AC signal to DC control

[gregfox151]

:-|
Hello all,
I’ve been working on a problem for a few days now, and had little success.
I have a timer with 3-24VAC output signals.
I need to send each signal to a 5VDC XOR gate.
What I did was to send each signal to full wave bridge rectifier (with load resistor-3.9K and 10Uf cap) and then to a 5V regulator and everything worked just fine.
The problem arose when I connected the grounds of each of the 3 rectifier circuits together. The scope shows an AC signal on the DC output, when the (AC)timer is off (BAD!), and 5VDC when the (AC) timer is on (GOOD!).
I would like to know if anyone could either suggest a better way to do this, or suggest a solution how to filter the unwanted AC.
Many thanks.

 

[kak111]

Safest way is to use optoisolators between 24Vac rectifier circuits and 5Vdc circuit
remember that 24Vac is 36Vp and 72 Vpp

 

[gregfox151]

Thanks, it worked...
Another question, I'm using an IL755 opti, and put AC to pin 1 via 330 ohm resistor, and the other AC leg to pint 2. I connected VCC (5VDC) to pin 5 via 1K ohm, and grounded pin 4.
Unfortunately I do not get a output voltage. Can anyone help?

 

[BradtheRad]

The led inside the optocoupler may have burned, if the 24 VAC had only a 330 ohm to limit current.

At each peak that's 109 mA through the led. Even if you average it, it still comes out to 36mA over a cycle. That's likely to cripple the led. Typical safe current is under 20 mA.

Additionally the led has a reverse voltage rating. The negative cycle of 24 VAC exposed it to 34V peak. This is greater than the typical led reverse V rating. So on that account it probably is damaged as well.

 

[gregfox151]

Yor are right, don't know what I was thinking....