Plugging a 7.5 Vdc Device Into a 12 Vdc Source

About the same as would happen to my 240Volt AC motor if I put it across the 415V supply.

Things might fail catastrophically, it might work for a little while and then fail, or it might survive forever due to some fortunate design consideration from the manufacturer.

The answer is in your hands!

(Don't know if this translates to the other side of the world, but the expression is used by a mobile phone network with a help line for their customers.)

Like wot PW says (#1)...with the added factor, it depends what current the 12v dc souce is capable of providing. If it's a low power output, then maybe the load would pull the output down befre damage could be done.
It's down to risk vs reward.
If you have some old components lying about and a meter, you could wire in a potentiometer and work out what was going on...but then, if you could do that, you could have built your own supply.
Del

Hi Ron;

Don't do it, first assume the worst posibility (no internal protection at all). then purchase a voltage regulator (#7808) that is capable of delivering the current drawn by your radio, (will cost cents); then you can chose from these two options:

1.-Build the regulator into your radio.

2.-Make an in-line drop-down regulator that accepts 12vdc in the input and gives you about 8vdc at the output.

When you see the schematic of the regulator, it'll become pretty clear to you and you'll need no help at all to build the circuit, it's a 15 minute task; 25 if your'e smoking and drinking cofee, wich I dont't advice since leaded solder is poisonous, but you can use "clean" solder (not so neat results, but is safer).

Yahlasit

Your radio will play it out loud. Don't know how many seconds. Secure it. Get a DC/DC voltage converter,

I have to get in on this one. I have an old favorite radio that the power cord got ruined I could not find another to buy or match up because the input plug on the radio was an unusual size so I simply found a converter at the correct voltage from a pile of old cellphone,cordless phones and other devices I had saved and cut the cords and reused my end and spliced it and it worked perfectly . In the future I will save any device that qualifies and keep my new set up away from the cat( he chewed the original cord) . The best part was re-useing what I had and I still love that radio and the memories that go with it.

Without analyzing the radio circuitry, you are going to have to determine a minimum voltage that will run the radio. Use a 110V heater ( CAUTION - UNPLUG THE HEATER BEFORE YOU START) as a large high wattage pot as follows:

Move the + power lead to the radio along the heater element, right to left ( high resistance to low resistance) until the radio starts playing. Vary the volume to be sure it will keep playing at this value. Measure the resistance of the heating element at this point. That is the value of power resistor you should put in series with your radio. The power resistor should be a fairly high wattage. Calculate the Watts with the formula:

W = V x A, where V is voltage drop across the heating element

and A (amps) = V/R ( resistance of element in ohms)

Good Answer, but, with reservations:-
Most of the radios power will depend on how loud it is, so your trying to hit a moving target.

I loved the "CAUTION".

What did you use to draw the picture?
How long did it take you?

My thought was to get the radio operating with no chance of burning it up. Then once current and voltage limits are known it would be possible to refine the power supply. The 7.5Vdc sounds like a wall cube which seldom are larger than 1 A. But, with the information given, who knows? The radio could have overvoltage protection, or maybe not. The answer could include a flaming fire in a possible dangerous location.

I drew the picture with a CAD program called DesignCAD that I bought for $500 in 1998. The picture took me about 5 min. I drew one loop of the heater coils, then used an array function to generate the others. The radio grille is done with a fill tool. For this forum, I have found that a JPG format is best.

The "CAUTION" is from long bitter experience.

Nice picture, but you explained why NOT good.

Unless you make it a whisper box, this doesn't work. Radios on batteries have a class C end stage, means : surely no constant power consumption, but loud = much and silent is very little current. Like your battery is on dope. Your pot works as a music crusher that flattens each attempt to listen to what an artist calls music.

Good answer,
Or you could use a single series 4v7 zenner . If its drawing 100ma that's only half a watt dissipation.
Del

Before experimenting with any of the self build proposals: how powerful is your radio? Watch out for diodes, watch out for Zeners, if not cooled and undersized.