# Changing DC voltage

Discussion in 'Electronic Basics' started by [email protected], May 16, 2006.

1. ### Guest

I know next to nothing about electronics, so my question may be very
basic, but here goes.

Is there an easy way to change DC voltage? For instance, if I have a 2
volt light and a 12 volt power source, I assume I would burn out the
bulb if hooked into the circuit. I know AC would use a transformer, but
not sure how it is done with DC.

2. ### ChrisGuest

Hi, Tom. The quick answer is, "Use a series resistor".

Here's how it works. Let's say your two volt bulb has a resistance
when hot of 2 ohms (view below in fixed font or M\$ Notepad):

|
| .--------------.
| | --------> |
| | 1 amp |
| | | 2 ohms
| +| .-.
| --- ( X )
| 2V - '-'
| | |
| | |
| | |
| '--------------'
|
| 2V / 2 ohms = 1 amp
|
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

Using Ohm's Law ( I = V / R ), we can infer that when the bulb is hot,
1 amp will be going through it.

You have seen that, if you connect the 12V battery to the 2V bulb, too
much current will go though it, causing it to burn out. We need to add
something else to the circuit so the total current going through the
bulb remains 1 amp.

That something is a series resistor. Again, using Ohm's law, we can
find the total resistance we need:

1 amp = 12 V / ? ohms

You can easily see that ? should be 12 ohms. That would mean we have
to add a 10 ohm resistor in series to make the total resistance 12
ohms, like this:

|
| ___
| .----|___|-----.
| | 10 ohms |
| | |
| | --------> |
| | 1 amp |
| | | 2 ohms
| +| .-.
| --- ( X )
| 2V - '-'
| | |
| | |
| | |
| '--------------'
|
| 12V /( 10 ohms + 2 ohms ) = 1 amp
|
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

If this isn't just a homework problem, you'd also have to size the
resistor properly for the power dissipation.

Good luck
Chris

3. ### Pooh BearGuest

But this is wasteful of energy. In this case only 1/6 of the power f om the
battery reaches the bulb.

A better solution would be a switching regulator but why not just use a 12V bulb
?

Where did you find a *2 Volt* light btw ?

Graham

4. ### Ralph MoweryGuest

To go from a higher voltage to a lower one there are 2 simple methods. If
the current is low and constant then a resistor would do it. A much beter
way is a voltage regulator. For currents up to about 1 amp you can use a 3
terminal voltage regulator. Look up a LM7805. I know of some 5 amp
regulator ICs. For greater current (up to 50 amps and more) you can use a
LM723 IC and a few transistors. The basic circuit has been around for 20 or
30 years. Still being used.

Start here and then look at some of the circuit diagrams.

http://www.repeater-builder.com/astron/astron-index.html

To get a higher DC voltage you usually convert it to an AC voltage , send it
to a transformer (which can be part of the DC to AC converter) , then change
it back to DC.

5. ### JeffMGuest

2 volt light and a 12 volt power source
This reminds me of the old joke
"Y'know, if they'd just hire taller girls
those ballerinas wouldn't need to stand on their toes."

6. ### ChrisGuest

Hi, Graham. It sounds like a homework or science fair question.
Considering the time of the year and the way the question is phrased, I
was guessing it might be a basic level high school physical science
question. Some high school sequences cover electricity toward the end
of the second semester.

Let's not scare 'em off too soon. ;-)

Cheers
Chris

7. ### Rikard BosnjakovicGuest

How do you convert DC to AC?

8. ### Pooh BearGuest

You 'chop' it.

Graham

9. ### John PopelishGuest

You use switches to reroute it in alternating directions.
Actually, you don't need to actually reverse the direction of current,
you can just interrupt it and use inductance and capacitance to work
with those changes to generate more voltage.

Here is a tutorial that covers some of the circuit physics involved:
http://www.national.com/appinfo/power/files/f5.pdf