MOSFET Relay

[guilio_2000]

What I want to do is take 9V batteries and run them in series, but I want to put relays in between each battery so that I can control what sets of batteries I want to put in series.

I could do this with a relay, but I would rather have FETs. I hooked this up, but it's not working the way I thought It would work. I have LEDs hooked up to show me which one turns off and on, but I'm not sure if I have them in the right place and was wondering if someone could draw them in.

I am using 2N2700 and a 2N4303. Are these the right ones to use?

The diagram is what I have set up so far.

**broken link removed**

Thanks for you help,
Bob

 

[guilio_2000]

Sorry, the link isn't working, but the photo is there.

Bob

 

[Roff]

Your schematic doesn't make any sense. Both of your batteries have one terminal floating. Where is the load? What are you trying to provide power to, and how much current does it draw?

 

[guilio_2000]

**broken link removed**

ok....try this

thanks

 

[Oznog]

What are you trying to do? Select one battery or the other, or is 18V a possibility?

 

[guilio_2000]

What I am trying to do is have the load run off a 9V battery when the input to the relay is low. When the input of the relay is high, the load will run off 18V.

I'm going to acutally be using 3 batteries, but if I can figure this part out, I shouldn't have a problem with the rest.

Thanks,
bob

 

[guilio_2000]

no takers?

Thanks,
bob

 

[Nigel Goodwin]

no takers?

Well I couldn't make much sense of what you want (or why?).

Try giving more details, and post a diagram of what you are wanting to do.

You could obviously use a comparator to switch a relay at a specific voltage.

 

[guilio_2000]

I'm having trouble trying to post this diagram so that everyone can see it...I thought that if you copied the link and entered it in the address line it would show, buts its asking for the password.
**broken link removed**

What I want to do is take 9V batteries and run them in series, but I want to put relays in between each battery so that I can control what sets of batteries I want to put in series. The comparator is what I planed on using to switch the relay on or off. The problem I'm having is with the relay. It makes sense in my mind, but I can't seem to figure out how to work it.

To give a quick application on how I want to use this:

I want 3 batteries. I want 2 batteries to be applied to the load. When the 2 batteries voltage level drop below the 3rd batteries not being applied to the load, I want 2 batters of a different combination of the 3 batteries to drive the load. Thats where the voltage comparator will come in to turn on and off the relays.

If someone can post the image, I would appreciate it.

Thanks,
bob

 

[Roff]

Here's my understanding of the original 2-battery circuit. You may have to pick a different MOSFET. I simulated the functional circuit with a 10 ohm load, and it seemed to work well. If the voltage on V4 drops below about 5v, the circuit will not work.
When the input is 0v, M1 is off and M2 is on, and all load current is provided by V3. When the input is 9v, M1 is on and M2 is off, and the load current (and voltage) is provided by V3 and V4 in series.
If this isn't what you want, let us know.

 

[guilio_2000]

nope...that is exactly what I wanted it to do. All I gotta do now is make it so that it is functional for 3 batteries and allow me to pick which of the 3 I want.

A few questions. What is the purpose of the diodes and BJT? Also, what kind of MOSFETS do you recommend?

Thanks,
bob

 

[Roff]

The diode and the zener form a level shifter which turns the NPN on when the input is 0v, and off when the input is 9v. The NPN forms the inverter (the triangle with the bubble on it) shown in the simplified diagram. The inverter is necessary because both the MOSFETs are N-channel, and only one should be on at a time.
The choice of MOSFET type depends on how much current you are drawing in the load. The ones on the schematic are rated for an ON resistance of 0.14 ohms. What is the nature of your load?

 

[guilio_2000]

Oh. I see now. I didnt' know you used 2 N channel....I thought you used one p and one n. Just wondering, How comes you didn't use a p channel?

Thanks,
bob