Odd wall wart voltage measurement - Page 1
Forum section: electronics / beginners

author: YurkshireLad
author_rank: Frequent Contributor
timestamp: February 23, 2021, 02:51:10 am
content: I have a wall wart that is marked as 9V DC output and 300mA. I used it to power a simple circuit on a breadboard with a DC barrel jack. I connected my multimeter to the +ve and ground pins of the barrel jack and got a reading of 15V. Is there a chance (likely!) that I'm doing something wrong or misunderstanding something, or should I not trust this wall wart?The circuit is a slide switch, a resistor and an LED.Thanks

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: February 23, 2021, 03:05:58 am
content: This is common for a lot of wall warts. It is an unregulated output that will float high if unloaded. You are using it without the load it is rated for. If you load it with 300 mA then the voltage will come down nearer 9V and with some power ripple on it.A regulated supply or wall wart will not do this.

author: YurkshireLad
author_rank: Frequent Contributor
timestamp: February 23, 2021, 03:08:25 am
content: Thanks for the helpful explanation.

author: Rick Law
author_rank: Super Contributor
timestamp: February 23, 2021, 03:08:34 am
content: May be, may be not.It could be that the wart is unregulated. As the word unregulated implies, the output voltage is NOT regulated. As you connect load to it, the voltage get pull down by the load.So, the 9V 300mA wart could output 15V when no load is connected and when you connect a 300mA load to it, it drops to 9V, there about.To find out, try putting a load to it and measure...

author: YurkshireLad
author_rank: Frequent Contributor
timestamp: February 23, 2021, 03:10:12 am
content: Thanks - I'll have to measure the load of the resistor and LED, and see if I can get a 300mA load.

author: MikeK
author_rank: Super Contributor
timestamp: February 23, 2021, 03:57:26 am
content: Don't count on getting the correct load. Set it up with a proper voltage regulator.

author: wizard69
author_rank: Super Contributor
timestamp: February 23, 2021, 04:33:50 am
content: The previous posts cover mos to the likely reasons but there are other things to consider.If the output is regulated then there is a good possibility that the wart is broken. As the name implies regulated means it should be close to the specified output.If the wart is unregulated you could very well simply need a load as others have mentioned. However it might not hurt to look at your AC line voltage to make sure that is reasonable. In the USA you should be fairly close to nominal, 120 VAC ±5 volts. However you can find systems out of spec.You could also have a plug pack designed for another country with a different voltage. The rated voltage should be stamped on the unit though.You will want to regulate this source before using it if it is unregulated. If it this far out and is supposedly a regulated supply then it is broken and should be replaced. The problem with trying to use an unregulated supply is that you can't be certain what the voltage will be in your system. This can be dealt with rather simply.

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 23, 2021, 06:10:43 am
content: While you got some good replies, they could be better if you could post photos. One of the unit overall and a close-up of the label, if there is one. Or, read on as there may be other ways to solve the mystery. Sometimes a brand name on the unit gives a big hint for its intended purpose.At 300 ma output rating, I’m guessing it’s an older unit with a transformer. With an output voltage that reads 15 volts open circuit, it could be for use as a battery charger.Since you have a meter, you can usually identify a battery charger by putting the meter on the AC scale and measure the wall wort output voltage. If you see see a high ac voltage, anything from half the dc rating to as high as the dc level you measured, you’ve got a battery charger without dc filter capacitors. It’s just a transformer and a diode rectifier. These were common with older ni-cad batteries. Obviously, it’s not going to work to power a project.A proper dc power supply will have a very low ac voltage content, usually less than 50 millivolts, especially when unloaded. With a high ac voltage measurement, it’s either a battery charger or the unit has suffered an internal failure. In either case, with a high ac voltage, it’s not suitable to use as a project power supply.FWIW, I purchase wall worts at the local thrift store. I load test them while measuring both the dc and ac output levels. With the no-load and full load measurements, you can calculate the “load regulation” spec for the unit. With the ac measurement, you’re measuring the “ac ripple” spec.I buy all the Nokia and Motorola 5 volt chargers I can find. They’re typically high quality, name brand switch mode power supplies inside, usually good for 700-1000ma with a Micro usb connector. With modern devices, the battery charging smarts are inside the device being charged, not the wall wort. Not a bad deal for a $1. YMMV.

author: Gyro
author_rank: Super Contributor
timestamp: February 23, 2021, 12:58:53 pm
content: Hold onto that wall wart, traditional transformer ones are getting hard to find these days.They're ideal (combined with a linear regulator IC) for applications where you want to avoid the high frequency noise of an SMPS one - and more importantly, to avoid the leakage current associated with 'Class Y' interference suppression capacitor in the SMPS types.