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

 

[Chris]

hens...@gmail.com wrote:

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

 

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?

 

[Andrew Holme]

henszey@gmail.com wrote:

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.

 

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

Thanks


petrus bitbyter wrote:

henszey@gmail.com> schreef in bericht
news:1113437187.407942.192560@g14g2000cwa.googlegroups.com...
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

 

henszey@gmail.com wrote:

... 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

 

Would I have to build a buck regulator like John Popelish described in
order to limit the current.




redbell...@yahoo.com wrote:

henszey@gmail.com wrote:
... 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

 

[redbelly]

henszey@gmail.com wrote:

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