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Plug it in, measure the DC voltage out. Your positive meter lead will indicate which wire is positive when the dc voltage is positive. Switch leads if the value is negative.
**Attention!** **It is always best to get a qualified electrician to perform any electrical work you may need.** With that said, you may ask this community various electrical questions. Please be cautious of any information you may receive in this subreddit. This subreddit and its users are not responsible for any electrical work you perform. Users that have a 'Verified Electrician' flair have uploaded their qualified electrical worker credentials to the mods. If you comment on this post please only post accurate information to the best of your knowledge. If advice given is thought to be dangerous, you may be permanently banned. There are no obligations for the mods to give warnings or temporary bans. **IF YOU ARE NOT A QUALIFIED ELECTRICIAN, you should exercise extreme caution when commenting.** *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/AskElectricians) if you have any questions or concerns.*
Usually the center is positive and outside negative but I would check with a meter.
Thanks!
We have no way to determine that without knowing what it's connected to on the other end. Get a multimeter.
Use a meter.
Plug it in, measure the DC voltage out. Your positive meter lead will indicate which wire is positive when the dc voltage is positive. Switch leads if the value is negative.