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How do I build a simple current switch

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EntradaTaco

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Before I begin, I think it is important to know, I can read a schematic and solder proficiently, and I haven't studied electronics or built a circuit since '98. That being said here's what I'm tying to do.

I'd like to build a circuit that switches a load on/off dependent on the current being drawn by that load. Here's the practical application. I have a DC computer charger in my truck (hardwired) that draws on the system even when the computer is not being charged. I'd like to have a circuit that will disable power to the charger unless a load above x amps is being applied.

I believe this is an easy circuite to build, maybe only a couple of components, but I have no idea where to begin... I should have paid more attention in electronics class in HS ;)
 
It's a little tricky to do that since the circuit has to periodically turn on the charger voltage to measure the current and determine if it is above or below x amps, and then either stay on or turn back off. So it would require a current sensor, a comparator, a timer, and a relay at a minimum.

Why not just wire the charger to the trucks ignition switch accessory circuit? I assume you don't care if the charger is wasting a bit of power when the engine is running(?).
 
Right now it's rigged up with a SPDT(?) switch ACC/OFF/PWR. I'm just trying to make it idiot proof, b/c I can be an idiot at times.
 
Right now it's rigged up with a SPDT(?) switch ACC/OFF/PWR. I'm just trying to make it idiot proof, b/c I can be an idiot at times.
So why not connect it to the accessory circuit of your truck? That way it's never on unless the engine is running or you have the ignition switch in the accessory position.
 
That is how it is currently wired. Depending on the position of the switch it's either ON when the key is in the ACC or START position, OFF, or always ON. The circuit is for the always ON side b/c once my computer is done charging, say when I'm camping and the vehicle isn't running, it's still drawing enough current to put a drain on the battery if it goes forgotten.
 
OK, now I understand.

So the simplest might be to sense the current and when it drops below the desired value it would cut off and stay off until reset. To reset it you would turn the ignition to off and then back on.

Does that sound satisfactory?

Do you know the current level that it normally uses, and at what level you want it to stop?
 
I do think there is a potentially easier solution. Something like the following:
1. Charge always if car is in the ACC position
2. When ACC goes low (IGN off), start timer for x hours
3. If button pressed, extend time for another x hours.

This, I think, would work potentially a lot better. Would it work for you?

Relatively easy to do with a microcontroller.
 
KISS, that seams like it could be easier... so for example, on the ON circuit, it runs for 2 hours until is clocks out or until a momentary switch or something is hit and it restarts the timer. How would I go about making that circuit?

BTW crutschow, I had to go buy a new multimeter today, that's why I didn't get back to you.
 
One way to do it is to use a small processor. Take a look at www.picaxe.com. I did buy a kit, but haven't spent much time with it. It's programmable in BASIC and it's relatively cheap.

You would have to start with a 12V to 5 or 3.3 V switching regulator and so a little spike suppression for the automobile environment. Boards are easily obtainable minus the suppression needed for the auto environment.

Turning it on might be tricky. I'm picturing a relay in a latch/hold kind of situation. Pushing the button will latch the relay and turn on power. One the timing cycle completes, another tiny relay would briefly open the holding contacts.

Take a look at the picaxe site.

Crutchow: Have any other ideas?
 
The picaxe kit looks like it would be a great way to control this and other components, but at $30 + shipping, that's outside my budget for this project. Any less exponsive ways to accomplish the same thing?
 
KISS's approach could be done with just a timer and an PMOS switch.

The long time delay could be done with a CD4045 or CD4060 counter with a 555 astable clock. At the start the PMOS switch is turned on by the counters low output. When the counter reaches the desired count and goes high, it turns off the PMOS switch and also is connected to inhibit further counting.

For example, a CD4060 has 14 stages so it's maximum count is 16,384. For two hours delay the clock frequency would need to be about 16,384/7200 = 2.3Hz, easily generated by a 555 astable. A power-on reset restarts the count.

These devices will operate from the 12V battery as long as a simple RC transient suppressor is added.
 
I'll be honest and say that I have no idea what these components are or how they work, but that's what google's for :D. Would the RC transient suppressor maintain a safe and consistant voltage even when the vehicle is running and the supply become ~14.7v?
 
As long as you keep the voltage below the maximum rated voltage of the CD4060 and the 555 you will be okay. The RC suppressor is to limit the high voltage spikes that can occur when you start the car.

If you take a look at those components and take a stab at understanding them, we can help with the design details.
 
Basically three components, a TVS diode rated at about 18V, a diode installed backwards and a fuse will do a great job of increasing the reliability of whatever device you use. It just turns out that the automobile environment is "hostile" with short high voltage +- spikes on the order of 50-200 Volts. The components are purely for reliability.
 
Thanks both of you for your assistance. I'll be unavailable for the next 8 days, in the middle of the desert, where computers cease to function. I hope that we can continue this discussion afterward. Have a great week.
 
hey, i dont know if this will help, but in my inverter AND inside my dashboard are low-voltage cutoff switches, so my chargers runs even when my devices are charged, (including for my jumpbox powerpack)

however i recommend this since you commented "idiot moments:" and during these times the dashboard will ensure to always have enough power avail for the starter, so the jumppack stops charging at a low battery then resumes after engine start,

my inverter is belkin300watt, idk price thou..

idk if this is chaper or simpler, definately less eductional,
 
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