Power supplies (Paranoid newbie strikes again)

Ugha

If you've read my past posts, then you know I'm a bit paranoid when it comes
to possibly damaging my BS2.

So here's another issue with me being paranoid...

I found a wall wart that fits into the BOE... but before I apply power to it I wanted
to verify that its completely compatible with both the BOE and the BS2.

Its stats look pretty identical to the one sold on the Parallax site (Item code 750-00008)
but I still would like one of you gurus to give me the OK on it.

It's from a rechargable flash light... its specs are:
120VAC/15W input
9VDC/300mA output

Its center positive and says its a "Class 2 power supply" whatever that means.

Please don't be annoyed by my smoke-a-stamp phobia [noparse];)[/noparse]

Mike Green

Well, you're not going to get very far by asking for "permission" all the time as opposed to learning the general principles yourself so you can make your own decisions understanding that mistakes happen and things don't always behave the way we expect them to. For example, you can spend oodles of time poring over power supply specs, then shuffle your feet on the carpet in an upper midwest winter when the humidity is really low and touch the "wrong" wire and ZAP, there goes your microprocessor or whatever.

Back to general rules about wall warts

The specs say how much current the supply can provide and what voltage will be present at that current (9V at 300mA). It says nothing about the voltage when the current drawn is lower. Unless the wall wart is well regulated (not the usual case), the voltage will be higher under light loads. Measure it under different loads if you're curious. The input rating tells you what the wall wart is designed for in terms of input (120VAC or, for some, 100-240VAC) along with the amount of power drawn maximally (15W).

Class 2 refers to operating conditions for testing. For example, a Class 2 power supply is not designed to work safely in an explosive atmosphere.

Again, when in doubt about a power supply, measure its output voltage under no load or low load conditions, then under full rated load. Parallax indicates on its boards and documentation what voltages its equipment is designed to operate with. Many of its boards are specified for a 6-9VDC power supply. If you try to operate it at lower voltages, the board may not be able to supply regulated 5V and the microprocessor may reset unexpectedly. If you try to operate it at higher voltages, the regulator may overheat or input filter capacitors may exceed their rating and explode (if the voltage is high enough and the current available is high enough). Often the regulators themselves will withstand 30+V without damage, but will overheat and shutdown if you try to operate the microprocessor at the higher voltages. Some of the regulators that Parallax uses will withstand reverse polarity power without damage, but other parts (like servos) that are directly connected to the power input will be damaged or destroyed.

Ugha

I guess its good I took your advice about testing the supplies... turns out the one I was planning on using put out 16v at no load.

Let me make sure I fully understand... the rating on the wall wart is the min value it'll put out at the stated amps? That makes it rather dangerous trying to match wall warts with projects, doesn't it?

I can't figure out how to test under full load because I don't have a barrel jack spare... is it needed to rule out a supply if the no-load is too high to start with?

I've gone around my house testing wall-warts left and right only to discover that their ratings are nothing like the values I get at no-load... a 6v ended up testing at 18volts!

Either I'm doing something wrong or I don't understand some basic fact... either way I'd greatly appreciate some guidance.

The only light of hope I've found so far are two supplies... one rated at 5v/1.5a that reads 5.17v at no-load and another 15 year-old one rated 9v/200ma and tests 8.77.

I don't want to try them til I'm 100% sure I understand everything.

I know questions like this are annoying but I won't know unless I ask, right?

Mike Green

Most wall warts are not regulated and it's a normal characteristic of unregulated power supplies that their no-load or lightly-loaded output voltage is higher (sometimes a lot higher) than their full load voltage. That's often not a problem because the no-load voltage is only present when the load is turned off. Within a few milliseconds of the load being turned on, the no-load voltage drops to something closer to what the device really expects. In the case of most Parallax gear, there is a voltage regulator that's rated for input voltages as high as 36V or sometimes higher. This is what the regulator will withstand without breaking (and taking out the rest of the device probably). I'm not certain, but I think most Parallax gear with a voltage regulator has input capacitors rated to maybe 50V or at least 35V. The problem with using actual input voltages near that is that the voltage regulator doesn't have much of a heatsink and all the excess voltage is dissipated as heat. The voltage regulator will overheat if more than a few tens of milliAmps is drawn and shut down to protect itself against further heat.

If you use power sources within the recommended range, the regulator will not overheat and it can withstand the peak voltages seen at very low loads.

The two supplies you mentioned, the 5V one and the 9V one may be regulated (have a regulator built-in). You want to make sure that's so rather than just assuming that you can use the 5V supply for logic circuitry without a regulator. The best way to do that is to find a manufacturer's name and part number and download the datasheet for the supply to check.

Ugha

From what you said... wouldn't the BOE's regulator protect the equipment even if the supplies aren't regulated?

Mike Green

Generally, yes, the BOE's regulator would protect circuitry "downstream". You still shouldn't use a supply voltage greater than 9V if a significant amount of current is being drawn and the voltage still isn't below 9V.