Hello,

I attach a simple circuit about an opto and a transistor. According to the simulation, base current is 1.92 mA while collector current is 2.53 mA because of the limiting resistor of 4.7k. How can I demonstrate that the base current is 1.92 mA from that circuit?

 

It looks like the U1 internal LED emitter and the D1 LED Red are wired backwards? Looks like circuit ground is above R3? Should that be the case the 4N25 will never turn on.

Ron

 

It looks like the U1 internal LED emitter and the D1 LED Red are wired backwards? Looks like circuit ground is above R3? Should that be the case the 4N25 will never turn on.

Ron

5V potential is above R3 and both diodes are connected in series.

 

The datasheet says that the minimum CTR for 4N25 is 20% and you have 9.01mA in the emitter (opto LED). The simulation is assuming a little better than minimum CTR.

 

It looks like the U1 internal LED emitter and the D1 LED Red are wired backwards? Looks like circuit ground is above R3? Should that be the case the 4N25 will never turn on.

Ron

The LED has the anode at the top and the cathode at the bottom. Not the easiest thing to see, but I don't think it can be otherwise.

 

The datasheet says that the minimum CTR for 4N25 is 20% and you have 9.01mA in the emitter (opto LED). The simulation is assuming a little better than minimum CTR.

Would it be the case in real life?

 

Hello,

I attach a simple circuit about an opto and a transistor. According to the simulation, base current is 1.92 mA while collector current is 2.53 mA because of the limiting resistor of 4.7k. How can I demonstrate that the base current is 1.92 mA from that circuit?

Well, let's see what a very simple analysis would predict.

Let's assume that the 4N25's saturation voltage is 0 V and that the Vbe of Q1 is 0.7 V. That would mean that the base current would be (12 V - 0.7 V) / 1.2 kΩ which is 9.4 mA, well in excess of the 1.92 mA your simulation shows.

But you don't give enough information to determine if it's because the Vce of the 4N25 is much higher than expected or if it's the Vbe of the 2N3904.

Since you are dropping 2.3 V across the base resistor, there is about 9 V unaccounted for.

The most likely explanation is that the assumption that the 4N25 is in saturation is a bad assumption.

Going out and looking at the data sheet for the 4N25, we immediately see that with a forward LED current of 10 mA, the collector current limits at right around 2.5 mA. So a collector current of 1.9 mA for a forward LED current of 9 mA doesn't seem all that unreasonable. It's also in rough agreement with the transfer ratio of about 30% that you expect in that operating neighborhood.

 

Would it be the case in real life?

In real life the device should be expected to operate within the specs presented in the data sheet.

But that can cover a lot of variability, so it is up to you to determine what kind of performance is necessary for your circuit to work acceptably well and then it is up to you to determine what kind of limitations you need to place on how the circuit is operated to keep it within a region where it performs in an acceptable manner.

 

In real life the device should be expected to operate within the specs presented in the data sheet.

But that can cover a lot of variability, so it is up to you to determine what kind of performance is necessary for your circuit to work acceptably well and then it is up to you to determine what kind of limitations you need to place on how the circuit is operated to keep it within a region where it performs in an acceptable manner.

I wanted to try that circuit in real life, exactly that one. So I was simulating it to observe its behavior. Let's say that's what I want, but I want to make sure that the design would not present problems in real life.

 

I wanted to try that circuit in real life, exactly that one. So I was simulating it to observe its behavior. Let's say that's what I want, but I want to make sure that the design would not present problems in real life.

Okay, let's say that.

So.... what?

What would constitute "problems in real life"?

What do you WANT the circuit to do under the conditions you are simulating?

Is what it IS doing acceptable or not?

Only YOU can determine that because you have not told us what your needs are.

 

The LED has the anode at the top and the cathode at the bottom. Not the easiest thing to see, but I don't think it can be otherwise.

Now I see it, thanks.

Ron

 

You still forgot to add the Rbe since the previous posts already told you that.