switching frequency of optoisolators

ericgibbs

Joined Jan 29, 2010
18,767
hi 48,
This table from the d/s shows the rise/fall times in uSec, so you can interpolate an approx frequency by adding the rise+fall times in uSec, add say 1uSec.
Using F=1/Ttr Sum will give you a guide.
E
 

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DickCappels

Joined Aug 21, 2008
10,152
The table on page 4 of the datasheet features a choice of two sets of component values. Use the times mentioned for the set of component values you would like to use.

The datasheet says in part "The TOSHIBA TLP785 consists of a silicone phototransistor..." Clearly a breakthrough! Imagine what can be done with a silicone transistor!
 

DickCappels

Joined Aug 21, 2008
10,152
Yes. The times will change if you change anything.

Notice the faster component value set which should reach to about 20 kHz if using a square wave.
 

crutschow

Joined Mar 14, 2008
34,285
If you need more speed there are faster optoisolators available.
For example, the 6N136 will go to 1 MHz typical.
 
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crutschow

Joined Mar 14, 2008
34,285
The 6N136 has a base pin.
For normal operation you leave the base connection open and connect the photodiode input to the plus supply voltage.

Below is its LTspice simulation, operating with a 100kHz square-wave input.
Note that it has a low CTR, so requires a 16mA input worst-case for a 2.6mA output.

upload_2019-5-12_22-10-52.png
 

MisterBill2

Joined Jan 23, 2018
18,179
Indeed the response times show that this device is "not fast", and so while it can provide isolation adequate for some applications it is not suitable for others. Thus a designer needs to understand the requirements of the design very well to choose the right parts. BUT also, the slower response means that it will not respond to some forms of noise, and so that will be useful in some applications.
 

MisterBill2

Joined Jan 23, 2018
18,179
It's not floating, it's connected to the photo-diode.
Many optos don't even bring out the base connection to a pin and they work fine.
Floating inputs on CMOS devices DO lead to problems, but this is not a CMOS device, and indeed the base is connected to the photo-diode and is not really open. The base connection is made available for those applications where some bias may be required, and it may also serve to allow production checking of the device, which may have been anticipated as needed at one time in the history of opto-isolator devices.
 

Janis59

Joined Aug 21, 2017
1,834
HBFR is the series of over 100 MHz opened sender/reciever with lightguide between them. At least 10 nsec front was rather good working with them
 
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