PC847 Optocoupler Alternative

Thread Starter


Joined Oct 9, 2011
Hi there - I am new to these forums, so please be gentle!

I am refurbishing an old 8085 microcontroller based product (simple production line controller). It uses a fair few Sharp PC847 (marked as PC817) opto isolators / opto couplers. These are quad versions of the Sharp PC817 opto.

According to the datasheet the device will degrade over time - given that this product is about 20 years old, I think it would be prudent to replace them. However RS only stock the PC817 single device, not the PC847. they do suggest an alternative: Siemens ILQ615. The comparison shows key differences are on maximum current tranfer ratio. The Sharp device goes up to 600% and the Siemens part goes to 125%. I do not properly understand CTF - can anyone advise if this is obviously ok?

Really appreciate any helpful advice which any can offer?

Many thanks,




Joined Apr 24, 2011
CTR is Current Transfer Ratio and measures how much output current you get for a given input currrent.

Example: If you drive a 200% CTR device with 10mA of base current you can expect 20mA out of the transistor.

So bigger is better, and without a careful analysis don't replace a 600% device with a 125% one.