# yanpsq (yet another newbie power supply question)

Discussion in 'Electronic Basics' started by Rusty Wright, Nov 9, 2004.

1. ### Rusty WrightGuest

I have a solderless breadboard, multimeter, etc. on order, getting
ready to dink around with some simple stuff to learn electronics.

I don't understand how to set up the power to the breadboard, for
example, if I'm using a 9v battery and I need 5v. On the surface it
seems like I could use a simple voltage divider, with 100 ohm and 80
ohm resistors in series which would give me 4v across the 100 ohm
resistor and 5v across the 80 ohm resistor. But then when I start to
connect things they'll be in parallel with the 80 ohm resistor and
that will lower the resistance and change the voltage drop. So that
doesn't seem like a good plan.

Or what if I want to use some wall wart I found in the dumpster? I
have one that's 20v.

Or should I be thinking about using a voltage regulator like the
LM2940T?

Confused and lost in newbie land,
Rusty

2. ### John PopelishGuest

Yes, you should invest in a voltage regulator. But an adjustable one
will be more versatile. Look for an LM317. It can be programmed with
a pair of resistors for any voltage between 1.2 volts and 2 volts less
than the input.

Check the data sheet for the capacitors required to guarantee
stability and for the formulas for the resistors.
http://www.national.com/ds/LM/LM117.pdf

3. ### Peter BennettGuest

No - you'll get the higher voltage across the higher value resistor -
remember Ohm's Law, and the fact that the current is the same in both
resistors.
Yes, it isn't
It would be best to use a wall wart and a voltage regulator -
otherwise, you'll go through a lot of batteries (and with batteries,
you'll still need a voltage regulator.)

You should look for a wall wart that will produce 3 - 4 volts above
the regulated voltage you want - otherwise, the regulator will get hot
dissipating all the extra voltage.

Even better that a random wall-wart and regulator would be a regulated
5 volt power supply

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

4. ### BobGardnerGuest

Or what if I want to use some wall wart I found in the dumpster? I
========================
Everybody builds a couple of power supplies at some time. The 20V wart would
run a 5v regulator no prob. BUT... just go buy a regulated 5v supply... use
this to run your projects. You test equipment needs to be working so you dont
need to fiddle with that to get the other stuff working.

5. ### Rusty WrightGuest

Thanks for all the help. We're moving our offices to another building
and people are throwing lots of stuff out; I now have several wall
warts and some are even 5v. I got two that were for external scsi
drives and use that round Mac ADB connector (which I'll just cut off)
and they put out both 5v and 12v. I'm ready to start cooking. Now if
only my breadboard and stuff would arrive...

Here's another question; for these dual output wall warts, what's the
best way to figure out which wires are which? I.e., is it safe to
just put the leads of my meter on 2 wires and see what it says? Or
should I put something like a 1k a resistor in series or parallel?
Should I expect it to have separate ground wires for each voltage or
one that's common?

6. ### Roger JohanssonGuest

Good thinking, the 1k resistor. But even better is knowledge about how a
multimeter works and how to use it.

A voltmeter needs to have a very high inner resistance, so you can
measure voltages without influencing the circuit you are masuring.

An amperemeter needs to have a very low inner resistance, because you use
it by inserting it into a circuit and masure the current throughthat part
of the circuit.

A resistance-meter works in a third way, it puts a voltage, usually
200mV, over a component, and measures how much current it can push
through it. That is how it finds out how much resistance the component
does against the current.

For voltmeters and ampmeters there is also a choice of DC and AC
measurement.

A multimeter combine all these instruments into one unit.

If you happen to set the MM to current and connect it to the mains it
will be destroyed, or at least you will have to replace the fuse inside
the MM. Work only with battery voltages until you know well how the MM
works. Read a manual on the usage of a multimeter carefully.

You can find out what function the output connections of a wallwart have
by plugging it in an carefully explore what voltages, both AC and DC,
there are on every connection.

With the wallwart disconnected from the mains you can test what
resistance there is between each possible pair of outputs. This will tell
you which wires are in contact with each other.

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