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#179130 - 06/27/08 01:03 PM AC Adapters
JValdes Offline
Member

Registered: 06/03/07
Posts: 308
Loc: South Carolina
I hope you all do not mind this type of question? But here goes.

I have a radio that will allow NIMH batteries to be charged on board. I need the AC adapter. They sell them, but I have one that fits the connector and is the correct voltage. However the MA rating is almost double that of what is recommended. It (radio) is rated for 450MA and the one I have is 1.25 amp. The tech at Marconi said to use less than 500MA. I don't get it. Current will only flow as needed. It would seem that a larger MA rating would be of no concern? But a lower MA rating may not provide enough power to charge the batteries?
Am I correct? Thanks in advance......John

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#179132 - 06/27/08 01:14 PM Re: AC Adapters [Re: JValdes]
Ann Brush Offline
Member

Registered: 12/27/05
Posts: 155
Loc: Ohia
You are correct - often tech just cover their "you know what". However, should there be some kind of board failure there will be extra current avail. and that may be problematic.
Cheers Ann

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#179147 - 06/27/08 06:53 PM Re: AC Adapters [Re: Ann Brush]
WESTUPLACE Offline
Member

Registered: 03/29/04
Posts: 252
Loc: Kingwood, TX USA
Many wall warts are unregulated, a 12vdc wall wart (WW) may have a 21vdc open ckt output. Fully loaded it will be near 12vdc. A WW rated a twice the current may present a higher voltage when lightly loaded than the one that goes with the equipment. Regulated supplies do not have this problem. Watch the polarity of the plug on the LV side. There are NO standards for these. All different voltages, currents & lots of different plugs. Those coaxial plugs was the worse invention. I have high quality equipment & tools with that cheezee coaxial jack on it. Robert

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#179167 - 06/28/08 08:38 PM Re: AC Adapters [Re: WESTUPLACE]
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: Chicago, Il.
I would need more info to decide if you're safe or not. What size and how many NiMH are in the radio and will it accept the same # of Alkaline batteries? As long as your lightly loaded supply doesn't exceed that voltage by much, the radio is probably safe. Then it comes down to the safe charging rate for the batteries and what you're willing to do with your adapter. Tweaking the adapter for a good charging rate could be as easy as inserting 1 or 2 forward biased diodes in series. 1N4001 series diodes are very common and fine if you keep the current < 1Amp. If you need 2 diode drops for the right charge rate, you can use one of those small cylindrical bridge rectifiers that I think Radio Shack still sells. Just use the "-" towards the supply "+" and the bridge "+" to the radio "+" input, while isolating the "AC" leads. The current rating would be twice that of the bridge because you would be using it as 2 pairs of series diodes in parallel. I would just try it and make sure that the batteries don't get too warm during charging.
Joe

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#179175 - 06/29/08 09:46 AM Re: AC Adapters [Re: JoeTestingEngr]
JValdes Offline
Member

Registered: 06/03/07
Posts: 308
Loc: South Carolina
Joe,
The battery requirement is 4 "D" cell as are the rechargeable batteries.
I am not a electronics tech so I really do not understand all of your questions and comments.
But the tech at Marconi indicated that there was a 1 amp diode in the charging circuit. He said 1125 MA would blow the diode.
The adapter I have is from an old battery charging station. Those were I believe double "A" Four charged at once. I can buy the adapter from Marconi for $11.00 plus shipping, or I can buy one here at my electronics store for $14.00 with multiple connector ends.

The tech also warned me about the polarity. He told me the center of the conn. was neg and the outer side was positive.
So I went and got my meter and could not read voltage on any of the ac adapters. So I cannot tell the polarity of the adapter. Does the adapter need a load to output voltage. I was checking with my meter set on DC volts and the positive and negative sign were jumping back and forth.
Let me know Joe. I sure do appreciate your answer and Ann's Cheers!!! Thanks

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#179183 - 06/29/08 06:41 PM Re: AC Adapters [Re: JValdes]
BigB Offline
Member

Registered: 03/31/04
Posts: 725
Loc: Tucson, AZ USA
The polarity is almost always indicated on the charger label with a small diagram, and on the unit to be charged with a raised (brail like)diagram near the jack.

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#179185 - 06/29/08 07:55 PM Re: AC Adapters [Re: BigB]
WESTUPLACE Offline
Member

Registered: 03/29/04
Posts: 252
Loc: Kingwood, TX USA
They make AC output adapters as well. And you do not have to have a load to check them.

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#179188 - 06/30/08 12:54 PM Re: AC Adapters [Re: WESTUPLACE]
JValdes Offline
Member

Registered: 06/03/07
Posts: 308
Loc: South Carolina
That was one of my questions. I cannot read volts. I am assuming that a load must be present.
If the shipping did not cost more than the adapter I would just by the one from the manufacturer.
I really do appreciate everyone's feedback....John

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#179200 - 07/01/08 03:42 AM Re: AC Adapters [Re: JValdes]
SteveFehr Offline
Member

Registered: 03/19/05
Posts: 1192
Loc: Chesapeake, VA
Wall warts are not constant current devices, they're simple rectifiers. Just because the AC/DC adapter is capable of 1.25A doesn't mean it will be supplying 1.25A; that current is still dependant on the resistance of the radio, and will be self-limiting. As noted above, a lot of the AC/DC adapters are cheaply designed and rely on a specific resistance to put out their specified voltage, but it's not like it's going to put out 80V or anything, you're just taking maybe 10V out of an 8V supply, etc.

Unless this is a really expensive radio, I wouldn't think twice about using this charger with it.

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#179210 - 07/01/08 08:11 AM Re: AC Adapters [Re: SteveFehr]
JValdes Offline
Member

Registered: 06/03/07
Posts: 308
Loc: South Carolina
Thanks Steve. I will use it asap. Those "D" cell NIMH batteries are expensive! At least I will not need a charger, the radio is the charger.
I guess it would be considered expensive as it cost me $150 plus shipping. It is a AM, FM, TV and weather radio. I use it mainly for AM talk stations that are out of the normal receiving distance. Sometimes I can get stations over a 1000 mile away at night.

I do not think the radio has a charge indicator light. It must be on their charger or there is no indicator at all. I guess I will just have to be careful not to leave it on charge and forget about it.
There is an icon on the LED screen for battery level, and I guess I can use this as my indicator? Now thinking, since I have never used a charger on this radio, the icon could be the indicator similar to a cell phone?


Edited by JValdes (07/01/08 08:15 AM)

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