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Need help - kinda complicated

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Newbie level 4
Jul 23, 2002
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I'm very new to all this, and have very little knowledge, but I need this for one project, so all your help is very appreciated.

I need a circuit which will pass the current only after some voltage. For example, I need something which will pass current through only when the voltage has reached let's say 2 volts. This would be a kind of like pressure valve. When pressure builds up sufficiently it let's it out.
This is the same. I'd like to connect this to the source, and only when voltage reaches 2 volts and more, I want it to let the current through so that it can be used for another electronic circuit.

Thank you very much for your help, and if anything is unclear, feel free to drop me a line.
Thanks again!

I dont have a specific circuit for you but it sounds like you could build this circuit in two stages. In the first stage I would use an op amp as a comparitor. This would then enable the second stage which could simply be a transistor that youa re turning on and off.

I just found this on the web. Again it probably is not exactly what you are looking for but hopefully will get you started.

**broken link removed**

R3, R4 and P1 set the level (Vref) that the comparator (U1) is going to switch on. Pin 2 of U1 is the input voltage that you are trying to measure. When the input voltage is less than Vref the output voltage will the lower rail voltage and when the input is higher than Vref the output will go to the higher rail. This will then turn on and off transistor T1. With T1 off no/little current with it on then we have current. Replace the relay in this diagram with whatever device it is that you are trying to supply current to. It will probably be necessary to add a resistor in series with this device to control the amount of current.

This may not be the best description of this circuit but it should get you started.

Also check out this site. This is where I got the circuit from.

**broken link removed**
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How about using a zener diode to pass current when the voltage has reached a specifit level

Well, I'm looking for the simplest solution possible, so this idea with zener diode would be great. But like I said I'm totally new to all this, so how would I do that?

Thanks for all your replies and suggestions!

The nice thing about using a zener is that it would make for a very simple circuit. There are a few reasons way this may be a bad idea. With a zener you will be using the reversed bias breakdown voltage for the control this is probably ok if the voltage that you need is available. (there are many different voltage levels so this may still be alright).

Also the device that you are driving current through must be equal to or preferably less than the current that the zener is capable of passing. I just looked at a couple of parts and found that they are able to supply about 20mA. Anything higher than that and the part will start to degrade or burn. (this is dependant on the way that the circuit is setup)

The benefits of the design that I showed you are:
1. Greater control over where the circuit switches on. especially if you use the pot that is shown.

2. The potential for being able to source a larger current.

Dont think that I'm telling you that a zener cant be used just be sure that it does fit the requirements. Here is a page with some information about zeners. If you find a part that you think might work take a good look at the datasheet since this will give you the specifics that you will need.

**broken link removed**
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Sorry I didnt offer any zener ideas. So here are the best I can think of. Maybe techie had something else in mind?..

Zener ideas:

Vin = Voltage in
Z<| = Zener diode (lets call it a 4.7V)
C = Your Circuit
GND = Ground


This setup would keep circuit C off untill the breakdown voltage of the zener is achieved. In this case 4.7V. The voltage that circuit C would see would be given by the equation:
Vc = Vin - 4.7

This might make for some unpredictable results when Vin is close to 4.7V. Since circuit C will have some inherent resistance that will create a voltage drop across the device:
Vc(drop) = I * Rc
As long as the voltage Vin swings high enough to keep you out of this range you are probably ok.

I dont believe that this will work because Vin will always be able to pass current through C. The impact that the zener will have is that it will cap the voltage available to C at 4.7V


I've just reread your requirements and now I'm not completely sure that I know what it is that you are looking for. Do you have a situation where you want circuit C to turn on only after a certain voltage level has been reached? Or after a certainvoltage level has been reached we want to siphon off some of the current. (not really a real situation). Please give me more details about what you are trying to do and I will continue to try to help.

When I first read your post I had the same idea that aglet1000 gave you to use a comparator. Depending on what you are supplying with current you may use a transistor or a relay, so please be more specific about the load you want to switch ON and OFF with this circuit (i.e. is it d.c. or a.c. what's its working voltage, the nominal current etc.). The circuit is pretty simple so you don't have to worry about that, I can design it for you if you wish, but just give me some info.

Well, this should be to fool a sensor on a car. The sensor returns voltage variing on the quantity injected. The main idea is to "remove" some current, and thus voltage, by connecting the feedback wire to earth via potentiometer, so you can regulate how much you want to loose (without any resistence you'd loose all, and it wouldn't work). And this works.
But the thing is that a car electronics has some routines for retaining idle speed which cause engine bucking after a bit lower resistance (when more signal is lost). So I want a circuit that'll loose current only after a specified voltage is reached (when there is more fuel injected than on idle, so that it doesn't have bucking at idle).

I'm sorry PTC, forgive me but I didn't understand completely what you tried to say. Plase try to explain again. I suggest you to make two explanations:
1 lets say mechanical, on what kind of problem are you having in real life. 2 what is the electrical part problem.
Please give as much detail as possible and, if possible, some parameters (voltage, current, resistance)


Let me see if I understand the problem. We have a sensor that measures the current and turns this measurement into a voltage. You want to measure this output voltage and when a specified voltage is met you want a circuit to be enabled that will then decrease the current that is measured by this sensor. Yes?


This is my best attempt at trying to get a circuit / block diagram to work here. Either imagine that the # doesn't exist or copy this to a text editor and remove them. Its a simple diagram.

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