design for 60 hz square wave TTL pulse from 9v ac line input

Thread Starter

roylesq

Joined Oct 19, 2016
8
I am using a 9v ac supply to power 1a 5v supply (fw bridge and 1000uc cap and 5v regulator) and using the same supply for generating square wave 60hz timing pulse. I have tried 741 op amp zero crossing but the low voltage output of 741 is at 1.2v, not enough to trigger a 74ls14 chip to high state. I am experimenting with a simple transistor circuit using 2n3904c but the ac input does not go below 5v no matter which side of the ac input I use.

this circuit has worked in the past. Is this a sign of shorted components?

upload_2017-6-5_10-54-17.png
 

crutschow

Joined Mar 14, 2008
25,130
Are you pulling the AC from in front of the bridge inputs?
You say 9Vac in the title but show 12vac in the schematic.
Which is it?
 

AlbertHall

Joined Jun 4, 2014
10,065
What's the purpose of a resistor across a low impedance AC supply? :confused:
The circuit as drawn is not quite correct from the TS description. The winding is connected to a bridge rectifier with the negative of the bridge connected to the ground of that circuit therefore the winding can supply a positive voltage to the circuit but it cannot pull down the input to the transistor and hence, with the help of the capacitor on its base, the transistor will always be on.
 

ErnieM

Joined Apr 24, 2011
8,046
Unless you line in an area services by a 30 Hz AC service you will not get your desired 60 Hz output from a full wave rectifier.

I do trust 120 Hz will suffice.

To enable outside help a schematic of the power supply section (9 VAC input thru bridge to filter cap) and the connections of your converter is necessary. This is a tricky connection to get correct without shorting out the bridge or driving the converter with DC.
 

ronv

Joined Nov 12, 2008
3,770
I am using a 9v ac supply to power 1a 5v supply (fw bridge and 1000uc cap and 5v regulator) and using the same supply for generating square wave 60hz timing pulse. I have tried 741 op amp zero crossing but the low voltage output of 741 is at 1.2v, not enough to trigger a 74ls14 chip to high state. I am experimenting with a simple transistor circuit using 2n3904c but the ac input does not go below 5v no matter which side of the ac input I use.

this circuit has worked in the past. Is this a sign of shorted components?

View attachment 128278
Here is another approach along with the transistor. The one thing that pops out is that 1000Ufd may not be quite enough depending on the regulator you use. Both circuits have some error from the true zero cross, but I don't know how close you need it.
upload_2017-6-5_20-43-46.png
 

Thread Starter

roylesq

Joined Oct 19, 2016
8
My schematic was in error. I was using a 9v supply (AC rms) in substitution for the 12 volt supply, it has about 11.4 V peak

Here is the corrected schematic.

BTW, I was unable to find a source for ELM440 which would be fine if I could find it.
 

Thread Starter

roylesq

Joined Oct 19, 2016
8
thanks for this tip. I Plan to try it. Elm is canadian company with some interesting products I was not aware of. (but then, I have been out of the electronics loop for almost 40 years so my skills are rusty and my knowledge is now ancient.


Unless you are dedicated to this design, ELM ic's have been around for a number of years, the ELM440 uses a common colour tv xtal.
https://www.elmelectronics.com/products/ics/bench/
Max.
 

Attachments

AlbertHall

Joined Jun 4, 2014
10,065
The circuit in post #12 will work but you need to add a resistor from the left hand end of the 6.4k resistor to ground as previously stated.
 

crutschow

Joined Mar 14, 2008
25,130
Below is an LTspice simulation of the circuit simplified.
I see no particular reason for the transistor input capacitor, since it distorts the output square-wave, and the base diode is not needed since the bridge diodes already clamp the AC input to ground.

upload_2017-6-6_9-57-42.png
 
Top