12v power source from Cordless tool batteries

[MrWindow]

Hi
Been lurking here for a little while and thought I'd pose a question for a project I am considering.

I have a Makita Am/Fm radio and I want to power a XM satalite radio to run off of it. The problem is I need a 12V source to plug in the adapter and none exists on the radio currently.

The radio will take different voltage batteries 9.6v to 24v, with my abilities this is out of my realm of knowledge to find the right area to tap into for the power.

The primary battery I will be using in this radio is the 18v 3amh version I use in my cordless tools.

My idea is to utilize these and add a second battery just to power the 12v source. I have the battery dock sourced and a location on the back of the radio to put it.

I need help in the area of proper electronic piece sizing.

The battery at a full state of charge is about 19.7 v.
The type of things I want to be able to run the in new 12v source would be 6v-5v 1000ml amp Cell phone chargers.Ipod, sat radio power etc.

I understand I will need a voltage reduction from 19.7 to 12v would a 7812 voltage regulator work or should I use an adjustable one?

What size capacitors should I use? how many and where?

I have read alot here and it seems fairly straight forward construction. I can follow instructions and read schematics a little.
I just need to know if Im going to burn down the house or not.

Thanks in advance
]James

 

[davenn]

hi James

ok, a 7812 doesnt have the current capability you need 750mA is about the best to get out of one of them before they start to shut down.
Next to consider is how many things are you wanting or may want to run off the 12V source at any one time ? That is what is going to determine your regulator requirements.

Probably an LM338 would handle most of your needs, it can do 5 Amps (when on a good heatsink). do a google search for the datasheet, there are plenty out there.

Although its a variable reg, you can use a pair of resistors to produce a fixed output
The datasheet also gives you all the capacitor and other component values needed Also a good number of example circuits. ALSO NOTE .... buy the version with the K suffix (LM338K) thats the steel cased one.

cheers
Dave

 

[MrWindow]

Dave
Thanks for the reply.
One item at a time. My thought is to use a common auto 12v reseptacle in the radio case.
Once I get this project figured out I'll try a stand alone battery 12v source for other things like a small inverter and such.
For now the goal is to power a sat radio.

 

[davenn]

ok thats cool, use the LM338K and at least you have the current capability if and when needed in the future

Dave

 

[MrWindow]

Ok I looked at the LM338k data sheet.

2 things come to mind. The case is out put does this make for a dangerous situation if you should touch it when energized? Do I need to take precutions to protect it? I would assume I will need a heat sink of some sort and will that not be energized as well?

I can regulate the voltage with lm338k but is it nescessary to regulate the current as well?

The battery is a 3.0amh 18v the lm338 is good up to 5 amps am I good?

The load side of things will be regulated by the adapter of each appliance no? Eg. 5v 1000mla sat radio 12v plug will only allow the currrent at that voltage to pass?

Man I have alot to learn! Thanks for your patiance
James

 

[barathbushan]

2 things come to mind. The case is out put does this make for a dangerous situation if you should touch it when energized?

if you are worried about a shock , just use an electric tester screw driver , to test if the point is energised w.r.t ground , if the light glows stay away from it

by doing this , you are safe from just one point w.r.t ground , AND NOT BETWEEN TWO POINTS


I just mentionned this from a generalized view of safety







however with 12v or 24v , you will never get shocked

That's because there is not enough potential for the current to flow through you ,
so don't be afraid of electronics

 

[MrWindow]

Barathbushan
Thanks for the tip.
Thinking in AC terms I guess. DC is different.

James

 

[barathbushan]

It doesn't matter if it's AC or DC . For both RMS AC and dc , you will look like a resistive load , and if there is enough voltage be it ac or dc , it get's dangerous .