Isolating the circuit.

jtgoulart · Joined: 13 Feb 2007 Posts: 10

Hi friends,
I need to isolate the pic circuit from a font of 30V. The pic will go to turn on or turn off the font. But I don't know how make it. I tried to use a photocoupler, however I can't lose the original waveform:

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

I don't know what a "font" is. Perhaps some kind of display using light bulbs ?
I don't know what the current is. But it's possible you could use a driver chip such as ULN2003A.

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

Your graphics are rather chopped up, but I guess you have a slow decay on the optoisolator. Does the isolator have a pull up resistor on the output? If so try a resistor 1/10 the resistance to speed things up. BTW how fast is this waveform? We have no idea from your info. Also you could try a logic output isolator instead of a transistor output one.
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Neutone · Joined: 08 Sep 2003 Posts: 839 Location: Houston

rnielsen · Joined: 23 Sep 2003 Posts: 852 Location: Utah

I've used photo-couplers in high speed applications and your standard ones, like 4N35 and such, are waaaay too slow. Find one that is rated for high speed applications and the wave form should square up.

Ronald

jtgoulart · Joined: 13 Feb 2007 Posts: 10

I need that the photocoupler answer at 30Khz. I'm using one of 80Khz, but the wave is deformed.

Humberto · Posted: Mon Mar 26, 2007 12:39 pm

Your "graphics" are very poor to get a good understanding of your problem.
For sure if you post an schematic drawing of the circuit involved you will get better help.

Humberto

Ttelmah · Guest

The 'rating' for the opto, in frequency terms, is the highest frequency it can pass, usually with 3dB of attenuation. Not the frequency it can reproduce with square edges. A square wave 'edge', at _any_ frequency, technically contains frequencies extending to infinity!. If (for instance), your oscilloscope is rated at 60MHz bandwidth, to pass the waveform with no more degradation than the scope introduces, will require an opto with this frequency rating. To reproduce a square wave 'reasonably well', requires handling several decades beyond the actual frequency involved. I'd guess something closer to a 1MHz opto, will be needed to maintain the edges you want.
An 80KHz opto, is very slow indeed.

Best Wishes

RolandAldridge · Joined: 26 Mar 2007 Posts: 7 Location: Southern California

It's a high speed isolator that can run at 110MBS, using Giant Magnetostrictive technology (whatever the hell that is). I've been using them for a while with virtually no problems. They also don't suck current like an optocoupler does.

Link: http://www.nve.com/Downloads/il71x.pdf

Roland
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