simple variable voltage regulator questions (about current)

Thread Starter

jorngensen

Joined Nov 25, 2019
2
hi -
i am learning about electronics and want to be extra cautious about power supplies, so i am posting here to make sure i understand this concept correctly.
i currently use a +/-15v power supply that delivers 3a of power.
i'd like to use a variable voltage regulators to provide +/- 5v to certain circuits.
it seems that the most readily available variable regulators are the LM337 and the LM317 - positive and negative voltage regulators, rated at 1.5a.
can i use these regulators, if the lower-voltage circuit itself doesn't draw more than 1.5 amps of power? or is the amperage rating more specifically rated to what is coming from the power supply, initially?

the issue is that i seem to be able to find a 3a voltage regulator for positive voltages (the LM350) - but i see in certain places that it is discontinued. also, i can't seem to find a negative voltage counterpart.
 

dl324

Joined Mar 30, 2015
9,321
Welcome to AAC!
it seems that the most readily available variable regulators are the LM337 and the LM317 - positive and negative voltage regulators, rated at 1.5a.
can i use these regulators, if the lower-voltage circuit itself doesn't draw more than 1.5 amps of power? or is the amperage rating more specifically rated to what is coming from the power supply, initially?
Yes, but the 1.5A value is with adequate heat sinking. Also note that you need to draw at least 10mA for the regulator to be guaranteed to regulate.

Since you're just starting out, it would be best for you to get and keep your terms straight. Amps is not a measure of power; that's watts (amps * volts), P = IV.
the issue is that i seem to be able to find a 3a voltage regulator for positive voltages (the LM350) - but i see in certain places that it is discontinued. also, i can't seem to find a negative voltage counterpart.
You can increase current with an LM317 by adding an external pass transistor.

Here's an example using LM195 (an integrated, virtually blowout proof, power transistor):
1574726129332.png
You can also use a power PNP transistor. Replace the 2N2905 with a power PNP, drop R2. Personally, I'd put a current divider on the input of the LM317 so that the regulator can operate at 1.5A under your maximum load. That will let the safe area protection in the regulator protect the pass transistor.
 
Last edited:

crutschow

Joined Mar 14, 2008
23,734
Lm317 and LM337 are common linear variable voltage regulators.
Note that the minimum output voltage for each is 1.25V (magnitude), but that's usually not a problem for most applications.

They will need to be mounted on a heat-sink, since they must dissipate power equal to the load current times the difference between the regulator input and output voltages.
For example, for a 5V, 1A output, the power dissipated would be 1A * (15V-5V) = 10 watts.
5V @ 3A output would dissipate 30 watts.
So the required heat-sink could be pretty substantial, depending upon the maximum output voltage and current you need.
 

dl324

Joined Mar 30, 2015
9,321
Here's something from a National Semiconductor Regulator Handbook regarding using a current divider on the input of the regulator:
1574730556392.png

Here's an example of how to use an external NPN power transistor:
1574730631427.png
 

ronsimpson

Joined Oct 7, 2019
325
can i use these regulators, if the lower-voltage circuit itself doesn't draw more than 1.5 amps of power?
This is a common thing to not understand. (1.5 amps maximum)
A 1.5A regulator will work just fine at 1A or 0.5A or 0.0003A or zero amps.

Same thing with house wiring. A over head light is likely on 15 or 20A breaker but only uses 1A. When the light is switched off it uses 0A. There is no requirement to use all of the available current.
 

SamR

Joined Mar 19, 2019
1,474
The flip side of the coin is 1.5A Max is the upper limit and just like my car can go 120MPH max I don't drive it that hard and if I did it wouldn't last long.
 

crutschow

Joined Mar 14, 2008
23,734
LM317 specifies a minimum load current of 10mA
That's typically provided by a 120 ohm resistor between the output and the Adj pin as part of the output voltage determining resistive divider circuit.
Since the voltage between those two points is a constant 1.25V, the minimum output current is always at least 10mA.
 
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