Yet another article (with pics) on QCX Mini regulator replacement #qcxmini #magicsmoke


PA1DMG
 

Hi,
I've created a blog post on how to replace the QCX Mini AMS1117 regulator (I've used a regular 7805 but the same procedure also applies to the 78M05 in SOT223 or So-252).

QCX Mini - Replacing the regulator

It's part of a series of posts, part 2 and 3 are here:

Part 2 - adding some cooling

Part 3 - building a power pack

I'm sure there will be more to add in the near future. These blog posts are not meant to be full tutorials, but I hope the information will help readers somewhat.

73,
Marc PA1DMG


Bob M.
 

Very nice "cooling" article. You probably should put some thermal compound between the FETs and the large solder pad on the circuit board, as that's the primary heat sink for those transistors. The washer just holds the FETs in good contact with the board and has minimal contact area due to the rounded backs of the FETs.


Hans Summers
 

Hi Bob

I agree... most people seem to think the washer is the heatsink but it is not. It is just the mechanical means to force the flats of the FETs against the PCB, which is the real heatsink. Thermal compound is harmless (as long as it is not electrically conductive!) but also pointless. The improvement in thermal conductivity is negligible in comparison to the thermal resistance from the BS170 junction inside the lump of plastic, and the exterior surface of the said lump of plastic. 

Marc PA1DMG did think well though, I noticed the heatsink has rounded grooves to match the body of the BS170s and get good thermal contact with them. So it looks like a nice mod! Though as Marc even himself suggests... perhaps not necessary :-D   

I still think anyone pushing a QCX-mini past 5W is going a bit wild...

73 Hans G0UPL
http://qrp-labs.com



On Mon, Feb 8, 2021 at 4:11 PM Bob M. <wa1mik@...> wrote:
Very nice "cooling" article. You probably should put some thermal compound between the FETs and the large solder pad on the circuit board, as that's the primary heat sink for those transistors. The washer just holds the FETs in good contact with the board and has minimal contact area due to the rounded backs of the FETs.


PA1DMG
 

Hans, Bob,
Thanks for the kind words!
The Cooling mod article is not meant to be a serious suggestion to improve the QCX Mini of course. But hey, the kit is cheap enough to experiment on and try out some things, even if their effectiveness is questionable.
Although English is not my first language, I did try to make my readers see the somewhat silly nature of the experiment :)

73,
Marc PA1DMG


ON7DQ Luc
 

Well done Marc!

and @G0UPL
You see Hans, I'm not the only one with a FUSE fetish, haha.

73,
Luc ON7DQ


ON7DQ Luc
 

Marc,

like minds have similar ideas it seems ;-)

I reported about my powerpack in this thread on the SOTA reflector ...
https://reflector.sota.org.uk/t/qcx-mini-part-2/24564/57

Only 3 cells in my case, but also a FUSE and an ON/OFF switch.

73,
Luc ON7DQ


PA1DMG
 

Hi Luc,

Brilliant enclosure that :)
BTW I totally agree on the fuse. Trust nothing, and leave nothing to chance. I mean, these same cells are also used in DIY spot welding devices. That should tell you something about the currents they are able to deliver. They're quite capable of burning a hole in your desk (or worse).


Chris Shaker, KJ7BLE
 

Yeah! That is exactly what I was suggesting. I am happy to see that it actually fits into the space available. I will be doing this same mod on the two kits I have to build.

Thank you for sharing this!

Chris Shaker, KJ7BLE

 


Chris Shaker, KJ7BLE
 

Sorry for being stupid. Should have read the blog post before I replied.

I had suggested using the through hole 7805, but flipping it over, so that the pinout matched, without having to shift it over.

Thank you for your excellent blog post.

Chris Shaker, KJ7BLE

 


Bob - K2KI
 

I have pigtails on all my gear with whatever the equipment takes for a connector on one end and Power Poles on the other end. In between, I have a fuse holder. For the smaller equipment, I use the holders that take the small Automotive "Blade" type fuses (Properly rated of course). For the larger equipment, I use the heave duty screw together fuse holders that take AGC style/type glass fuses.
So, I guess I am a little OCD about fusing and grounding too. :-)

--
73/72 . .
Bob de k2ki
k2ki@...
k2ki.bb@... (Goes to my cell)
Grid: FN34LW
NEQRP#: 911
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FP#: 4214
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On 2/8/2021 1:08 PM, ON7DQ Luc wrote:

and @G0UPL
You see Hans, I'm not the only one with a FUSE fetish, haha.
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Hans Summers
 

Hello to all those with fuse fetishes, proud members of the Society Of Safe operating (SOS operating), fuse-o-files and fuse-a-holics... from the deepest darkest charred ashes infested wilderness of the ignorant fuse-o-phobic, fuse-what-fuse-anonymous... I have a question... 

