Stereo amp EMF protect

[Klaus Jensen]

I am using a 25W home stereo amp to experimentally drive various types
of step up transformers, ie. auto ignition coils, rewound flybacks,
etc. Of course, I will use an appropriate value load resistor, if
required, but no external coupling cap.

In this application, what kind of protection would be advised so that
back EMF, etc. does not damage the amp's output stages.

I am looking for something that will not affect the drive signal's
waveshape.

Thank you for any advice.

Klaus Jensen

 

[Joerg]

Hello Klaus,

I am using a 25W home stereo amp to experimentally drive various types
of step up transformers, ie. auto ignition coils, rewound flybacks,
etc. Of course, I will use an appropriate value load resistor, if
required, but no external coupling cap.

In this application, what kind of protection would be advised so that
back EMF, etc. does not damage the amp's output stages.

I am looking for something that will not affect the drive signal's
waveshape.

Hard to say without any schematic. Large diodes to the rails could help
some. However, if it is a pnp/npn pair that is driven from a hard source
exceeding base current or reverse Vbe limits could already blow the
transistors.

Maybe you should find good spare transistors and keep a whole lot of
them in a bin for replacement ;-)

 

[Phil Allison]

"Klaus Jensen"

I am using a 25W home stereo amp to experimentally drive various types
of step up transformers, ie. auto ignition coils, rewound flybacks,
etc. Of course, I will use an appropriate value load resistor, if
required, but no external coupling cap.

** That is mighty serious abuse for a domestic audio amplifier !!

Only by having a series resistor of at least 4 ohms from the output AT ALL
TIMES could a domestic audio amp be expected survive driving low such
impedance and saturating magnetic loads.

In this application, what kind of protection would be advised so that
back EMF, etc. does not damage the amp's output stages.

** Back emfs are not your problem, the low output impedance of the amp will
take care of them.

If the amp has VI limiting protection built in for its output devices, then
it will also have clamping diodes from the output to each supply rail. These
are needed for use with ordinary loudspeakers.

I am looking for something that will not affect the drive signal's
waveshape.

** My god - what horrendous input signals is this clown using ??




........ Phil

 

[Tim Wescott]

Hello Klaus,



Hard to say without any schematic. Large diodes to the rails could help
some. However, if it is a pnp/npn pair that is driven from a hard source
exceeding base current or reverse Vbe limits could already blow the
transistors.

Maybe you should find good spare transistors and keep a whole lot of
them in a bin for replacement ;-)

Or a great big series resistor.

I have seen 60Hz resonant electromechanical loads driven by a regular
old Archer PA amplifier -- this is a load that consumes 60W at 12Vrms
during 'normal' operation, and has an instantaneous impedance that's
well below the apparent 2.4 ohms, and that will bite you if you ramp up
the amplitude too fast. AFAIK the amp is still in working condition,
buried in a lab someplace, ready to be dug out and used when it's needed.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Tim Shoppa]

"Klaus Jensen"

I am using a 25W home stereo amp to experimentally drive various types
[...]
I am looking for something that will not affect the drive signal's
waveshape.

** My god - what horrendous input signals is this clown using ??

There is a long and deep tradition of using home audio equipment in
scientific experiments.

When I got started, tube based consumer amps were being used to drive
everything from voice coil shake tables to piezo transducers.

At the ultrasonic end, it didn't matter what you put in, what you got
out didn't look like it . And the shake tables were at the other
extreme.

When (not IF) they blew up in the lab, I got to fix 'em .

For the cost of a single output transformer back then, you could buy a
dozen brand new solid-state amps of ten times the power capacity, any
of which will do a much better job at driving the bizarre loads than
the old tube/transformer amps (especially outside the AF range).

Tim.