I have a couple of these wall warts that claim to be 9V on the label but my meter shows them as 18V. They both do the same thing. So, I don’t think they’re bad. Anyone got an explanation for this? Maybe they need a load before they show the right voltage? Would these be safe for a Cerbuino that has a max 9V input? I’m going to try adding a load next and see what it measures during use but I’m concerned about some sort of initial voltage surge when used with a Cerbuino.
You will need to put them under load for the voltage to drop.
And boy did they… I hooked them up to a 12V fan and now they measure 6.6V. So, you don’t see any problem using these with a Cerbuino that expects 6-9V input?
Yes. Well, maybe.
The main reason the 9v upper limit is applied to those devices is because of the heat generated in the VREG by using a higher voltage, If you don’t draw much current though (so if you have minimal additional module requirements) then you may be fine,
The main reason I wouldn’t use it for a Cerb is that they’re unregulated so you can have wildly varying output. They are basically designed for a single purpose, say charging a phone that has a known load, and unless your load matches that they will not be within the expected output.
[quote=“Brett”]The main reason I wouldn’t use it for a Cerb is that they’re unregulated so you can have wildly varying output. They are basically designed for a single purpose, say charging a phone that has a known load, and unless your load matches that they will not be within the expected output.
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But, wouldn’t the regulator on the Cerbuino smooth that out?
But, wouldn’t the regulator on the Cerbuino smooth that out?
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The problem is, as Brett pointed out, these devices are very poor voltage regulators in themselves and desiged to operate with a fixed load so if you have a small load the voltage can increase to well above your 9v.
You would be better off finding a switched mode wall wart of one with a better designed regulator. I suspect that your current one is nothing more than a transformer, bridge rectifier & capacitor.
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@ Sprigo - OK. You convinced me. They go to the scrap heap… 
But, wouldn’t the regulator on the Cerbuino smooth that out?
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yes, but it could be needing to regulate 15v, or 8v, not the marked voltage it’s label says. And when something else in your circuit turns on, it could be regulating from 6v, or 5.9v or 5.8v, which is too close to the Cerb’s VREG dropout point and you go out of tolerance.
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The LM1117 on the Cerb is rated for up to 15v input, and if I’m seeing correctly (the circle obscures the marking) these are only rated for 200mA output, so just about any load (such as a dim LED even) is going to drop the input down well under 15v.
At 200mA rated output, though, you probably don’t have enough to run the Cerb and much of anything else, if it even works. The Cerb itself consumes ~100mA, and that doesn’t count any power LEDs, or the waste heat produced by the two linear regulators.
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[quote=“godefroi”]
The LM1117 on the Cerb is rated for up to 15v input, and if I’m seeing correctly (the circle obscures the marking) these are only rated for 200mA output, so just about any load (such as a dim LED even) is going to drop the input down well under 15v.[/quote]
I haven’t looked but I suspect the power supply circuit on the Cerbuino is the same as is used on the EP power module. I accidentally tested the EP with more than 9V when I first got it and realized it didn’t have the same limits as the DP module. One of the caps became very unhappy and turned into flames… So, it may be that the LM1117 isn’t the limiting component here. This 6-9V range has definitely cut into my choices when digging through my box of scrap wall warts Anyway, these particular ones are going to the “useless” pile.
As was said earlier, unregulated wall warts are really only useful in very specific circumstances. That, and 200mA wall warts aren’t generally particularly useful at all
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ian, what sort of a ridiculous power supply goes to 6V from 18V when connected to a load. They should’ve been in scrap the moment you saw the figure of 6 on your multimeter. Anyways, usually power supplies do have a higher value when they are open ended but there can be a difference of a few milli volts perhaps in order for someone to understand that. It’s as if they haven’t connected any resistors on the output stage of their supply.
An unregulated supply is exactly that, unregulated. The scenarios they are useful for is where you design explicitly for a specific load that you expect (the mobile phone you’re charging for example). This is not uncommon and it’s not something you can bring back “into spec” with resistors, because then adding the load will push them out of spec again… it’s the fact that they are simple and cheap makes designers use them. In the real world, particularly with the ubiquity of USB charging and the global focus on efficiency, the logical solution here is always a SMPS capable of being efficient across a range of output currents to support faster charging on capable devices, which plays perfectly into the “reuse for tinkering” camp like we’re playing in… but unregulated supplies are just dodgy 
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