# Use a postive linear regulator for regulating a negative voltage

Can I use an LT78L05, a postive voltage regulator for regulating 5V, for regulating a -5V in this configuration :

simulate this circuit – Schematic created using CircuitLab

In this way, it sources current.

Thank you :)

## 5 Answers

You mention that you want to use this -10 V as the negative supply of an opamp.

That will not work as such a positive regulator can only source (output) current but not sink a current, see circuit on the left.

simulate this circuit – Schematic created using CircuitLab

A (not so elegant) way to solve this is to make sure that the L780L05 will always output a current, we can do that by pulling more current using a resistor, R1 in the right schematic.

Suppose the opamp will have a (negative) supply current up to 1 mA, then we need to make R1 such that is will draw at least 1 mA so 5 V / 1 mA = 5 kohm. But it is better to have some margin so I would use 3.9 kohm.

But you really would do better by using a negative regulator as that is what they're designed for and also, regulators provide a constant voltage at their output relative to their ground pin. If you "misuse" a positive regulator like this, any ripple on the -15 V line is not suppressed. A negative regulator would suppress that same ripple.

• Just what I was thinking. Well-added.
– jonk
Commented Mar 2, 2021 at 9:16
• Thank you for this comment :)
– Jess
Commented Mar 2, 2021 at 9:38

The 5V output ends up attached to the -15V rail instead of the ground rail, so relative to ground you get -10V, and the regulation will only be as good as the -15V regulation.

If you have 5V loads that you either want attached this way or that don't interact with the rest of the circuit, it's fine.

• I'd like to stress this: that circuit gives you -10V, not -5 V as claimed by OP!
– mmmm
Commented Mar 2, 2021 at 8:20
• You re rigth .... I should use the L78L10 which regulates a 10V output ! Actually this just for having a negative supply for an opamp ? Do you think that I could just use a zener diode ? The output op amp no sink or source a lot of current (<1mA). Actually It should always source current. I just neeed a negative voltage supply for increase the common mode range
– Jess
Commented Mar 2, 2021 at 8:34
• It might be possible to use a zener, but I find it awkward getting them close to the accuracy of a voltage regulator. Best thing to do is probably find an actual -5V regulator(7905 for example) and use that.
– K H
Commented Mar 2, 2021 at 8:46
• Hm, what about an opamp that works with the supply range you have? Surely, the cost difference to the one you're currently using can't justify adding another supply. Commented Mar 2, 2021 at 9:52

Firstly, it's not a 5 volts regulator; it will produce -10 volt relative to ground when Vin is -15 volts: -

And, given that the regulator internally is a simple pass transistor, it won't be able to handle load currents from the ground node to the output i.e. it won't regulate.

So, if you chose a 10 volt regulator and the input supply is -15 volts, you would appear to get -5 volts between Vout and ground but, there's another problem. If the -15 volt supply fluctuated to -16 volts, the Vout voltage relative to ground becomes -6 volts.

In other words, it's not a regulator circuit any more. I advise strongly against using this circuit.

Other answers address this case:

• You have an existing ground-referenced DC supply of too-large negative voltage, and you want to regulate it down to a smaller ground-referenced negative voltage. The quick answer is No.

This answer addresses a different case:

• You have an existing floating DC supply which you want to regulate to give a negative ground-referenced output.

In this case, YES, you can use a 7805 to provide a regulated negative voltage. The key is that the unregulated source must be floating, meaning that no part of the supply is referenced to ground. This kind of DC supply is most-often generated from an AC source winding of a transformer, followed by rectifier diode(s) and filter capacitor:

simulate this circuit – Schematic created using CircuitLab

Can I use an LT78L05 (https://www.st.com/resource/en/datasheet/l78l.pdf), a postive voltage regulator for regulating 5V, for regulating a -5V in this configuration

Not with this configuration. The other answers already showed it very clearly. But this post is worth seeing:

Is this negative regulator really meant to be an inverted positive regulator?

The key is the floating 0V reference.