The invisible kill-joy

Yesterday, around 3 o’clock in the afternoon, I had some time to finish the job with the MS-35 LED strip controller. The LED strips were installed in the hallway, the wires had been made almost invisible, the software side of things was ready, so all that was left to do was putting the MS-35 and EZL-70 in an enclosure and connect it to the LED strips and the LAN switch under the stairs in the hallway. I thought I should be able to finish this before dinner, but in the end it turned out I needed until half past 10 to get it working because an invisible phenomenon spoiled everything..

During the last few days I changed a few things to the setup. The EZL-70 needs 5V DC to operate but I didn’t want to use 2 adapters – a 12V DC adapter for the MS-35 LED controller and LED strips and a 5V adapter for the EZL-70. So I searched for a circuit to convert 12V DC to 5V DC. That shouldn’t be too hard; I’ve done this quite a lot on breadboards with a LM7805, so I took a small piece of perfboard and soldered the circuit shown below:

12V DC to 5V DC

 

Two additional capacitors and a diode should do just what I needed. I tested it with a small 12V DC adapter and it worked fine – great, now I can ‘steal’ some power from the 12V LED adapter and stop using the additional 5V adapter for the EZL-70.

But somehow, after I installed all the components (MS-35, 12 to 5V converter and the EZL-70) in the enclosure and secured them with some drops of hot glue, it didn’t work anymore.

After I put the power cord of the 12V LED adapter into a socket, I saw the LEDs on the RJ-45 Ethernet connector blinking a bit strange; not the way they usually blink. And my 5-port LAN switch on my desk started making strange sounds as well… something’s not right here. And not a single ping succeeded either… did the EZL-70 die, maybe the heat from the hot glue destroyed something? Loose contacts in 12V to 5V converter circuit? Auch, the LM7805 is hot! What on earth is going on here?? It worked before, so why not now? Why is the LM7805 getting hot (and sometimes not)?

So I started disconnecting everything I soldered together inside the enclosure again… what a mess. First I took the EZL-70 out and tested it with a 5V adapter; no problems. I checked the 12V to 5V circuit stand-alone and this also worked as before. The MS-35 was working as expected too..

Just before I was at the point of pulling my hairs out because I couldn’t think of anything else to check and re-check, I realized that I used a different 12V DC adapter while testing it all here in the ‘lab’. This test-adapter was an old adapter I picked randomly out of a big box full of old adapters I have – the label said 12V DC 1.8A, so that should be OK. However, the new and not-working setup was powered by the original Quintezz power adapter that came with the LED strip… would that be the cause? I couldn’t believe it, but decided to give it a try – I was out of debugging resources anyway… and YES, after swapping the Quintezz power adapter for the one I used earlier, everything worked again!

Right… what is going on here? 12V DC != 12V DC I guess…but why??

I probably need a scope to look at the 12V output signal of the Quintezz adapter and compare that to the output of the ‘better’ adapter to see what’s going on. I suspect that the Quintezz power adapter output is only good enough for powering LED strips and nothing else..

Because the original LED power adapter is not good enough and will probably never be used again, I opened it up (an old habit). As you can see it’s mostly heatsink inside.. I see diodes, a transformer, some capacitors (one of them is very huge), an NTC, fuse, a CS5N60F what seems to be a voltage regulator and a 2CZ10100… on the other side of the PCB are a bunch of SMD resistors and capacitors, that’s it… and I can’t make cheese of this 😉

12V power adapter

The big question here is:  what can be done to prevent me from making this mistake again in the future, how can I easily distinguish the good from the bad ? Do I really need a scope for that, or … ?

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9 Responses to The invisible kill-joy

  1. Kyle Gordon says:

    Could it be that the 12v supply wasn’t putting out 12v 100% of the time? Odd situation! 🙂

    For what it’s worth, I find these to be great drop in replacements for the LM7805. Runs cold, and should save itself in electricity costs over the lifetime of it http://uk.rs-online.com/web/p/products/672-7155/

    • I checked the power supply and I’m sure it was 12V all the time; odd situation indeed.
      Another lesson learned: check the setup you’re going to use with all the final components you’re going to use, even with those of which you think they won’t make any difference..
      Thanks for the 7805 alternative, I’ll keep it in mind.

      • Jeff says:

        But, why a 470uF cap? Recommended values are much lower.

        Anyway car USB adapters are also handy for getting 5V from 12V– usually real buck converters that never get hot like linear voltage regulators. But, in a mixed 12V/5V system, hopefully there’s not much 5V current so a 7805 should be fine.

        • That’s because I have to rely on circuits drawn by others – I don’t have the right knowledge to say what is too high and what’s not. I’ve seen more examples with 470uF so I thought that must be OK… The current (according to the EZL-70 manual) is about 65 mA.

          • Stoat says:

            Back in the 80s 78xx regulators used to be notorious for oscillating, especially if the input wasn’t 100% clean.

            The best precaution is small ceramic caps (0.1uF or so) to ground on the input and output. Electrolytics are good power stores but they’re useless for high frequency shunting.

            That comment about car USB adaptors reminds me of times long gone when I rigged up a linear 5V supply for an early-generation portable CDplayer in a car – after about 5 minutes the case was too hot to touch. It turned out the CD player was drawing upwards of 1.5A.

  2. OWK says:

    I can think of 2 reasons the 7805 is hot:

    Input voltage to high
    High load

    So if you think of it from the output: wasn’t there a shortcircuit that was temporary? Or is the problem still reproducable?

    • Input voltage was 12.7V, load 64 mA (didn’t measure that myself but from the EZL70 manual). Yes, it was reproducable over and over again; at some point I decided to move on and finish the thing – it’s in use by now with a different power supply (an old power supply for an Asus R2H) and working great.

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  4. Saad Ismail says:

    What voltage are best to purchase capacitors for this project? Will 50v 1uf & 50v 470uf will work?

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