# Thread: I want to install a switch

Thanks, got one today. So you can use AC switches with DC? What is the difference between an AC and DC switch?

2. This is the closest topic to a question I have... this way I don't get 'd and harassed

I used a SPDT relay to short my laptop's power switch to resume it from standby when I turn the ignition on. For some reason it would work intermittently and then it stopped working altogether.

I used the12volt.com method to give the power switch a nice .5 second shorting when the ignition is turned on (the12volt.com) and pretty sure I did things right.

Now that I took that out and threw it away, I'm ready to try again. So I made a MS Paint picture to describe what I'm picturing...

I have 2 capacitors rated at 3300 uF / 6.3V a piece that I can put in series for 1650 uF, but what is the voltage rating telling me? If I put them in parallel I know I end up with 6600 uF, which may give me too long of a time delay.

Using Ohm's law, the circuit WITHOUT any additional resistors or capacitors produces the needed 60 mA to actuate the switch, but now how am I affecting it by adding in the cap + resistor to cause the 1 second pulse?

If any of this is unclear, let me know please I want to get this done by this weekend. Thanks guys!

Btw:

Infiniti FX35 w/ 7" Lilliput touch-screen, Dell Inspiron 1100 laptop, inverter, RR, etc.

3. Anyone? Relay? Capacitor(s) + Resistors in series with the relay coil?

4. Kinda confused to what you're looking for. From looking at that link on 12volt.com for that circuit (nice find by the way) it looks like what you have above is correct. Not sure why you're trying to calculate Ohm's Law stuff or see the affect of the capacitor and resistor here....?

5. Well you see... I built the circuit pictured above, using the values from the12volt.com, but it didn't always work and then it just completely stopped working. My main concern is the voltage rating on the capacitor since it's not stated on the12volt.com page. What's also not mentioned is the rating of the SPDT relay...

I'm not exactly sure what kind of currents I'm working with when dealing with an RLC circuit like this... and I don't know if I'm sending TOO much current through the relay or if there's just not enough...

6. Originally Posted by Cube
...

I have 2 capacitors rated at 3300 uF / 6.3V a piece that I can put in series for 1650 uF, but what is the voltage rating telling me? If I put them in parallel I know I end up with 6600 uF, which may give me too long of a time delay.

Using Ohm's law, the circuit WITHOUT any additional resistors or capacitors produces the needed 60 mA to actuate the switch, but now how am I affecting it by adding in the cap + resistor to cause the 1 second pulse?...
When you first turn on, the cap has 0V and the relay has ~12V which pulls it in. As the cap charges, its voltage goes up and the relay coil voltage decreases. At some point the relay doesn't have enough voltage to keep it energized and your "Pulse" times out. The cap keeps charging though, going up to 12V. When you turn off, the cap would hold its charge if not for the resistor. You need it to discharge to "Reset" the circuit so that you can trigger a new pulse again. So here is the effect of the values:

Cap voltage should be 25 volts or greater. Capacitance value determines length of turn on pulse, higher value means longer pulse. Resistor value determines reset time, higher value means longer recovery time needed. Your relays' coil resistance also has some effect on the timings.

My suggestions to try: C= 470uF 35VDC, R=4.7k ohms

Add a diode across the coil of the relay, too. Here is a thread with a bunch of timing circuits similar to what you are looking for; see posts 6, 9, & 32

Schematics/Pin Outs/Diagrams

7. Awesome, thanks Jimmy. I understand the whole concept... just wasn't sure about cap ratings I should be using. Will try to get those values you mentioned, but instead of 4.7k ohm resistor I'm just going to put 2 10k in parallel. How about capacitors? I happen to have 2 1000 uF 6.3V capacitors... in series the capacitance is 500 uF, but will the voltage be alright? If not I'll just go pick up a single capacitor rated for 25V

Thanks again.

8. Originally Posted by Cube
Awesome, thanks Jimmy. I understand the whole concept... just wasn't sure about cap ratings I should be using. Will try to get those values you mentioned, but instead of 4.7k ohm resistor I'm just going to put 2 10k in parallel. How about capacitors? I happen to have 2 1000 uF 6.3V capacitors... in series the capacitance is 500 uF, but will the voltage be alright? If not I'll just go pick up a single capacitor rated for 35V

Thanks again.
Those 6.3V caps won't last very long with surges from your alternator on them. Caps are cheap. Get a new one. You might have to experiment with the values.

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