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Simple Circuit

Hello,

This is my first time posting to USENET. I was wondering if anyone
could help me figure out if I have the correct circuit for what I am
trying to do. I have followed other peoples schematics before, and this
is the first time that I have tried to make something for myself.

You can see the schematics that I drew up here:
http://img68.echo.cx/my.php?image=schem1wn.png

I want to run 4 things off of one power supply. The 15V 4.5A is an old
laptop power adapter. The two fans run at 12V and .01A-.02A. The LED
runs from 3.5V-4V. The peltier will run upto 15V and 15A. I did'nt have
any images for the fans and peltier so I just used some random ones.

What I don't understand is does a device draw as many amps as it needs?
like the fans and the LED or does that need to be limited some how?

Thanks
 
C

Chris

Jan 1, 1970
0
Hello,

This is my first time posting to USENET. I was wondering if anyone
could help me figure out if I have the correct circuit for what I am
trying to do. I have followed other peoples schematics before, and this
is the first time that I have tried to make something for myself.

You can see the schematics that I drew up here:
http://img68.echo.cx/my.php?image=schem1wn.png

I want to run 4 things off of one power supply. The 15V 4.5A is an old
laptop power adapter. The two fans run at 12V and .01A-.02A. The LED
runs from 3.5V-4V. The peltier will run upto 15V and 15A. I did'nt have
any images for the fans and peltier so I just used some random ones.

What I don't understand is does a device draw as many amps as it needs?
like the fans and the LED or does that need to be limited some how?

Thanks

Hi. You might want to replace the LM317T with a simple
current-limiting resistor. Diodes are current-driven devices. Your
white LED may have 3.5V across it if you put 20 mA through it, and it
will only have 3.7V across it if it has a lethal 50 mA. By the time it
gets to 3.9V, you might just have a smoking blob instead of an LED.
Just use a 560 ohm, 1/2 watt resistor in series with the LED, and you
should be fine. That will give you about 20mA through the LED.

Using an LM7812 as a series regulator for the fans is a good idea.
Your schematic doesn't show a connect between the GND pin of the 7812
and GND -- I'd guess that's just an oversight. You should place a
small (10uF) cap from input to GND, and from output to GND, to avoid
oscillations which might cause problems.

The part that's bothersome, though, is the peltier cooler. You say
you've got a cooler that needs 15V at up to 15A, but your power supply
is only capable of 15V at 4.5A. If that's not a typo, you're in
trouble. Choose a power supply that will provide the voltage you want,
but make sure it can provide enough current.

If you've got a voltage source, it's rated to provide a certain amount
of current. That means you can't exceed that amount, or something
won't work right. It might be automatic shutdown, current limiting
(the power supply drops down to a lower voltage to keep current below
maximum), or the power supply might be damaged or even smoke. A good
power supply will be able to provide a steady DC voltage over a wide
range of current, as long as it's less than the maximum. As an extra
note, you should know that some switching power supplies require a
minimum load current, too. It's best to ask before specifying.

Good luck
Chris


Good luck
Chris
 
A

Andrew Holme

Jan 1, 1970
0
Hello,

This is my first time posting to USENET. I was wondering if anyone
could help me figure out if I have the correct circuit for what I am
trying to do. I have followed other peoples schematics before, and
this is the first time that I have tried to make something for myself.

You can see the schematics that I drew up here:
http://img68.echo.cx/my.php?image=schem1wn.png

I want to run 4 things off of one power supply. The 15V 4.5A is an old
laptop power adapter. The two fans run at 12V and .01A-.02A. The LED
runs from 3.5V-4V. The peltier will run upto 15V and 15A. I did'nt
have any images for the fans and peltier so I just used some random
ones.

What I don't understand is does a device draw as many amps as it
needs? like the fans and the LED or does that need to be limited some
how?

Thanks

With the fans, you're OK - they'll "draw as many amps as they need," but you
need a current-limiting resistor in series with LED. You would be
better-off connecting the LED, in series with a resistor, across the 12V or
15V supply. Forget about VREG1 altogether. Calculate the LED current using
Ohm's law:

I = (Vs - Vf) / Rs

Vs = Supply voltage
Vf = Approximate forward voltage drop across LED (3.5V ish)
Rs = Series resistor

You need decoupling capacitors (try 100nF) on the input and output of the
other (remaining) voltage regulator.
 
