Cheap wallwart? Beware.

Just this weekend I went to a seminar on AC measurements. The presenter kept talking about CCC. He gave away a door prize to the person who could guess what CCC means. I blurted out "Cheap Chinese Crap" and was awarded one of these:


Just don't take one of these outside and try to make a measurement with direct sun falling on the case. The readings become erratic in direct sunshine because the chip inside is covered with glop that doesn't block IR. The case doesn't block IR, either.

Before the 1970s that would have been a multimeter beyond your wildest dreams.

spec

Why would anyone want to but a cheap unregulated wallwart these days ?

0.6366 of 19V=12.1V is the DC content of a full wave rectifed sinewave.

Hydro One usually operates well above 120V to get more revenue. (125 here)
But if it was rated for worst case 100Vac then it could be 5/6 of 12V or 10V or 9VDC/280mA might be possible with more transformer losses.

with an linear load of 9/0.28A=32Ω needs a cap of RC=8T for 10% ripple where I define T=1/f using 60Hz (or 16T for 10%Vpp if you use the rectified frequency 120Hz

thus my Rule of Thumb is the above resulting in C=8/fR =8/(60Hz*32Ω) = 4166 uF

Caps of 4.2mF @25V cost $1 in bulk TH parts

So you helped save the factory in a $1 cost reduction or $10k per 10K units.

Hi,

I have seen cheap wall warts used as chargers for NiCd batteries before. This type has a transformer and four rectifier diodes but no filter cap. I've also seen one for something else but cant remember what it was, but i think it was another charger for something.
The reason they might use this kind of wall wart for charging is because for charging some types of batteries we only need a pulsating DC we dont need a smooth DC and it was even believed that the pulsing DC would charge the batteries better than smooth DC. That's when the only charger component is just the wall wart itself and maybe one series resistor.
I have not seen one of these in quite a while now, but then again i wasnt checking many wall warts that came with products like trimmers, shavers, etc., in the more recent years.

At one time I used to fit regulators to the old linear wall warts which were relatively expensive, but nowadays it is not necessary as most wall warts are pretty good and dirt cheap too. They even run cool, unlike the early linear type that you literally could not touch after a few minutes of running.

spec

Haha, always gotta be careful.

Reminds me of this:

I've also been burnt (figuratively) by 3rd party PSU's, with all their so called approvals.

Deliberately designed in an embedded PSU by MeanWell because it had all its emissions and immunity approvals, so that would cut away a huge chunk of R&D time. Except it didn't! On test I ended up pinpointing the PSU as the noisy culprit causing me to fail, even with a straight resistive load.
Went back to MW who criticised my test setup. I then went back to the test facility and retested their PSU with their suggested setup (not what they expected I'm sure) and it failed dismally anyway!!
The MW response was along the lines of, "Oh yes, you're right, sorry", and nothing else.

These companies fudge everything. It makes a mockery of the entire industry, especially the regulation and approvals system.

/rant over

JimB said:

China makes what we ask them to make and pay for them to make.

If we ask for it dirt cheap, that is what we get.

If we ask for some quality, are prepared to pay for it, and just as with any western manufacturer do quality control, then China will make good stuff.

JimB

Click to expand...

My experience with low-cost far east vendors, goes back when things were made in Taiwan or British Hong Kong or even Malaysia.

You can get both low (not the lowest) cost and good (not the best) quality if you are prepared to pay for it and continuously monitor it.
Never let your guard down, test continuously, don't allow undocumented process or raw material changes. Have a qualified second source in waiting, and make sure your primary source knows that orders will go there if they start cheating.
It also helps if you have an in-country office, where local-speaking engineers working in the same time zone can visit and audit them quickly and cheaply.

Unfortunately, you have to be a large multinational company to have these capabilities.