# Help with circuit of laser diode driver

#### U_K

Joined Jul 13, 2021
5
Hello. I restored a driver circuit for the laser diode using driver's PCB and Diptrace. Laser is Thorlabs S1FC635. I have some questions about circuit. I attached some pictures of this circuit.

1)I would like to understand role of the U2.3. "U2.3In-" is signal from drain of Q2.2. When I press "Enable" button on front panel of laser - laser is on and Q2.2 close. So it's some positive voltage on drain. When I increase power of laser(I see actual power (mW) on indicator in front panel), voltage on drain decrease(power of 1mW -> 3.665V on drain, 0mW -> 4.2V on drain). I don't understand what's going on. So I understand that U2.3 is inverting ampilifier, I try to calculate output voltage on U2.3, but in point of "virtual ground" of op-amp is some voltage(not 0V) because there is zener diode in feedback of op-amp, so I don't understand how to calculate VOUT of U2.3.
P.S. maybe Q2.2. performs the function of slow start (standard in good laser drivers)

2)Why two signals (from U1.4 output and from U2.3 output) connected together without resistors? Are their output voltages is summed up? How to calculate voltage in point of connection of two outputs op-amps?

3)What role of optocoupler U5? I think it use for disable of gain in U2.4. But I want to know resistance of emmiter-collector when optocoupler is opened or how to calculate gain with using optocoupler in feedback.

That's all. I will be glad to any help. Thanks in advance

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#### ericgibbs

Joined Jan 29, 2010
14,718
2)Why two signals (from U1.4 output and from U2.3 output) connected together without resistors? Are their output voltages is summed up? How to calculate voltage in point of connection of two outputs op-amps?
hi U_K,
Welcome to AAC.

U1.4 output and from U2.3 output are not linked directly together, the two Zener's act as voltage clamps to control the drive to the U2.4 and the Q4 MOSFET.

By varying the voltage on the two OPA's Outputs ,U1.4 and U2. the operating point/voltage on the two Zener's can be controlled, so U2.4 can be limited/controlled.

I would say the Opto coupler acts as level shifter. [cannot see where Q11 & 12 Sources are connected]

E

#### U_K

Joined Jul 13, 2021
5
I would say the Opto coupler acts as level shifter. [cannot see where Q11 & 12 Sources are connected]
Thanks for help. Q1.1 and Q2.2's gate source is from non-inverting output of D-trigger - when I press "Enable" button on front panel of laser , on D-trigger's output set logic "1" (4.98V) and transistors open. So you can see in attached picture.

But still U2.3 is inverting ampifier with R22 (feedback resistor) and R33(input resistor) or not if there is zener diode for voltage clamp?

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#### ericgibbs

Joined Jan 29, 2010
14,718
Hi,
I have partially created a LTSpice circuit of the first OPA section you posted.

If you could add the measured voltages at U1_3out [ PD inp] also the U3_3In, also the ‘APC’ connection, I could try a LTS simulation of that section.

E
BTW: What are supply voltages of the OPA's.?

Update:

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#### U_K

Joined Jul 13, 2021
5
Hi,
I have partially created a LTSpice circuit of the first OPA section you posted.

If you could add the measured voltages at U1_3out [ PD inp] also the U3_3In, also the ‘APC’ connection, I could try a LTS simulation of that section.

E
BTW: What are supply voltages of the OPA's.?
If I set 1mW of power in laser:
U1.3out = -0.633V
U2.3In- = +3.665V
U1.4In+(APC connection) = -0.635V
Supply voltages of the OPA's are +5V and -5V(from LDO fixed regulators)

#### ericgibbs

Joined Jan 29, 2010
14,718
hi U_K,
This is what we have so far.
Please confirm the actual Zener diode voltage ratings and I will add them to the simulation.
E

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#### U_K

Joined Jul 13, 2021
5
Please confirm the actual Zener diode voltage ratings and I will add them to the simulation.
So Zener diodes in SOD-80 MiniMELF package, there is not title on diodes. I unsoldered one of diodes from PCB , but I still couldn't determine the stabilization voltage when I applied a voltage of 0 ... 9V through the resistor (6.8k). When I applied 9V in cathode through resistor, it was also 9V at the output of the zener diode. Further I didn't increase voltage so as not to burn it. I think there are small signal Zener Diodes in circuit(1N5221, 1N5222 and etc.)

P.S. I also try to model (in Multisim) but I don't have enough knowledges to interpreate some voltages in circuit.

I see your circuit and there is no capacitor (C1) in integrator U1.4. Also in point of cathodes D3 and D5(when "ILIM" in my first circuit) there is -0.941V (when laser power is 1mW)

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#### ericgibbs

Joined Jan 29, 2010
14,718
Hi,
This is what I see.

For U1_3 I have use a 0.5V 10Hz sine wave with a .65V offset, in order to see the effect.
The Zeners do conduct when the o1 and o2 [ OPA outputs a re pulled low]

E