how i want reduce 12v 6ah to 5v, 250mah

[plutoo]

hi all, i would like to ask about how i can reduce the voltage car battery to suitable with my micro controller pic 18f452. im read alot about voltage regulator lm 7805 wit using heatsink..

1 more question, i would like to know how the capture function can operate starting frequency 0hz and above. my osc frequency 20Mhz in im using 8 prescale on timer 1. base on my calculation, the slowest frequency i can use is 9.54Hz. im desing this for my digital speedometer with TTL output from sensor. so i just want calculate the TTL output by using Capture mood.

there is another option for it?

 

[bychon]

The 7805 regulator is an excellent choice. Data sheet attached.

As 4 the capture function, not my ability. Wait for the next helper.

 

[jimlovell777]

If you're using the 7805 for an automotive project make sure it's one that goes off with the key. A 7805 that isn't disconnected when the car is off will drain your battery faster than you think. That's the voice of inexperienced youth talking.

 

[plutoo]

tq guys.. ur answer is very helpfull

 

[Hero999]

Are you running any 12V appliances from the battery?

If not you could use a 6V 6Ah battery (which is half the size and mass) and use a low dropout regulator.

Alternatively you could use a switching regulator, which will make the battery last much longer and has the advantage of not requiring a heat sink. You can build a switching regulator using a couple of transistors.
2-transistor Black Regulator

 

[plutoo]

then, how im going to reduce the current, if 6ah goes directly to my microcontroller, sure it will burn my microcontroller..
base on the datasheet pic18f452:
Maximum current into VDD pin ..............................................................................................................................250 mA
im still have problem with the current =.=

 

[Hero999]

No, it won't burn out the microcontroller.

Apart from the fact that the LM7805 is current limited to 1A and the Black regulator can be current limited to any desired current, if won't happen because the MCU will only draw as much current as it needs, see the datasheet.

 

[plutoo]

im still begginer with the microcontroller.. need help alot.
thank you guys for your help..^^

 

[bychon]

Oh boy...another one that thinks if a battery can supply 300 amp hours it will push 300 amps through anything that is connected. How do people design electronic circuits when they don't know ohm's law?

 

[Hero999]

Oh boy...another one that thinks if a battery can supply 300 amp hours it will push 300 amps through anything that is connected. How do people design electronic circuits when they don't know ohm's law?

I know, I get frustrated too, I don't know how many times I tell people that.

I think the most annoying thing is I have never ever thought that, even then I was a total nube and was 5 years old.

I think it's because people are told not to use a larger rated fuse than it says on an appliance's label without understanding what a fuse actually does.

The maximum voltage ratings on capacitors and transistors is another classic: can I use a 50V capacitor in a 5V circuit? Arrgh!

Perhaps there should be a FAQ with really simple stuff like that, we could just post a link to it?

EDIT;
Even better there should be a RTFFAQ button which just posts a link to it when it's clicked on.

 

[plutoo]

beginer nub try to do something interesting..

 

[Hero999]

beginer nub try to do something interesting..

I hope you're not offended.

Without meaning to seem patronising, you'll understand what I mean when you learn more about electronics and have to answer other people's silly questions.

 

[plutoo]

no problem with that..
im glad u all help me ^^..
in learning, i need to accept everything..
just think positive..

 

[TechnoGilles]

What you need to understand, Plutoo, is that tension (vots) is "pushed" to your circuit, but courrent (amperes) is not. It is "made available" to your circuit. Don't worry, your PIC will only take what it needs !!

This is a very basic (yet VERY important) principle in electronic. It links the tension (V), current (I), and load (or resistance) of your circuit (R). It is called Ohm's law and is expresed: I = V/R (or V = RI).

It is probably the most important key (along with a few other principles) to the world of electronic so that's why our electronic Masters are puzzled as to how you entered the world without the key !!!

As for the "h" in "6ah" is means that your battery will last 1 hour if you require 6 amps from it. So if you ask only 3A from it, it will last 2 hours... and so forth.

 

[bychon]

That is a brilliant explanation about why the masters are puzzled when people show up without the first key to the kingdom of electronics.

Thank you!

 

[TechnoGilles]

Welcome !!

 

[jimlovell777]

I know, I get frustrated too, I don't know how many times I tell people that.

I think the most annoying thing is I have never ever thought that, even then I was a total nube and was 5 years old.

Embarrassingly I must admit that at one point I was one such person. I can tell you why though. As a n00b if you wire something wrong and your circuit is starved for current things might not get damaged and you have a second chance to make things right. If you're using a beefy supply and screw up you open the valve on the magic smoke. Adolescent instinct says if that didn't happen last time it must be the current that made the difference. Not true of course, it just made your mistake more obvious... and permanent.

 

[plutoo]

now i know.. im not really expert in this field..
that y i need asking alot.
dun care what people saying but finally i got the answer..
tq all

 

[Hero999]

Embarrassingly I must admit that at one point I was one such person. I can tell you why though. As a n00b if you wire something wrong and your circuit is starved for current things might not get damaged and you have a second chance to make things right. If you're using a beefy supply and screw up you open the valve on the magic smoke. Adolescent instinct says if that didn't happen last time it must be the current that made the difference. Not true of course, it just made your mistake more obvious... and permanent.

Part of it I think is caused by anxiety, they might think it's all right to use 5V 1A PSU to power a 100mA circuit but they don't feel that confident so seek confirmation.