Slight deviation from the thread's theme, but...We were also taught about signals passing through the ether I just could not figure what the either was and how it worked. That's because the either does not exist. There is only air or a vacuum
As you quite rightly say R2 (2K2) serves no purpose in this circuit configuration. The current through the opto transistor and Q1 is already limited by R3 (10K). But R2 will do no harm and will not stop the circuit from working.
The PMOSFET has a few problems:
(1) It is a 20V max VSD device and it is being used with 24V VSD
(2) The source and drain connections are swapped
(3) The IRF7404 PMOSFET is not suitable for driving a motor- it is not man enough
Once the above points are fixed the circuit will work OK but it would be operating the opto receiver transistor with a VCE lower than is specified in the characteristics defined by the data sheet.
This is a light on function. ie when light is shining on the opto receiver transistor, the motor will run.
The reason why Q1 (BC546B) is overheating is that there is nothing to limit its collector current and hence the base current of Q2 (BD140).
The opto reciever transistor can conduct 18ma for a really good sample. Q1 (BC546B) can have an hFE of 300 so potentially Q1 collector current could be 18mA * 300= 5.4 Amps. The voltage across Q1 collector emitter is 24V, so Q1 could possibly be dissipating, 5.4A * 24V = 129.6W. If you had a power supply with a current capability of 5.4 A and the Opto reciever transistor were particularly good and Q1 had an hFE within its data sheet specification there would be an explosion and all that would be left of Q1 would be the collector, emitter, and base leads.
To fix the problem all you need to do is put a 2K2 resistor between Q1 collector and Q2 base. This will limit Q1 collector current to around 24V/2K2 Ohms= 10.9mA, slightly different to 5.4A
This circuit will work with the addition of the 2K2 resistor, but once again it is relying on parameters not given in the dara sheet.
Sorry. Much of the schematics I posted did not reflect what I actually did so I have to start again.
I picked up an IRF9540N P-Channel Mosfet and wired it like **broken link removed**with 24V directly to the source and the drain to the load (24V relay). The result was that the Mosfet got quite hot and the relay was on all the time though the voltage at the gate was changing (with the dark and light operation of the photo transistor) from 17.35V to somewhere around 2V if my memory serves me right. I believe that mosfet is damaged .
So I put a new Mosfet and wired it the same way with the addition of a 10K resistor between the 24V and the source then connected an LED to the drain. The LED is on all the time with high voltage of 17.3 and low of 0.033. Which I don't get. I find it so confusing that I thought I got the pinout wrong but all my search resulted in the same pinout configuration.
To your points:
1) I put anything in the schematic that was P-Channel just to work on the wiring but I hope that the IRF9540 is man enough
2) You mean connecting the 24V to the drain instead of the source and the load to the source? I just tried that too and the LED is on all the time.
But I don;t get the last line about the opto receiver. The opto receiver is connected to the BC547 and that did not change when I replaced the N-Channel with the P-Channel. How is it being affected by the Mosfet?
**broken link removed**Hi Kal,
The solution is to use one of the circuits that I have suggested, which also use the opto sensor as defined in the data sheet.
**broken link removed**
At the moment, when Q1 is conducting, its collector will be at 0V, so the gate of the PMOSFET (call it Q2) will also be at 0V. The source of Q2 is connected to 24V, so this means the there will be 24V across the gate/source terminals of Q2. This will turn Q2 hard on and will probably damage Q2 because the limit of 20V across the gate/source voltage will have been exceeded by 4V.
The solution is to use one of the circuits that I have suggested, which also use the opto sensor as defined in the data sheet.
Ok I finally got it. I will retry today.
Although I'm looking to build this circuit my goal is to learn so that's why keep messing around with different versions of things.
If I may ask you again and I do appreciate your patience, how's the opto sensor is affected by the way I wired it. I understand it has an effect on the mosfet but not clear on how it will affect the opto.
Thanks a lot
Kal
I think I understand a bit of this too and how it affects the opto receive. So as you stated because of the misplacement of the zener diode no I'm getting 24V across the gate/source of Q2 and the gate of Q2 is connected to the Collector of Q1 which at that same moment is also conducting so the current will flow through Q1's base which is connected to the collector of the opto receiver. I'm actually going to recreate that for fun and to see the effect in real time. There will be lots of smoke
But the opto at that moment is off with no path to ground so it should not be affected except for the fraction of time as the light is turning on and it starts to conduct it will be getting current from across the path I described above which will be soon turned off by the receiver conducting. Am I on the right track?
