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.

comapator

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.

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!

Does the switched load have to be via a mechanical relay ? Is the load
even
connected to the same ac supply ?

Does the switched load have to be via a mechanical relay ? Is the load
even connected to the same ac supply ?

No, it doesn't have to use a mechanical switch like a relay. Yes, the load
is connected to the same power supply. Here's the jyst of it

I'm replacing regular incendescent stage lights with home made LED lamps. I
have a light controller that controls the various color lamps. Since the
LEDs are much more sensitive to voltage and current than regular lights are,
I've found out that the controller actually puts out 90-something volts when
the lamp should be "off". This works fine with regular lights. But with
LED lamps, the lamps go dim rather than switch completely off. Thus, I'm
trying to come up with some sort of a gate circuit that will detect the
undervoltage and electrically (or mechanically) cut the LED string off.

The LED string simply consists of a string of LEDs connected in series with
a resistor in front and in back of the circuit, then connected directly to
the light controller that puts out 120 VAC when "on". And anywhere from
75-90VAC when it's supposed to be "off".

I decided to put a switching transistor in the circuit, that would switch
the LEDs on and off. But the problem I ran into with that was I couldn't
figure out how to bias the base of the transistor in relation to the voltage
change between 90 VAC and 120 VAC.

It seems to me that your controller is the problem though since it doesn't
really switch off. Is there any way to modify that ?

It seems to me that your controller is the problem though since it
doesn't
really switch off. Is there any way to modify that ?

Yes, it's definitely the problem. It's a DMX light controller, very
commonly
used for stage light controlling. Definitely not made to drive LED
lights.
;)

The LED string simply consists of a string of LEDs connected in series with
a resistor in front and in back of the circuit, then connected directly to
the light controller that puts out 120 VAC when "on". And anywhere from
75-90VAC when it's supposed to be "off".

I wonder if the light controller is getting confused by the
characteristics of the LED light. Have you tried putting a normal
incandescent lamp in parallel with the LED lamp? If things work that
way, you could then use a suitable resistor in place of the
incandescent lamp (although this would increase the power consumption
somewhat.)

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!

Dr. Phibes wrote:
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

The LED string simply consists of a string of LEDs connected in series
with
a resistor in front and in back of the circuit, then connected directly to
the light controller that puts out 120 VAC when "on". And anywhere from
75-90VAC when it's supposed to be "off".

Are these back to back LEDs or do you use a diode in series?

Your suggested solution seems very ass backwards - there must be a better
way.

BTW, why are you using LED lamps for this? Seems odd.