Is this wall wart suitable ? - Page 1
Forum section: electronics / projects

author: netdudeuk
author_rank: Frequent Contributor
timestamp: August 29, 2015, 03:18:32 pm
content: HiMy wife has a Roberts R250 Revival analogue radio and I bought her a new wall wart advertised specifically for this radio. On arrival, I found that it was in fact the wrong item, having the wrong polarity on the plug. So, they swapped it for one with the correct polarity.The radio was designed for one of those chunky 9V batteries. I've connected it to the DP832 and it doesn't draw more than 0.1A and with the volume at a reasonable level, about half that. The question is, will the 9V / 2A wall wart that actually shows 11.5V when connected only to a DMM be ok with the radio when 9V is specified, with not much current being drawn ?Thanks

author: Kalvin
author_rank: Super Contributor
timestamp: August 29, 2015, 04:11:01 pm
content: Add 3-4 diodes in series with the wallwart. For example one 1N4001 diode will give about 0.7V voltage drop. Four connected in series will give about 3V voltage drop.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 29, 2015, 04:30:15 pm
content: It is expected that a common wall-wart will show a significantly higher no-load voltage than under normal operating conditions.From what you have revealed here, it sounds like you have a perfectly adequate power source for the radio.I would NOT start inserting diodes before confirming the voltage UNDER LOAD.

author: ion
author_rank: Regular Contributor
timestamp: August 29, 2015, 04:34:48 pm
content: Best test it with a load, I've tested some that weren't anywhere near the rated voltage until they were pushing close to their maximum current rating.A 220 ohm resistor should be adequate.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: August 29, 2015, 04:41:42 pm
content: Thanks guys. I had thought that it would be great to have a DC load. I've been watching a video today about one and seen Dave's.I know that the voltage generally drops with a load (hence my mention of theDMM above) and know that I could measure the voltage with the power to the radio on but then I could be blowing it up in the processAre we saying that a normal 1/4W 220 ohm resistor as a load would be ok for a quick test with the DMM across the supply ?

author: ion
author_rank: Regular Contributor
timestamp: August 29, 2015, 05:19:10 pm
content: Quote from: netdudeuk on August 29, 2015, 04:41:42 pmAssuming the voltage doesn't drop much at 50mA you'd want a 0.5W resistor.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2015, 05:30:53 pm
content: I doubt it would cause any problem at all. The radio should be fine on 11.5V.

author: SeanB
author_rank: Super Contributor
timestamp: August 30, 2015, 09:02:38 am
content: Only ever had one pop a cap from running on 11V, but it was a $2 cheapie, where the battery costs more than the radio. They had used a cheap 10V no name brand capacitor in the power side ( could not even spring for a CapXon or any poor brand) which popped after a hour on 11V. I just left it there as it had no real effect with the wall wart connected.

author: calexanian
author_rank: Super Contributor
timestamp: August 30, 2015, 07:01:35 pm
content: Agreed with Richard. It is common for DC output wall wart supplies to have an unloaded voltage well above rated voltage at minimal or no load. They are just bridge or full wave rectified and filtered. They most likely have a 9 volt transformer in there and you are seeing your peak rectified and filtered voltage less diode drops, etc etc on the output. If you put a heavy load on it you will most likely see it drop. I would not worry. If the device is running it should be fine. If it keeps you up at night get a regulated output type power supply. 9 volt ones are common on ebay and the like these days.

author: Kalvin
author_rank: Super Contributor
timestamp: August 30, 2015, 07:11:02 pm
content: Or place a simple three-legged low-voltage-drop (adjustable or fixed) voltage regulator in series with the wallwart's output.