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LM317 as current limiter for audio amplifier

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darryl_co

New Member
Hi,
I was planning to use LM317 as a current limiter to a transistorized audio amplifier that would limit the current to 500mA.It is understood that the current would be limited by adjusting the output voltage i.e. drop more or less voltage across the LM317 to regulate the current.
My query is if the input signal level is low would the LM317 provide more voltage to the amplifier to maintain the current?
If the speaker shorts or the speaker terminals short accidentally would it still keep providing the 500mA which would result in blowing off the power transistors?
 
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simonbramble

Active Member
I have used this part many times, but never as a constant current source. The part adjusts its output voltage to maintain 1.25V across the output resistor. If the load demands more current, the voltage on the right hand side of the resistor will drop and the output voltage of the regulator will go up to compensate for the voltage drop across the resistor. The part needs at least 3V from input to output to maintain regulation, so if your input voltage is lower than that, the output voltage will start to drop. It looks like if you short the load, the part will continue to try to source 500mA, although I am open to comment from people who have actually used this part as a current source to correct me
 

3v0

Coop Build Coordinator
Forum Supporter
If you set up the LM317 to limit the current to 500mA that is the most current it will allow. The actual current used is determined by the circuit as per usual. The LM317 will not try to maintain a constant 500mA.

If you want the supply to cut out when the current reaches a max value you will need use other circuitry to provide the short circuit protection.

Thinking an comparator driving a transistor to cut off the supply to the amp.
 
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unclejed613

Well-Known Member
Most Helpful Member
you could use output stage VI limiters as many amplifiers use. if properly done, they won't activate until a real problem exists. or use SOA (Safe Operating Area) detection and a speaker relay.

you can find out more about amp protection methods here: **broken link removed** and here: **broken link removed**

the second one is about dc protection. i will post an SOA detector and associated relay circuit later when i have access to my computer at home.
 
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darryl_co

New Member
3v0 :
I do not want the supply to cut out when the current reaches a max value instead limit the max current to 500mA
unclejed613 :
Using limiters in the output or speaker relay is not useful as relay switching would cause clicks in the speaker and why use something that come in use if only there is a problem
My query still remains unsolved
 

unclejed613

Well-Known Member
Most Helpful Member
using a constant current source as a supply rail means the supply voltage will drop out when you hit the current limit. so when you hit the current limit it will distort like crazy as the rail voltage collapses. you seem to want something that is inaudible when it protects, but no matter which method you use, there will be audible results. the VI limiter seems to be the best bet, since it will be barely audible if done right.
 

darryl_co

New Member
If you have questions you need to restate them because I looks like everything you have asked about has been answered.

My query is if the input signal level is low would the LM317 provide more voltage to the amplifier to maintain the current? who has answered this

If the speaker shorts or the speaker terminals short accidentally would it still keep providing the 500mA which would result in blowing off the power transistors?
simonbramble says "It looks like if you short the load, the part will continue to try to source 500mA, although I am open to comment from people who have actually used this part as a current source to correct me", but he is not sure
 

3v0

Coop Build Coordinator
Forum Supporter
My query is if the input signal level is low would the LM317 provide more voltage to the amplifier to maintain the current? who has answered this

NO, the actual current used is determined by the circuit as per usual. The LM317 will not try to maintain a constant 500mA (when the circuit is requiring less).

If the speaker shorts or the speaker terminals short accidentally would it still keep providing the 500mA which would result in blowing off the power transistors?

YES, if you want the supply to cut out when the current reaches a max value you will need use other circuitry to provide the short circuit protection.

In a later post you wrote
I do not want the supply to cut out when the current reaches a max value instead limit the max current to 500mA

That contradicts you desire not to blow the power transistors.
 

unclejed613

Well-Known Member
Most Helpful Member
a constant current source feeding an amplifier output stage is not exactly a good solution for protecting output devices. a constant current source is a high impedance node, and for an amplifier to work properly, it's power supply needs to be a low impedance node. i am currently working on a way to illustrate this for you, and i will post it when i am finished.
 

unclejed613

Well-Known Member
Most Helpful Member
so here is a way to see the problems when you use a constant current source as a supply for an amp output stage.the schematic shows an amp output stage with 30V rails. Q1 and Q4 are the output transistors. Q2, Q3 are the current sources set for 500mA. the input waveform is a 35.35Vrms sine wave (50Vpeak). as can be seen in the second picture, the output is a distorted waveform with 4V peaks (blue=input, green=output). the third picture shows what's going on at the output stage collectors. green is the output waveform, blue is the collector current, which gets limited to 500mA, and the red waveform is the output transistor Vcc, which dips down when the current limiting is active. this circuit would work great if the desired result were soft clipping of the type made by a tube amp. but for a solid state amp for listening to music, the performance of this output stage is horrible.
 
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unclejed613

Well-Known Member
Most Helpful Member
maybe so, but it does have some interesting applications where the distortion is a wanted side effect...
 
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