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Adapting an adaptor

 
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ReseTim
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Joined: 26 Dec 2008
Posts: 4
Location: Minneapolis, MN

PostPosted: Fri Dec 26, 2008 10:36 pm    Post subject: Adapting an adaptor Reply with quote

Good news: new toy. Bad news: it runs through batteries real fast. Good news: it comes with an adaptor. Bad news: it's a car adaptor, and I'm not using it in the car. Worse news: it has a funny connector, so I can't use the universal adaptor from RS. Good news: I pull off the cigarette lighter plug, hook the wire up to a 3V AC adaptor, and new toy works fine. Bad news: a permanent solution would involve cutting wires, stripping wires, and soldering. I need this 3V adaptor for it's original purpose. Good news: lots of other old AC adaptors laying around. Bad news: none are 3V.

I'm thinking I can add a resistor, using the same formula you use for LEDs:

resistor impedance = (source voltage - device working voltage) / device working ampage.

But is this a good way to blow up my new toy?
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Alan
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Joined: 29 Jan 2006
Posts: 1399
Location: Winnipeg, MB

PostPosted: Sat Dec 27, 2008 10:53 am    Post subject: Re: Adapting an adaptor Reply with quote

ReseTim wrote:
Good news: new toy. Bad news: it runs through batteries real fast. Good news: it comes with an adaptor. Bad news: it's a car adaptor, and I'm not using it in the car. Worse news: it has a funny connector, so I can't use the universal adaptor from RS. Good news: I pull off the cigarette lighter plug, hook the wire up to a 3V AC adaptor, and new toy works fine. Bad news: a permanent solution would involve cutting wires, stripping wires, and soldering. I need this 3V adaptor for it's original purpose. Good news: lots of other old AC adaptors laying around. Bad news: none are 3V.

I'm thinking I can add a resistor, using the same formula you use for LEDs:

resistor impedance = (source voltage - device working voltage) / device working ampage.

But is this a good way to blow up my new toy?


What is your new toy (not that it matters, just curious). 12 Volt DC adapters are very common, you probably have one or can purchase one very cheap. I would simply feed in 12 volts into the car adapter. You can just connect the adapter to the plug for a quick fix or purchase a Y adapter and cut off one of the female ends for your use.
http://www.amazon.com/gp/product/B0006OC2MY

When on vacation this summer I saw lots of them for $5.00 at gas stations so there should be no issue finding a cheap model.

Note that the center pin is positive in the male cigarette lighter plug.
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ReseTim
Newbie


Joined: 26 Dec 2008
Posts: 4
Location: Minneapolis, MN

PostPosted: Sat Dec 27, 2008 10:21 pm    Post subject: Reply with quote

That can work. I was at Axman's in St Paul today, and they had heaps o' 12V adaptors, and not a single 3V.

I didn't mention the type of toy, because I was looking for a generic solution that I could use in a lot of different situations. (It would be nice to know if the formula works.) The toy is an FM transmitter. These are used to transmit the music on an MP3 player or CD player to the car stereo. I'm using it to transmit from one radio to another. Radio A is an HD radio. Radios B through E are not HD. I suppose I could turn the volume way up on Radio A, but that would be ill advised at 6:00 AM in an apartment building.
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Alan
Site Admin


Joined: 29 Jan 2006
Posts: 1399
Location: Winnipeg, MB

PostPosted: Sun Dec 28, 2008 12:57 pm    Post subject: Reply with quote

ReseTim wrote:
That can work. I was at Axman's in St Paul today, and they had heaps o' 12V adaptors, and not a single 3V.

It would be nice to know if the formula works.


Unfortunately it doesn't usually work well for complex devices since an LED will continue to draw the same amount of current once it has been setup with the correct current limiting resistor and a stable voltage. Your transmitter will want a stable voltage source as opposed to the LED that wants a constant current. The transmitter will probably have functions that can drastically change the current draw such as a backlight or low and high power modes, if a simple resistor was used the voltage will raise and lower as the unit changes its current demands.

For this application a voltage regulator is your best bet, that is what would be in the shell that plugs into your car and provides the 3 volts that the device needs.

The LM317 device is a variable device where you set the voltage that you want.
http://www.national.com/mpf/LM/LM317.html
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ReseTim
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Joined: 26 Dec 2008
Posts: 4
Location: Minneapolis, MN

PostPosted: Sun Dec 28, 2008 6:51 pm    Post subject: Reply with quote

Life isn't simple? Darn!

There was probably a voltage regulator in the electronics that I pulled out of that plug that I dissented a couple days ago. It just looked like a capacitor and a transister.
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