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Author Topic: Logikey K3 Keyer PS question (LM340 regulator)  (Read 982 times)

Posts: 42

« on: March 23, 2002, 06:07:02 PM »

I purchased a Logikey K3 keyer from another ham. All I got was the keyer (no manual, power supply, etc.). No biggy, the rear power connector is labled 12 v and there is an LM340 (5v) used as a voltage regulator at the input.

In the meantime I've been in contact with the company (Idiom Press). They're being VERY adamant that I should NOT use a 12v wall wart unless it is REGULATED and FILTERED (they're emphasis, not mine). Instead, I should only use an external, regulated 12VDC power supply ("Do you have a 12 volt supply in your shack, perhaps to power a 2 meter rig or the like? That would be ideal").

Seems like overkill to have to power a simple TTL (albeit microprocessor) based CW keyer with a big 'ol external PS to me... especially since the input is already regulated via the LM340. Is there any reason to be concerned the LM340 can't handle regulating voltage/current from a decent wall wart? Before I got the warnings I already hooked a 12v 500ma wall wart to it and it seems to work just fine (although not yet connected to the rig). Is there something else I'm missing here?  Inquiring minds want to know!

Posts: 3585

« Reply #1 on: March 23, 2002, 06:41:42 PM »

HI: I SUSPECT the mfr's concern is the notoriously high percentage of wall warts that put out far more than their "label voltage," or that have no filtering at all.

Too much voltage can certainly damage a circuit, and there's no telling what rectified but unfiltered DC would do in a keyer. Give you six dits instead of two, perhaps.

While I have several warts that are close to spec, every one came with a fairly high priced piece of gear. An HT, a frequency counter, or something of that sort.

And I have a box full that are 25 to 75 percent above their label voltage. Like this one labeled 12.6 VDC. 117 V in gets 18.8 VDC. Pretty well regulated, too. This one says 6 VDC, and it puts out 10.7 VAC and no DC! And so on for another ten or twelve of the things.

IF you actually have a wart that puts out well filtered 12 V DC, measured, I would use it. If not - I would be cautious and hook it to a good source of filtered DC.

73  Pete Allen  AC5E


Posts: 8

« Reply #2 on: March 23, 2002, 08:10:12 PM »

I agree with the other posts. Many wall warts are rated at the voltage and current indicated on the wall wart. If you lightly load the wall wart it will probably put out a lot of AC on top of the DC (if you want to call it DC).
If you got a scope, prove it to yourself and connect it up without a load and you'll notice that the output is ripple with 1.4 times the rated DC rating.
You could always put some more rectification and a big cap before the LM340. I do and it works out fine.
Since the Logikey probably only needs about 10-30 ma, you won't have to worry about a very big cap.
Have fun, hope this helps

Posts: 17418

« Reply #3 on: March 25, 2002, 11:49:56 AM »

It doesn't sound like you will be stressing the LM340 regulator -
most are rated up to at least 30 volts input.  But there could be
an electrolytic capacitor or some other circuitry across the input
line that can't handle over 15 volts or so.  (Also, the LM340 will
dissipate more heat when the input voltage is higher, but at this
low current level it is difficult to imagine that being a problem.)

One simple approach is to measure the actual output of each
wall wart under the expected load current.  Sounds like a 270 ohm
resistor is about right to simulate the load.  Check the output on
a scope if you have one, as some don't have any filtering.  And
you may find that a wart rated at 9V gives closer to 12V at this
load level.

I am using a wall wart from a battery charger with my keyer.  Had
all sorts of problems at first with extra dots.  Then I added a
1000uF capacitor across the line and it cleaned things up nicely.
Turns out the wart had no filtering after the rectifier, so the output
went to zero volts 120 times per second.  The keyer seemed to
interpret this as the key being closed.

As long as you make sure there is some filtering, and check the
output voltage (AFTER adding the filtering capacitor), then the
wall wart should work for you.
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