Posted by
Don Wilhelm-3 on
Jul 24, 2005; 7:05pm
URL: http://elecraft.85.s1.nabble.com/voltmeter-offset-and-accuracy-in-K2-tp380698p380706.html
Jim,
You are exactly right - it will read a bit high for all voltages. I don't
believe that would be a problem for most folks since they seldom use the
voltmeter for anything other than monitoring the K2 voltage. If you use the
K2 metering circuit in its "EXT" mode by changing the jumper and using a
probe, you will find those readings a bit high after making the change.
Proper compensation COULD be done in firmware - but the firmware would have
to know exactly the diode drop and that varies from diode to diode, and it
is not a good solution for those using the EXT voltmeter function either
because there is no input to the firmware to indicate when the voltmeter is
in the INT or the EXT position.
I hope I did not give the wrong impression - I am not advocating nor
recommending this as a change, I was simply responding to those folks who
wanted to know how to do it.
73,
Don W3FPR
> -----Original Message-----
>
> Please forgive me for putting on my "physics teacher hat" and doubly
> forgive the algebra which is to come :^))
>
> I'd argue against trying to recalibrate the K2 internal voltmeter to
> compensate for the diode drop in series with the supply. How come?
> Because changing the resistor values in the voltage divider is a
> multiplier correction, while the problem is an additive offset.
>
> The algebra:
>
> if the supply voltage is Vs and the metered voltage is Vm then
>
> Vm = Vs - Vf where Vf is the forward drop of a diode
>
> if you fiddle with the voltage divider you can get
>
> Vm = k (Vs - Vf) where k is the factor for the "new"
> divider.
>
> Now, for any particular Vs, and knowing that Vf is pretty much constant,
> you can choose a k that will make the meter read perfectly, that is Vm =
> Vs if k = Vs/(Vs-Vf)
>
> But if Vs changes, you'll need a new k. Granted, since Vs will probably
> vary over only a small range, say 11 V to 15 V, the difference won't be
> large. But ... the diference now won't be known, whereas before there was
> a known, predictable offset. The k value will only need to change by 2%
> or so, but 2% of 14V is a couple of tenths.
>
> You went from *knowing* the reading was about a half volt too low, to
> having a reading that could be a couple of tenths high or low depending
> on where in the range you are.
>
> What you really needed to do was add Vf to both sides:
>
> Vm (new) = Vm + Vf = (Vs - Vf) +Vf
>
> And that you can't do by changing the divider ratio. Could it be done in
> the firmware? Sure, just add in the requisite counts to put back the
> diode offset. But I'll betcha Wayne and Eric figured it was better to
> know the actual voltage the rig is seeing (after the protection diode AND
> any voltage drop in the power cord).
>
> OK, physics teacher hat off, QRPer hat back on.... If it ain't broke....
>
> 73
> Jim N5IB
>
>
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