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RC Circuit Isolation

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smithn

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I need to isolate a Resistor Capacitor circuit from an input. The goal is to provide power for a short period of time to an input and LED but, I don't want the resistance of the input to effect the discharge time, since this resistance may be unknown. Also the RC charge time has to be very short on the order of uS but the discharge time needs to be on the order of Seconds. Can this be achieved with an optoisolator?
 
Hi,

What kind of power supply voltages are available?
 
Hi,

Can you use that power supply to power the LED?

I ask because what you seem to need probably requires a power supply and circuit rather than just a simple RC. It is too hard to charge a capacitor large enough to run an LED for several seconds in only a couple microseconds, but a small circuit can easily do that.
 
I am doing this right now with a large capacitor 1000uF. The goal is to provide 85%-100% of the 24VDC to an input and LED as, a series of mechanical relays switch. The capacitor is charged with very little resistance and charges very rapidly. However if one of the relay contacts opens the capacitor is no longer receiving 24vDC and begins to discharge fairly slowly through an unknown resistance, greater then 1 sec time constant. When the contact is restored to a closed state 24VDC is restored to the LED and input and the capacitor is charged again.

What I need to do is isolate the LED and input from the RC circuit and put my own known resistance in their so the discharge time can become a known value, since the resistance of the input may change from one application to another.

I agree something like a 555 timer maybe more appropriate for this application but, it cannot be used.
 
Hi,

Can you draw up a diagram of your connections? That would make it easier to understand what exactly you need.
Also, what is wrong with using something like a 555 timer, can a LM339 be used then, or how about a transistor or two?
 
Hi,

Can you draw up a diagram of your connections? That would make it easier to understand what exactly you need.
Also, what is wrong with using something like a 555 timer, can a LM339 be used then, or how about a transistor or two?

Right now 555 timers and LM339's are not approved devices for this application. Right now only certain Relays, Resistors, Capacitors, Opto isolators, LEDs are approved for this application. I don't know about transistors, even though the Opto isolator is composed of transistors.

I think opto isolators are approved due to the fact they are considered a galvanic isolating device where a plain transistor circuit is not.
 

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Hi,

I see one resistor is 1600 ohms, is the other resistor approximately 1 ohm or something else?

Also, what point in the circuit needs to be isolated?

When i mentioned the transistor, that would not be for isolation that would just be to aid the function we would need here. It would not affect the isolation, we would still need an opto coupler for example.
 
Its approximately 1 ohm I cant remember if I have a 1 ohm or a 0 ohm in there now.


In between the LED and RC portion.
 
Hi again,

So the 1000uf cap just has to keep the LED on for a second or two, is that the only reason for the 1000uf cap?

If so, you can use a diode. Disconnect the top of the 1 ohm resistor from the relay, insert a diode cathode to the 1 ohm resistor anode to the original connection (relay). Disconnect the top of the 1600 ohm resistor from the relay, connect it instead to the cathode of the diode.

Now when the relay turns off, the capacitor only supplies current to the LED not the node at the far right. When the relay turns on, the capacitor charges again.

If the 1000uf cap also has to filter the output from the relay so the far right node gets a cleaner supply, then simply add another 1000uf cap at the far right node from the node to ground.

Is that what you needed?
 
I am using the capacitor as a kind of "time out function". So the two relay contacts shown on the left need to switch states at the same time, or very close to it within ~1 second. So the capacitor still supplies power to both the LED and Input for that short amount of time. if the relays fail to switch at the same time due to a fault the capacitor should completely discharge no longer supplying power to the LED or Input.
 
Hello again,

Ok, it looks like you'll have to clarify what it is you are trying to do with this circuit pretty much in it's entirety. For one thing, the relay contacts are drawn such that it appears that when one set of contacts opens the other set closes, so there's never a break in power to the capacitor, or only for the time it takes for the contacts to switch over which would be some milliseconds.

Did the circuit work 'sometimes' as it was originally?

Is one set of contacts normally closed and the other set normally open?

Did you try the diode and additional capacitor?
 
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