AC loss detector with HCPL3700

Gang,
What am I missing here? I can’t get a low out of this thing to show good AC coming in.
HCPL3700V datasheet & application note.
https://www.fairchildsemi.com/datasheets/HC/HCPL3700.pdf
http://www.avagotech.com/docs/AV02-3699EN

I have 120V connected thru 30K resistors to pins 1 & 4, 10uF electrolytic cap across pins 2 & 3, Vcc at 5V with 0.1uF cap to Gnd, Gnd to Gnd, and Vo pin 6 to an Arduino input with internal pullup. Output stays at 5V.
I measure 59VAC at pins 1,2,3,4.

What do I need for a low? I am stumped.
AC_loss.jpg

Can you forget about the 120vac for the moment and apply, say DC 12v at the chip input to see what you get?

.

I have 5.08VAC across pins 1 & 4, and 3.25VDC across pins 2 & 3.

Altho on a scope and with a meter relative to Gnd they all look like 60VAC.

LarryD,
By chip input, do you mean +12/Gnd on pins 2(+) & 3(Gnd) ?

I'll have set up a 12VDC supply, too late to start that now, can barely see with these dry contacts.
Tomorrow after work I'll set that up.

Would I need some current limiting with the 12V?

I was thinking something like fig 5 bottom page 3 on the application sheet 1004.

I have things soldered up, didn't want loose parts with the live AC.
I have ~29K resistors in series now. Re-reading page 6 for input AC filtering, it looks like I could have used two 15K resistors instead for a total Rx of 30K. I can add a 30K in parallel to each one to get each leg down to 15K, also has power dissipation benefit.

Have to wait until I get home to try this after fencing class.

Got it figured out - just need to lower the resistance in the AC lines from 30K to 15K (Rx/2, which I was not reading correctly in the datasheet).

Need to determine Rs for 24VAC also but dont' have a 24VAC source to test with yet.
Need to poke thru my pile of old printer power supplies & stuff.

CrossRoads:
Got it figured out - just need to lower the resistance in the AC lines from 30K to 15K (Rx/2, which I was not reading correctly in the datasheet).

Need to determine Rs for 24VAC also but dont' have a 24VAC source to test with yet.
Need to poke thru my pile of old printer power supplies & stuff.

@CrossRoads...hi. What is the purpose of the 10 uF cap across the two DC input terminals #2 and #3 on the HCPL3700 (C11)?

Hi,
Are you happy with the 24Vac being somehow connected to 120Vac?

I would use two separate 3700 ICs.

At the moment you need BOTH 120Vac and 24Vac to fail to change the 3700 output?

Tom.... :slight_smile: