Pages: [1]   Go Down
Author Topic: How do you supply an opamp with -12 volts?  (Read 834 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Jr. Member
**
Karma: 0
Posts: 88
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

How do you supply a regular opamp (lm324) with -12V if you do not have a center tapped transformer? (ie regular  240 to 12 volt stepdown transformer with a single secondary coil)

Thank you in advance
Regards Voidugu
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 602
Posts: 33379
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Use two power supplies and connect the + of one to the - of the other to form the ground. Then you have the unconnected + of one to be +12V and the unconnected - of the other to give you -12V.

Alternatively use an op amp that can work off a 5V single rail supply.
Logged

Offline Offline
Jr. Member
**
Karma: 0
Posts: 88
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Should I go for a rail to rail opamp or a single supply one?
Logged

Offline Offline
Edison Member
*
Karma: 64
Posts: 2465
Now, More Than Ever
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Your LM324 will do fine.
Logged

"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

SW Scotland
Offline Offline
Edison Member
*
Karma: 17
Posts: 1376
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Or use an isolated DC - to - DC module which will operate off 12 volts and give 12 volts out.   You can then connect the 12 v output to the common 0volt line and the output 0v pin will now sit at -12 volts.

If you use a non-isolated version then you must use one which is designed to give -12 volts output.  ie you connect the 0v output pin to the common 0volt line.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 602
Posts: 33379
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Should I go for a rail to rail opamp or a single supply one?
Those choices are not mutually exclusive.

With a single supply one that works from 5V that is 2.5V per rail as you have to make a signal ground with two 1K resistors in a potential divider. Put a cap 0.1uF across each.
Then it is best to go for a rail to rail one to give you some swing to play with.
Logged

Offline Offline
Jr. Member
**
Karma: 0
Posts: 88
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Jackrae can you be a bit more explanatory please. I don't seem to understand what you are saying.

Mike thanks for the help man.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 602
Posts: 33379
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Jackrae is saying that another choice is to use a DC to DC converter. There are lots of them http://uk.farnell.com/dc-dc-converters
Basically there are two sorts.
1) Isolated - where there is no connection between the input voltage and the output voltage. With these you can then connect the +ve output to the -ve input to get your split supply.
2) non isolated - if you tried the above it would short out but you can get voltage mirrors, that is they will output the negitave of what you put in.

DC-DC converters tend to come as small modules and can be expensive.
Logged

SW Scotland
Offline Offline
Edison Member
*
Karma: 17
Posts: 1376
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Not too expensive I hope ?
http://www.ebay.co.uk/itm/12V-to-12V-1W-DC-DC-Converter-isolated-single-output-New-Qty-1-/121013364355?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item1c2cf56683

Data sheet here :  http://www.datasheetcatalog.org/datasheet/candd/NME0515S.pdf
« Last Edit: May 12, 2013, 03:39:46 am by jackrae » Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 602
Posts: 33379
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Indeed it looks a bargain. 
Logged

Offline Offline
Jr. Member
**
Karma: 0
Posts: 88
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Thank you very very much for the help people. It turns out that i only need the opamp to swing its output from 0 to +5 volts (no negative voltage involved). Is though the lm324 able to go as low as 0 volts and as high as +5 volts while being connected to +5volts and ground? I guess if not then i would have to raise the 5 volts and supply it with a negative voltage as well, or go for another opamp or implement your suggestions by using a DC to DC converter or a second center tapped transformer.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 602
Posts: 33379
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
Is though the lm324 able to go as low as 0 volts and as high as +5 volts while being connected to +5volts and ground?
No.
The data sheet says:-
Quote
Large Output Voltage Swing: 0V to VCC -1.5V
so it will only go to 3.5V
Use a MCP602
Logged

Rapa Nui
Offline Offline
Edison Member
*
Karma: 60
Posts: 2066
Pukao hats cleaning services
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

..google for beyond the rail and rail to rail opamps. There are dozens of such available today. Most work @5V.
No opamp can work rail-rail output, however. Afaik there are always few mVolts (5-100mV) left, based on the load..
Are you going to use it with your RTD sensor?
Logged

Offline Offline
Jr. Member
**
Karma: 0
Posts: 88
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Yes pito smiley I am currently designing the circuit for a temperature controlled soldering station and i want to get everything sorted out. I guess i am going to go with the lm324 since its cheaper. I will get it to swing to +5V by supplying it with more than 7 volts. By the way the lm324 contains 4 opamps. Do you guys know the package if there is one that contains one instead of all 4 opamps?
Logged

Rapa Nui
Offline Offline
Edison Member
*
Karma: 60
Posts: 2066
Pukao hats cleaning services
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

LM321
Logged

Pages: [1]   Go Up
Jump to: