[Elecraft] Adjusting Samlex PS for proper battery charge voltage

[Don Wilhelm]

Folks,
Don (the other one) is correct here - the diode drop is from the same diode
in both cases - I didn't have the correct schematic orientation in my head
when I wrote that.

There is one other consideration to be aware of - the voltage should be set
with a fully or almost fully charged battery because of there is still some
resistance in the wires and connectors coming from the power supply.  Even a
0.1 ohm resistance is significant and not abnormal - (a 5 ft. length of #16
wire = 0.04 ohms and for #18 is 0.06 ohms - gotta count both wire lengths!).
If the battery voltage is at 12 volts, and the voltage on the input side of
the 6.2 ohm resistor is 13.8 volts, the battery circuit will draw 290 ma.
and the K2 in receive will draw about 240 ma or so.  The total current from
the power supply is 530 ma. and the voltage drop through the 0.1 ohm
resistance will be 0.053 volts.  While that is not likely a problem, if the
resistance is 0.2 ohms (likely for some wires and connectors), we end up 0.1
volt too high because the battery was not fully charged when we made the
measurement - and if the battery terminal voltage is 11 volts rather than 12
(not unusual), the error is greater still.

73,
Don W3FPR

----- Original Message -----
From: "Don Brown" <DolfinDon@msn.com>
To: <elecraft@mailman.qth.net>
Sent: Saturday, August 02, 2003 12:16 PM
Subject: Re: [Elecraft] Adjusting Samlex PS for proper battery charge
voltage


Hi

The battery blocking diodes voltage drop does not come into play in this
circuit only D10 in the K2. When charging the internal battery the battery
blocking diode is reversed biased so does not conduct. The charge current is
limited by the 6.2 ohm resistor across the diode. The blocking diode in the
K2 (D10) is in series with the power supply and the battery so the drop
across it will show up on the display. The battery will charge up to the
applied voltage less the diode drop in D10. The early <3000 s/n K2's had a
standard diode in D10 so the drop is .6-.7 volts. The newer >3000 K2's have
a low drop diode with a .2-.3 volt drop so the power supply will need to be
adjusted slightly different for the older or the newer radios. The A to B
mod kit also changes out D10 to the low drop diode. The easiest way to set
the power supply to the correct charging voltage is to measure the voltage
at the aux 12 volt connector. So you don't short anything out connect the
negative lead to one of the RF board grounds and carefully touch the
positive probe to the back of the red wire crimp terminal on the battery
plug connected to the AUX 12 volt connector. Adjust the power supply for a
reading of 13.8 volts. At this point you will read the charging voltage
including the drop of D10 but not the drop across the limit resistor because
of any current into the battery. If you don't want to go to all this trouble
then just adjust the power supply to 14.4 volts for the older K2's or 14.0
for the newer measured at the power connector for the K2. Then tap display
and read the voltage on the K2 display. Make note of the read voltage and
always set the power supply to that voltage.

Don Brown
KD5NDB