how to make floating source by using optocoupler

[elahetal]

Hi guys,
I have a digital pulse generated by a DAQ and is connected to a pulse generator IC.
I need to have a floating voltage so I was thinking about adding an optocoupler after digital pulse or in the output of the IC.
Either way, I do not know how to use the optocoupler for this purpose. Do you have any help or suggestion for me?


Thanks,
Ela

 

[IanP]

A floating output requires floating power ..
The pulse can be easily transferred via an optocoupler, but the output stage of the optocoupler has to be fed from something – whether you use an external power source or a small dc/dc module – it’s your call ..

eg. NME0505
https://pdf1.alldatasheet.co.kr/datasheet-pdf/view/78564/ETC/NME0505.html


IanP
:|

 

[elahetal]

Thanks a lot IanP,
For my understanding, can you tell me if the pulse in second part is a floating one with referenced to its GND(GND2)?
I have another question, If I want to simulated the my circuit in Spice, do you know how can I apply GND1 and GND2, I mean if spice consider these two GNDs to be different ?

 

[IanP]

The output is of course referenced to GND2 ..

See: EDAboard.com Forum Index -> Power Electronics > how to do isolation in spice:
#1297193

You can always reference both sides to one common ground, test the transfer function of the circuit, and keep in mind that when in the real circuit the grounds are separated the output is referenced to it's local ground, that is GND2 ..


IanP

 

[keith1200rs]

In Spice there is only one ground - node zero. Your second ground can be called anything you like and you can use it as a ground. Spice will complain though if anything is truly floating so you will need to link your floating ground to the real ground with a high value resistor. I tend to use 1G ohm.

Keith

 

[elahetal]

Hi IanP,

I have a bizarre problem, I get the pulse from drain-source but not from source!! I know it seems strange but I do not know....why!

 

[IanP]

I get the pulse from drain-source but not from source

Can you explain a little bit more what do you mean by that ..
Maybe a drawing of the circuit that you tested ..

IanP
:|

 

[elahetal]

HI IanP,

Thanks for your answer,
I attach my schematic and the results. I may made a mistake in drawing or understanding your point. please have a look and provide me with your comments.

Thanks,
Ela

 

[keith1200rs]

By drain I assume you mean collector and source you mean emitter.

It looks like the transistor doesn't turn on fully. You will never manage to get the emitter to go to 10V though. More drive current to the LED would help, or a larger value for R4. Using the opto as you are doing can result in a <100% current transfer ratio depending on the opto. If you want the widest output voltage swiing you need to put the load resistor in the collector not the emitter.

Keith.

 

[elahetal]

Hey keith,
Thanks for reply,
Yea, I meant collector and emitter just a blunder....!
I tested more drive current but the result is the same...I want to transfer the input pulse, I do not care about swing and..., just I do it for ground isolation...but the result I see here is inverted input(green signal).... and the blue one is a level-shifted signal

Ela

 

[keith1200rs]

You get the signal inversion because that is the way you have connected it. If you ground the emitter, put the load on the collector you will reverse the logic.

Keith