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
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.
Should I go for a rail to rail opamp or a single supply one?
Your LM324 will do fine.
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.
Voidugu:
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.
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.
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.
- 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.
- 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.
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 : C&D Technologies NME0515S datasheet pdf
Indeed it looks a bargain.
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.
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:-
Large Output Voltage Swing: 0V to VCC -1.5V
so it will only go to 3.5V
Use a MCP602
..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?
Yes pito 
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?
LM321