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  #1  
Old 12-25-2004, 11:52 AM
hyjyljyj hyjyljyj is offline
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DC Adapter Question

Hi Electrodopers,

I got a radio which calls for a 9V, 600mA adapter with a positive center.

I already have a positive center adapter rated at 11 volts, 600mA. Is it OK to use, or am I going to burn down the house?

Thanks and Merry Xmas
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  #2  
Old 12-25-2004, 11:58 AM
astro astro is offline
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With something as relatively simple a a radio, unless it's some hypersensitive unit you can probably have it work fine, but bear in mind that you will be slighly overstressing the electronics and this may lead to a somewhat shortened life over time. If it's an expensive radio I'd just get the rright adapter. If it's a cheapie live dangerously.

BTW make sure the 11V is DC not AC out.
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Old 12-25-2004, 11:59 AM
Finagle Finagle is offline
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Very likely Not OK to use. It's possible the manufacturer built the radio of robust components that can handle an over voltage, but I wouldn't count on it. You're not likely to start a fire with those kinds of voltages and amperages, but you are likely to burn out a few components in the radio as they attempt to handle more current than they were built for. At the very least, the components will run hotter than they should and will shorten the life of your new gadget.
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Old 12-25-2004, 12:00 PM
Q.E.D. Q.E.D. is offline
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Tough call. Some things can tolerate a fair amount of overvoltage, though they may run hotter than normal. Other devices have voltage regulators and can tolerate quite a large range of input voltage with no adverse effects, other than a hot regulator, if the voltage is towards the high end. Many less expensive devices, however, will be destroyed by even a small excess voltage. So, while you might be able to get away with it, you should probably wait to get the correct adaptor. Knowing the risks, you may choose to try it, but realize that there's a chance you'll destroy your radio.
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Old 12-25-2004, 12:15 PM
hyjyljyj hyjyljyj is offline
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Thank you very much for all the replies. Man, you guys are sharp and FAST.

It sounded fine but since it's a brand-new radio I'll just get the 9V DC adapter. Thanks again!
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  #6  
Old 12-25-2004, 12:41 PM
Joe Random Joe Random is offline
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Quote:
Originally Posted by hyjyljyj
since it's a brand-new radio I'll just get the 9V DC adapter.
If you're going to buy an adapter anyway, you may want to consider a universal adapter that has multiple ends, and allows you to manually select the polarity and voltage. They're very handy to have around.
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  #7  
Old 12-25-2004, 03:03 PM
jnglmassiv jnglmassiv is offline
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Quote:
Originally Posted by Joe Random
...you may want to consider a universal adapter that has multiple ends...
I've always liked the idea of these but I've never had one last longer than a few months. They just kinda stop working.

As for the OP, while others here have expressed the usual caution, I'd use the 11v one you have without any worries. Often, those rated adapter voltages are "very nominal" meaning pretty rough as accuracy goes. They are usually unregulated and may have different voltages for different loads. For example, it may measure 10.0volts with the tuner and volume turned low and 8.5volts at high volume and using the cassette motor.
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  #8  
Old 12-25-2004, 04:23 PM
Nanoda Nanoda is offline
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If your universal adapters are malfunctioning, check the stats on them.
Mine worked great for years, until I tried to use it to power a small printer last month. That's when I noticed that the incredibly tiny print on the printer said 1.1A, wheras the adapter was rated for 500mA. Whoops!

I wouldn't feel good using a too-small amperage adapter (likely to overheat), but I'd go overvoltage by 2V at 9V without much worry. If you've got a multimeter you'll notice that, especially with no load, adapters don't jive 100% with their rated stats. The people making the hardware would certainly expect that.
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  #9  
Old 12-25-2004, 05:46 PM
Crafter_Man Crafter_Man is offline
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Im surprised no one has whipped out their calculator yet.

Lets assume the radio employs a 7806 6V regulator w/ TO-220 package. A TO-220 package typically has a RJC of 5 C/W and a RCA of 65 C/W. If we assume a 9V/600 mA (unregulated) wallwart has an output voltage of 9V, and if we assume the current draw is 600 mA, then the regulator would dissipate 1.8 W and the junction temperature would be 161 C when the ambient temperature is 35 C. This exceeds the 150 C limit typically specified for 78XX regulators. Therefore, I would bet the 7806 regulator in the radio has a heat sink attached to it.

So whats the RCA with the heat sink? Without knowing the particular heat sink thats being used its impossible to know. Lets assume RCA = 20 C/W when the heat sink is used, and thus the overall thermal resistance is 25 C/W.

If we use a 9V/600 mA wallwart, and we (again) assume an ambient temperature of 35 C, the power dissipation would be 1.8 W and the junction temperature would be 80 C. This is O.K. given the 150 C limit typically specified for 78XX regulators.

If we use a 11V/600 mA wallwart, the power dissipation would be 3 W and the junction temperature would be 110 C, which is a temperature increase of a whopping 30 C. Even though its within the 150 C limit typically specified for 78XX regulators, this temperature rise would make me uncomfortable. Besides, how do you know the wallwarts output voltage will be 11V? Because most wallwarts are unregulated, the output voltage could be significantly higher than the nominal rated voltage, especially if you draw a lot less current than the maximum (which may not be the case in your situation). But just to be safe, lets assume the 11V/600 mA wallwart has an output voltage of 12 V. This will make the junction temperature 125 C, which (in my opinion) is too close for comfort.

BTW: If we assume the radio uses a 5 V regulator the situation is even worse. This is another reason for going with the right wallwart. On the other hand, if the radio uses a switching regulator then (for the most part) all of this was a waste of time.

Anyway, the message here is that you should use a 9V wallwart. Mouser Electronics sells them real cheap.
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