:

http://www.skyko.com/isource/DIODEVOLT.JPG

Here is a pic of the breadboarded circuit (note the output transistor and sense
resistor are soldered to a piece of circuit board to help save the breadboard


http://www.skyko.com/isource/CIRCUIT.JPG


All in all, a pretty good start. If I had some small gate capacitance nfets I
would try replacing Q2 (and possibly Q3) with them. By the way, thanks to Win
Hill and Tony Williams for the hints, tips and circuits.

Rick

Try this





+---+ V+
.-. |
| | |
| | |
'-' +
| |
|<
|-|
___ |/ |\
+|___|-| +
| |> |
| | |
___ | ___ | |
V- o---|10R|---o-------|-|_R_|--+-+-+
| | |
| |\| + |
'--|-\ | .-.
| >-+ | |
+++|+/ |Rs
| |/| '-'
____ + ___ |
Vin o--|10R|--o--------|_R_|-----o Iout
|
V
- Laser
|

V-
I out =Vin/10Rs
If R = or> 10 K it will have insignificant effect on I out at 2A

 

[Rick]

Try this





+---+ V+
.-. |
| | |
| | |
'-' +
| |
|<
|-|
___ |/ |\
+|___|-| +
| |> |
| | |
___ | ___ | |
V- o---|10R|---o-------|-|_R_|--+-+-+
| | |
| |\| + |
'--|-\ | .-.
| >-+ | |
+++|+/ |Rs
| |/| '-'
____ + ___ |
Vin o--|10R|--o--------|_R_|-----o Iout
|
V
- Laser
|

V-
I out =Vin/10Rs
If R = or> 10 K it will have insignificant effect on I out at 2A

It works, but if you make R 10K (so 10R = 100K) and use a negative feedback
capacitor, it has to be unrealistically small (less than 10pF). If you make R
1K, then you need a buffer op-amp anyway for Vin to have low impedance. If you
leave out the feedback cap, you get spikes of 50mA or more when you modulate
Vin.

Thanks,

Rick

 

[Winfield Hill]

Rick wrote...

GPG wrote, Try this
+---+ V+
.-. |
| | |
| | |
'-' +
| |
|<
|-|
___ |/ |\
+|___|-| +
| |> |
| | |
___ | ___ | |
V- o---|10R|---o-------|-|_R_|--+-+-+
| | |
| |\| + |
'--|-\ | .-.
| >-+ | |
+++|+/ |Rs
| |/| '-'
____ + ___ |
Vin o--|10R|--o--------|_R_|-----o Iout
|
V
- Laser
|

V-
I out =Vin/10Rs
If R = or> 10 K it will have insignificant effect on I out at 2A

It works, but if you make R 10K (so 10R = 100K) and use a negative
feedback capacitor, it has to be unrealistically small (less than
10pF). If you make R 1K, then you need a buffer op-amp anyway for
Vin to have low [high?] impedance. If you leave out the feedback
cap, you get spikes of 50mA or more when you modulate Vin.

Hah, 10pF is not so small. Use Rs = 0.1 ohms as you like, and
R = 2.00k ohms and "10R" fifty times larger = 100k, for the same
control setup you have now. Note: a feedback cap of only one
or two pF is not too small (keep stray paths far away or shield
them), and most signal sources won't mind 25 or 100k loads. Then
you can compare the two circuit's response speed, drift, etc.

One thing I'd add to both circuits is a series resistor to use
up the output voltage-compliance range at the maximum current
rating of the laser. This current-limiting will prevent misuse
of the laser and help protect it.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[Rick]

Hah, 10pF is not so small. Use Rs = 0.1 ohms as you like, and
R = 2.00k ohms and "10R" fifty times larger = 100k, for the same
control setup you have now. Note: a feedback cap of only one
or two pF is not too small (keep stray paths far away or shield
them), and most signal sources won't mind 25 or 100k loads. Then
you can compare the two circuit's response speed, drift, etc.

One thing I'd add to both circuits is a series resistor to use
up the output voltage-compliance range at the maximum current
rating of the laser. This current-limiting will prevent misuse
of the laser and help protect it.

Thanks,
- Win

whill_at_picovolt-dot-com

Thanks, I will experiment with that.

Rick

 

[GPG]

Thanks, I will experiment with that.

Rick

10pF, conveniently small
Yes, Vin needs to be low impedance sorce to maintain linearity,
not so important for digital sig. Could use fet for speed:









V+

|
V Laser
-
|
|
___ ___ |
V- o---|10R|---+---------|_R_|----+ Isink
| |
| |\| |
+--|-\ ___ .-.
| >|-|Rg_|--+ | |
+--|+/ | |Rs
| |/| | '-'
____ | ___ | |
Vin o--|10R|--+--------|_R_|--|--+--+
| |
| |
| ||-+
| ||<-
+--||-+
|

V-
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de
RgCg will give rolloff.