18V Batt. to 5V USB Port and 12V CIgarette Lighter

[brandon102696]

i am beginner in electrical engineering and am doing it has more of a hobby, therefore i have no books to study from. but i am trying to power a 5 Volt USB Hub and a 12 Volt Cigarette Lighter from a car, with an 18 Volt Battery. Can anyone show me an example of how to set this circuit up, including the resistors, or volt regulators i need?

 

[AjithC]

Use 7805 and 7812.

 

[uart]

18V is an unusual battery voltage. What type of battery (pack) are you using and is there any particular reason for that choice.

 

[brandon102696]

AjithC - thank you but i know this much. I am wondering if someone could give me an example of how the circuit should look. i noticed the 7805's and 7812's have 3 connectors, how would i go about setting these up? which part goes to what?

uart- yes, i know it is. it is a DeWalt drill battery pack and I am using it because that is the source of power my friend requested to be used on this project

 

[AjithC]

Follow the attached pdf datasheet and go to page number 21.

 

[cjameshuff]

Use 7805 and 7812.

Don't overlook the current limitations and heat issues. Linear regulators convert a lot of the power to heat, especially when there's a large difference between input and output voltage. For 18 V input and 5 V output, for every 5 watts output, 13 watts of heat are produced in the regulator. It's not as bad for the 12 V output...only a third of the total power is dissipated in the regulator.

A USB hub with stuff attached to it might well draw an amp of current (5 W output), which at 18 V input may overheat the regulator and will certainly require a good heatsink. A cigarette lighter would most likely draw several amps, and those regulators aren't rated for more than 2.2 amps. Cigarette lighters won't be very sensitive to excessive voltage though...the USB hub needs a regulated supply, the cigarette lighter just needs to get hot and not burn out. And it'll be hooked up to a smaller than usual battery that might itself not be able to deliver the full current it would draw at 18 V...it may be fine hooked directly to the battery, especially if for brief periods of time.

There's switch-mode DC-DC converters that can handle higher currents and waste far less power as heat...which not only means longer battery life, but also means less cooling required. But also be aware that this battery might not be able to simultaneously run a USB hub and get a cigarette lighter fully heated up, and that it probably won't do either for very long.

 

[Voltux]

i am beginner in electrical engineering and am doing it has more of a hobby, therefore i have no books to study from. but i am trying to power a 5 Volt USB Hub and a 12 Volt Cigarette Lighter from a car, with an 18 Volt Battery. Can anyone show me an example of how to set this circuit up, including the resistors, or volt regulators i need?

Hey Brandon,

Glad to see you have an interest in EE, Cars are especially tough to work with they can fry your electronics if not done properly such as when you start the car with sensitive electronics plugged in all sorts of voltage spikes and such.

There are really two main power regulators one is Switching and one is Linear. There are thousands of different ones out there for many different applications and as cjameshuff pointed out Linear regulators get freaking HOT with a load (For example I was messing with one just for fun running from 12v to about 5v with an LM317 and I was able to get it up to 200F without a heat-sink (TO-3 Package). This is because they waste the voltage difference as heat, Linear regulators typically are not very efficient with efficiencies topping about 50%. Linear regulator can give less noise than switching regulators and can be better in some applications but I won't overwhelm you with all of that. I encourage you to look at switching regulators such as the "Jellybean" MC34063 or the LT1302 used in the Mintyboost (5volt 500ma).

Perhaps if you tell us more about what you need we can better assist you in building your project. When you look for a regulator you look at your needs such as:

1. What voltage do I need?

2. How much ripple is allowed? (In other words am I driving things that need low ripple such as expensive electronics or am I just driving a small hobby motor?)

3. How much current will be used? (Standard USB devices are limited to 500ma and some can pull more check out the Mintyboost project for more about that http://www.ladyada.net/make/mintyboost/)

4. How will the regulator be powered? Wall socket? Car? Battery pack?

For your first regulator you can use a Linear one to get the concept down however you won't be able to put much current through one without a large heat-sink, If possible look for a TO-3 Package, The 7805 is a 5v regulator, LM317 is a variable linear regulator controlled with two resistors. Aka Voltage Divider

If you'd like to learn more about regulators check out the Wikipedia page
http://en.wikipedia.org/wiki/Switched-mode_power_supply and if you want a fantastic tutorial on the MC34063 (Switching Regulator) Dave Jones at the EEVBlog has a great video tutorial on it explaining everything you can check out his other videos where he explains a lot of stuff you may be interested in.

http://www.eevblog.com/2010/09/10/eevblog-110-lets-design-a-dc-to-dc-switchmode-converter/

Hope this helps, and again glad to see you have some interest in Electronics.

Oh, and before I forget don't try to measure current off a car battery and make sure if your doing any measuring that you have your leads in the voltage jack and not the current.