# Isolated circuit output

#### pinkyponky

Joined Nov 28, 2019
235
Hi,

I have chosen the component (LP2748 is attached) for isolate the signal between the two circuits.

If I apply 24V to pin1 and 5V to pin6, then what is the value at pin5 (Vo)?.

If I apply 0V to pin1 and 5V to pin6, then what is the value at pin5 (Vo)?.

Do we need to use any resistors to pin1 and pin5?. If so, what values need to be used?.

#### Attachments

• 1.1 MB Views: 13
Last edited:

#### click_here

Joined Sep 22, 2020
471
"I_f" on is recommended to be between 2mA and 10mA, so you'll need to put a resistor in series with that

24V/2mA = 12k
24V/10mA = 2k4

I'd probably use an 8k2 because it is close to the testing conditions that they use (3mA) - But you'll also need to check the thing driving this can provide 3mA.

You also want to minimise output current to 2.1mA, but you need to look at the load to see if that needs a resistor...

You will need a 100nF bypass resistor between pins 6 and 4

#### crutschow

Joined Mar 14, 2008
28,546
Do we need to use any resistors to pin1 and pin5?
Definitely for pin 1 otherwise you let out the magic smoke.
The normal input diode current should be from 2ma to 10mA.
For a nominal 5mA with a 24Vdc input and a 1.5V nominal diode voltage, you should put about a 4.5kΩ in series between the +24Vdc and pin 1.

Pin 5 needs no resistor unless the load on that pin would draw more than 50mA.

Note that for true isolation, the ground for the 24Vdc cannot be common with the ground for the 5Vdc, otherwise you defeat the purpose of the isolator.

#### pinkyponky

Joined Nov 28, 2019
235
What will be the output if I apply 24V or 0V?

Is it necessary to apply the 5V to pin6?, because, some of the tutorials saying that the pin6 left floating.

#### click_here

Joined Sep 22, 2020
471
Vcc has a minimum of 4.5V in the recommended operating conditions

#### click_here

Joined Sep 22, 2020
471
The input/output conditions are shown here

#### pinkyponky

Joined Nov 28, 2019
235
What will be the output of "Optoisolators - Transistor, Photovoltaic Output" and "Optoisolators - Logic Output"?. As I said I will apply 24V or 0V to the optocoupler (using for control the power supplies), so which type of optocoupler is correct for this application?.

#### pinkyponky

Joined Nov 28, 2019
235
Vcc has a minimum of 4.5V in the recommended operating conditions
Hi click,
Then, What will be the output if I apply 24V or 0V?. I have asked this question 2 time no one answer this. please can you?.

#### click_here

Joined Sep 22, 2020
471
The input is *current*, not voltage

The graph that I shared shows the input current vs Vout

The current you supply is determined by a series resistor.

#### click_here

Joined Sep 22, 2020
471
Have a look at post #2 for resistor calculations

#### pinkyponky

Joined Nov 28, 2019
235
The input is *current*, not voltage

The graph that I shared shows the input current vs Vout

The current you supply is determined by a series resistor.
Ok. That is fine. But, what will be output voltage?. If you applied current 'x'mA and 0mA.

#### click_here

Joined Sep 22, 2020
471
... Look at the y-axis of the graph in post #6

#### click_here

Joined Sep 22, 2020
471

#### pinkyponky

Joined Nov 28, 2019
235
In general, for example, If we apply 2mA current to the LED. Then, does we will get the same amount of current (2mA) at the collector of photo transistor?. Or Is there any current will lose?.

Last edited:

#### click_here

Joined Sep 22, 2020
471
In an optocoupler with a transistor output, current gain has to do with the CTR.

Here is a great explanation http://electronicsbeliever.com/opto-coupler-ctr-explanation/

If you want to check any particular device for what the CTR at a particular I_f is, the datasheet tells you.

All that said: This doesn't apply to the device you are using. The TLP2748 is specified in I_f vs
V_out, not CTR (current in vs out).

Last edited: