Vce(sat) for opto transistor is too high

[treez]

The following opto has a vce(sat) of 0.4V (max) at 1mA (as on page 3 of datasheet). Can we be sure that it will be much less than this when the current in the transistor is much less than 1mA?

VOS618A opto datasheet
https://www.vishay.com/docs/83465/vos618a.pdf

 

[Audioguru]

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.

 

[treez]

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?

 

[D.A.(Tony)Stewart]

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

 

[Audioguru]

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, 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.