Vce(sat) for opto transistor is too high

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The maximum saturation voltage of 0.4V is when the LED has 1mA and the collector current is only 0.25mA.
It is because a photo-transistor gets hardly any base current. Its base current is just a tiny leakage current from collector to base caused by light or IR from the LED.

It is easy to make the saturation voltage less, simply give the LED more current.
 
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but presumably the older and hotter the opto gets, the more led current you have to give to get the VCE(sat) voltage to fall?
 

YES

Vce(Sat)= Vth + Ic*ESR is a linear approximation.
for Ic=20mA Vth = 0.04V and
for Ic=1mA Vth= 0.1V

then slope of curve will determine actual voltage drop above threshold,
We can call this slope Rce or ESR

So for IC=1mA ESR= 80Ω , as long as If=1mA. Note the CTR is 100% typ. not worst case over all temperatures and this affects Vce.

thus 80mV above Vth=0.1 approx or 0.18V for Vce

 
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treez said:
but presumably the older and hotter the opto gets, the more led current you have to give to get the VCE(sat) voltage to fall?
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Yes, an LED operating at a high current makes it hot and makes it get dimmer and dimmer. But this opto is operating at a very low current (its maximum allowed LED current is 50mA) so it will dim extremely slowly and will last a long time.
 
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