# Power Supply Protections.

Discussion in 'Homework Help' started by Theory, Nov 6, 2015.

1. ### Theory Thread Starter New Member

Nov 3, 2015
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I would like to ask for help, my professor gave me a project to make a power supply (variable) with overcurrent protection and short circuit protection. I designed a working power supply already using LM317, but i don't have any idea about the protection circuits. Can u guide me somehow?

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2. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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According to the datasheet, the LM317 has built-in overload (current and temperature) protection.

A fuse would also meet the criteria for over-current and short-circuit protection .

3. ### john hauton New Member

Dec 19, 2009
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3
Hi, The only things that immediately spring to mind are, there is no fuse on the input to you bridge rectifier. Although the 317 does have some protection, what happens if your pass transistor goes short circuit? Overvoltage protection is a real necessity with modern low voltage circuits.
regards john

Theory likes this.
4. ### Theory Thread Starter New Member

Nov 3, 2015
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W
The thing is that my prof want a circuit that could protect the power supply without using fuse. I would like to make a supply wherein it will stop to function once it detects overcurrent and when the probes are shorted.

5. ### crutschow Expert

Mar 14, 2008
12,535
3,071
The circuit you designed has the booster 2N3055 outside the feedback loop so the voltage regulation of the supply due to load and temperature changes will be compromised.
You need to use one of the booster circuits shown in the LM317 data sheet which does not have that deficiency.
The may still require an added current limit circuit.
How much current output do you need?

6. ### Theory Thread Starter New Member

Nov 3, 2015
28
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My prof restricts us using fuse sir. He want us to make a circuit where it will make the supply stop functioning once it detects over current and when the probes are shorted.

Nov 3, 2015
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8. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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You need to decide what action to take in an over-current situation e.g. reduce output current to zero, maintain current but at a safe level, manual or automatic reset when over-current condition is removed. Also, consider how quickly the protection should operate. It's common for loads to draw high, but brief, starting currents and if the protection operates too quickly the power-supply will never start. You may also want to add over-temperature sensing on the output transistor.

Then, choose a means of current sensing e.g. series resistor, hall effect sensor, current transformer (on the incoming AC) etc. and devise the means to control the current (according to whatever method you decided upon) when an overload is detected.

9. ### Theory Thread Starter New Member

Nov 3, 2015
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Im planning to have a sensing Resistor (Rs), and want to stop the transistor to operate. Meaning i want to give 0v in Vbe of my 2n3055. Will that be okay ?

10. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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Here's a simple, classic current limit sensing circuit based around TR1 (ignore the switching controller to the left of the transistor):

when the current reaches the limit (around 6.5A in this case), TR1 collector pulls high. You could use this output to trigger an SCR which would quickly remove the base current to the output transistor (or pull the adj pin of the LM317 low) and it would remain in that state until the supply is restarted. A small capacitor across R3 would delay the response so that the protection does not trigger on brief current peaks e.g. when the load is switched on.

Last edited: Nov 6, 2015
11. ### crutschow Expert

Mar 14, 2008
12,535
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Here are some ideas for current limiting some of which you could use in front of an LM317 boosted regulator.

Last edited: Nov 6, 2015

Nov 3, 2015
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13. ### Theory Thread Starter New Member

Nov 3, 2015
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@crutschow Thanks a lot for your guidance sir.

Nov 3, 2015
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15. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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Mod edit :
This is a homework, the links was deleted.

recklessrog likes this.
16. ### ScottWang Moderator

Aug 23, 2012
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If you wish to using the 2N3055 then you may need at least 4 2N3055 in parallel and four limited current resistors to complete the job.