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Protection circuitry reference
Protection circuitry reference
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Old 3rd September 2009, 06:41 PM   #1
lt cdr data is offline lt cdr data  Wales
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Join Date: Dec 2003
Location: earth
Default Protection circuitry reference

Inspired by a computer power supply, doug self's book, I thought I would bring this together, its always been a little obtuse to me, so here's a new way of looking at it.

I hope this brings a fairly unmethodical subject and gives it a newer intellectual rigour of classification.

I am not an expert, and comments are welcome

Some if not all the methods overlap between what could be called protection and sound engineering design decision, eg thermal and bias decisions.

1/ overvoltage protection

what's at risk in power amps? fet outputs by diode clamping the inputs

and clipping the dc power rails, use clip detectors...comparators?

I wonder if you can use a compressor circuit to protect here?

This is not often attended to in commercial hifi amplifiers

2/ overcurrent protection, aka underimpedance

very commonly implemented,

what's at risk...

the amplifier by the speaker impedance dropping to low

solution, current limiting methods

sometimes also VAS stage is protected here from too much drive?

3/ overpower
...hmm combination of overvolt and current, nothing to say here

4/ short circuit protection,
one of the biggies, infinite current,

psu can be at risk by caps, rectifiers and transformers going short

short on output transistors puts speakers at risk

solutions, fuses and relay protection mostly

also, I will add, inrush protection here, toroids and power supply caps can sometimes be effective short circuits that can cause some problems.

5/ overtemperature

and fan cooling are all decent design decisions here

mains transformers are also at risk,

thermisistors used to monitor heatsink temperature, perhaps IC's too

6/ undervolts
, here I will call it AC powering on and off, muting circuitry.

7/ overfrequency protection

design decision, related to 1/ oscillation/stability techniques

rf and input slew limiting are used, too.

8/ underfrequency,
here one could say DC protection as in shorted speaker leads or output transistors

and an input band limiting filter will protect what is following from preceeding DC, over excursion etc.

finally, miscellaneous circuits I am not sure where to put into my scheme are 3:

output clamping diodes
zobel network
and the series inductor output network, all for excessively reactive loads, perhpas over and under impedance protection?

Last edited by lt cdr data; 3rd September 2009 at 06:48 PM.
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