[Elecraft] Re: 12 VDC Output

[wayne burdick]

Joe,

We just measured the voltage at the 12V ACC output on three K3s. DC 
into the K3 APP connector was 13.80 in all cases (measured at the APP 
connector, K3 powered ON, in receive mode).

No load output:  13.1 VDC in all cases.

Loaded output (500mA): very close to 12.0 VDC on all three units 
tested. It should be a lot higher at the typical current required for 
one relay (< 100 mA).

So the total drop at maximum rated load is about 1.8 V. Circuit drops 
include two series protection diodes, two self-resetting fuses, and the 
PMOS FET on/off switch (Q2). These should not be bypassed or removed, 
as that would compromise the safety of the ACC output.

You could replace D34 (1N4007) with a 1N5817/18/19 or SB520/530 to get 
a few tenths of a volt lower drop if required. You could get another 
couple of tenths by replacing both D6 and D34 with 95SQ015's, although 
they're very expensive.

Sounds like OM7ZZ's output was loaded too heavily, or that he had the 
radio switched off when measured it, which would provide only leakage 
current. OTOH he could have tripped F2, the self-resetting fuse for the 
ACC output.

73,
Wayne
N6KR

> What is the spec for the 12 VDC accessory output?  OM7ZZ ran into
> some issues in interfacing with the K3 from the VP6DX group because
> the accessory output was only 10.8 volts even with a well regulated
> 13.8V supply to the transceiver.
>
> 10.8 would be marginal depending on cable resistance for a remote
> relay and is certainly too low for most of the commercial antenna
> switches that use 12 V relays.
>
> 73,
>
>    ... Joe, W4TV
>
>
>

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