I suppose we all agree that the fastest fuse is the one on three legs (transistor) and that a fuse doesn't do much (if anything), to protect the semiconductors in our equipment. 

My question is... if I were ever tempted to leave the dark side and seek the light... why would you put fuses in the devices themselves? Wouldn't it make more sense to put a fuse in the power supplies? Since the number of devices may be expected to usually exceed the number of power supplies, and most power supplies already contain a fuse... there would be some economies on this approach. Secondly the fuse would protect against a short in the cable between power supply and device, thereby providing more protection than having a fuse in the device? Or is it a just a case of: we love fuses, the more fuses the better, you can never have too many fuses, if one doesn't blow then another will, belt and braces, let's not take any chances?

73 Hans G0UPL


On Tue, Feb 9, 2021, 02:26 Bob - K2KI <k2ki@...> wrote:
I have pigtails on all my gear with whatever the equipment takes for a
connector on one end and Power Poles on the other end. In between, I
have a fuse holder. For the smaller equipment, I use the holders that
take the small Automotive "Blade" type fuses (Properly rated of course).
For the larger equipment, I use the heave duty screw together fuse
holders that take AGC style/type glass fuses.
So, I guess I am a little OCD about fusing and grounding too. :-)

--
73/72 . .
Bob de k2ki
k2ki@...
k2ki.bb@... (Goes to my cell)
Grid: FN34LW
NEQRP#: 911
4SQRP#: 2292
FP#: 4214
SKCC#: 12195
NAQCC#: 2337
100WattsID#: 4675

On 2/8/2021 1:08 PM, ON7DQ Luc wrote:
>
> and @G0UPL
> You see Hans, I'm not the only one with a FUSE fetish, haha.


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Alan G4ZFQ
 

I suppose we all agree that the fastest fuse is the one on three legs (transistor) and that a fuse doesn't do much (if anything), to protect the semiconductors in our equipment.
Hans,

Yes.
And several times I've noticed my PSU that continuously powers various small items like U3s and Softrocks is running very hot due to a short in one of those connections.
I have fuses in those connections but the PSU devices go into current limit before they blow.
Many fuses, especially general purpose automotive ones will not blow until their rating is greatly exceeded for a relatively long time.
I have intended to fit foldback limiting, maybe one day..

73 Alan G4ZFQ


Zdenek OK2BQN
 

Hello everyone,
I give TRANSIL to all my DC sources. Then, even if the voltage increases, the fuse will blow. On the test PCB, I simulated the destruction of the stabilizer and the operational amplifier behind the transil survived. Therefore I put TRANSIL 5V6 (measured 5V4) on output NCP1117ST50T3G (1A, 18V). But maybe my test was a coincidence and it won't help. Only time will tell. So far, I have made many connections with the 16V power supply and everything seems OK.
73
Zdenek


PA1DMG
 

Hi Hans,

I've been in the electronics repair business for for over 25 years and I've never seen a fuse fast enough to protect electronic components. However, a properly rated fuse will protect internal and external wiring, the PCB itself, you, your desk and/or your house. I'm hardly a Fuse-o-Phile (great name btw!) but just one fuse can make all the difference.
Where to put that fuse? No matter where you put it, there will always be people who will try to bridge it or stick a 20 amp fuse in, so in QCX - type projects I'd put it in the DC power supply line (or add a fuse holder on the QCX+ board). Specify a sensible recommendation in the manual (I went for 1.25AT for the QCX Mini) and that's it.

