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optocoupler voltage drop

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mamech

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hello

I am trying to make isolation from low voltage side to high voltage side. the low voltage side sends pwm signal that has 1-5 volts range, the problem is that the optocoupler 4N35 that I am using have already around 0.4 volts, and I need to maintain the same range 1-5 volts on the other side.

is there workaround to make the output to be linear in 1-5 volts range ?
 

Hi,

show your circuit... or at list a draft of what you want to achieve.

Are you talking about analog signals or digital signals?

Klaus
 

sorry for late reply, I just did not develop this part in the circuit for a while, and now I am into it again.

the circuit is shown below:


my problem is, when the pwm output from microcontroller is around 5 volts (full pwm output), the output on the other side of the optocoupler is less than 5 (it was about 4.7 or so) because of drop of the optocoupler itself.

I want to reduce or eliminate this effect, so I get a linear scaling (5 volts on optocoupler output when I give 5 volts on pwm side, and 0 volts on optocoupler output when I give 0 volts on pwm side)

are there any suggestions how can I achieve this?
 

Hi,

Read about CTR of 4N35.
The output is simply not in saturation.

--> either use more drive curent. Or less load corrent.

Klaus
 
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    mamech

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KlausST said:
Hi,

Read about CTR of 4N35.
The output is simply not in saturation.

--> either use more drive curent. Or less load corrent.

Klaus
Click to expand...

I have looked in datasheet and looked at CTR curves, but I have some questions so I become sure that I got it:
1-you told me use more drive current, so this means to use resistor less than 1k that is used in pwm side of optocoupler , right?
2- you told me to use less load current, this means to use resistor less than 1k that I used with optocoupler emitter?
 

mamech said:
sorry for late reply, I just did not develop this part in the circuit for a while, and now I am into it again.

the circuit is shown below:


my problem is, when the pwm output from microcontroller is around 5 volts (full pwm output), the output on the other side of the optocoupler is less than 5 (it was about 4.7 or so) because of drop of the optocoupler itself.

I want to reduce or eliminate this effect, so I get a linear scaling (5 volts on optocoupler output when I give 5 volts on pwm side, and 0 volts on optocoupler output when I give 0 volts on pwm side)

are there any suggestions how can I achieve this?
Click to expand...

Have the optocoupler drive a low RDSon Mosfet. But if your PWM is fast, the output may not be a proper square wave. Plus, it will be inverted.
The best solution would be to use a CD4050 CMOS non-inverting buffer after the optocoupler.
 

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