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VOD207T Optocoupler CTR couldbe as low as 0.55?

zenerbjt

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Do you agree the attached schematic for the VOD207T opto is in error. The pullup resistor in the collector is too low value.
The VOD207T has a CTR of 100-200
However, fig 6, page 5 of the datasheet shows that when Vce=0.4V, then CTR could be as low as 0.55.

In the attached, I(diode) = 8.1mA
..therefore i(collector) can be a maximum of only 4.45mA. As such, V(collector) could be as high as 1.21V…..this is above the logic low threshold of the 3V3 fed micro.

https://www.mouser.co.uk/datasheet/2/427/vod205t-1767048.pdf
 

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KlausST

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Hi,

No, I don't agree.
I don't agree, because the intention (of the schematic) is not clear.

There is no output, no function, no specification...

Klaus
 

zenerbjt

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..I see your point...though i think you would agree that the CTR of the VOD207T can be as low as 0.55?

Spec wise, we would like the opto_NPN to be able to pull down so much current so its in saturation (VCE ~ 0.2).....and we believe it cannot in this unfortunate case.
 

KlausST

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Hi,

I agree that you can't expect V_CE to go down to 0.4V with the given circuit.
It won't work reliably when one expects logic level input and logic level output.

But using the circuit combined with a comparator should work.

It may give valid logic output voltage levels when the output pullup resistor is 10kOhms, but then it might be too slow...
--> It all depends on what the circuit is meant to do.
Give clear specifications: voltages@currents, timing ....

Klaus
 

zenerbjt

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We need this circuit to pull down to <0.5V at the collector. (its just a rough indicator and timing [ie fall time] is not critical)
I agree that it clearly wont let its VCE fall below 0.5v.
It seems strange because i cant find any other opto datasheets that suffer such a fall in CTR with fall in VCE

Why is the VOD207T so poor?.....i cant see how it made it onto the market.

EG the CNY17-2 doesnt suffer it...
 

KlausST

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Hi,

I agree they look different at the charts.

But in the table both show the same:
* Collector emitter saturation voltage IF = 10 mA, IC = 2.5 mA VCEsat 0.4 V

Klaus
 

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It's easy to design a circuit with inappropriate parameters and complain about poor component performance. Non-zero saturation voltage of bipolar transistors is known behavior, not specific to opto couplers. Although you find opto couplers with lower saturation voltage, you won't apply this kind of circuit dimensioning anyway. It has no margin for CTR reduction over lifetime and CTR temperature coefficient.
 

zenerbjt

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To be honest, i think the CNY17-2 datasheet shows no degradation of CTR with falling VCE...
Looking at figs 11 and 12 on page 6 of the datasheet.
CNY17 opto

I wonder if others would agree with my conclusion that the VOD207T is potentially a "nightmare part" for the unwary, with serious degradation of CTR as VCE falls....this is just not normal for an opto
Fig 6 page 5 shows the gory truth with VOD207T opto...
 
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zenerbjt

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..ah yes thankyou it does...just that the VOD207T suffers much higher CTR degradation at higher VCE voltages. It is this feature which marks out the VOD207T as a very poor part with more limited applications.
 

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