Advanced wallwart dickery - Page 1
Forum section: electronics / projects

author: con-f-use
author_rank: Unknown
timestamp: August 11, 2014, 09:04:09 am
content: A friend gave me something to repair. The device is operating from just the strangest wall wart. The wart looks fairly standard, like any generic charger for a mobile. On the outside it says 6V 330mA. This one died and doesn't put out any voltage anymore. Inside the wall wart, it seems like the unmarked IC controlling the switching gave up. Must be a micro of some sort.I tried to replace it with a generic cheapo that can switch the voltage between 3,5,6,9 and 12 V. Didn't work.....until I accidentally set the new generic power adaptor to 9 V instead of six volt (because I'm dyslexic or something). I realized my mistake and quickly set it back to 6V. To my surprise the device started operating normally. Turns out this is reproducible. To make the device work, I have to feed it with 9V for a small time (no more than two seconds or it won't turn on) and than switch it back to 6V. I assume this is so one needs to buy the expensive power adaptor from the device's manufacturer? Do you know of something else that could cause this behaviour? Have you encountered something like that?Also what is the best way to shoehorn a board that puts out 9V when first connected for about a second and then 6V while powered from 6V? I'd like to have a good solution for that from standard components I have lying around (maybe a 555). The device doesn't draw more than 10 mA in the 9V phase, and starts drawing its regular current of about 150 mA once in the 6V phase.But mostly I'm curious as to why this device behaves so weirdly. As to the nature of the device, it's an expensive toy.

author: mariush
author_rank: Super Contributor
timestamp: August 11, 2014, 09:23:52 am
content: How about you start by posting some pictures of the wallwart and the connector, and if it's not clear in the picture maybe say what's written on the main chips in the wallwart.Just some guesses...Have you checked the connector? It could be 3 wires, ground and voltage and one wire used for identification (connection to eeprom)Have you checked if the adapter is AC or DC? If it's 6v AC the device may have a bridge rectifier inside and some capacitors that were supposed to convert that 6v AC to about 10v DC.... with your 6v DC and the additional voltage drop on the diodes of the bridge rectifier, there may be an undervoltage at start, when the capacitor inside fills up and everything tries to start using some current.After initial startup, current demands may be much lower and the device may still work at 5v DC or whatever you have left.

author: con-f-use
author_rank: Unknown
timestamp: August 11, 2014, 10:01:07 am
content: Okay, thanks, but I thought I gave all the details. It's just your standard wall wart from the outside. Has a regular barrel jack connecter to the device it's powering: two terminals, +V and GND. Its broken so I can't say much more. On the outside is the CE label and the rating "Input 250-110VAC, 0.1 A Output 6VDC 330 mA". Inside is just the usual stuff you'd expect from a switching supply plus an unmarked controller chip, that I'd guess is a broken micro controller. No extra wire to the device.I use a wall wart with selectable output voltage now. I have to switch it to 9V and than to 6V in order for the device to power on. The initial 9V just seems to be a signal to the device to power up - it only draws 10 mA in that stage. If I apply 9V for a second and than switch to 6V the device powers on and works normally. If I apply the 9V for say 3s and than switch to 6V, the device does not turn on at all. So it times the the 9V somehow.My plan is to power it from a 9V wall wart, with a board in series, that steps down the voltage after a second.

author: amyk
author_rank: Super Contributor
timestamp: August 11, 2014, 10:29:04 am
content: What's in the device itself? The power input circuitry in the device would probably give a good clue as to why, as well as the voltage tolerance.

author: Maxlor
author_rank: Frequent Contributor
timestamp: August 11, 2014, 10:49:24 am
content: I wonder whether you're just hitting some weird edge case, like your 6V on the second walwart being slightly too low, and your device not being tolerant of that during startup. Might be that with 6.5V it works just fine without any voltage switching weirdness. Try it with a lab power supply?

