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G8MNY > TECH 07.01.06 01:55l 53 Lines 2307 Bytes #999 (0) @ WW
BID : 12073_GB7CIP
Read: GUEST DL1LCA OE7FMI
Subj: Stopping HF Tx/Rx SMPSU QRM
Path: DB0FHN<DB0FOR<DB0SIF<DB0EA<DB0ACC<DB0GOS<DB0RES<DK0WUE<I0TVL<ED1ZAC<
GB7YKS<GB7SYP<GB7COV<GB7YFS<GB7CIP
Sent: 060107/0029Z @:GB7CIP.#32.GBR.EU #:12073 [Caterham] $:12073_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Sep 05)
Where a 13.8V SMPSU is used directly to power an HF Tx/Rx the normal mains & DC
filtering is often inadequate to remove all the switch mode spogies that a
sensitive Rx can pick up. Here is one method that stop all QRM on Rx for a 1
rig set up.
The strategy is to stop the SMPSU from running while on Rx, by using a small
linear PSU for Rx instead. Stopping the SMPSU running is normally possible on
all SMPSU if the O/P goes slightly higher than the nominal 13.8V by only a few
100mV. Note that too much overvoltage may trigger a crowbar overvoltage circuit
if that sort of protection is built in (I recommend U add one if not).
On a good insulated Heatsink
inÚÄÄÄÄÄÄÄ¿out
ÚÄÄÄÂÄÄÄÄÄÂÄÄ´ 7812 ÃÄÄÂÄÄ´>ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄ> +Tx/Rx
_³_ _³_ ³ ÀÄÄÄÂÄÄÄÙ ³ ³
MAINS /_\ /_\ ³ ³ ³ --------³---
ÄÄ¿ ÚÄÄ´ ³ ³ ³ ³ | Ä´>ÃÄ´ | 14V Rx < 1A
)³³( ³ ³4700u³+ ÃÄÄ2K2Ä´ ÄÄ| TX ³+ | 13.8V Tx > 1A
)³³( ³ ³ 25V=== Pot ³ MAINS | SMPSU === |
)³³ ÀÄÄ)ÄÄÄ´ ³ 1K === ÄÄÄ| 25A ³ |
ÄÄÙ 16V _³_ _³_ ³ ³ u1 ³ --------³---
1.5A /_\ /_\ ³ ³ ³ ³
ÀÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄ> -ve
Linear Rx PSU
All diodes 1N4001 or better.
The rating of the current limiting linear PSU needs to be just enough to run
the Rx at normal volume etc. Normally 1A will do, but 2 x 1A regs could be used
in parallel (with 0.22ê Rs in O/P) if 1A is not enough, or some other
regulator.
Before under taking a SMPSU modification, test the principle works OK with a
variable bench PSU first. If the test is OK & not too high a voltage is needed
(watch those dial lamps if over 15V), then look for room inside the SMPSU or
add a bolt on box to the SMPSU unit if you like.
Other than eliminating the SMPSU QRM there is 1A more current available on Tx.
The reg must be the current limiting type & well heatsinked, as it will be
working very hard all the time!
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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