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G0FTD > FT817 27.04.07 08:28l 44 Lines 1389 Bytes #999 (0) @ WW
BID : 080314116PZT
Read: GUEST DL1RX VE7HFY DG4IAK
Subj: Re: Max current from mic 5v pi
Path: DB0FHN<DB0FOR<DB0SIF<DB0ROF<DB0ACH<DB0MKA<DB0LJ<DB0RES<ON0AR<GB7SYP<
GB7PZT
Sent: 070427/0458Z @:GB7PZT.#24.GBR.EU [Kidderminster] #:31408 XSERV410h
Date: Fri, 27 Apr 2007 05:58:43 +0000
From: g0ftd@gb7pzt.#24.gbr.eu (Andy)
To: ft817@ww
Subject: Re: Max current from mic 5v pi
M0PZT (the rare dx I spoke to on GB3EK t'other day) wrote:-
>> I wanna build a mic preamp into existing mic case, that's why.
>
> The +5v line on the RJ45 mic connector is fed via a 10ohm resistor from
> Q1088 which is a BA05FP regulator. Appears to have an absolute max rating
> of 1amp according to the datasheet.
Phew, I wouldn't think the SMD resisitor would like a whole ampere.
> http://dw.ham.dmz.ro:8080/downloads/yaesu/ham.dmz.ro.sch_ft-817.zip
Cheers, gottit now.
And then from Neil ZL1ANM:-
> The MIC jack is J1014. Looking at the '817 circuit diagram, we see
> that regulator Q1088 supplies +5v to multiple stages and to the 5v
> MIC pin (pin 6) via 10-ohm 1/16W resistor R1414.
>
> It's not possible to calculate the loading on Q1088, and for this
> reason I wouldn't attempt to draw more than 10, or at most 20mA
> from pin 6. Resistor R1414 should be OK at 20mA, since it will be
> dissipating only 4 milliwatts. You should easily be able to design
> a pre-amp that consumes 10mA or less. Aim for the least.
Sounds more than reasonable - thanks Neil.
And all I wanna do is improve the AM tx mod.
Plenty of AM left on 160/80m, and quite a few of use it on VHF/UHF
just because we feel like it ;-)
- Andy -
G0FTD @ GB7PZT
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