Thanks
Kal
Hy kal,
Firstly let me assure you that the above circuit is correct and will work as intended. I suspect that you have the PMOSFET incorrectly wired. I also suspect that that your PMOSFET is blown. The IRF7404 type is not suitable for this application anyway.
P Type MOSFET Operation
A PMOSFET is the compliment of an NMOSFET. This means that all the operating voltages are reversed. Under normal operating conditions the drain of a PMOSFET is more negative than the source. If the gate and drain are at the same potential no drain current will flow. If the gate is more negative than the source drain current will flow. So in the opto sensor the PMOSFET source would be connected to 24V and the drain would be connected to the top of the motor.
Circuit Function light on
Light impinges on the Opto Receiving Transistor (ORT) this causes an ORT collector current to flow from the 24V supply line through the emitter base junction of Q17 (consider R7 to be a short circuit at the currents concerned).
The current flowing through Q17 base emitter will generate a Q17 collector current of hFE Q17 * Ib to flow in the collector of Q17, but this current is limited by R5 (2K2) to 24V/2K2 = 10.9 mA.
As the whole supply line voltage is dropped across R5, the collector of Q17 will be at 24V. This voltage will also be on the gate of PMOSFET Q16 (R8 has no effect). As the drain of Q16 is also at 24V there is OV between the gate and source, so Q16 is turned off.
Circuit Function light off
No light impinges on the ORT so only a small dark current flows in the ORT collector. This small current flows through R24 (10K) and does not generate enough voltage (600mV) to turn Q17 on.
As Q17 is off R5 tries to drag Q17 collector down to 0V but Zener D8 prevents this and starts conducting at 24V-18V = 6V. The net result is that gate of PMOSFET Q16 is at 6V and its source is at 24V so the gate is 18V more negative than the drain. This means that Q16 is fully turned on.
Zener Diode
You ask what the Zener diode is for: simply to protect Q16. It serves no other purpose. The data sheet for the PMOSFET says that the gate should never be more than +-20V with respect to the source. Without the Zener the gate could be 24V more negative than the source which could potentially destroy the PMOSFET.
What does R7 (2K2) do?
R7 in the base of Q7 probably looks a bit odd but it is there to protect the collector of the ORT and Q17 base from destructive current flow from the 24V supply rail to 0V. You might say but the data sheet says only a maximum of 18 mA will flow in the ORT and you would be right, except:
(1) Radio frequency interference could cause the ORT to conduct heavily
(2) Electrostatic discharge could cause the ORT to conduct heavily.
(3) High energy radiation could cause the ORT to conduct heavily (Gamma, X, etc)
Also an arrangement like that is like a loaded gun waiting to go off and even if an excess current could not possibly flow in theory, you would still protect against it as a matter of good design.
Slight deviation from the thread's theme, but...
The definition of the "ether" is often nebulous at best, if not downright drifting into the realm of "magic smoke". So don't feel bad. I grappled with the concept as well.
But to add to your concept, RF wave propagation (of any type) is also affected, to a greater and lesser degree, by solar "winds", as they exist outside of the Earth's atmosphere. A pretty comprehensive piece on the subject can be found here: https://deepspace.jpl.nasa.gov/dsndocs/810-005/106/106B.pdf
Hello Spec, Hope you are well and don't mind me picking your brains a little more on the same topic.
I put the circuit together on a bread board and of course it worked like a charm. Then I got to thinking that I'm actually going to have two of those sensors positioned at 90 degrees out of phase. Only one will be on at a time.
So I **broken link removed**one of your circuits (the one with the BD140 I was using for an LED as I decided I don't need a Mosfet after all because the PLC input requires only 4mA to turn fully on) by adding a another BC557 PNP.
I tried it on a bread board and nothing burnt but buy now I learned that doesn't mean it's correct so I thought I'd double check.
Another thing I was thinking of is that although the BD140 preformed wonderfully and did not burn like the Mosfets with all my abuse I could also just use another BC557 as it will be required to conduct a few milliamps. What do you think?
Oh and one more thingwhat happens if I use the BD140 almost as a **broken link removed** so that it would switch the PLC input and shares only ground with the PLC . I'm not quite sure if current would flow the same way form V+ of one supply into the other.
I'm not inclined to do it this way but curious as to the effect.
Cheers
Kal
Excellent,The final result of the project is complete, tested and in action. I made an **broken link removed**with dual connection to connect two sensors and there by have a "Quadrature AB encoder"
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