Dr. Phibes said:
I want to trigger a relay so that when the input voltage is at 95 VAC (134 V
peak), the relay is off. When the input voltage is at a full 120 VAC (170 V
peak), or it goes beyond the 95 VAC threshold, then the relay is energized.
This circuit needs to be discrete enough to where I can put it on a small PC
board inside of an enclosure. So I can't be using hulky 120 VAC relays or
anything like that. The only source voltage is the input voltage that
switches between 95 and 120 VAC. The relay will be switching on a seperate
circuit that uses less than an amp of current (a series of LEDs). Thus,
another reason why the relay doesn't have to be hulking.
I'm a newbie to this board, so I hope someone out there can help.
Thanks in advance!
I made safety the top priority for this circuit, so it uses
a very small transformer and an opto triac driver. This will
do it for you if you have room on your PC board for the parts.
The transformer needs a bit less than 1" sq and 1" high. The
opto is a 6 pin dip, the TL431 is a TO-92. The trimpot is 5K.
Opto = Sharp PC3SD12NTZAF Mouser # 852-PC3SD12NTZAF
TL431 - Mouser #512-LM431ACZ
Transformer Mouser # 823-BV020-5384
+------------------------------------------------+
| |
| ----- D1+ |
AC-+-|Trans|-->|--+----+------+---[1.5K]---+ |
| | |+ |+ | | 1 | 4
| 12V | [D2][470uF] | [Opto] [OptoOut]
| | | | | | 2 | 6
AC-+-| |------+----+ P --- |
| ----- | 5K O<--[1.5K]--/_\TL431 |
| | T | |
| | | | [LEDCircuit]
| +------+------------+ |
| |
+------------------------------------------------+
Here's a drawing of the opto:
----
1| |6 Pin 1 is + input
2|Opto|. Pin 2 is - input
3| |4 Pins 4 & 6 is nonpolarized output
---- There is no pin 5
The opto specified normally drives a triac, but in
this circuit driving LEDs the current is assumed to
be low enough that it can be used directly. It will
handle 100 mA on the output side, and your LEDs should
be configured to run at less than 30 mA. If your LEDs
draw appreciably more, a different output circuit is
needed.
The TL431 acts like an adjustable zener. When it "sees"
about 2.5 volts on the adjustment pin it turns on. You
adjust it to turn off at ~ 95 volts. You said you had
a control signal that could be either 95V or 120V, so
no hysterisis is needed and you can make the adjustment
using your control signal. Set it ~half way between the
point where the LEDs go out at 95 volts and turn on at
120 volts.
How it works:
The voltage across the 10 uF cap will be ~ 16.3V when the
line voltage is 120. When line voltage drops to 95, the
voltage across the cap will be about 12.8V. The small
5K trimpot is a voltage divider which sets the voltage
at the TL431 adj pin. Say it happens to be set to provide
2.77V to the adj pin when the control signal is 120.
That will turn the TL431 on, which in turn makes the
opto output conduct. When the control signal drops to
95 volts, the voltage at the adj pin will be 2.17 volts,
and the TL431 will turn off, which turns the opto off.
Ed