How do I make a -5v rail ?

[Screech]

Looked for hours on the net, with no success.
Even the 7905 data sheet is useless to me.

I nead to make a +-5volt supply. (5volts positive and 5 volts negative).

I know how to use a positive volts regulator.
You feed it's input with a voltage of about 3 volts higher.

But what about the -5 volt regulator.?
Does the input of the -5 volt regulator have to be -8volts, or -2volts or 5volts or 8volts, or can it be all?

Ideally, I would like to power these two regulators from a single 9volt source/battery.

Thanks

 

[Someone Electro]

The easyest way wod be to center tap the 9V batery to get +4,5V and -4,5V.

The other option wod be an switch mode power suply runing on that batery.

 

[ThermalRunaway]

You can't center tap a single battery, but you could use that technique on two batteries.

Brian

 

[Oznog]

Back to the old question- what do you want to do? Can't answer without a context. We don't know how much current you need or other critical details. Many times people asking that sort of questions may not even need negative voltage.

 

[eblc1388]

Does the input of the -5 volt regulator have to be -8volts, or -2volts or 5volts or 8volts, or can it be all?

For 7905, it can be -8V or more, meaning from -8V to -35V, anything else like -4V or less, or positive voltage like +2V, +9V etc...won't work.

Ideally, I would like to power these two regulators from a single 9volt source/battery.

No problem using the 7805. However, you do not have any negative voltage for the 7905 to regulate. The usual method is to use a 555 or other ICs oscillators to generate a square wave and rectify it into a negative voltage. Now you have the negative voltage higher than -5V so you can use the 7905 to give you regulated -5V output.

 

[Someone Electro]

You fogot to mentchon the coil in the 555 timer oscilator.Since it feeds it the negative kickback of the coil that is made becose of the sqare wave.

But this method produces a lot of electrical noise.

 

[checkmate]

I know how to use a positive volts regulator.
You feed it's input with a voltage of about 3 volts higher.

There are 2 main methods for getting negative regulated voltage. You either supply a negatve unregulated voltage (with additional negative voltage clearance for dropout), as others have mentioned, or you use dc-to-dc convertors, which are based on oscillators. The second method is mainly for low current appliucations. One common chip used for this is the 7660.

 

[eblc1388]

You fogot to mentchon the coil in the 555 timer oscilator.Since it feeds it the negative kickback of the coil that is made becose of the sqare wave. But this method produces a lot of electrical noise.

Not forgot, but coil/inductor not required in 555 design for negative voltage generator. Try Google for "555 negative voltage" and see designs that use only capacitors and diodes.

 

[Screech]

Back to the old question- what do you want to do? Can't answer without a context. We don't know how much current you need or other critical details. Many times people asking that sort of questions may not even need negative voltage.

I want to supply an op-amp with +5volts and -5volts.
The op-amp is an TL072 JFET OP-AMP. recommended by Nigel.

The project is Automotive, so, primary supply is 12Volts DC.

 

[Someone Electro]

Thats why i use norton op amps that dont need an dual suply.But then agen there output cant go negative whithout an dual suply.

do you need an negative output?

 

[Screech]

do you need an negative output?

just nead a negative 5v to op-amps negative supply. Thats all, nothing else.


I book I have talks about a 7660 voltage converter. It converts any positive voltage from 0 to 12 volts to a negative0-12volts.
I catalog I have ,also talks about dc-dc charge pumps, with that chip.

Maybe I'm onto somthing purpose made for op-amps?

thanks.

 

[Someone Electro]

Most prpole use 2 baterys to do it.

 

[Nigel Goodwin]

do you need an negative output?

just nead a negative 5v to op-amps negative supply. Thats all, nothing else.


I book I have talks about a 7660 voltage converter. It converts any positive voltage from 0 to 12 volts to a negative0-12volts.
I catalog I have ,also talks about dc-dc charge pumps, with that chip.

Maybe I'm onto somthing purpose made for op-amps?

In the other thread I advised you to check my analogue PIC tutorial!, haven't you done that? - seeing as it answers this question for you - and gives both a circuit diagram and veroboard layout!.

 

[Dr.EM]

**broken link removed**

This and your regulators should work ok, or;

**broken link removed**

 

[Screech]

Nigel Said

In the other thread I advised you to check my analogue PIC tutorial!, haven't you done that? - seeing as it answers this question for you - and gives both a circuit diagram and veroboard layout!.

I did go but,
WinPicProg PIC Tutorial
ERROR
The requested URL's could not be retrieved
so, I gave up trying.

Just now I went to your other link (forgot you had two websites).
"http://www.lpilsley.uklinux.net/picprog" <this site works, and yes I found the 7660 chip. Thanks for getting on my back about it.

 

[Nigel Goodwin]

Nigel Said

In the other thread I advised you to check my analogue PIC tutorial!, haven't you done that? - seeing as it answers this question for you - and gives both a circuit diagram and veroboard layout!.

WinPicProg PIC Tutorial
ERROR
The requested URL's could not be retrieved

All working fine here!, try the actual page for the hardware .

 

[Screech]

How did you get that link to work.?

Anyway, I edited my prier post, have a read.
Thanks Nigel.
All is worked out.

 

[audioguru]

Opamps don't need a negative supply, unless you need them to produce a negative output voltage swing. Just bias the opamp to half the supply voltage, and connect the negative supply pin to ground.
Its output DC voltage will swing around half supply voltage.
Capacitor-couple to its input and from its output if you want.

A 7805 won't work from a little 9V battery for much time. The battery's voltage quickly drops to about 7.5V which is too low for a 7805 to regulate properly.
You need a low-dropout regulator so the battery's voltage can drop to less than 6V over its life and the regulator works fine. :lol:

 

[audioguru]

BTW, a TL072's minimum recommended total supply voltage is 7V, so won't work well on a used 9V battery anyway, certainly not if it is regulated at a lower voltage.
A TLC072 is also low-noise and its minimum recommended total supply voltage is only 4.5V. :lol:

 

[Nigel Goodwin]

Opamps don't need a negative supply, unless you need them to produce a negative output voltage swing. Just bias the opamp to half the supply voltage, and connect the negative supply pin to ground.
Its output DC voltage will swing around half supply voltage.
Capacitor-couple to its input and from its output if you want.

You don't appear to have read the thread at all?, it's required to buffer the DC input to the A2D of a PIC, so it needs to swing from 0V to 5V, or (in the case of my tutorial) from 0V to 2.5V. By adding a neagtive supply rail it guarantees the swing to 0V, and it increases the supply to be high enough for a TL072.

A 7805 won't work from a little 9V battery for much time. The battery's voltage quickly drops to about 7.5V which is too low for a 7805 to regulate properly.
You need a low-dropout regulator so the battery's voltage can drop to less than 6V over its life and the regulator works fine. :lol:

I've never had any problem using a 7805 with a 9V battery, the obvious thing is to use a decent quality Alkaline one - if you use a cheap rubbish battery it's no good for anything, as my old discharge curves at show. From the Alkaline discharge curves it's fairly obvious you get a good long life from the battery, using most of it's capacity - I would suggest a 9V battery discharged to 7V is FLAT, and should be thrown away!.