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MOSFET switching optocoupler

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yassin.kraouch

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Hi,

the Vout of my circuit is between 300 mV and 3V, i would like that my Vout is between 0 and 3V, how can i do this ?? please help!!!



best regards
 

alexxx

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Hi!

There is a transistor characteristic called "Collector−emitter saturation voltage" (VCEsat). This is the voltage drop accross collector-emitter when the transistor is saturated. So if you bias the transistor correctly and it goes deep into saturation, the lowest voltage you can get is VCEsat. I don't know if someone ever used special circuits to drop this voltage from VCEsat to 0V, but I don't think that you really need it anyway.

Normally and talking about a general purpose transistor, you increase Ib so that VCE will decrease, until it reaches VCEsat. Here we have a phototransistor, so the only thing you could test is to increase photodiode's current and maybe the phototransistor will go deeper into saturation. But again I think that 300mV is OK, it is clearly a '0' state.

Alex
 

yassin.kraouch

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Hi!

There is a transistor characteristic called "Collector−emitter saturation voltage" (VCEsat). This is the voltage drop accross collector-emitter when the transistor is saturated. So if you bias the transistor correctly and it goes deep into saturation, the lowest voltage you can get is VCEsat. I don't know if someone ever used special circuits to drop this voltage from VCEsat to 0V, but I don't think that you really need it anyway.

Normally and talking about a general purpose transistor, you increase Ib so that VCE will decrease, until it reaches VCEsat. Here we have a phototransistor, so the only thing you could test is to increase photodiode's current and maybe the phototransistor will go deeper into saturation. But again I think that 300mV is OK, it is clearly a '0' state.

Alex
You are talking abour JGBT transistor but i used MOSFET transistor !!
 

alexan_e

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He is talking about the output transistor of the optocoupler, that is where you are getting the drop not the mosfet
You can either use a lower resistor in the input diode of the optocoupler (assuming that you don't go above the max current) ot use a higher value resistor in the transistor output, you will get values closer to 0 but still not 0

Alex
 

alexxx

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yassin.kraouch said:
You are talking abour JGBT transistor but i used MOSFET transistor !!
Yes you used a MOSFET as a switch for the photodiode's current. When the photodiode emmits light, then the optocoupler's phototransistor is biased (U1 - OPTOCOUPLER NPN). So I am talking about this npn which is a BJT. Your output is Vout, shorted directly to npn's collector and this output's '0' state obviously depends on this npn's VCEsat.

Hope that helped.
 

alexan_e

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Where will you feed the output and why does it matter that the voltage doesn't go down to 0?

Alex
 

yassin.kraouch

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ah ok thank you, please look at this picture will i obtain the same PWM ??

 

alexan_e

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The emitter of U2 and U3 are not connected so you can't get any output (you will get always V+ because of the pullups).
If you connect the emitters then for the 10K resistors you will get a low value closer to 0 than with the 350 ohm but on the other hand you will get less current in the positive output.
All input diodes will share the same current so they will turn on/off together.

Alex
 

alexxx

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12V are enough for the photodiodes to conduct, but you also need to connect ground to U2 and U3. I suggest you place electrolytic capacitors to the collectors of the phototransistors, 10uF should be OK.

If you plan to implement this circuit on the real world, then R1 consumes over 500mW, so you need at least 1W resistor. However I don't see any practical use of it, since you produce the same signal three times.

Regards,
Alexis
 

yassin.kraouch

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ok Alex, so i change just the value of 10k to 350 ohm and connect the emitter to the ground, and i will obtain the same signal, is there any condition related to the diod ? for example current or voltage ?

---------- Post added at 22:00 ---------- Previous post was at 21:59 ----------

12V are enough for the photodiodes to conduct, but you also need to connect ground to U2 and U3. I suggest you place electrolytic capacitors to the collectors of the phototransistors, 10uF should be OK.

If you plan to implement this circuit on the real world, then R1 consumes over 500mW, so you need at least 1W resistor. However I don't see any practical use of it, since you produce the same signal three times.

Regards,
Alexis
connect the resistor will limite the current that pass throught diode
 

alexan_e

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is there any condition related to the diod ? for example current or voltage ?
If you mean in relation to the output then yes, the current through the diode is the equivalent of a base current in a normal transistor, for each specific optocoupler you will find in the datasheet the current relation between diode and transistor to saturate the output

Alex
 

alexan_e

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and the voltage U3A is there any condition ?
What is the voltage U1A?

You mean the output voltage of the transistor, yes this will be affected, the more current you give to the diode the more current you will be able to sink in the transistor output while keeping a low Vce drop

Alex
 

yassin.kraouch

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Yes but ho i can calculate the current to give to the diode, what is the limit of the current that i should give ? and how to limit it ?

---------- Post added at 22:38 ---------- Previous post was at 22:35 ----------

the voltage U3A is the voltage between the three diode if you look the schematic
 

alexan_e

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You are using a generic model so I can't give you an answer.
In a real optocoupler the datasheet has the max allowed diode current and Vf the led (forward voltage drop) so for three optocouplers you can use (12v-(Vf*3))/I to find the resistor value to use

Alex

---------- Post added at 00:41 ---------- Previous post was at 00:39 ----------


U3A doesn't matter as long as it is above Vf*3 , then depending on the voltage level you use an appropriate resistor

Alex
 

yassin.kraouch

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so the current that a diode should pass is in the datasheet?
 

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The max current that the diode can handle is always in the datasheet, it is up to you to use a lower current if you want.

Alex
 

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