I have came across a Idea that I want to pursue....

Basically what I want to do is take a set of car batteries...lets say 3....I want to hook 2 of them together (24V) and use the third one to compare....now I know if I hook this up to a voltage comparator, its going to burn it up...my question, is how do I lower the voltage and current of these car batteries so I can use IC, transistors, etc...?

Thanks,
space

 

What do you want to compare?

 

I'm wanting to compare the 2 batteries that I hook up together to the one battery left....

When the two batteries aren't putting out what the 3rd one is, then the system shuts off....

I'm just looking for a way to lower the voltage and current of a car battery so I can use it in a circuit...

space

 

I hate to sound like a nag, but why are you wanting to do this? What is your application? I'm asking because the solution is frequently simpler if we are not constrained by your preconceived notions of how you have to solve the problem. Are you drawing power from all the batteries, or is the 12 volt one simply for use as a reference?

 

I agree with Ron, you say it will burn up... that has yet to be seen! :-)
Did you know that you can find ICs and certainly transistors that can work with 24V? It would be very helpful if you could explain further what you are trying to do. But if nothing else, you can always use a resistive divider to reduce voltage and limit current on the front end of your comparing circuits.

 

Do yourself a favour and rip the PSU from old PC (they are usually free and don't need load to start). You will get plenty of power and usable (commin) voltages. Actually power supply is good start project. Before you start making gadgets etc. build some bench equipment that will help you with your work. Just to name few: PSU, Logic probe, Function generator, Signal tracer, Capacitance meter (if you don't have it on your multimeter) etc.

 

ATX power supplies from computers usually require a load on the 5v line to function properly. Some power supplies even have the minimum load listed next to the max load of each rail, but a 10ohm 10 watt resistor on the 5v line to ground should be fine.

 

I'm sure there are transistors that can handle 24V...I'm more worried about the current that is going to be going through it....are there transistors that can handle this much current?

space

 

I agree with everyone else, you need to say what you are trying to do, the current handling requirements depend entirely on the load - which you've never mentioned. Transistors are available which handle substantial currents, and you can always use more than one - but you would tend to need large heatsinks.

 

You know, I always wanted to build a variable power supply (would like to make it digitally controlled too) for myself so that I would have one available. I never thought 'bout makin a func. gen or cap meter.... those would be nice to have too Thanks!

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I did mention what I wanted to do...I'm wanting to compare the 2 batteries that I hook up together to the one battery left....

When the two batteries aren't putting out what the 3rd one is, then the system shuts off....

space

 

The reason I'm asking is we can use the 12 volt battery as a power supply for the comparator and the switch if that is allowable.

 

oppps...didnt see that last part of your post Ron...

No....I want to still use 3 car batteries...I want to be able to alternate them....like battery 1+2 compared with battery 3, then when that is down, battery 1+3 compared with 2, etc....

hope this helps

Travis

 

Travis, I don't mean to be nosy, but whut the heck are you powering with these batteries? And do you really plan to electronically switch to the different combinations? What do you have that will run over the range of 24 volts all the way down to 12 volts?

 

if i understand the question correctly, you want to lower the voltage of the two car batteries that are connected in series? this would defeat the purpose of connecting them in series because series gives you the higher voltage level. if you want a voltage that is between 12 and 24, or lower than 12, you can make a simple voltage divider circuit using as little as 2 resistors (want to use resistors that can handle high power). just connect the resistors in series with the battery and the required voltage is seen across one resistor (i can give some resistor calculations if this is what you are talking about)

as for the current, that is totally dependent on the load you are driving. if you want to lower the current that is drawn from the battery, add another resistor to your load. this will lower the current as needed

i don't know if this is what you mean or not
just some ideas

adam