Current Transfer Ratio in Photocoupler

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bert_kak

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Guys,

Can you explain to me what is a current transfer ratio in photocoupler and what is my basis for choosing current transfer ratio because I saw a photocoupler datasheet and there are lots of variants for CTR.

Thank you.
 

A 100% current transfer ratio means if you put 5mA into the LED you will get 5mA out of the transistor. The exact test conditions will be in the data sheet.

Choosing the current transfer ratio probably isn't critical. It is more likely to be the case that you just use the data to choose the LED drive current and the phototransistor load.

Keith
 


does it mean if i have CTR of 200% i will get 20mA out of the transistor.

so i will based my choice depending on the current requirement for the transistor side circuit? am i correct.

Thank you.
 

If you have a 200% transfer ratio then it means you will get 10mA out for 5mA in, although you need to check the data sheet for the specific conditions. For example, if the figures are quoted at 5mA in, it doesn't mean the transfer ratio will be the same if you only put 1mA in. Be careful to make sure you design to allow for both the minimum and maximum current transfer ratio, not just the typical.

Keith.
 
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