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Opto coupler erratic behaviour

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dark

Member
Hi ,
I use 4N35 to isolate a Relay . The opto's transistor's base is pulled down with a 1M resistor ,The problem is when this resistor is even touched slightly with finger the relays chatter . Here is the sch;

Thanks
-D
 

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ericgibbs

Well-Known Member
Most Helpful Member
Hi ,
I use 4N35 to isolate a Relay . The opto's transistor's base is pulled down with a 1M resistor ,The problem is when this resistor is even touched slightly with finger the relays chatter . Here is the sch;

Thanks
-D
hi,
The 1M resistor is connected to the Base of the opto transistor, touching it will connect into the opto the background 'mains' and other electro magnetic radiation also any electrostatic field thats being picked up by your body, which acts as a receiving aerial.:)
 
Last edited:

dark

Member
Hi,

sheldonstv , thanks for the ckt , but mine is a finalized PCB .
eric , yes , it chatters like a 50Hz humm 'only on touching it' otherwise not . Is it ok ? I pulled the base down to avoid any spurious triggering . But the PCB is going to be inside a closed cabinet . What to do with the base , should this be left open ckt.


Thanks
 
Last edited:

ericgibbs

Well-Known Member
Most Helpful Member
Hi,

sheldonstv , thanks for the ckt , but mine is a finalized PCB .
eric , yes , it chatters like a 50Hz humm 'only on touching it' otherwise not . Is it ok ? I pulled the base down to avoid any spurious triggering . But the PCB is going to be inside a closed cabinet . What to do with the base , should this be left open ckt.


Thanks
hi,
I would leave the base open, keep any track to the base as short as possible.

Look at this pdf, may help with future projects.:)
 

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dark

Member
hi,
I would leave the base open, keep any track to the base as short as possible.

Look at this pdf, may help with future projects.:)
hi, thanks for the info . How about clipping it right from the die.


-D
 

dark

Member
Just remove the 1meg resistor.
I just de soldered all the 1megs but still after touching the opto pin the Relays chatter. FYI, I use mosfet to drive relays after opto.


-D
 
Last edited:

ericgibbs

Well-Known Member
Most Helpful Member
I just de soldered all the 1megs but still after touching the opto pin the Relays chatter. FYI, I use mosfet to drive relays after opto.


-D
hi
It will do, touching most transistor bases will introduce mains hum.
 

crutschow

Well-Known Member
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Don't touch the pin.
 

MikeMl

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I would just leave out the 1meg, and not touch the pin. If you are really anal, solder a 100nF capacitor where the 1meg used to be.
 

Hero999

Banned
I don't think anyone's told him why he should remove the 1M: connecting the base to the emitter via 1M will greatly reduce the gain of the transistor.
 

ericgibbs

Well-Known Member
Most Helpful Member
I don't think anyone's told him why he should remove the 1M: connecting the base to the emitter via 1M will greatly reduce the gain of the transistor.
Its not the 1meg thats the problem, its the OP's over active index finger.

Or as we say in the trade 'finger trouble'.
 

dark

Member
MikeMl I,Thanks for the idea , 100nF capacitor is too good shunting all the noise like a charm.


-D
 

ericgibbs

Well-Known Member
Most Helpful Member
MikeMl I,Thanks for the idea , 100nF capacitor is too good shunting all the noise like a charm.


-D
hi,
I dont know what type of signals you are coupling thru the opto, but be aware adding a 100nF from the base to 0v will significantly reduce the rise and fall times of the signals.!!

EDIT: As you are using a FET to drive the relays, the heat dissipation in the FET's will be increased, also the switching point/times will be delayed.
IMHO it dosnt require a 100nF on the base of the opto, just dont touch the base.
 
Last edited:

dark

Member
hi,
I dont know what type of signals you are coupling thru the opto, but be aware adding a 100nF from the base to 0v will significantly reduce the rise and fall times of the signals.!!

EDIT: As you are using a FET to drive the relays, the heat dissipation in the FET's will be increased, also the switching point/times will be delayed.
IMHO it dosnt require a 100nF on the base of the opto, just dont touch the base.
Hi ,
I could still see the humm even with the 100nF . Anyways I removed the Capacitor .

-D
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi ,
I could still see the humm even with the 100nF . Anyways I removed the Capacitor .

-D
hi dark,
If you dont feel easy with the performance of the opto, post the circuit fragment that shows the opto driver and signal, also the FET and relay part.

I can check it with an LTspice simulation.
 

dark

Member
hi dark,
If you dont feel easy with the performance of the opto, post the circuit fragment that shows the opto driver and signal, also the FET and relay part.

I can check it with an LTspice simulation.
Hi,
Its just driving a bigger contactor , not very particular regarding switching speed.

Thanks
 
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