author: Psi
author_rank: Super Contributor
timestamp: August 11, 2014, 10:53:38 am
content: My guess is that the device has some bad caps which caused an increase in current which killed the PSU.When you feed it with extra voltage for a short time you can coax the device to start despite the bad caps.Sort of like when a PC motherboard has failing caps and begins to crash under heavy load from the excessive noise on the power rails. Unplug a few extra devices like CDROM / i/o cards and the PC runs stable again for a while.Except in this case your not reducing load but increasing voltage which also has the effect of scaling back the voltage to noise ratio.

author: con-f-use
author_rank: Unknown
timestamp: August 11, 2014, 11:54:28 am
content: So the device is sealed and quite expensive. On top it doesn't belong to me, so I won't crack it open.Indeed, I tried it with a lab power supply now. It won't turn on without the drop, i.e. any constant voltage between 6 and 9 V. It needs the drop from 9V to anything between 6.5 to 5.5 V. And the drop seems to need to occur after anything between .8 and 1.9 seconds after power was applied.Quote from: Psi on August 11, 2014, 10:53:38 amThat sound's reasonable. Just seems odd that the behaviour is so repeatable, but I've seen stranger faults in equipment. I'll build my voltage dropper board and see how long the device will last on that.

author: Precipice
author_rank: Frequent Contributor
timestamp: August 11, 2014, 12:05:22 pm
content: An expensive silicone-clad toy, huh? Fair enough.Are you charging batteries in it, or running it directly from the power supply? If the latter, is jiggling the settings a bench power supply not going to be a bit distracting?

author: con-f-use
author_rank: Unknown
timestamp: August 11, 2014, 12:42:08 pm
content: Quote

author: German_EE
author_rank: Super Contributor
timestamp: August 11, 2014, 04:26:21 pm
content: So what you are saying is that you want a 6V DC power supply with a 50% switch on surge, as engineers we normally try to avoid this sort of thingPlease see the following page:http://www.rason.org/Projects/transwit/transwit.htmAt the top you will see a power switch that uses a 2N3906 PNP transistor and a 12V supply. Firstly you supply 9V to the circuit then test it by grounding the base resistor R1, you should see the transistor switch on.Now please make two alterations to the circuit. Firstly increase R1 until 6V is supplied to the load whilst still working at a 9V supply, you will need to experiment here to find the correct value. Secondly please add across R1 a 10uF capacitor. When you ground the capacitor it briefly appears as a direct short circuit and turns the transistor full on, supplying 9V. As the capacitor charges the output voltage falls to the value set by R1 (6V) and stays there.

author: free_electron
author_rank: Super Contributor
timestamp: August 11, 2014, 05:15:07 pm
content: those supplies could simply regulate so bad that they send out 9 volts and as soon a load is applied the voltage collapses tot 6 volts...

author: Precipice
author_rank: Frequent Contributor
timestamp: August 11, 2014, 05:21:37 pm
content: Or maybe there's a little switcher in there, and it drags down the external PSU when it (tries to) start. Stiffening it up (snurk) with, say, 1000uF might let it start at 6V?

author: SL4P
author_rank: Super Contributor
timestamp: August 12, 2014, 01:38:21 am
content: I'm not convinced it's an*expensive* piece of kit...If they supply it with an oddball 6V @ 330mA plug-pack...It may have cost a lot of money to buy, but thinking it was a lot less to make!It sounds like Wan Hung-Lo, with a 600% markup.Just out of interest, what is the current drawn during startup & operation?(And what is the piece of gear?)

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 12, 2014, 02:22:44 am
content: Quote from: free_electron on August 11, 2014, 05:15:07 pmThat is exactly what I thought of when I read the original scenario. Some of those wall-warts put out a much higher voltage without a load.Maybe the mystery load requires that "kick" to get started. Seems improbable. But I've seen improbable several times before.

author: turbo!
author_rank: Regular Contributor
timestamp: August 12, 2014, 07:23:47 am
content: What's the adapter for?I've noticed that some battery packs will use CCCV adapter. When you plug it in, it will limit current to the rating within a range of load (not necessarily against a dead short) and switch to CV mode once it reaches the set point on constant current.