Sunday, 7 February 2021

QRP Labs QCX Mini - Part 1: replacing the 5 Volt regulator

Many folks in the amateur radio community may already have heard of the latest wonder from QRP Labs: The QCX Mini! An amazingly small and versatile mini CW - only transceiver for amateur radio. Read all about it here:

QRP Labs Kits website

Nice! So why this article? You definitely don't need another 'putting together the QCX Mini' article; there's enough of them already and the official building instructions are high-quality; they leave nothing to chance.

The story of the fragile QCX Mini Voltage regulator

Although most builders seem to have had no problems so far, some have reported failure of the 5Volt voltage regulator. When this part fails, all 3.3Volt and 5Volt parts will receive the full power supply voltage. Needless to say this causes the magic smoke to escape from many parts inside, some of which are very small and hard to reach and replace...

There's fixes, and then there's fixes

The cause was quickly identified by Hans Summers, the owner of QRP Labs and designer of this kit. It appears that a inadequate capacitor was placed on the board by the manufacturer, leading to excessive ringing when power was interrupted in quick succession. The fix was to add a capacitor over the regulator power input like this:

This takes care of the issue without having to replace SMD parts. The diodes in the picture were added by me. They are a pair of 5.6 Volt zeners that will hopefully act as a 'crowbar', tripping the power supply fuse if the output voltage ever rises to dangerous levels.

You do have a fuse in the power supply line, right? RIGHT???

The problem with this?

It still leaves the original AMS1117 regulator in place, which has a maximum input voltage of 18 Volts (absolute maximum input). This left me a bit uncomfortable, as I was planning to use a 4-cell LiFePo battery which puts out over 15 Volts when freshly charged. That's a bit too close for comfort for me.

Several QCX Mini builders have therefore replaced the AMS1117 with a more sturdy / reliable part, usually of the 78M05 variety. The 78M05 is available in SOT223 and TO252, both of which will fit with minor modifications. I only had the bigger brother in stock unfortunately, the 7805 in TO220F. But hey, proper tools will make everything fit :)

Done! A bit more tweaking was necessary to make the 7805 fit:

The pin layout for 78xxx type regulators is different from the AMS1117, so to make the part work the connections will need to shift to the left:

 The leftmost pin is then connected to the original input protection diode:

Last but not least, a 1uF tantalum capacitor is added across the input pins (the 7805 requires at least 330nF according to the datasheet). The original SMD cap (C38) I've removed as it apparently was rated for 10 Volts so why take the chance?

And it all fits! More importantly, the 7805 has a 35 Volts maximum input voltage, which is enough to end my worries.

Do you need to do this?

Absolutely not! Many builders are using the QCX mini with only the recommended extra 10uF capacitor fitted and as far as I know that's working fine. Also, only the first batch of Mini's will benefit from this mod. Later revisions will have an 78M05 fitted as standard.
This article was created as a guide only for OCD sufferers. Like me :)

Are you missing anything in this article? Need more details? Please drop me a line below!

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