How do I make a -5v rail ?

[audioguru]

BTW again,
I reviewed your other topic and found that you need an opamp follower to buffer a signal from 0V to 5V.
A TL072 doesn't work if its input voltage swings near its negative supply voltage (ground in a single supply system) and a TLC072 doesn't work if its input swings near its negative or positive supply voltage.
So you do need a negative supply voltage for the opamp to work with an input voltage from 0V to 5V. The max recommended supply voltage for the TLC072 is only 16V.
If you need only a single opamp, use a TL071, powered by two 9V batteries. It probably doesn't need regulation of its supply voltages. :lol:

 

[audioguru]

Hi Nigel,
My 9V alkaline battery powered projects work fine until the battery discharges to only 5.5V when I use a little LM2931-5.0 low-dropout regulator.
Energizer rate the capacity of their 9V alkaline batteries to when the voltage has dropped to a "completely flat" 4.8V. :lol:

 

[eblc1388]

Hi Audio,

Isn't an old fashioned LM358 supplied with 9V suitable for use in this case?

Its common input voltage ranges from 0-7.5V and output goes truely to ground, according to the datasheet.

 

[Nigel Goodwin]

Energizer rate the capacity of their 9V alkaline batteries to when the voltage has dropped to a "completely flat" 4.8V. :lol:

Advertisers generally exaggerate their products :lol:

Although that's probably not as bad as in-car speaker and amplifier manufacturers? - or your '200W' computer speakers, that have a 12V 800mA power supply?.

 

[audioguru]

Hi Eblc,
The LM358 is a good choice if you don't use high audio frequencies and don't care that its output can go down to within 20mV above ground with a 10k load to ground. The datasheet lies on the 1st page, it should say near ground.

Are you the person with a 600k sensor? The max input bias current of an LM358 from a 600k source causes a positive error of up to 150mV. This can be reduced by adjusting an added resistor in series with its negative input.

Its input is spec'd to work at -2V from its positive supply, so a 9V battery must be replaced at 7V.
It uses low supply current so a 9V alkaline battery will power it for many hours before the voltage reaches 7V. :lol:

 

[eblc1388]

Hi Eblc,
The LM358 is a good choice if you don't use high audio frequencies and don't care that its output can go down to within 20mV above ground with a 10k load to ground. The datasheet lies on the 1st page, it should say near ground.

Really, then insert a diode between opamp ouput and feedback resistor/connection.

 

[audioguru]

Really, then insert a diode between opamp ouput and feedback resistor/connection.

Good point! Let the load swing the diode's output down to exactly 0V when required, and the open loop gain of the opamp erases the non-linearity of the diode. :lol:

 

[Oznog]

BTW again,
I reviewed your other topic and found that you need an opamp follower to buffer a signal from 0V to 5V.
A TL072 doesn't work if its input voltage swings near its negative supply voltage (ground in a single supply system) and a TLC072 doesn't work if its input swings near its negative or positive supply voltage.
So you do need a negative supply voltage for the opamp to work with an input voltage from 0V to 5V. The max recommended supply voltage for the TLC072 is only 16V.
If you need only a single opamp, use a TL071, powered by two 9V batteries. It probably doesn't need regulation of its supply voltages. :lol:

I don't know what the original thread was but this looks like an inappropriate op amp. If you're just trying to buffer it to fix a high impedance you generally want a rail-to-rail input/output or at least an output. Also there are many amps not rail-to-rail because they don't swing to the + rail but can swing to ground.

Just go on Digikey, search Op Amps, go under "Features", select all the rail-to-rail stuff. Example:
https://www.electro-tech-online.com/custompdfs/2005/09/TLV23707172737475.pdf

 

[audioguru]

The rail-to-rail opamps can have their output swing up to the positive rail only when they don't have a load to ground. Therefore they can't be used here without a higher than 5V positive supply.
A rail-to-rail opamp could be used here and need only a single 9V battery for its positive supply.

 

[eblc1388]

Swing to ground is not good enough for a voltage follower, I bet it is more likely 10mV above ground instead of true ground as in the above LM358 example.

Can single supply rail to rail Opamp's output provides zero volt for zero volt input?

 

[Nigel Goodwin]

Swing to ground is not good enough for a voltage follower, I bet it is more likely 10mV above ground instead of true ground as in the above LM358 example.

Can single supply rail to rail Opamp's output provides zero volt for zero volt input?

I'm highly dubious about it?, and also to them reaching completely to the positive rail?, which is why my analogue tutorial uses a negative rail (so you CAN get to 0V), and a 2.5V reference voltage, so you only have to go 2.5V positive, which doesn't present a problem.

 

[audioguru]

Can a single supply rail to rail Opamp's output provide zero volts for a zero volt input?

Only when it doesn't have any load current (for it to sink) and only when its input offset voltage is nulled.

An LM358 has a some idle current in its push-pull output transistors, so its output voltage can never go all the way down to ground.

A rail-to-rail opamp's problem in this circuit is that its output voltage drops when loaded with the resistance of the A-to-D resistor ladder to ground. So with a 5V supply and a 5V input, its output might be only 4.85V.

 

[TekNoir]

Would it be possible to wire together a circuit such as the one detailed **broken link removed** and then regulate it on both outputs with a 78L05, 79L05 regulator (or better)?

 

[audioguru]

Charge pumps oscillate and might cause interference to the measurment.
In a simple opamp buffer circuit, the opamp doesn't need its supplies regulated.

 

[jkmadsci]

LMC7660IN this ic is a good negative voltage generator. Ive used it several times and its cheap!

 

[bryan1]

Hiya Screech,
Eh mate in this months siliconchip is a circuit that uses a 741 and zener to produce -5.4 volts. It's the first time I've seen anything like this and here's an excerpt explaining it " Because IC5 needs a balanced bipolar supply, IC1 and ZD1 are used to give the 10.8 volt supply an active centre tap, which is connected to the circuits earth. The two main supplies thus become +5.4 and -5.4 nominal to ground" Pretty neat I reckon.

Hope this helps and what do you other guy's think???

Cheers Bryan

 

[Nigel Goodwin]

Hiya Screech,
Eh mate in this months siliconchip is a circuit that uses a 741 and zener to produce -5.4 volts. It's the first time I've seen anything like this and here's an excerpt explaining it " Because IC5 needs a balanced bipolar supply, IC1 and ZD1 are used to give the 10.8 volt supply an active centre tap, which is connected to the circuits earth. The two main supplies thus become +5.4 and -5.4 nominal to ground" Pretty neat I reckon.

Hope this helps and what do you other guy's think???

It's a standard technique, it's being used for years and years!.

It's NOT providing a negative voltage, it's providing a psuedo 0V point between the existing -ve and +ve of the supply.

I also wouldn't consider it a very good example?, because it uses a zener on the opamp input - so one supply rail will vary as the incoming supply changes. It's usual to just use two resistors, so BOTH rails will vary with the supply, keeping the 0V point in the middle.

It's essentially just using two resistors across the supply to give a midpoint, but by using an opamp to amplify it, it compensates for differences in the loads on the two rails.