CB Radio power supply?

[David Sewell]

Hi
I've got this power supply from an old piece of computer equipment. One of
the outputs reads about 14.6v dc. I tried a resistance of about 16 ohm to
load it. The voltage remained about the same. I think it should be
powerful enough to run my 4watt cb radio. But, how can I easily drop the
voltage down to (what to, anyway?) 13.8v for the cb?
Thanks,
David

 

[Sal Brisindi]

David,
If you are using a car (mobile) CB 14.6 volts is ok as the alternator in a car
will sometimes put out a higher voltage than 13.8

Sal

 

[Stan]

}I've got this power supply from an old piece of computer equipment. One of
}the outputs reads about 14.6v dc. I tried a resistance of about 16 ohm to
}load it. The voltage remained about the same.

That might be the function of a voltage regulator circuit in the power supply.

}But, how can I easily drop the
}voltage down to (what to, anyway?) 13.8v for the cb?

Got diodes? Silicon diodes have a 0.6 voltage drop...2 in series will
drop 14.6 to 13.2 volts...just be sure that they're rated for 2 amps
or better.

Stan.

 

[David Sewell]

}I've got this power supply from an old piece of computer equipment. One of
}the outputs reads about 14.6v dc. I tried a resistance of about 16 ohm to
}load it. The voltage remained about the same.

That might be the function of a voltage regulator circuit in the power supply.

}But, how can I easily drop the
}voltage down to (what to, anyway?) 13.8v for the cb?

Got diodes? Silicon diodes have a 0.6 voltage drop...2 in series will
drop 14.6 to 13.2 volts...just be sure that they're rated for 2 amps
or better.

Stan.

Thanks, I think I will use diodes then, just to be on the safe side.
Why is it 13.8 volts anyway. And not 12 volts?

David

 

[Franc Zabkar]

Thanks, I think I will use diodes then, just to be on the safe side.
Why is it 13.8 volts anyway. And not 12 volts?

Because, as was stated elsewhere, an automobile charging system
delivers approx 14V (depending on temperature).


- Franc Zabkar

 

[Rich.Andrews]

Hi
I've got this power supply from an old piece of computer equipment. One
of the outputs reads about 14.6v dc. I tried a resistance of about 16
ohm to load it. The voltage remained about the same. I think it should
be powerful enough to run my 4watt cb radio. But, how can I easily drop
the voltage down to (what to, anyway?) 13.8v for the cb?
Thanks,
David

A car will have 14.4 to 14.6 across the battery when the car is running.
You are good to go.



r

 

[Art]

Simple Answer: Yes Gov, It will work fine!! Suggest fusing either the supply
and/or the radio with appropriate device though.

 

[Rich.Andrews]

Simple Answer: Yes Gov, It will work fine!! Suggest fusing either the
supply and/or the radio with appropriate device though.

A car will have 14.4 to 14.6 across the battery when the car is
running. You are good to go.



r

On the plus side, opertaing the radio at 14.4 to 14.6 VDC as opposed to
13.8 will result in greater power output when transmitting.

r

 

[Allodoxaphobia]

On the plus side, opertaing the radio at 14.4 to 14.6 VDC as opposed to
13.8 will result in greater power output when transmitting.

Also on the plus side, operating at 14.4 to 14.6 VDC as opposed
to 13.8 VDC will result in a shorter life span for a CB radio.

Jonesy

 

[David Sewell]

Also on the plus side, operating at 14.4 to 14.6 VDC as opposed
to 13.8 VDC will result in a shorter life span for a CB radio.

Jonesy

Thanks,
Now i'm confused. Anyway, I have just picked up the cb I intend to use with
this power supply, but I need to get a mike and aerial for it. On the back
of the cb is stamped 13.2 volt. I think I will definitely go with what Stan
recommends above and use two diodes in parallel to drop down to about 13.2
volt. This cb was made in 1981 I do not want to push my luck.
Thanks again,
David

 

[TCS]

Also on the plus side, operating at 14.4 to 14.6 VDC as opposed
to 13.8 VDC will result in a shorter life span for a CB radio.

cite?

 

[exray]

cite?

I don't know squat about CB radios but don't they have any voltage
regulation inside?

-Bill

 

[David Sewell]

Correction, diodes in series!

 

[Rich.Andrews]

Thanks,
Now i'm confused. Anyway, I have just picked up the cb I intend to use
with this power supply, but I need to get a mike and aerial for it. On
the back of the cb is stamped 13.2 volt. I think I will definitely go
with what Stan recommends above and use two diodes in parallel to drop
down to about 13.2 volt. This cb was made in 1981 I do not want to push
my luck. Thanks again,
David

You can't help but run it at 14.4 to 14.6 when installed in a car. I have
tried running a radio with diodes in series with the power supply. It
worked OK until one actually tries to transmit. The resultant IR losses
are so great as to prevent the radio from operating correctly.
Undervoltage will do more harm than overvoltage. Have had some radios
lose their audio amp IC due to undervoltage.

