Power Supply Negative/Positive

[joeyla]

Hi there. I'm pretty new to electronics and have question.

I have a project using Analog switches and I need +5v and -5v.

I have tried using ICL7660, but I need to supply over 100mA to both
supplies and this device does not work. I tried using a potential
divider and a 7805 and 7905 tapping the center as ground,with no
success either.

How can I get +5 and -5 volts to my circuit without 2 power supplies??

Thanks in advance,
joeyla

 

[Ban]

Hi there. I'm pretty new to electronics and have question.

I have a project using Analog switches and I need +5v and -5v.

I have tried using ICL7660, but I need to supply over 100mA to both
supplies and this device does not work. I tried using a potential
divider and a 7805 and 7905 tapping the center as ground,with no
success either.

How can I get +5 and -5 volts to my circuit without 2 power supplies??

Thanks in advance,
joeyla

Here is a small schematic for a transformer without center tab. The current
capability is around 150mA per side. View\fixed font

____
| |
+-----+---|7805|---+----------------o
| | |____| | +5V
1N4001 - | | |
63mA ^ | + | ---
____ | ### | ---
o-|_--_|-. ,---+ --- | |220n
)|( | |1000u|16V |
120Vac )|( | | | |
o--------' '---)-----+-----+------+------+---------o
8V/500mA | | + | | 220n GND
- ### | ---
1N4001 ^ --- _|__ ---
| |1000u/16V | | |
+-----+----------|7905|---+---------o
|____| -5V
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

 

[Ryan Wheeler]

Hi there. I'm pretty new to electronics and have question.

I have a project using Analog switches and I need +5v and -5v.

I have tried using ICL7660, but I need to supply over 100mA to both
supplies and this device does not work. I tried using a potential
divider and a 7805 and 7905 tapping the center as ground,with no
success either.

How can I get +5 and -5 volts to my circuit without 2 power supplies??

Thanks in advance,
joeyla

look for some designs in here:

http://www.reed-electronics.com/ednmag/index.asp?layout=news&spacedesc=d
esignIdeas&starting=1

 

[Jamie]

use a transformer with a center tap and a bridge rectifier.
for your 5 volts you will need at least a 12 Vac with CT.
the center tap goes to your common/ground/
both the - and + outs of the bridge should have a E-Cap on them
around 1k or more uf. each goes to ground for the common connection.
beware of polarity, they are placed as indicated on the package.
the + to the + and the - to the - etc..

the outer leads of the secondary goes to the ~ ac inputs of the
bridge rectifier.
the - out goes to the input side of the 7905 which is the #2/case
and the + out goes to the #1 leg of the 7805.
the outs of both the 79 and 78 #3 legs are your -/+ lines with the
common/ground for either.

 

[joeyla]

Cheers guys, I have plenty of ideas here to work from.

Thanks for your great advice.

Joeyla

 

[Ken Smith]

Hi there. I'm pretty new to electronics and have question.

I have a project using Analog switches and I need +5v and -5v.

I have tried using ICL7660, but I need to supply over 100mA to both
supplies and this device does not work. I tried using a potential
divider and a 7805 and 7905 tapping the center as ground,with no
success either.

How can I get +5 and -5 volts to my circuit without 2 power supplies??

You can't really but the supplies can share parts such as the others have
suggested.


If you have an isolated power supply to run the circuit on, perhaps you
can do it like this:

Schematic just to get the idea:

----------
------! LM7805 !------------- Sort of 5V
----------
!
!------------------ Sort of GND
/---/
^ 5V zener
!
------------------------------ Sort of -5V


If the input power supply is isolated, you can hook the "Sort of GND" to
the real ground. If not, perhaps you can keep the whole thing +5V above
the real ground.

Now for some details:

There must be more load on the +5V than the -5V. If there isn't naturally
you need to add a resistor to make it such.

The 5V zener can be a bit of trouble because you may need a fairly high
power one. If this is the case, you can help it out with a
transistor.



-------------------
! !
/---/ 4.3V !
^ !
! !/ c
---------! NPN power transistor
! !\ e
/ !
\ !
/ 100R !
\ !
! !

 

[roma]

[email protected] (joeyla) wrote in

Hi there. I'm pretty new to electronics and have question.

I have a project using Analog switches and I need +5v and -5v.

I have tried using ICL7660, but I need to supply over 100mA to both
supplies and this device does not work. I tried using a potential
divider and a 7805 and 7905 tapping the center as ground,with no
success either.

How can I get +5 and -5 volts to my circuit without 2 power supplies??

Thanks in advance,
joeyla

Your answers is here http://members.shaw.ca/roma/supply.html

 

[Norm Dresner]

[email protected] (joeyla) wrote inBuy one. For one-off projects, consider the surplus market. Some vendors
are
www.allelectronics.com
www.mpja.com
www.bgmicro.com
also new stuff at
www.jameco.com

Norm

 

[Pinchy]

Buy one. For one-off projects, consider the surplus market. Some vendors
are
www.allelectronics.com
www.mpja.com
www.bgmicro.com
also new stuff at
www.jameco.com

Norm

This is quite basic. First buy a transformer with a tap in the middle
on the secondary. This will become the ground. Connect a
diodebridge between the other 2 ends of the secondary using the marks
indicating AC . On the (+) of the bridghe, connect a capacitor to the
ground en repeat this for the (-) of the bridge side. Watch polarity
!!

Now you get 2 seperated circuits with a ground in the middle. Connect
the 7805 on (+) & ground resulting in +5V. Repeat for the 7905
from(-) to ground resulting in -5V

one remark: pinlayout for 7805 and 7905 are not corresponding, use a
datasheet !

Succes (draw a schematic first)

 

[[email protected]]

Hi Guys, thanks again for your help and suggestions.

I am plesently supprised at how helpful people can be out there, it's
great
for a beginner like me to have access to more experienced minds.
Cheers.

I've made some progress with my project, which is a dart's scoreboard
by the way. I need the +5 and -5 volts to power a set of six 4inch
7-segment displays which is controled by a MAX7219.

I have managed to get -4.4v from a 5volt power supply by using a
MAX660 (2 acutally in parallel). I won't be happy until I get the full
-5v though as the Dot Point on the 7-segment display is too dull. So I
will try out some of the methods posted here until I succeed, I want
the display as bright as possible.

Thanks again,
Joeyla