Basic Electronics Help with Testing Circuits for Niles

[Niles]

Hi

First I would like to say that I am not an electrical engineering student. I study physics, and unfortunately I lack a lot of basic electrical know-how. I know the theory behind e.g. a low-pass/high-pass filter and stuff like that, but when it comes to implementing things and - specifically - testing that my circuit works, then I fail. My wish is to get better at these things

I have attached an example of a circuit that I found online. Say I have made the circuit, and now I want to test with a multimeter that everything works.

1) In the top right corner is "Supply". If I measure the resistance between "Supply 1" and "Laser diode 1", then I go through 2 resistors in series of 5.6 Ohm, 1 resistor of 39 Ohm in parallel and 1 inductor in series. Assuming that the inductor has no resistance (it is DC, so I guess I can neglegt it?), then is it correct that I would measure something close to R = 5.6Ω + 5.6Ω + 39Ω = 50.2Ω?

2) Say I measure from "Modulation 2" to "Laser diode 2". I have 1 resistor and 4 diodes in parallel with each other + a resistor of 10kΩ in series. The arrow on the diode I guess indicates that I can't go through it. What should I expect to measure here?

I would be very happy to get some feedback.


Niles.

 

[Kholdstare]

1) You can not simplify just like that. There a MOSFET in there. Depending on the bias on the MOSFET terminals, the voltage at the point between two 5.6Ohm resistance will be different. In that case you can not assume that those two 5.6Ohm resistances are just in series. The current can also flow through the MOSFET drain terminal.

You have to look at the gate bias and determine how much current might flow through the drain.

2) Those four diodes are provided to limit the voltage level at the gate. The 280Ohm resistance limits the current(accidental high current). The two 10K resistance provides the bias voltage of the gate. You should try to measure small signal voltage at the gate.

 

[Niles]

Thanks. Regarding the MOSFET, then I have found two data sheets:

1) http://www.eng.yale.edu/ee-labs/morse/compo/datasheets/2n5460-d.pdf
2) http://www.fairchildsemi.com/ds/2N/2N5460.pdf

They do not tell me the same thing. In (1), leg #1 is S, whereas (2) tells me leg #1 is D. How do I know which one to choose?Niles.

 

[Kholdstare]

Both of them can be used in any circuits. They just differ in manufacturer. Just recognize the source, drain and gate pin and connect them to the appropriate wire in the circuit.

 

[alphysicist]

Hello Niles,

It's great that you are interested in learning more about electronics and testing circuits. I can offer some guidance on testing circuits and understanding the results.

1) Your calculation for the resistance between "Supply 1" and "Laser diode 1" is correct. You are correct in assuming that the inductor can be neglected for DC measurements. However, it's always a good idea to double check the resistance value by measuring it with a multimeter.

2) When measuring from "Modulation 2" to "Laser diode 2", you should expect to measure a resistance value of 10kΩ. This is because the four diodes in parallel will act as one diode with a lower resistance, and the 10kΩ resistor will add to that resistance.

In general, when testing circuits, it's important to have a basic understanding of the components and how they are connected. This will help you make sense of the measurements and troubleshoot any issues that may arise. It's also a good idea to have a circuit diagram or schematic to refer to while testing.

I hope this helps and good luck with your circuit testing!