R

Rheilly Phoull

Jan 1, 1970
0
One day [email protected] got dressed and committed to text
Hello,

This is my first time posting to USENET. I was wondering if anyone
could help me figure out if I have the correct circuit for what I am
trying to do. I have followed other peoples schematics before, and
this is the first time that I have tried to make something for myself.

You can see the schematics that I drew up here:
http://img68.echo.cx/my.php?image=schem1wn.png

I want to run 4 things off of one power supply. The 15V 4.5A is an old
laptop power adapter. The two fans run at 12V and .01A-.02A. The LED
runs from 3.5V-4V. The peltier will run upto 15V and 15A. I did'nt
have any images for the fans and peltier so I just used some random
ones.

What I don't understand is does a device draw as many amps as it
needs? like the fans and the LED or does that need to be limited some
how?

Thanks

Yup, things that are designated as 12v 15v etc will only take what current
they need.
An LED needs to have the current limited, your schematic would likely
'terminate' the LED. While LED's have a forward voltage you have to deduct
that from the supply voltage and then calculate a resistance that will limit
the current. Say you have taken 3.3v as the LED forward voltage, 15v - 3.3v
= 11.7v .
So to limit the current to 20mA a resistor of 11.7v / .02mA is required,
which would be 585ohms (use 680ohms). No need for the voltreg here just the
resistor. Note that you can use a lower current for very little loss of LED
output, if power is an issue 10mA or less will still give a good output
(particularly with high intensity LED')s.
 
L

Lord Garth

Jan 1, 1970
0
Here is the revised version of my circuit:
http://img91.echo.cx/my.php?image=schem26xj.png

What do decoupling capacitors do? I know that a capacitor stores a
charge, but what purpose does the decoupling capacitor serve?

When digital circuits change states, there is a very brief time when both
output devices are conducting. That creates a spike on the power rails
which you can easily see on a scope. The caps absorb much of this by
providing a local source of power.
 
P

petrus bitbyter

Jan 1, 1970
0
Here is the revised version of my circuit:
http://img91.echo.cx/my.php?image=schem26xj.png

What do decoupling capacitors do? I know that a capacitor stores a
charge, but what purpose does the decoupling capacitor serve?

In this situation the caps restrain the regulator from oscillating.

Once more, switching on the Peltier will switch off or blow your power
supply.

petrus bitbyter
 
A

Andrew Holme

Jan 1, 1970
0
Here is the revised version of my circuit:
http://img91.echo.cx/my.php?image=schem26xj.png

What do decoupling capacitors do? I know that a capacitor stores a
charge, but what purpose does the decoupling capacitor serve?

Since it contains a feedback loop, the regulator has the potential to
oscillate - depending on the load impedance. It is standard practice
to decouple the regulator inputs and outputs. Look at the typical
application circuit on the datasheet. The capacitors must be mounted
close to the regulator to be effective.
 
L

Lord Garth

Jan 1, 1970
0
How do I keep the peltier from trying pull more amps than the
powersupply can handle with out droping the voltage?

Thanks

You don't! You make sure your power supply has the necessary head room and
let the peltier take what it needs.
 
J

John Popelish

Jan 1, 1970
0
How do I keep the peltier from trying pull more amps than the
powersupply can handle with out droping the voltage?
If you built a switching buck regulator, you could lower the supply
voltage to the peltier so it would draw less current (and move less
heat). The current demand is roughly proportional to the supply
voltage. So, for instance, if you lowered the 15 volt supply to 10
volts, the peltier current would fall from 15 amps to 10 for a total
power to the peltier of 10 volts * 10 amperes = 100 watts.

Assuming the efficiency of the buck regulator was 90%, its input
current would be (100 watts * 100/90%)/ 15 volts = 7.4 amps.
 
... The peltier will run upto 15V and 15A.

Are those the maximum ratings for the Peltier? In practice, their
efficiency drops very low as you approach the maximum voltage and
current, as they will internally generate more heat than they are
pumping.

Best to run it at roughly half of its voltage and current rating, or
less.

Mark
 
R

redbelly

Jan 1, 1970
0
Would I have to build a buck regulator like John Popelish described in
order to limit the current.

A buck regulator is just one way to do this. You could also try getting
another power supply.

Mark
 
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