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How to use MOSFET to control AC current of speaker output?

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mhtplsh

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http://www.geofex.com/Article_Folders/ampprot/dcprot.htm

Here they hv suggested to use mosfet in place of relay. Now there r three pictures of circuits.
I want to use mosfet configration from circuit 1 with mosfet driver circuit from circuit 3. but i want to use single power supply.
Most interesting part i want is : i want to use this as speaker protector. Hence first the mosfet will conduct full.
then as the driving circuit voltage increases the current through the mosfet switch must reduce. So during overload
mosfet will work like bulb protector i.e. it will conduct less.
Is this possible?
If yes, then guide me how to do this?
 

nxtech

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Re: Is this possible?

You really dont want to use a MOSFET for a protection device. First off a MOSFET is a voltage controlled device. So if you need current protection its not a good choice.

In addition, overvoltage and overcurrent protection devices are designed to open or short (depending on the type of device) faster than any MOSFET can.

E
 

A.Anand Srinivasan

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Re: Is this possible?

actually MOSFET is a voltage controlled current source and so you can use it to control the current.... you may even effect automatic gain control using it....

talking of switching MOSFETs can switch better than mechanical relays... the main thing they rely on while using relays is that the output can bear overpower for a short time...
 

mhtplsh

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Re: Is this possible?

A.Anand Srinivasan,
Thanks 4 ur reply. Can u suggest how to use above circuit to control ac current of soeaker output. The control device in circuit 1 is not available easily. Hence can i use optp coupler instead? Or any other suggestion welcome.
 

A.Anand Srinivasan

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Re: Is this possible?

actually the control circuit PV1050N used there is a optocoupler only.... the word photovoltaic isolater is used that is all... it seems there is no equivalent part for that component(PV1050N) so when you use some other optocoupler please see to that you change the sensitivity externally using resistor to make the circuit work as you desire.....
 

mhtplsh

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Re: Is this possible?

A.Anand Srinivasan, ,
Thanks 4 ur reply.
I hv mentioned above about my requirement. Can u draw some circuit as suggested? I will assemble & then finetune it as required.
 

A.Anand Srinivasan

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Re: Is this possible?

circuitry without physical realisation is hard to make and especially for that in your case i would be needing the output level of the audio system that drives the optocoupler and also the input levels of the MOSFET,...

also if a fault occurs it would be hard for you or me to correct.... you post your proceedings in the circuit and i'll let u know what all i know....
 

mhtplsh

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Re: Is this possible?

The requirements r almost same as mentioned in the first circuit.

http://www.geofex.com/Article_Folders/ampprot/dcprot.htm

Some more details :
Amplifier output to my protection circuit will be max. 25V ac. It will vary as the output varies. so i hv told u the max AC voltage output of the amplifier to tweeter.
This AC will be rectified to DC & used as control circuit power supply driving opto coupler. The mosfet will control the 25V AC instead of relay. I want the mosfet to be in full conduction at low voltage. As the voltage goes high it should resist ac 25v from flowing. Hope u understand.
 

A.Anand Srinivasan

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Re: Is this possible?

why are you rectifying and making the signal DC... the signal has negative voltage too so the optocoupler would have two LEDs in opposite directions i guess....

how are you going to rectify a fast varying signal to DC.... especially tweeter is useful only for high frequency components....
 

mhtplsh

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Re: Is this possible?

how are you going to rectify a fast varying signal to DC.... especially tweeter is useful only for high frequency components
I will use slow recovery diodes for rectification of ac signal from amplifier, but after crossover of the tweeter. Many tweeter protection circuits r designed this way.
 

A.Anand Srinivasan

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Re: Is this possible?

my question is what are you going to do by rectifying the signal to DC because the speaker responds only to AC....
 

mhtplsh

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Re: Is this possible?

Now let me explain u.
As per the circuit1 in the above mentioned link, the 2mosfets will be used as a switch for AC signal. To drive this circuit through opto will require some driving circuit. This circuit will require some dc supply.
As done in this post https://www.edaboard.com/ftopic263843.html .
This way ac signal going to speakers will be controlled by mosfets & dc supply required for the circuit to drive mosfets will get by rectifying the speakers ac signal.
This topic of webpage in the starting post is also the same.
 

A.Anand Srinivasan

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Re: Is this possible?

that circuit you have mentioned to be a tweeter saver.... there the signal must be rectified for the sake of protecting the tweeter from damage due to overvoltage...

also i dont see what signal you are using to drive the optocoupler and other details....

correct me if i'm wrong....
 

mhtplsh

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Re: Is this possible?

From the rectified signal only we will drive optocoupler to control mosfet.
 

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