[Elecraft] K3-Front panel shut down question.

[Vic K2VCO]

There will be about 0.6v drop across each diode. That won't be a problem for the power 
supply. It will reduce the amount of time the battery will operate the K3 a bit, but the 
K3 will work down to a pretty low voltage to start with.

Obviously the diodes have to be rated properly and heatsinked if necessary.

Float-charging the battery is nice, but you have to be very careful. Too much voltage can 
destroy the battery, too little will shorten its life. There is also the problem of 
producing hydrogen if it's not a sealed battery, etc.

I just wanted to provide the simplest solution possible for the original poster's question.

On 8/17/2011 6:17 PM, Fred Townsend wrote:
> Vic, what you are describing is called a diode 'OR' gate. That works well
> with logic but it has some problems with power. If the diode is not ideal it
> will drop voltage across the diode. That can amount to considerable power
> loss and destroy the diode if you are not carful. That's why you see such
> devices sold on heat sinks (called battery isolators). A better solution if
> you can set the power supply to the float voltage of the battery, then float
> the battery across the power connection to the K3. This has a bonus that the
> battery acts as a filter.
> Remember the power supply voltage must match the battery float voltage which
> is around 13.85v +/- 50 mv.
>
> 73
> de Fred, AE6QL
>
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Vic K2VCO
> Sent: Wednesday, August 17, 2011 5:12 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3-Front panel shut down question.
>
> You could connect a battery and a power supply to the K3 power input, if you
> put a diode in series with the battery and one in series with the power
> supply. The cathodes of the diodes would go toward the K3. Then as long as
> the voltage of the PS was greater than that from the battery, the power
> would come from the PS. If the PS went off it would come from the battery.
> The diodes would prevent any current from flowing between the PS and the
> battery.
>
> Of course a UPS is an idea, but many UPS' generate a lot of radio noise.
> After all, they contain inverters, which are switching devices.
>
> On 8/17/2011 3:12 PM, kf4clo wrote:
>>    My K2 is set up with a power supply to the 100w connection and a
>> battery to the low power connection, If commercial power cuts off
>> without warning(and it does sometimes)the K2 stays on with battery
>> power.
>> I am looking for a way to do this with my K3 so I can at least shut
>> down from the front panel when this happens. I understand the K3 don't
>> like it when shutdown other than the front panel. Maybe a battery
>> between the power suppley and the K3?(or is there a better way).
>
> --
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/