[Amps] Good engineering

[David Cutter]

Heat can be a friend but it's usually our enemy in so many ways.  Yet, to 
prevent melt-downs and dangerous conditions it is the easiest indicator of 
failure to detect and manage.

A carefully placed ptc resistor embedded into a transformer primary or in 
other hot zones works wonders and makes compliance with safety laws very 
much easier.  They are cheap and reliable and very popular over here.  For 
the sake of a $1 that Alpha and any other amplifier would have powered-down 
safely, well before any damage could have occurred.  Cycle the power and you 
are back in business immediately.  Magic devices.

In our safety standards it has been a mandatory test to put 15% excess mains 
supply voltage on an article, prevent or restrict cooling, put all controls 
into the worst condition and then measure mains transformer primary 
temperature rise (usually the resistance method) after a suitable 
stabilising period.  That temperature must not exceed figures laid down for 
the type of wire used in the primary.

It's likely that a pa will exceed proper temperature limits before the mains 
transformer gets to its limit, and it could be controlled with a simple 
thermostat, thermal fuse or ptc resistor in a low level circuit.

Is this done in amateur linear amplifiers?

A responsible manufacturer of any equipment places safety first and 
compliance law is written that way.  It should make equipment practically 
fool proof.

David
G3UNA




>
> Any fool can overbuild a product. Good engineers know when to stop,
> and good engineers get paid good money to get it right.
>
> 73, Bill W6WRT
 A buddy accidently had his cat fall asleep on his footswitch for 2 hrs.His 
87-A  was pumping out a 1.5 kw FM cxr on 29.650 mhz.   He had the optional 
cooling fan.The RF deck had turned into molten glop !   Jim   VE7RF