We have a switchable signal source. With the switch in position 1 there is an AC signal that fluctuates from 0V to 12V at about 60Hz (it does not reverse polarity). If the switch is in position 2 the signal is 12VDC constant.
The signal is connected to a standard electromagnetic relay. We would like for the relay to turn on ONLY when the 12VDC signal is applied and NOT when the AC signal is applied. Currently, the relay reacts fast enough that when the 60Hz signal is applied, it will switch on and off at 60Hz.
Will this work with a suitable sized inductor in series with the relay coil? If so, how to calculate the proper size? If not, what is the best method?
The series inductor may or may not work. The AC signal you describe has some DC component; if the waveform is sinusoidal, for example, or if it alternates between 0 and +12 with equal "on" and "off" times, the DC component is 6 volts, and if that is enough to pull in the relay, an inductance alone won't do the job. I can imagine a circuit that should be able to do it, if the inductor alone is not enough: an inductor in series between the switched source and the relay coil, a capacitor across the relay coil, and a diode in parallel with the inductor, cathode to the switched source side. That way, the capacitor across the relay coil will be discharged each time the input voltage goes to zero, and depending on the part values, you can keep the relay coil voltage from ever getting very large. Instead of an expensive inductor, you could probably use just a series resistor instead, assuming it's OK to put a somewhat heavier load on the input AC signal and that the relay will pull in well below 12 volts. Assuming a relay coil of 1000 ohms and a maximum (high temperature is usually worst-case) pull-in voltage of 9V, I'd suggest a 100 ohm resistor, and something like a 470uF 16V electrolytic capacitor, and a
1N4001 (or 4002 or 4004 or...) diode. The 42 millisecond time constant of the resistor+coil and the capacitor should do a decent job of killing the 60Hz on the coil. If the 60Hz source is very "stiff," it may be advisable to put a small resistance in series with the diode to control peak currents.
Wow! Thanks for the great ideas! I think your method is way better than the inductor alone...
A few questions though:
1) If the source never goes to zero, but instead went from say 4V to
12VDC would this still work?
2) The relay coil (automotive relay) impedance is about 100 ohms, what formula did you use to calculate R and C?
Connect a retriggerable monopulser to the source. The time constant should be longer than the cycle time of your AC. So long as there is AC present the MP output will be a constant 1. When the source switches to DC the MP output will revert to a 0 within a time constant. Operate the relay when the MP output is a 0.
You don't even need a "real " MP; just a diode, capacitor, resistor and FET.
There is no best way because most people don't work themselves into a situation like this. Re-select the switch to be a DPDT: View in a fixed-width font such as Courier.
. . . . DPDT SW . -------------- . | | . | 1a | . Fluct------------- o | . | \\ coma | . | o -------> some other crap . | | . 12VDC-+----------- o | . | | 2a | . | | | . | | | . | | 1b | . | | o | .-|
On Feb 13, 9:13 pm, snipped-for-privacy@hotmail.com wrote:
Well, to do things right, you need to characterize the relay a bit better. At what voltage does it pull in, minimum? (This will set the max you can allow across the relay coil to insure it stays in the de- energized state.) At what voltage is it guaranteed to pull in? (This will set the maximum series resistance you can allow.) The R-C time constant should be long compared with the cycle time of the pulsating AC. R is the parallel combination of the relay coil and the resistor, and C is just whatever size capacitor you use. R*C ought to be at least a couple times the cycle time of the slowest wave-form. The value of the resistor itself must be small enough so that it provides enough current to let the relay pull in reliably: for example, if the relay pulls in reliably at 8 volts, and you can guarantee that you supply 12V DC minimum in the DC state, and the relay coil is max 100 ohms (when hot: worst case), the resistor must be no more than 50 ohms: 8V = 12V* 100/(100+50). 47 ohms is a common value that should work in such a case. Power dissipation in the resistor would be R*I^2 = R*(Vin/(R+CoilR))^2 = 50*(12/150)^2 = 0.32 watts; in this case, I'd recommend using at very least a 1/2 watt resistor, but 1 watt would be better, to allow for higher excitation voltage and lower relay coil resistance than I used there. Then 47 ohms in parallel with 100 ohms is about 30 ohms, and to get an R*C time constant of, say, 50 milliseconds (about 3 cycles of your ~60Hz waveform), you'd need
1500uF or so--2200uF would be a common value, easy to find, but starting to get physically big.
Use Tam's way by adding an FET for a more efficient circuit and one that can handle the 4-12V pulsating signal more easily. Here's a variation on that theme: N-channel FET, source to common, drain to "bottom" of relay coil. Other side of relay coil to DC/pulsating AC source. Capacitor from FET gate to common. Resistor from FET gate to common. Another resistor from FET gate to the DC/pulsating AC source. Adjust the ratio of the resistors so that with the DC input, the FET gate is positive enough to turn it on, and with the pulsating AC, it's not. R*C, as above, should be longer than the cycle time of the slowest pulsating wave form. You could also get fancy and rectify the pulsating AC part of the waveform to get a negative bias to _really_ keep the FET off, but probably don't need to go to that trouble. Big advantages: using the FET, you can get by with a much smaller capacitor value, since the gate current is practically zero and you can then use high value resistances; the FET will turn on with very low resistance, so the relay coil gets very nearly the full effect of the 12V DC, and with the pulsating AC, it's very "off" so the relay coil won't have any significant residual voltage across it. Also, this way doesn't significantly load the pulsating AC source, as the diode could in my initial suggested circuit. The relatively stable and narrow range of gate voltage between "on" and "off" for a given part is another plus; I assume this is one-off and you can select the resistors to match the particular FET's gate turn-on voltage. It's a simple circuit, using inexpensive small parts that should be quite reliable. One thing to beware about: any possibility of excess voltage on the FET gate.
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