Spannungsregler beim QCX Mini - Update zu C38

Hallo zusammen,

es gibt neue Infos zum Thema Spannungsregler beim QCX Mini:

Offensichtlich wurde bei der Fertigung der Leiterplatten durch die Bestückungsfirma die Stücklistenspezifikation von C38 nicht beachtet - es liegt wohl definitiv ein Fertigungsfehler vor.

An Stelle des spezifizierten 25V Kondensators wurde versehentlich ein 10V Typ verbaut. Die Kapazität eines X5R-Kondensators ist spannungsabhängig, die Kapazität verringert sich mit steigender angelegter Spannung. Daher ist Hans Summers nun der Meinung, dass die Kapazität von C deutlich unter 10uF liegt und daher nicht ausreicht, um den Spannungsregler 1117 zu schützen. Er rät davon ab, den 1117 auszutauschen, da dies normalerweise nicht nötig sei und gewisse Risiken für die Beschädigung der Leiterplatte birgt.

Allerdings gibt es gerade die Überlegung, bei der nächsten Fertigungsscharge einen 7805 an Stelle des AMS1117 einzusetzen um bei der Eingangsspannung mit etwas Reserve auf der sicheren Seite zu sein.

Hier die Original-Info von Hans Summers:

Hi all

I thought you may be interested to see this photo attached. I dug into the records of the assembly facility which soldered the PCBA in the QCX-mini, and managed to get the actual photo of the reel that was installed in the machine for the QCX-mini assembly. It's very nice that such photos are captured and retained.

Note the manufacturer Samsung (should be OK) and the part number CL21A106KPFNNNE. You can find the datasheet for this capacitor here: https://datasheet.lcsc.com/szl…CL21A106KOQNNNE_C1713.pdf

At page 4 you can see an explanation of the part code from which we can see:

• CL = Multi Layer Ceramic Capacitor series
• 21 = size, SMD 0805
• A = Class II dielectric X5R
• 106 = Capacitance 10uF
• K = Tolerance +/- 10%
• P = rated voltage 10V
• F = Thickness code 1.25mm
• N = normal product Ni/Cu/Ni battier/ Sn 100% termination plating
• N = Normal product code
• N = Control code (reserved for future use)
• E = Packaging code, Embossed 7" reel

What should certainly be noted is the 10V rating of this capacitor. An X5R capacitor exhibits capacitance dependent on applied voltage, capacitance DECREASES as the applied voltage increases. This capacitor is already beyond its rated 10V and therefore the capacitance would be expected to be significantly under 10uF, and therefore inadequate to protect the '1117 voltage regulator.

The manufacturing error was therefore at the assembly facility who did not follow my BOM specification (25V rated capacitor) and used this lower voltage capacitor reel.

I also believe this further reinforces with a sound theoretical reasoning, the recommended solution of fitting a 10uF electrolytic or tantalum capacitor at the voltage regulator input. It remains the case that there is no evidence to suspect that the AMS1117-5.0 voltage regulator is at fault or is in need of replacement, and replacement is NOT recommended since it will risk damaging the PCB unless you are quite experienced in such matters.

73 Hans G0UPL

http://qrp-labs.com

Hallo Dietmar,

laut offizieller Stellungnahme von QRPLabs kann C38 drin bleiben, siehe mein nächster Beitrag

Hello Evan, all

I have done some detailed testing of this 10uF 10V MLCC capacitor...

First of all, this is a good quality MLCC 10uF capacitor, made by Samsung. I have used Samsung Galaxy phones for the last 5 years so I like Samsung. But this is merely coincidence. Anyway. Here's what I found.

1. Capacitance vs Voltage

Refer to the attached graph which shows my measured capacitance vs voltage for each of the three capacitors under consideration:

• 10uF 10V MLCC Samsung 0805 SMD, which is installed on the board.
• 10uF 16V electrolytic through-hole, used in the QCX+ and QCX kits, and which is a recommended modification capacitor
• 10uF 35V tantalum through-hole, which will be supplied with some QCX-mini kits and is a recommended mod capacitor

Experimental method: I wired a tactile push-button switch in series with the cap, and a 100K resistor across the capacitor; then push the button to charge the capacitor (from a variable voltage supply) then measure the voltage decay on my oscilloscope and use the oscilloscope cursors to measure the time required for a 10% decay in voltage. Use the formula for capacitance voltage decay to calculate the capacitance. Repeat for various supply voltages and plot the graph.

