Ducky_Doug said:
I need a simple if not a single device to turn on a relay at about 10 volts
or so. Idle voltage is about 6 volt but when the voltage rises above 10-11
volts I would like the it conduct and turn the relay on. The 12 volt relay
approx 65 ohms DC. I don't want any idle voltage to the relay.
The device normally used to make such decisions is called a
comparator. They have two voltage inputs labeled + and -. When the
one labeled - is more positive than the one labeled +, the output
turns on and pulls down to the negative supply rail. However, to make
a comparator based decision like you need, you also need some sort of
voltage reference to compare the supply voltage to, and a switching
device able to boost the few milliampere output current to a current
capable of driving the relay coil
(65 ohms / 10 V =154mA in this case).
Here is the kind of thing I am talking about: (View with font set to
something fixed width per character, like Courier)
+-----+-------+------+--------+---+--- +
| | | / | |
| | 22k | .-. o o | |
| .-. | | | |( - 1N4148
z | | | | | |( ^
A | | | '-' |(_ |
| '-' | |10k +--+
| | | | |
| |\|LM393 | |
+-----|-----|-\ | |<
| | | >-----+-------| 2N4401
| +--+--|+/ | |\
| | .-. |/| | |
| | | | | | |
.-. .-.| | | | |
| | | |'-' | | |
| | | | |220k| | |
'-' '-' +----|------+ |
|10k |10k | |
+-----+-------+----------------+------ -
(created by AACircuit v1.28.4 beta 13/12/04
www.tech-chat.de)
For all voltages lower than 10, the zener diode will drop more voltage
than the 22k resistor, but for voltages higher than 11, the 22k will
drop more. So for voltages higher than 11, the - input of the
comparator will be more positive than the + input and the output of
the comparator (actually, the LM393 contains 3 comparators, so you van
make 2 of these with one chip) will pull down to the - supply rail,
turning the 2N4401 transistor on, and energizing the relay. The diode
across the coil prevents a large voltage spike when the comparator
turns off. The 200k feedback resistor lowers the switching threshold
voltage once the relay is on, to make sure the decision doesn't chatter.