BS2 External Power Supply
Archiver
Posts: 46,084
I was wondering the maximum current that the VDD pin on the BS2 can handle
from an external power source. I found the schematic in an old Radio Shack
electronics book and it uses a 120V primary to center tapped 25V secondary.
My second question was whether or not a full wave bridge rectifier would
increase this voltage to a level unusable by a 7805 regulator. If so, would I
have to use the center tap to instead deliver 12.5V to the rectifier which
doubles that and sends 25V to the regulator? Also, what would the current
rating of the full wave rectifier need to be? I'm trying to get involved with
this sort of thing, but no classes are available to me and I have not been
able to completely understand Ohm's law (I have not had many good educational
opportunities, oh well). Any help would be very useful, although this is not
especially urgent.
-Thanks
from an external power source. I found the schematic in an old Radio Shack
electronics book and it uses a 120V primary to center tapped 25V secondary.
My second question was whether or not a full wave bridge rectifier would
increase this voltage to a level unusable by a 7805 regulator. If so, would I
have to use the center tap to instead deliver 12.5V to the rectifier which
doubles that and sends 25V to the regulator? Also, what would the current
rating of the full wave rectifier need to be? I'm trying to get involved with
this sort of thing, but no classes are available to me and I have not been
able to completely understand Ohm's law (I have not had many good educational
opportunities, oh well). Any help would be very useful, although this is not
especially urgent.
-Thanks
Comments
you will have more voltage than a 7805 will regulate (and probably a 7812
also). You either need to use one secondary winding and the centertap with a
full-wave bridge rectifiers, or use the the centertap and the secondary
windings with couple of diodes in what I believe is referred to as a
half-wave recifier. The downside is it will take more filter caps to smooth
out the ripple using the two diodes.
As far as the current, the BS2 will only draw as much as it and whatever the
pins are driving require. Voltage is what you need to worry about.
A 2 amp bridge recitifer would probably do. The standard 7805 is capable of
about 1 amp with a heat sink.
Original Message
> I was wondering the maximum current that the VDD pin on the BS2 can handle
> from an external power source. I found the schematic in an old Radio
Shack
> electronics book and it uses a 120V primary to center tapped 25V
secondary.
> My second question was whether or not a full wave bridge rectifier would
> increase this voltage to a level unusable by a 7805 regulator. If so,
would I
> have to use the center tap to instead deliver 12.5V to the rectifier which
> doubles that and sends 25V to the regulator? Also, what would the current
> rating of the full wave rectifier need to be? I'm trying to get involved
with
> this sort of thing, but no classes are available to me and I have not been
> able to completely understand Ohm's law (I have not had many good
educational
> opportunities, oh well). Any help would be very useful, although this is
not
> especially urgent.
battery? There are a lot of easier ways such as purchasing an adapter from
Parallax or Radio Shack or a surplus electronics supplier like Mendelson
Electronics (MECI). It's not worth killing yourself with the 120VAC to save
a couple dollars. You can buy a DC Wall Transformer that will work for
under $4.
http://www.meci.com/default.asp?mode=getitems&category=216
Building power supplies that connect to a 120VAC outlet is DANGEROUS and you
can potentially kill yourself. Please be careful.
- Brice
Educational & Personal Robots
http://www.cyberbound.com
Original Message
From: <eolxe@a...>
To: <basicstamps@yahoogroups.com>
Sent: Sunday, July 15, 2001 10:23 PM
Subject: [noparse][[/noparse]basicstamps] BS2 External Power Supply
> I was wondering the maximum current that the VDD pin on the BS2 can handle
> from an external power source. I found the schematic in an old Radio
Shack
> electronics book and it uses a 120V primary to center tapped 25V
secondary.
> My second question was whether or not a full wave bridge rectifier would
> increase this voltage to a level unusable by a 7805 regulator. If so,
would I
> have to use the center tap to instead deliver 12.5V to the rectifier which
> doubles that and sends 25V to the regulator? Also, what would the current
> rating of the full wave rectifier need to be? I'm trying to get involved
with
> this sort of thing, but no classes are available to me and I have not been
> able to completely understand Ohm's law (I have not had many good
educational
> opportunities, oh well). Any help would be very useful, although this is
not
> especially urgent.
> -Thanks
>
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>
>
xfmr leads each through a 1N4002 type diode with the
+ side of the diodes [noparse][[/noparse]marked with ring] to capacitor
+ and connect the capacitor negative to the center
tap: [noparse][[/noparse]assuming a 1 ampere or less transformer]
Transformer secondary:
>|----|
( |
) CT - + |
(
||
|-> to 7805 + input [noparse][[/noparse]- goes to center tap].
( |
) |
>|----|
This makes a full wave rectifier with a peak voltage of
12.5 x 1.414 = 17.7 VDC on the capacitor [noparse][[/noparse]no load].
use a 2,200uf 25 volt capacitor and note that you are
running about 12 volts over the output voltage so
the power to dissipate in the 7805 is 12 x current
being used - if you use a lot of current you'll need
a heat sink on the 7805.
And practice ohm's law until you can do it while asleep!
Have fun... take a look around a site I referenced
earlier for a lot of educational stuff -
http://www3.ncsu.edu/ECE480/480_555.htm go to home page and
links.
Don't be afraid to make mistakes but stay out of the
way of flying sparks [noparse][[/noparse]be sure you have a capacitor
of adequate voltage rating and get the polarity
right - they explode if you don'T :-)]
Have fun and keep at it... premena
>>
Date: Sun, 15 Jul 2001 23:23:36 EDT
From: eolxe@a...
I was wondering the maximum current that the VDD pin
on the BS2 can handle from an external power source.
I found the schematic in an old Radio Shack electronics
book and it uses a 120V primary to center tapped 25V secondary.
My second question was whether or not a full wave bridge
rectifier would increase this voltage to a level unusable
by a 7805 regulator. If so, would I have to use the center
tap to instead deliver 12.5V to the rectifier which
doubles that and sends 25V to the regulator?
Also, what would the current rating of the full wave rectifier
need to be? I'm trying to get involved with this sort of thing,
but no classes are available to me and I have not been
able to completely understand Ohm's law (I have not had many
good educational opportunities, oh well).
<<
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