Lower Voltage with Resistor

[Yoa01]

Hi all,

I have a kind of noob question for you today. I'm trying to get 4-5V out of a 9V battery. At first I tried a large resistor (1Kohm), but only ended up with 9V. When I tried a small (330ohm) resistor, I got what I needed.

I may have been measuring wrong. Here's my setup:

The resistor is 330ohms and the voltmeter was measuring 4V.

Did I mess up, or does less resistance mean less power running through them? Try not to bring up Ohm's law if you can, because every time I try to measure it I blow a fuse on my meter. Obviously, calculating it is easy, .012 A (or .027, not sure if I should use 4 or 9 V to calulate that).

Thanks!

 

[Yoa01]

True, but I don't need a voltage divider (though they do come in handy, espeially as potentiometers). Like I say, I just need to lower the voltage, not split it or anything.

 

[KrisBlueNZ]

You DO need to split the voltage. You have a 9V battery, and you want a voltage of 4.5V. You can't reduce the voltage across the battery; it will always be 9V. All you can do is to "tap off" a portion of it.

A voltage divider will do that, but any load (e.g. another resistor, or anything else that draws CURRENT) that you connect to the voltage divider will affect the voltage. Voltage dividers are used when the current drawn out of them is zero, or small, compared to the current flowing through the voltage divider.

If you want to use your 4.5V to supply power to something, you need a regulator, not a voltage divider. Depending on what you need to power. you can use a simple circuit with a transistor, a linear regulator such as an LM317, or a switching regulator.

What do you want to do with the 4.5V?

 

[Yoa01]

I recently acquired 20 PT2399's (digital delay chips) and they need 5V to operate. I was thinking, since my little resistor setup seemed to work, I could just put a resistor on the positive end of the battery to get my 5V.

but any load (e.g. another resistor, or anything else that draws CURRENT) that you connect to the voltage divider will affect the voltage.

So, since resistors draw current (and can be used as voltage-current converters from what I've read), I would have to, what, attach a resistor to the voltage divider, or could I just use a divider? Or just a resistor? Damn, I'm confusing myself.

 

[KrisBlueNZ]

Resistors are, pretty much, voltage-to-current and current-to-voltage converters, yes.

In your case, you should use a 5V regulator. These are very cheap and common. If you only need 100 mA of current out of it, you can use a 78L05, which comes in a TO-92 package like a small-signal transistor. If you need more current, you can use a 7805, which can supply up to 1A. Both of these should be readily available from any electronic component store.

These regulators should have decoupling capacitors connected across them, for stability, so you should also get some 0.1 uF ceramic capacitors.

For details, see http://en.wikipedia.org/wiki/78xx or just Google 7805 regulator.

 

[Yoa01]

I need a bunch of .1u caps anyway; the chip needs a ton of them.

Hopefully, I'll be getting a whole slew of those and other components for Christmas (my dad loves the fact that I'm into electronics. $10 goes for a LOOOONG time), so I can try then.

So, just so I know, a mere resistor from the battery positive to the 5V input isn't a good idea, but would work, right?

 

[davenn]

.............
So, just so I know, a mere resistor from the battery positive to the 5V input isn't a good idea, but would work, right?

No as said, you need 2 resistors to form a voltage divider OR a voltage regulator chip

Dave

 

[Yoa01]

Alrighty. Thanks everyone! Incredibly helpful, as always!