What the f*** is the problem with "cheap" power supplies... :(

I am doing it on a regular basis, trying to use a computer AC adapter, or any other adapter with a barrel plug that fits the USB-DP or elsewhere.

Quite a few times I have nuked my system, or if lucky, if have only had lots of weird interrupt triggers, or strange events in the system.

Can anyone explain why that happens, and what the real problem is with these supplies??

The real question should be… “Since I have proven to myself multiple times that cheap power supplies are crap and not worth the risk, why the f*** am I still using them??” :wall:


@ ianlee74 - Good point. ::slight_smile:

Do you plug them into a line conditioner? Does your facility have stable mains voltage etc? Do these power supplies have active power factor controller?

Are they regulated, or unregulated power supplies? Unregulated ones require a minimum amount of current to keep the voltage in an acceptable range, and could easily be exceeding what is acceptable on the input.

@ Mr. John Smith - Nope, no line conditioner. The voltage in DK is pretty stable. How do you see if a given supply has active power factor controller?

@ njbuch - its written on the box. Other than that I dunno.

I fully agree that a regulated supply is the only way to be sure that 12v (or 9v) means 9v, and I’ve gone away from buying non-regulated supplies - and in most cases it’s not significantly more expensive to buy a regulated one anyway.

But that may not really be your problem

@ Neils can you elaborate on what you have actually had happen? A DP module for example should isolate the mainboard and attached peripherals from voltage fluctuations, assuming you don’t exceed the 30V the DP module supports. If you use an unregulated 12v supply, that was producing say 19v, that should still work via the DP. Perhaps you actually have a grounding issue more than anything, using an earth reference ground on one and not on another that caused issues. Since the other active thread today has been about your motor driving experience, it’s possibly more to do with trying to use multiple power sources that’s caused the issue.

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@ Brett - Hmm, now we are talking. In most setups I am using one or more modules that needs seperate power, where I normally just use an AC adapter. So in my setup with the motor driver, I am using the usb plug to power the board while debugging, and the motor driver is both attached via a Gadgeteer cable and the power from the AC.

I guess thats a terrible thing to do and the two GNDs should be connected or what…?

The biggest problem is that I have fried several drivers without knowing what the problem was, but on top of that the systems I am working on are unreliable with weird interrupts and other strange things happening…

This has to be a common challenge… :think:

With respect to multiple power supplies in the system, the Grounds should all be tied together inherently, as the load modules Gnd pin is connected back to the main board.

Now, that said, I have seen a case in the past, where we were using control transformers to provide 12v ac to chain two motors together, so if one started the second started. I don’t remember the exact setup, but I did see where voltage would come out of a wire on one transformer, and complete the circuit by being connected to a wire from the second transformer 30’ away in a completely different enclosure / power source. A better way to picture this, is cut all but one pin off of two power supplies, plug them into the wall, and have them work by connecting both to the same circuit. Converting to DC normally avoids this issue, but depending on the quality of the power supply, just a thought.

This would be similar to having two car batteries, connecting only the positive terminal of one of them, to the negative terminal of the second one, and having them melt down because they were short circuiting.

The other scenario that I’ve had, for example when debugging, watching buttons being pressed that are NOT being pressed, is reboot the computer. I’ve just gotten to the point where when I’m deploying on site software, I reboot as I arrive on site. I’ve seen this happen about 5 times over the last couple of years, where I spend a couple hours trying to figure out what I was doing wrong, to have the solution be a restart.

I still don’t fully understand what power supply you’re using. Bring it back to simple terms, is there one or two AC->DC power packs in use? Is there two or three pins on the AC plug? And are you connected to a laptop or desktop PC, that is earth referenced GND?

Here’s a Dave Jones video about mains earth reference measurements - entitled how not to blow up your oscilloscope. EEVblog #279 - How NOT To Blow Up Your Oscilloscope! - YouTube

This might help explain where you may be falling foul of.

so that may be true in the general Gadgeteer module scenario, but you should validate that there is a connection between GNDs. There are good reasons why some products may use separate power sources that are unconnected; things like significant current use that can cause voltage drop that isn’t overly important to something like a motor, that someone wants to keep separated from the microprocessor power.

@ Brett - i have one ac wall adapter with no ground giving 12v in two wires connected to the motor driver and a USB cable to the mainboard from my computer which has no ground.

EDIT: very informative video Brett!

I assume the 12v goes into the DP module to power the device, as well as the motor module?

theoretically I don’t see you having differing potentials that should blow stuff up. I’d next look closer at the 12v power supply and see if that’s the source of your problem

This is why I advocate having a proper bench power supply with current limiting etc especially for your initial testing. Yes the cost is much higher than an AC-DC brick but consider the cost in time and hardware blown up and you can pretty much see the advantage.

I think your issue with things blowing up is a bad ground. Ground faults can cause your simple 12V supply to appear much higher.

Your issue with things behaving erratic is probably down to bad filtering. Any time you add a device to your design, make sure it has decoupling capacitors. Anything like the gadgeteer modules with a cable connection should really have as a minimum a 4.7uF Cap on the power supply input to the board and local decoupling on each device.


Why not use this:
And this:
And be done with it

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