Current limiting before voltage regulator

In summary, the conversation discusses the use of an LM338K voltage regulator to regulate a variable voltage from 2 to 30 volts with current limiting up to 5 amps. The question is raised about whether limiting the current before the regulator will cause a short circuit on the outputs, but it is noted that the regulator has built-in protection. The National Datasheet for the LM338 is recommended for further information. The conversation also suggests using the TO-3 version of the LM338 for better thermal characteristics. Additionally, the use of a current limiter before the voltage regulator is mentioned, with the caveat that it will require additional supply voltage. The use of dedicated regulators such as the LM723 is also suggested for better current limiting and voltage
  • #1
AdamK9
8
0
I am making a power supply and am using a lm338k to regulate a variable voltage from 2 to 30 volts. I also want to put in current limiting, preferably variable, up to 5 amps since that's what the lm338k is rated at. Since the resistances in the current limiting circuit need to vary when the voltage varies i want to limit the current before the voltage regulator where the voltage stays constant. I was wondering if limiting the current before the regulator will cause a short circuit on the outputs to allow more than 5 amps to flow through the voltage regulator. I don't see how it can since the regulator drawing an excess of 5 amps will still cause the transistor in the current limiter to shut down which will keep the regulator safe, but I've definitely been wrong before.
 
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  • #2
Take a look at the National Datasheet for the LM338 (page 12):
http://www.national.com/ds/LM/LM138.pdf

I think some voltage regulators with short circuit protection have built-in self-resetting fuses. If you look at the first set of graphs, you can see how the device operates near the limits of its operation.

If you can, buy the steel can (TO-3) version (STMicroelectronics also makes one, but you'll have to compare and contrast the datasheets in case there are some unexpected nasty surprises). Much better thermal characteristics, and easier to heat sink. Also pricier than the TO-220, but worth it.
 
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  • #3
You can put a current limiter before the voltage regulator and you might do it if you were using a three terminal regulator for voltage regulation.
Don't forget that this will chew up some of your supply voltage and you will need two or three volts above the output voltage for the Voltage regulator. So, you could have to supply 16 volts just to get 5 volts out.

However, dedicated regulators like the LM723 have built in foldback current limiting as well as sensing their voltage directly across the load (so you don't get poor regulation due to the current sensing).
They also have superior voltage vs temperature characteristics and allow you to use high powered transistors to carry high currents.
 
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1. What is current limiting before voltage regulator?

Current limiting before voltage regulator is a technique used to limit the amount of current that flows into a voltage regulator. This is done to protect the voltage regulator from being overloaded and potentially damaged.

2. Why is current limiting important before a voltage regulator?

Current limiting is important before a voltage regulator because voltage regulators are designed to handle a specific amount of current. If too much current flows into the regulator, it can overheat and fail. By limiting the current, the voltage regulator remains within its safe operating range.

3. How does current limiting work before a voltage regulator?

Current limiting before a voltage regulator is typically achieved by using a resistor or a current-limiting circuit in series with the input to the voltage regulator. This resistor or circuit restricts the amount of current that can flow into the regulator, keeping it within its safe operating range.

4. What are the benefits of current limiting before a voltage regulator?

The main benefit of current limiting before a voltage regulator is the protection it provides to the regulator. By limiting the current, the regulator is less likely to overheat and fail. This can help to increase the lifespan of the regulator and prevent costly repairs or replacements.

5. Are there any downsides to current limiting before a voltage regulator?

One potential downside of current limiting before a voltage regulator is that it can reduce the available output voltage from the regulator. This is because the current-limiting resistor or circuit will cause a voltage drop. However, this can be managed by selecting an appropriate resistor or circuit with a suitable voltage rating.

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