Running at 13.8 or 14.6 will not make any difference in life span. As a
matter of fact most will withstand 18VDC or more. Consider what happens
when the voltage regulator of the vehicles charging system fails and it
goes overvoltage. Does all of the electronics of the car fail? No! If
they did, there would be one hell of a class action lawsuit. Same thing
with CB radios. Had radios come in with failed reverse polarivy diodes
blown, terminals burnt, etc. many times but never had one that failed due
to over voltage. 24 volt systems will cook it as when the vehicle is
running the voltage across the batteries is something like 28-29 volts.
Had one come in like that. It was a total loss. The internal voltage
regulation for the freq. synt had failed premitting +28 volt where only +5
should have been. The audio output was gone as was the RF output, driver,
and a few other miscellaneous pieces.

In all my years of repairing CB radios, Ham rigs, etc, never have I had
one show up that failed due to failure of the voltage regulator on the
car.

Without knowing what kind of CB it is, it is possible that it would quit
the next time you keyed the mic. Some of the radios of that era were
junk. Toshiba based units were expecially nasty. Then there was
Teaberry, Royce, EF Johnson (faily rugged but crap design), but the worst
was Sparkomatic. The cream of the crop was anything made by Uniden. That
includes Cobra, President, some Midland, and a few select others.

It was amazing the junk that was being peddled for big bucks during those
days. It was also amazing what one could get for performance out of some
of the units.

r

 

[Rich.Andrews]

I don't know squat about CB radios but don't they have any voltage
regulation inside?

-Bill

Bill,

The RF final stages don't have any regulated B+ supplying them. Other
areas do such as the PLL or crystal oscillator stages. There may be other
areas that are require regulation depending on the design. Never have I
seen a RF output or driver regulated. Current limited with a resistor or
choke, yes, but not regulated.

r

 

[David Sewell]

You can't help but run it at 14.4 to 14.6 when installed in a car. I have
tried running a radio with diodes in series with the power supply. It
worked OK until one actually tries to transmit. The resultant IR losses
are so great as to prevent the radio from operating correctly.
Undervoltage will do more harm than overvoltage. Have had some radios
lose their audio amp IC due to undervoltage.

Running at 13.8 or 14.6 will not make any difference in life span. As a
matter of fact most will withstand 18VDC or more. Consider what happens
when the voltage regulator of the vehicles charging system fails and it
goes overvoltage. Does all of the electronics of the car fail? No! If
they did, there would be one hell of a class action lawsuit. Same thing
with CB radios. Had radios come in with failed reverse polarivy diodes
blown, terminals burnt, etc. many times but never had one that failed due
to over voltage. 24 volt systems will cook it as when the vehicle is
running the voltage across the batteries is something like 28-29 volts.
Had one come in like that. It was a total loss. The internal voltage
regulation for the freq. synt had failed premitting +28 volt where only +5
should have been. The audio output was gone as was the RF output, driver,
and a few other miscellaneous pieces.

In all my years of repairing CB radios, Ham rigs, etc, never have I had
one show up that failed due to failure of the voltage regulator on the
car.

Without knowing what kind of CB it is, it is possible that it would quit
the next time you keyed the mic. Some of the radios of that era were
junk. Toshiba based units were expecially nasty. Then there was
Teaberry, Royce, EF Johnson (faily rugged but crap design), but the worst
was Sparkomatic. The cream of the crop was anything made by Uniden. That
includes Cobra, President, some Midland, and a few select others.

It was amazing the junk that was being peddled for big bucks during those
days. It was also amazing what one could get for performance out of some
of the units.

r

This is a commtron cb40f. I have tried it on receive only at present and it
is dropping the 14.67v supply down to 13.02v with two byx70 diodes in
series. Think I might take one out....or both....I'm scratching my head
now!!

 

[Rich.Andrews]

This is a commtron cb40f. I have tried it on receive only at present
and it is dropping the 14.67v supply down to 13.02v with two byx70
diodes in series. Think I might take one out....or both....I'm
scratching my head now!!

Are you saying that the supply itself is failing to regulate or that there
is excessive voltage drop through the diodes?

Commtron was not a popular brand in my area. I cannot offer any info
regarding it.

Keep in mind that many of the older radios had no high SWR protection.
Many times the radio would work for a while but after extended use the
output would fail. They would be very prone to failure when operated into
an improper load if the unit had been tuned or "peaked".

r

 

[Stan]

}Correction, diodes in series!

Thank you...I started banging furiously on my keyboard to correct you
when I noticed this second posting.

In all likelyhood, you probably would not cause damage to the CB.

But better safe than sorry.

BTW, if you have a schematic of the power supply, you could probably
lower the voltage internally through modification.

Heck, if you're lucky, it might even have an adjustment inside just
for that purpose.

Stan.

 

[Stan]

}This is a commtron cb40f. I have tried it on receive only at present and it
}is dropping the 14.67v supply down to 13.02v with two byx70 diodes in
}series. Think I might take one out....or both....I'm scratching my head
}now!!

13.02 V is fine...just check it in XMIT with antenna connected to make sure
you have enough V under full load.

Stan.

 

[Peter]

I don't know squat about CB radios but don't they have any voltage
regulation inside?

Some do, some don't.
Most of the ICs used in the UK sets can operate to at least 20 volts, it's
the electrolytic capacitor across the supply that may let someone know if
they have gone too far.
About the most common cause of such over-voltage is probably truckers
overloading their dropper, which then goes faulty - putting the 24 volt
supply straight to the output and into the radio.
Nobody ever told them not to run a 300 Watt RF amplifier from that 5 amp
dropper.