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I would like to power a table radio with 12V DC instead of 110V AC

oragex

Apr 11, 2022
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Hi everyone, I'm new on the forum
I'm not that skilled with electronics and I would like to power a radio (it's a GE Superadio model) from a car battery. The radio circuit has 12V but the radio works either on 110V or on batteries. I was thinking about soldering wires after the transformer to feed directly with 12V DC while the radio is unplugged. Could this be possible ?
 

bertus

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Nov 8, 2019
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Hello,

Looking at the page, the used battery seems to be 9 Volts.
Are there 6 C or D type batteries in the battery compartiment?

Bertus
 

Alec_t

Jul 7, 2015
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The schematic for the power supply in the post #3 link shows a 9V battery. You would need to check that all components in the radio are rated to handle a minimum of 12V. You also might need to adjust transistor bias voltages at various points in the circuit to cope with raising the supply from ~9V to 12V. Note that 12V is only a nominal value for a car battery: it might well be around 14V when the battery is in use in a car with engine running.
 

oragex

Apr 11, 2022
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It takes indeed 6 batteries. I also didn't realize that a car battery would have variable voltage. I'm thinking of a way to power the radio for longer periods at a remote location where there is no electricity, if possible without its batteries.
 

Alec_t

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You could use an analogue voltage regulator, or switch-mode DC-DC converter, to drop the car battery voltage (say12-15V) down to a fixed 9V to run the radio safely within its existing ratings. Such a DC-DC converter, if of low quality, might introduce undesirable noise though.
 

oragex

Apr 11, 2022
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I went looking to a 12v-20v to 9v converter, it says max output power 18W, would this be a parameter related to the power of the speaker ?
 

dave9

Mar 5, 2017
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There are lots of buck converters capable of more than 18W. 18W might even be enough, but it's always nice to have more margin, especially if it's a generic chinese module with optimistic ratings which don't correlate with the minimal heatsinking they provide on a tiny PCB.

One example: https://www.ebay.com/itm/123928792557

If a SMPS (like linked above) is too noisy, maybe throw together a 3A LDO regulator circuit, though then you need to shoehorn in enough heatsink, i mean ideally it'd be in the battery compartment so it's out of the way.
 

CircutScoper

Mar 29, 2022
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Hi everyone, I'm new on the forum
I'm not that skilled with electronics and I would like to power a radio (it's a GE Superadio model) from a car battery. The radio circuit has 12V but the radio works either on 110V or on batteries. I was thinking about soldering wires after the transformer to feed directly with 12V DC while the radio is unplugged. Could this be possible ?

So called "inverters" that convert 12VDC to 120VAC are pretty cheap.

https://www.amazon.com/BESTEK-Inver...la-758680743983&ref=&adgrpid=70591845793&th=1
 

oragex

Apr 11, 2022
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Much appreciated the answers, I'll look into a converter to attach and wire directly to the rear battery compartment as suggested.
 

oragex

Apr 11, 2022
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A small question, as the input voltage drops - the car battery will keep discharging, will the output voltage stay constant ? Do all these buck converters keep the output voltage constant ? Assuming there will still be a difference between in/out voltage
 

dave9

Mar 5, 2017
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^ Yes that is the point of a regulation circuit, though once the battery drops below a certain voltage, the forward drop of the regulator (suppose around 2V) will stop stable operation. For example with a 12.6V battery and a 2V forward drop, it should be stable down to at least 11V with the regulator set to 9V output. It would be wise to put a low voltage cutout circuit in series before the regulator to save the battery from excessive drain, particularly with lead acid or (Li-Ion) batteries as they don't like to be drained too low.

I'd want the cutout circuit to open the circuit before it gets to 11V with a lead acid ~12V battery, possibly even above 12.0V depending on the discharge rate. The lower the rate the higher the cutoff voltage should be. You can find discharge rate vs voltage graphs with a web search, then determine acceptable battery capacity drain % based on type of battery or consulting battery manufacturer datasheet.
 
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oragex

Apr 11, 2022
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Thanks. Is it a concern if the output voltage drops below 9v ? Would it affect the radio circuitry, or simply the radio will stop working ?
 

dave9

Mar 5, 2017
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^ I've added more to my prior post.

I can't predict the minimum voltage the radio needs to operate correctly but it is bound to also be a factor of how loud you want the sound playing. I mean it would probably work fine down at 8V and even lower, considering it must do this to get reasonable life out of the 6X primary cells it was designed to use.

You could experimentally determine this, but as stated previously I wouldn't want to drain a lead acid battery below 11V anyway at this slight drain rate as it will reduce its lifespan.
 

CircutScoper

Mar 29, 2022
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Much appreciated the answers, I'll look into a converter to attach and wire directly to the rear battery compartment as suggested.

Hold the phone!

Looking at the schematics you've provided, it turns out that your radio already has extensive power conditioning built in, including a voltage limiter (Q7 etc.) and a switching converter (U2 etc.). Nothing further should be necessary and a direct connection to your car battery (e.g. to the "+" end of C66) should work fine.

sr3_pwr.gif
 

dave9

Mar 5, 2017
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^ I'd still want a low voltage cutoff circuit to not excessively drain the battery.
 

oragex

Apr 11, 2022
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Thanks for the answers. I am going to purchase a buck converter as I will also try to power up the radio from a solar panel. I'm adding to the list a 12V Battery Low Voltage Cut Off Switch.
 
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