The notable result is that the 10uF electrolytic and tantalum capacitors measure very close to 10uF and show little if any variation with supply voltage. What slight variation there is, could be measurement error. The MLCC 0805 SMD 10uF 10V capacitor shows an enormous variation in capacitance with voltage; at 14V supply the capacitance is 1.3uF. I believe this is normal and the rated "10uF" value is the small-signal value.

For this application of a capacitor at the supply voltage input to the voltage regulator, if an MLCC were to be used then the rated voltage ought to be many times higher than the operating voltage. A more suitable capacitor for this position in the circuit is electrolytic or tantalum.

2. 10uF 10V 0805 Samsung SMD capacitor failure mode and voltage

I applied voltages up to 31.6V (the upper limit of my 30V 5A linear variable bench power supply), each time for several minutes (steady) and rapid pulsed on/off operation and I could not force this capacitor to fail at all. I also found in the capacitor datasheet some mention of "withstand voltage" testing at 2.5x the rated voltage.

I do have two PSUs so theoretically I could wire them in series and continue up in voltage to try and force failure. But given 31.6V is so so far above the likely operating range of the radio I think this is unnecessary and would cause some considerable complication in my lab setup here (e.g. is it correct and safe to wire the two PSU outputs in series etc).

It leads me to the unprovable conclusion that the capacitor is quite unlikely to fail at normal QCX-mini voltages in the 12-14V range (or even outside that).

I know you will say that if it is operated beyond its specified rating then its lifetime will be reduced... I have no way of testing that in a reasonable timescale, but I think the fact it doesn't fail at 2.5x the voltages we want to use it at, provides a significant and sufficiently safety margin.

3. AMS1117 voltage regulator failure voltage

During this testing I also had an AMS1117 voltage regulator IC connected to the same supply voltage. It was connected permanently, not downstream of the push-button that was used for the capacitor experiments. As I gradually ramped up the voltage for the 10uF 10V capacitor capacitance value measurement graph, I also observed the state of the AMS1117. At 28V, the AMS1117 failed. I imagine it would fail at lower voltages under pulse conditions such as power-on. The failure mode was as usual, a short-circuit which is nasty, but I don't think it is unique to the AMS variant of the 1117.

4. AMS1117-5.0 source

I also investigated with the PCB A and supply chain and was informed that the AMS1117-5.0 was procured from a reliable and reputable source, and has been used many times before in various PCBA without issue. I do not believe the AMS1117 is at fault here, it is not a fake or a poor quality device.

CONCLUSIONS

1. There is no necessity at all to change the AMS1117 voltage regulator. It is adequate for the job and I do not think there will be any problem operating it with supply voltages up to 15V or perhaps a bit more. Changing the AMS1117 is likely to incur risk of PCB damage. Changing it to a different brand 1117 will not solve the problem or provide any benefits.
2. The problem is clearly the 10uF capacitor C38. It isn't a bad quality component (Samsung), it's just the WRONG component. Somewhere in the supply chain the 10V rating was used and it should have been a 25V device. Even then, it is probably a design error to use MLCC at this point, it should have been an Electrolytic or Tantalum.
3. The recommended solution remains to install the 10uF electrolytic through-hole or tantalum capacitor in parallel with C38, and not to remove C38. C38 does not seem likely to fail so can be left in place. Removing C38 and replacing it with something else (should be electrolytic or tantalum), or replacing the voltage regulator with something else, will risk damaging the PCB and you will be in uncharted, untested territory. Do it if you want, it's your radio and we are all experimenters... but don't cry later if it doesn't go well

Executive summary:

Just add a 10uF electrolytic or tantalum and BE HAPPY.

73 Hans G0UPL

http://qrp-labs.com

Hallo zusammen,

hier noch eine gute Nachricht von Hans Summers, für alle die den 1117 nicht (oder nicht mehr, Hi!) mögen:

Wie ja Hans sehr akribisch dokumentiert hat ist nicht der verwendete 1117, sondern ein falsch bestückter Kondensator C38 die Ursache allen Übels.

Trotzdem wird ab der nächsten Produktionscharge der 1117 ersetzt durch einen 78M05 in einem TO-252 Gehäuse.

Dadurch wird zusätzlich etwas mehr Sicherheit erreicht, wenn versehentlich kurzzeitig mal eine etwas höhere als die spezifizierte Versorgungsspannung angelegt wird (was dann die Endstufe mit derzeit schon ca. 4,5 W bei 12V allerdings dann auch nicht lange vertragen dürfte).

Ich finde, das ist im Sinne einer Produktverbesserung eine sehr gute Lösung, die gleichzeitig eine schnelle Reaktionsfähigkeit von QRPLabs zeigt.