LM338 current Regulator allowing modulation

[Chris Daly]

Hi
I need help with an interesting question, I am currently examining a AM 1Kw transmitter which currently uses
6 amp fuses for fault /DC protection- 1 per module.

I am assessing whether just fuses are the best answer
for this output stage. I think two LM338 devices acting
as a current regulators will provide another layer of
protection.

Also to consider is that AM modulation is switch mode added at a sine -wave frequency of 125Khz across the DC rail ( where the fuses are ) into a bridge 4 device per module Fet network

Will a LM338 arranged as a current regulator Vref/R
allow this modulation frequency to pass as well. ? or
does a LM338 attempt to block frequency content
preferring to live in a DC environment.

Has anyone had experience with frequency and LM338
LM317 family of devices ?

My other option is to design current regulation within
a DC part of the board before modulation is added.

Thank You

 

[Chris Daly]

A few additional points:
The idea is to current limit the circuit to approx 5 amps per module - which should be within the 338's capability
rather than just relying on 6amp fuses.

The fuses will remain to protect the circuitry in case of outright abuse.

 

[AndrewT]

Hi,
125kHz is well beyond the knee in the frequency response of the regulator. It does not work very well at this high a frequency.
That may lead to some serious performance shortfall.

 

[Chris Daly]

Hi Andrew

Thanks very much, it begs the question what is the
frequency response of a fast blow fuse ?

I can design this limiting earlier which should solve
the reliability issues. Many Thanks

Cheers / Chris