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QRP 12V Power Pack

A perfect companion for your Elecraft K1 or any other (not power hungry...) QRP rig

Here is a very useful device when you are portable with your K1, it works, automatically, in these two modes:

- With a 12V power connected to the input, this power is supplied directly to the rig and used to charge the battery with a current of about 130mA at maximum; the 12V source can be leaved connected for hours or days because the charger will supply no more than 13.8V.

- Without a 12V power connected, the battery is used to power the rig

I built mine using a perf-board (in Italy, we call it "thousand holes") and the parts fits perfectly in a commercial metal box, as you can see in these pictures (click to enlarge the pictures).


Here follows few notes:

- Don't connect the battery until you have checked the circuit and adjusted the trimmer!!!

- The DC-DC converter must be ISOLATED and it must convert 12Vdc to 5Vdc with, at least 250mA and it is used to boost the 12V to 17V. I used a Traco TEM 2-1211 (2W - 5V / 400mA)

- The LM317T must be on a ML24 or similar heat sink

- As NPN and PNP transistor you can use any generic TO92, like 2N3904 and 2N3906

- I added a resettable fuse, the Polyswitch RXE160, to limit the current at 1.6A and protect the battery or the power supply.


The trimmer is a multiturn cermet type and it must adjusted in this mode (after you plug 12V to the input jack, verified that no smoke or bad smelling is present and  that you have 17V on LM317 input...):

- Don't connect the battery and put a 1.2 KOHM resistor in place of battery

- Put a digital multimeter across the resistor and turn the trimmer until you have 13.8V

- Remove the resistor and connect one 9.1V / 3W zener (or two 5.1V / 1.3W zeners in series) with an ammeter in series and check that the current is limited to 100-150mA. Remove the zener and ammeter and connect the battery

- You can adjust it better: after you have connected a fully charged battery, turn the trimmer until you have 13.8V across the battery


You can use this circuit with more powerful batteries and you can sink more current, the actual limits are:

- 1.6A is the maximum current of  RXE160, you can increase the supplied current changing this resettable fuse with a bigger one

- 400mA is the maximum current of TEM 2-1211 and about 120mA is the limited current of the circuit, you can increase the charging current changing the 4.7 OHM resistor with a smaller one (for example, 1.5 OHM for 400mA) but you must use also a bigger heat-sink to dissipate the increased power


Sorry for these very scarce details and notes (and for my bad English...), if you have any question or need some help write me...

Ciao and good work

Guido, ik2bcp / iu2r / ab9dg


Any question and/or comment to ik2bcp AT