Checking Polarity

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  #1  
Old 08-27-07, 08:42 AM
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Checking Polarity

I bought a Radio Shack 9 volt power supply 1.5 amp output. The power supply has the wrong adaptor at the end. It wont fit into the device for whick I bought it for. I cut the adaptor off and found another adaptor to solder on to the Radio Shack power supply. My question is how do I check for polarity I need to have polarity correct when I solder the adaptor on to the two wires from the Radio Shack power supply. I have a digital multimeter to use for this purpose which I believe I will need. The end of the adaptor which plugs into the device is barrel shaped - I will be soldering it to the Radio Shack power supply.


Thank you in advance.

Le
 
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  #2  
Old 08-27-07, 08:47 AM
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I don't understand the question.

I am assuming this is 9 volts DC, since AC has no polarity. Set the meter to DC volts and connect the probes to the output. You will measure positive or negative voltage.
 
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Old 08-27-07, 08:55 AM
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If you look the voltage is applied to the inner part, the ground is on the outer shell.
 
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Old 08-27-07, 09:38 AM
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Not necessarily. Some devices are set up pin positive, others pin negative. Most tell you which way it should be by virtue of a symbol next to the power jack.
 
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Old 08-27-07, 09:48 AM
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Take your DMM and set it to DC Voltage at some scale greater than 9 volts. The jacks on your DMM where you plug the leads into will be labeled + and - or + and ground. Touch the leads to the wire from the power adapter. The DMM will read either 9volts or -9 volts. If it's negative, switch the leads and try again.

Once you get 9 volts (positive), the lead that's connected to the + terminal on the DMM is your positive lead from the power supply.

As stated, _usually_ the center pin is positive, but not always - so it's good to always check first.

Also, as an FYI, most electronics have built-in diodes to protect against mixed up polarity. Usually, the piece of equipment simply won't work if wired backwards (or the batteries are installed backwards).
 
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Old 08-27-07, 10:09 AM
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Another word of caution. Many power converters will not read the expected voltage except under load. With no load the voltage may be much higher than, in this case, 9 volts.
 
  #7  
Old 08-27-07, 01:35 PM
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Checking Polarity

Thank you Racraft, Zorfdt, Eclipse and Pcboss.

I will go now and try out your advice.

Thanks again!!

Le
 
  #8  
Old 08-28-07, 07:52 AM
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Checking Polarity

Racraft, Zorfdt, Eclipse, Pcboss :

I attached the two wires from the Radio Shack power supply to the two wires from the barrel shaped adaptor. I plugged in the power supply. Took the multimeter and inserted the positive red probe into the barrel and the black probe onto the outer shell and the meter read 9.27 volts dc positive.

I believe I connected the wires correctly(polarity correct) since the meter reads "positive" . The diagram on the device reads middle pin positive.

How does this sound to you guys?


Le
 
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Old 08-28-07, 08:00 AM
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Sounds like you're good to go.
 
  #10  
Old 08-28-07, 08:13 AM
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Yes, that sounds correct.
 
  #11  
Old 08-28-07, 09:35 AM
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Checking Polarity

You guys are the best. I really appreciate it!

Le
 
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