# Negative Voltage Regulators

#### PCBoy

Joined Dec 14, 2011
26
Hi.

I'm looking for a negative voltage regulator that has an output current that's higher than 2A.

So far, I've found LM333 but realized it's been discontinued. Any suggestions?

Specifically looking for a fixed negative 15 regulator. But a negative variable could work as well as long as it can be set to -15.

#### SgtWookie

Joined Jul 17, 2007
22,230
What is your input voltage (minimum and maximum), and available current?

What is the maximum output current that you need at -15v?

#### PCBoy

Joined Dec 14, 2011
26

My input voltage should be at maximum 50V. And output current at least 2.5A or 3A.

#### russ_hensel

Joined Jan 11, 2009
825
A common approach is to use a std. regulator with an external pass transistor(s). It should not be hard to find a circuit ( using the usual resources, no need to post a question ).

#### PCBoy

Joined Dec 14, 2011
26
I have thought of that but our instructor specifically instructed us not to use transistors.

#### k7elp60

Joined Nov 4, 2008
562
You could possibly us a + regulator in the ground lead such as the ruff diagram posted.

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#### SgtWookie

Joined Jul 17, 2007
22,230
You could possibly us a + regulator in the ground lead such as the ruff diagram posted.
I can guarantee that this will NOT work.

PCBoy,
You say a maximum of 50v; do you mean +50v or -50v? Since you did not specify a polarity, I am assuming you mean +50v.

What is the MINIMUM input voltage and polarity?

If your input is indeed positive, you will need a switch mode power supply to generate a -15v supply.

If your input is actually -50v, and you require 3A @ -15v out, that means you would have to dissipate (-50v - (-15v)) * 3A = 35 * 3 = 105 Watts of power in a linear regulator. You would need a very large heat sink to get rid of that much heat, and even then it probably won't be enough to keep the junction temperature within the safe operating region.

You're going to need a switching regulator whatever way you look at it.

Is 3A really the maximum output current that you need? You don't seem terribly certain about that point.

#### bloguetronica

Joined Apr 27, 2007
1,541
If you need a negative voltage regulator, you can use, for instance, a LM79xx (LM7905, LM7909, LM7912) or a LM337 coupled to a 2N3055 transistor (used here as pass transistor). With an adequate heatsink on that transistor, it should give 2A. There are lots of circuit examples that use positive voltage regulators with MJ2955 pass transistors. You circuit will probably be very similar, except it will be inverted and it will use the corresponding complementary transistors).

Here are some good links (for positive regulators):
http://www.fairchildsemi.com/ds/LM/LM7805.pdf (contains interesting schematics on page 24)
http://www.teachers.ash.org.au/jfuller/electronics/regulators.htm

#### SgtWookie

Joined Jul 17, 2007
22,230
If you need a negative voltage regulator, you can use, for instance, a LM79xx (LM7905, LM7909, LM7912) or a LM337 coupled to an 2N3055 transistor. <snip>
See our OP's reply #5; transistors are not allowed.

#### bloguetronica

Joined Apr 27, 2007
1,541

#### SgtWookie

Joined Jul 17, 2007
22,230
I see that in Figure 21. I believe that is someone's wishful thinking. Ask yourself how can a negative voltage be regulated, when current is sourced from pin 5?

There is no terminal on an L200 that sinks current, unless you wish to consider the input pin 1 as a current sink. In figure 21, they have the lower L200's pin 5 connected to ground. I don't see this as being a very happy situation.

#### k7elp60

Joined Nov 4, 2008
562
I can guarantee that this will NOT work.

PCBoy,
You say a maximum of 50v; do you mean +50v or -50v? Since you did not specify a polarity, I am assuming you mean +50v.

What is the MINIMUM input voltage and polarity?

If your input is indeed positive, you will need a switch mode power supply to generate a -15v supply.

If your input is actually -50v, and you require 3A @ -15v out, that means you would have to dissipate (-50v - (-15v)) * 3A = 35 * 3 = 105 Watts of power in a linear regulator. You would need a very large heat sink to get rid of that much heat, and even then it probably won't be enough to keep the junction temperature within the safe operating region.

You're going to need a switching regulator whatever way you look at it.

Is 3A really the maximum output current that you need? You don't seem terribly certain about that point.
If the + input to regulator is isolated from ground and the output is grounded it will work. If both inputs - and + are isolated from ground it will work.

#### bloguetronica

Joined Apr 27, 2007
1,541
I see that in Figure 21. I believe that is someone's wishful thinking. Ask yourself how can a negative voltage be regulated, when current is sourced from pin 5?
Though pin 5 is sourcing current, the voltage ends up being regulated. The difference is that you consider pin 5 ground, and thus the common (pin 3) of the regulator turns up to be negative compared to ground. After all, the regulator regulates the voltage between pin 5 and pin 3.

I don't like this solution either, but I can't find an adequate negative voltage regulator capable of delivering 2A or more.