That's my $0.02. Now let the Fuse wars begin! :)

73,
Marc PA1DMG


David Poole
 

Hans, a fuse is to protect wiring if too much current is drawn, so you don't burn the place down. A fuse on a power supply will protect the wiring of the supply. You can imagine a 20amp power supply can safely carry 20amps. A small item or items connected to the power supply may draw only a small amount of current, maybe 1 amp or less, so a 20 amp fuse on the power supply will let that item burn to the ground with only 2 or 3 amps flowing and will never blow the 20 amp fuse on the power supply. So small rated items must be fused with a fuse rated to protect them. If it normally runs at 1amp then maybe a 1.5 amp fuse will blow before the whole thing bursts into flame. If you had 2 or 3 small items all powered from the large supply and they all failed with 5 amps flowing, the 20 amp fuse in the supply would not blow with power still running to the faulty items and probably catching fire.
Dave
VK5PL


Roelof Bakker
 

Alan,

You might have a look at polymere self-resetting fuses.
These don't break the circuit, but develop a high resistance when a short happens.
It limits the current and protects the power supply.

I have been using them for ages and never have to bother for spare fuses when operating portable.
Actually, that was when I learned to appreciate them.

73,
Roelof, pa0rdt


Alan G4ZFQ
 

You might have a look at polymere self-resetting fuses.
Roelof,

Thanks for reminding me. I did get some 800mA ones but have not yet remembered to test them to see how practical they are.

I'm very slow to do things here, not supercharged like Hans!

73 Alan G4ZFQ


Steven Dick
 

If it were me, the first step would be to review components and select them to fail in a benign condition (i.e. fail open rather than fail short) if failing short could cause major damage down stream. Otherwise I would consider a low cost solid state circuit for a fold-back current limiter. It adds complexity and may have unacceptable voltage drops for simple circuits, but if it saves having to rework multiple components and even the PC board it is sometimes worth it.  Fuses are often too late to save what's past them, including the resettable type fuses. In complex systems, fusing must be distributed to properly fuse subsystems at the lower level.  In my former life, I've seen some serious damage caused by inadequate fusing or current limiting coupled with a failed component that failed short in a mission critical application.

A simple, commonly used current limiter consists of two transistors and two resistors but has a voltage drop of 1.5 volts (see article below).  This can be improved a bit with component selection.

Here's an article on current limiters just to generate some creative juices flowing:

https://www.electronicdesign.com/power-management/article/21801456/current-limiter-offers-circuit-protection-with-low-voltage-drop

-Steve K1RF.

------ Original Message ------
From: "Hans Summers" <hans.summers@...>
Sent: 2/9/2021 3:08:04 AM
Subject: Re: [QRPLabs] Yet another article (with pics) on QCX Mini regulator replacement #qcxmini #magicsmoke

Hello to all those with fuse fetishes, proud members of the Society Of Safe operating (SOS operating), fuse-o-files and fuse-a-holics... from the deepest darkest charred ashes infested wilderness of the ignorant fuse-o-phobic, fuse-what-fuse-anonymous... I have a question... 

I suppose we all agree that the fastest fuse is the one on three legs (transistor) and that a fuse doesn't do much (if anything), to protect the semiconductors in our equipment. 

My question is... if I were ever tempted to leave the dark side and seek the light... why would you put fuses in the devices themselves? Wouldn't it make more sense to put a fuse in the power supplies? Since the number of devices may be expected to usually exceed the number of power supplies, and most power supplies already contain a fuse... there would be some economies on this approach. Secondly the fuse would protect against a short in the cable between power supply and device, thereby providing more protection than having a fuse in the device? Or is it a just a case of: we love fuses, the more fuses the better, you can never have too many fuses, if one doesn't blow then another will, belt and braces, let's not take any chances?

73 Hans G0UPL


On Tue, Feb 9, 2021, 02:26 Bob - K2KI <k2ki@...> wrote:
I have pigtails on all my gear with whatever the equipment takes for a
connector on one end and Power Poles on the other end. In between, I
have a fuse holder. For the smaller equipment, I use the holders that
take the small Automotive "Blade" type fuses (Properly rated of course).
For the larger equipment, I use the heave duty screw together fuse
holders that take AGC style/type glass fuses.
So, I guess I am a little OCD about fusing and grounding too. :-)

--
73/72 . .
Bob de k2ki
k2ki@...
k2ki.bb@... (Goes to my cell)
Grid: FN34LW
NEQRP#: 911
4SQRP#: 2292
FP#: 4214
SKCC#: 12195
NAQCC#: 2337
100WattsID#: 4675

On 2/8/2021 1:08 PM, ON7DQ Luc wrote:
>
> and @G0UPL
> You see Hans, I'm not the only one with a FUSE fetish, haha.


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