two switching power supplies connected together

[cosminnci]

I have a circuit with an optical isolation, one side powered form a switching power supply and one through a usb from pc.
the middle piece is ft232rl
when connected so the ftdi chip is not recognized
when I power the circuit from a battery all works great.

so I suppose there is something happening with the power from the 2 supplies the one from pc and the switching one, in combination with the optical isolation.

what could be the cause?

thank you,
regards,
Cosmin


 

[Stephen37]

You need to power everything from a single power supply or use optocoupler + power transistor on the other side if you want to drive something that requires another power supply.
The PC side with the USB should work fine and power an optocoupler.

[cosminnci]

I use it for i2c communication, the usb powers an ftdi chip, but when there is noise from ac especially fluorescent bulbs the ftdi resets.
when I power one of them from battery all works fine.

[tautech]

Check the ripple from the SMPS supply AT the chip it powers.
Being SMPS a scope will be needed to see it at those frequencies, probably ~40 KHz
Some can have some horrendus ripple and if this value on top of the DC value is close to a threshold mayhem ensues.
Some extra bulk capacitance on that rail and a inductor in series should get the ripple under control.