Speaker protection using LM311 - diyAudio
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Old 18th March 2015, 05:10 AM   #1
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Default Speaker protection using LM311

  1. Hi all.
    I am using LM311 comparator to design a speaker protection circuit. I read quite a few data sheet of LM311. Seems like it's such an old IC that they assume people know the IC already. I happen to be a first time user and I am not sure I design it right. Mainly strobe pin #6. From reading, it seems like if I use a transistor to pull it down, the output transistor turns off regardless and become open circuit pulled up by the pull up resistor. Also the output transistor how I can use it off ground. Anyone that familiar with the LM311, please confirm I designed the circuit correctly.

    Also, this is a speaker protection circuit for SS amp that can drive a DC voltage if something gone wrong. This circuit will:

    1) monitor the output by using a low pass filter to filter out the audio frequency, only the very very low frequency or DC passes through. Two window comparators using LM311(U1 & U3, U2 & U4) are used to monitor the DC and turn off the transistor if over 0.7V DC is detected.
    2) monitor circuit to monitor +/-24V supply (R3, D3 and relay S1). If total voltage drop by say 5V, the relay opens.
    3) Power on delay (Q1 C15). The drain of the Q1 connects to pin 6 of all the comparators. when Q1 is on and pull all the pin 6 to ground, the LM311 turns off.

    The /OUTPUT EN are use to turn on the speaker output relay when it is pulled to ground.

    I am connecting the two LM311 output transistors in series with the relay contact, so if anyone opens, the chain opens and loss the ground, the speaker relay will be turned off. Only if both NPN output transistor of LM311 and the relay are on, then the /OUTPUT EN will be low to turn on the speaker relay. Please confirm that I am doing everything correctly.

    Thanks

    Alan
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Old 18th March 2015, 09:09 AM   #2
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First of all, you do not need two channel circuit for stereo. After all, if one of the amp speaker is disabled, you do not want to listen to the other channel.

I don't see any low pass filter for detecting DC.

Gajanan Phadte

Edit:Even if you are disabling both the relays, you don't need double the circuit.

Last edited by gmphadte; 18th March 2015 at 09:11 AM.
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Old 18th March 2015, 09:28 AM   #3
sajti is online now sajti  Hungary
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Quote:
Originally Posted by gmphadte View Post

I don't see any low pass filter for detecting DC.

Gajanan Phadte

Edit:Even if you are disabling both the relays, you don't need double the circuit.
R4+R5+C5

Sajti
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Old 18th March 2015, 10:07 AM   #4
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Originally Posted by gmphadte View Post
First of all, you do not need two channel circuit for stereo. After all, if one of the amp speaker is disabled, you do not want to listen to the other channel.

I don't see any low pass filter for detecting DC.
Maybe I can sum the two output into one set of window detector.

thank
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Old 18th March 2015, 10:34 AM   #5
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Elegant approach - using separate RC filter for each channel and then using two small signal diodes (e.g. 1N4148) per channel for connecting those filters to the same window comparator. This way you can connect as many channels if you like without interference between them. Any channel, introducing DC offset outside the window, will trip the protection.

In this case - compare to "Ground". 0.7V will fall on those diodes.

Cheers,
Valery
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Old 18th March 2015, 04:20 PM   #6
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Originally Posted by vzaichenko View Post
Elegant approach - using separate RC filter for each channel and then using two small signal diodes (e.g. 1N4148) per channel for connecting those filters to the same window comparator. This way you can connect as many channels if you like without interference between them. Any channel, introducing DC offset outside the window, will trip the protection.

In this case - compare to "Ground". 0.7V will fall on those diodes.

Cheers,
Valery
Thanks

Good idea.

How about the way use the pin #6 for power on delay?
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Old 18th March 2015, 04:31 PM   #7
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I got a suggestion from Physics Forums that actually simpler and seems like it works for everything I want.

C1 is the start up delay, C2 is the low pass filter. If +12V failed, the MOSFET loss gate drive. If -12V failed, the relay loss power.

This got to be the simplest circuit I've seen.
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Last edited by Alan0354; 18th March 2015 at 04:33 PM.
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Old 18th March 2015, 05:45 PM   #8
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I've used this one a couple of times before. Works 100%
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Old 18th March 2015, 06:52 PM   #9
sbrads is offline sbrads  United Kingdom
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Quote:
Originally Posted by Alan0354 View Post
I got a suggestion from Physics Forums that actually simpler and seems like it works for everything I want.

C1 is the start up delay, C2 is the low pass filter. If +12V failed, the MOSFET loss gate drive. If -12V failed, the relay loss power.

This got to be the simplest circuit I've seen.
Polarised cap C2 has a 50% chance of getting a reverse bias after an output stage blowup, so may fail.
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Old 18th March 2015, 07:15 PM   #10
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Originally Posted by Vrystaat View Post
I've used this one a couple of times before. Works 100%
Have you any pcb for this?
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