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Old 20th April 2007, 06:19 AM   #1
diyAudio Member
Join Date: Sep 2004
Location: New Zealand
Default LM338 current Regulator allowing modulation

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

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
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Old 20th April 2007, 06:37 AM   #2
diyAudio Member
Join Date: Sep 2004
Location: New Zealand
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.
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Old 20th April 2007, 06:49 AM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
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.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 20th April 2007, 06:58 AM   #4
diyAudio Member
Join Date: Sep 2004
Location: New Zealand
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
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