One 9v adapter gives power, others don't. Opposite readings on multimeter. WTH.

[StompboxAH]

After finishing my test pedal, I tried to power it on with a 9v wall wart I have. It powered on with no problem. I tried two other power sources, another wall wart and a 9v battery, but neither powered the pedal on.

I took out my multimeter and did a Load test. My readings were:

9v 1200mA wall wart = 12.74

9v 1000mA wall wart = - 9.29

9v Battery = - 9.30


Why are my values negative for the latter two? What is going on?


Edit: mA not mAH

[UKToecutter]

My first thought is that the first adaptor has different polarity. 
i.e. One might be centre negative and the other centre positive.

Your battery would seem to be wired the same polarity as the second adaptor, therefore, if you switched the + & - then the battery and the second adaptor will work and the first adaptor won't.

[antonis]

Propably you didn't include a polarity protection diode to your pedal's power supply input...

Good for the second wall wart and the battery but, maybe, not so good for the circuit...

[duck_arse]

when you say "a load test", how much load did you use? 12V from the first looks unloaded and unregulated. 9V from the second suggestes unregulated and loaded to 1Amp, or regulated, and current below 1A doesn't matter.

and the battery suggests you have your red/black and +/- mixed.

[StompboxAH]

when you say "a load test", how much load did you use? 12V from the first looks unloaded and unregulated. 9V from the second suggestes unregulated and loaded to 1Amp, or regulated, and current below 1A doesn't matter.

and the battery suggests you have your red/black and +/- mixed.

Ok well I figured out that the first power supply was center negative while the other two were center positive.

I used the 9v load test on my multimeter. What do you mean they look unregulated?

[cloudscapes]

Ok well I figured out that the first power supply was center negative while the other two were center positive.

A battery isn't center positive. It's however you've got it wired up.

[StompboxAH]

Ok well I figured out that the first power supply was center negative while the other two were center positive.

A battery isn't center positive. It's however you've got it wired up.

I didnt wire the battery directly. I used a 9v adapter with barrel jack.

[Ice-9]

Case Solved then, you simply used the wrong PSU type. You might also want to swap the wires around on your battery snap to barrel adapter for future use.

[StompboxAH]

Case Solved then, you simply used the wrong PSU type. You might also want to swap the wires around on your battery snap to barrel adapter for future use.

yes definitely. I was just wondering what the above poster was talking about when he said my readings looked unregulated.

[PRR]

> took out my multimeter and did a Load test

Not sure what this is. Most electronics-work multimeters do not have a "Load" setting. Battery-testers often do. A 9V batt may be 9.4V fully-fresh and 9.3V half-dead but rested. Since this difference is hard to see (and the fresh voltage varies with chemistry and temperature), a good Battery Tester includes a Load, a typical drain for the battery type being tested. A fresh batt might sag to 9.1V, a half-dead batt will sag below 8V under "typical" load. The "typical load" a tester uses is often not specified.

"9V adapters" can be found in either polarity, and either "stable 9 Volts" or "12V sags toward about 9V".

> 9v 1200mah wall wart = 12.74
> 9v 1000mah wall wart = - 9.29

Your first problem is Wrong Polarity. Transistors and e-Caps only work one way. When reversed they may DIE. You need to be sure of your polarity before you connect.

Yes, in "guitar world" there is an informal standard for barrel connectors. In the broader world of answering machines, LED clocks, cassette players, there is NOT a standard. In fact sometimes it seems the makers use every possible variation of length diameter and polarity to foil replacement wart use. If the wart is not clearly a Guitar World brand, assume it may be wrong for your uses.

You say these are "9V adapters" but one shows over 12 Volts. Also 1200 mA (not mAH!!) is far-far more than any 9V pocket-radio battery can supply, so your "Load" test (whatever it is) is probably "no load" for this lump. All power sources sag. This one may sag near 9V when loaded in 1200 mA, but un-sags toward 13V when not loaded at all. This type of saggy supply is very common. The device it was made for may not be very fussy, or may have additional voltage-regulation inside the device. An answering machine may run full raw 9V-13V on the speaker-amp chip, and regulate a steady 5V for the digital bits.

In 9V pedal work, many designs *assume* you will use a 9.4V battery or careful equivalent. Putting 12V or 13V on them may do damage, may not work right, and may work fine. A 9V pedal "can" use a 10 Volt rated power capacitor, which will burst in hours at 13V. The popular LPB is very fussy about its bias, is usually scaled to work well from 9.3V down to 8.0V, and may cut-off if fed 13V.

People casually say "Go to GoodWill and get a wart". But you really need to test it for polarity and voltage before you go plugging it into pedals.

[StompboxAH]

Paul,

My multimeter has a battery load test feature that can test 9v batteries.

I already figured out the polarity thing. Luckily, this was just a test pedal and did not have any caps or transistors.

As far as the 12V reading from the first wall wart, I'm literally quoting what the labels say. The label clearly says "OUTPUT: 9V 1200mA (not mAH, thank you). Not really sure what the deal is with that. I've been using this wart to power my pedals for a while now with no problem. Any guesses as to why I'm reading 12V?

[davent]

A couple typical pedals will only have a load of maybe a few 10's of  mA's, not enough load to drop the voltage to 9v. As i understand it, with a 1200mA load you'll get 9v's out. It's not regulated to 9v, it depends on the load get there.

PS, Your initial post does say 9v 1200mah

[StompboxAH]

A couple typical pedals will only have a load of maybe a few 10's of  mA's, not enough load to drop the voltage to 9v. As i understand it, with a 1200mA load you'll get 9v's out. It's not regulated to 9v, it depends on the load get there.

PS, You initial post does say 9v 1200mah

Yes I know that....That's why i thanked PRR for the correction....

[ashcat_lt]

Case Solved then, you simply used the wrong PSU type. You might also want to swap the wires around on your battery snap to barrel adapter for future use.

Edit - Sorry, most of my post here was exactly backwards, so I'm deleting it, leaving the only good part:
...remember to always(!!!!!!!!!!) check the polarity of any wallwart you're thinking of plugging into anything.  Wrong polarity breaks things.  AC where it should be DC breaks things.  "9V" is nowhere near enough information.

[PRR]

> Any guesses as to why I'm reading 12V?

I thought I touched on that?

All power sources sag. This one may sag near 9V when loaded in 1200 mA, but un-sags toward 13V when not loaded at all. This type of saggy supply is very common. The device it was made for may not be very fussy,......

> label clearly says "OUTPUT: 9V 1200mA

OK, so try the *rated* load, 1200mA.

Known-current loads are awkward. Resistors are more handy.

Ass-uming they are right and it sags to 9V under 1200mA (1.2A) load, that is a 9V/1.2A= 7.5 Ohm resistor. Note also that 9V*1.2A is 10.8 Watts of power... can't try this with common 1/4W resistors. Interestingly, Radio Shack used to stock a 8 Ohm 20 Watt resistor, if you care to go there, if they still stock it. And if the label is just bogus, and it will put 12V in 8 Ohms, that's 18 Watts and the RS part will take that fine.

Or just remember that labels usually give full-load voltage, rarely say the no-load (or light-load) voltage. On a regulated wart the no- and full-load voltages will be about the same; on un-regulated warts they will be different, often 20%-40% higher.

[Ice-9]

If your 9v power supply is reading 12v under no or light load then it is UN REGULATED, as was said in the above post. (or a faulty regulated psu).