[Elecraft] K-2 Power Supply

Wed May 5 09:47:01 2004

Hi

The K2 power input circuit has a low drop blocking diode (D10) in series =
with the radio and the battery. The battery circuit has a 6.2 ohm =
limiting resistor for charging and another low drop diode in parallel =
with the resistor that provides full power to the radio. To provide the =
13.6-13.8 volts required to float charge the SLA battery the input =
voltage needs to be about 14-14.1 volts. The diode will drop about .2-.4 =
volts and the resistor will limit the charge current if the battery is =
discharged to a low level.=20

Because of the above described circuitry many smart battery chargers or =
monitored chargers will not work when connected to the K2 power jack. =
Also many of these chargers supply unfiltered DC for the charging =
current and will cause hum on the receive and transmit of the K2 if the =
radio is used while the battery is charging. If you are going to use one =
of these chargers you must provide a fused connector on the K2 that =
connects directly to the battery so the charger can sense the correct =
terminal voltage on the battery.

The simplest solution is to use a well regulated and filtered DC supply =
adjusted to 14.0. This will allow the battery to be float charged with =
no damage and the radio to operate with the optimum supply for maximum =
power out. The K2 will automatically switch to the battery if the power =
fails. A good Astron, MFJ, Radio Shack or equivalent adjustable power =
supply will do the job. Some of the supplies have a front panel adjuster =
others will have the adjustment inside.=20

One other note is the K2 can operate from a much smaller power supply =
than you might think if it has the battery option. You could use a .5 to =
1 amp 14 volt power supply made from a transformer, rectifier, filter =
cap, a three terminal regulator like a LM117, LM350 or LM338 and a =
couple of resistors. The ARRL Handbook has several supplies you could =
home brew using this setup. The power supply would keep the battery on =
float and the battery would supply the large current for transmit when =
needed. With the low duty cycle of transmitting the power supply could =
keep the battery charged even running 10-12 watts. Another option is a =
solar panel and controller connected directly to the battery through a =
fuse and connector on the back panel of the K2. Now that the transverter =
jacks have been moved to the lower rear panel this leaves the upper =
transverter holes available for use as a power jack to connect to the =
battery.=20

Don Brown
  ----- Original Message -----=20
  From: Kenneth Cooperstein<mailto:[email protected]>=20
  To: [email protected]<mailto:[email protected]>=20
  Sent: Wednesday, May 05, 2004 7:35 AM
  Subject: Re: [Elecraft] K-2 Power Supply

  >Thom R. Lacosta wrote:
   >Can the output voltage be changed to something above that conforms=20
  with the voltage required to charge the internal gel cell?

  I offered a tiny 12.0 VDC switcher and Thom asked the above question.  =

  The answer is Probably, but you would have to destroy the case to open =

  it up.

  I see from the K2 manual that you need to apply 14.0 VDC at the jack =
to=20
  charge the optional internal SLA battery.  Does the K2 have circuitry=20
  that regulates this voltage down to float level (13.4 VDC) when the=20
  battery has been fully charged?  If not, you will ruin the battery if=20
  you don't closely monitor the charging.

  <SNIP>

--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
The reason this message is shown is because the post was in HTML
or had an attachment.  Attachments are not allowed.  To learn how
to post in Plain-Text go to: http://www.expita.com/nomime.html  ---