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Posted By: JValdes AC Adapters - 06/27/08 08:03 PM
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
Posted By: Ann Brush Re: AC Adapters - 06/27/08 08:14 PM
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
Posted By: WESTUPLACE Re: AC Adapters - 06/28/08 01:53 AM
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
Posted By: JoeTestingEngr Re: AC Adapters - 06/29/08 03:38 AM
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.
Posted By: JValdes Re: AC Adapters - 06/29/08 04:46 PM
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
Posted By: BigB Re: AC Adapters - 06/30/08 01:41 AM
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.
Posted By: WESTUPLACE Re: AC Adapters - 06/30/08 02:55 AM
They make AC output adapters as well. And you do not have to have a load to check them.
Posted By: JValdes Re: AC Adapters - 06/30/08 07:54 PM
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
Posted By: SteveFehr Re: AC Adapters - 07/01/08 10:42 AM
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.
Posted By: JValdes Re: AC Adapters - 07/01/08 03:11 PM
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?
Posted By: JoeTestingEngr Re: AC Adapters - 07/02/08 12:11 AM
I would be very careful about hooking that adapter up to your radio. It sounds like it might be one of many wall warts that don't have a rectifier and cap built in, just a transformer. If so, that diode the techie described makes all the difference in the world. If it is in series with the coaxial jack, it would accept the half wave where it was forward biased. Your unfiltered supply would be Ripley, believe it or not.<G> Many supplies however, use a reversed biased diode across the input to protect against polarity reversal. An AC output adapter will be shorted every half cycle in that case. The diode might short out if the transformer can deliver more than the diode's rated repetitive current. The transformer could open up a winding or protection or just be loaded down.

You said that you had your meter set on DC. Did you check it on AC? Your best best is about a 6 Volt DC output center negative on a coax plug of the right id and od. Going to the store and guessing those darn diameters is always the biggest problem for me. Are you sure that you really have one that fits? I just built a regulated car supply/charger for my little one's DVD player and went through 5 or 6 plugs in my drawer o' stuff, before realizing I had to head over to Radio Shack fot the biggest size that they sell. If you don't have the - on the center or a reversible plug, and don't feel like using dikes, a soldering iron, and heat shrink, just buy it from them. Save the money that all your time is worth.
Posted By: JValdes Re: AC Adapters - 07/02/08 05:32 PM
You right Joe. The last thing I want to do is fry my radio. The adapter is 120 volt AC in and 6 Volt DC out. So I must assume the rectifier is present?
The plug fits is what I should have said. I have no idea if it will make correct contact when it is plugged in, or if the polarity is correct, but it does snap in quite securely into the jack.
I think I will just buy theirs and forget about the $9.00 shipping. That was the only reason I did not order it. I guess I should buy the NIMH batteries from them too, since the shipping charge will be the same.
I really do appreciate your suggestions......Thanks John
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