With the circuit shown can someone tell me, and show derivation of, the output current the optocoupler can support?

I am only interested in circuit as shown and do not need variations or suggestions on how to change it.

The KB814 is being driven to saturation and is acting as switch to control the 12V supply to the relay.

Thanks in advance for any help given

 

Check the coupler’s data sheet says about CTR (current transfer ratio).

The current the collector-emitter can provide is >= LED current x CTR

 

Hi Alan,
With that circuit, the relay is always energised and the opto shorts out the 12V supply?
E

 

Shorting the supply seems a very inefficient way to turn the relay off. It might mean the optocoupler has a short life too.

 

Sorry should have indicated that 5V is switchable logic 5V/0V.

 

Move the diode and relay to 12V and 1.5K with LED in series with the phototransistor of optocoupler.

Edited.

 

The KB814 will not like having 12 V applied to the photo-transistor.

 

The KB814 will not like having 12 V applied to the photo-transistor.

Can you explain, wrt KB814 datasheet. Thanks.

 

The CTR (Current Transfer Ratio) is not likely to be a well controlled parameter of the device and it wiil degrade over time. Over a period of several years it may go from an initial value to maybe 10% of tha original value.

 

View attachment 340614

With the circuit shown can someone tell me, and show derivation of, the output current the optocoupler can support?

I am only interested in circuit as shown and do not need variations or suggestions on how to change it.

The KB814 is being driven to saturation and is acting as switch to control the 12V supply to the relay.

Thanks in advance for any help given

My understanding is that linear relationship where CTR = Ic/If only holds in unsatuarted region and that it breaks down in opto saturated region. Graph below (not for KB814) shows this.

 

My understanding is that linear relationship where CTR = Ic/If only holds in unsatuarted region and that it breaks down in opto saturated region. Graph below (not for KB814) shows this.

View attachment 340626

Linear on a log scale is not really linear. CTR definitely gets worse as you increase the LED current beyond the nominal testing point which is usually 10 mA. Here is a typical simulation of CTR for a fairly "hot" version of the PC817

 

Can you explain, wrt KB814 datasheet. Thanks.

The output transistor is wired across the 12 V supply. The transistor has no load or anything to limit the current. As soon as the transistor is turned on, it will die a sudden death.

 

Sorry my mistake with diagram. Will post corrected version asap. Sorry to have wasted peoples time.

 

View attachment 340614

With the circuit shown can someone tell me, and show derivation of, the output current the optocoupler can support?

I am only interested in circuit as shown and do not need variations or suggestions on how to change it.

The KB814 is being driven to saturation and is acting as switch to control the 12V supply to the relay.

Thanks in advance for any help given

Ironically, opening post was already flawed in the first place.

 

Have posted amended diagram to first post.

Mod: added the modified image.

 

Have posted amended diagram to first post.

Don’t do that! You invalidated all of the comments that were made about your original post. If you make a correction to a pist that has already been discussed, make it inly in a new post.

I found the thread very confusing when I first read it after the edit.

 

Looking at the corrected circuit and the copy in post #15, a crude approximation is 12.5 mA for the relay and about 12 mA for the LED assuming it is a red LED, and assuming that the transistor in the opto-isolator reaches full saturation. The exact numbers will be a bit less, probably.

 

Looking at the corrected circuit and the copy in post #15, a crude approximation is 12.5 mA for the relay and about 12 mA for the LED assuming it is a red LED, and assuming that the transistor in the opto-isolator reaches full saturation. The exact numbers will be a bit less, probably.

What I don't know is if the opto at suturation can support that output current. On some sites I have read you should, for a saturated opto, use its lowest CTR value, which here is 20%. The forward current through the opto is 21mA. 20% of 21mA would only support a collector current of about 4mA.

 

As noted by the member in post #16, you must not edit the post because it negates the discussion that follows it.
The original post has been restored.

 

Using the minimum CTR as a design point is good practice, as that is the value the device can hold as the internal components age. And, your math is correct. 20% is a relatively poor CTR for an optocoupler these days. If you are stuck with that part number, the solution is a driver transistor between the opto and the relay.

ak