Saving Radios from Reverse Polarity

[FrankB]

On any DC operated TV, radio, home or car, receiver or transmitter, just install a bridge rectifier of the correct voltage and current rating on the battery connector leads. No matter which way you put DC into it, the radio won't be damaged, due to the bridge rectifier.
Any DC radio I work on gets one installed on it, especially the car radios.
yes, its a few cents, but then it sure saves a lot of time troubleshooting.
(And I deliberately RP'd one of my own radios I did this to, and it worked just great!)

[trh01uk]

The bridge approach will work very well - provided you can live with the loss of voltage to the equipment, which is about 1.4V in all due to two diode drops.

There is a more complex solution which does not require any voltage drop at all, but is not quite as flexible in that you have to get the DC polarity right. It does totally protect against reverse polarity though.

And that is to use a relay and a diode in series with its coil. To save describing it, look here for a schematic. (That's just a random example of many, which I refer to save myself the bother of drawing it!)

If you want to reduce the power to the relay coil when the equipment is on, then you can put a resistor in series with the relay coil, which is shorted out by a second relay contact when the relay is not powered. This works because a relay needs more power to to operate than it needs to stay in the 'on' state.

It is possible to cut the power draw of the circuit to zero if a MOSFET is substituted......I leave readers to look that up.


Richard

[mhennessy]

A single Schottky diode is a way to minimise forward voltage drops - around 0.3 volts, which in percentage terms is a decent improvement on the 0.6 or 0.7 volts of standard diodes. Nothing stopping you building a bridge with them, which would still only drop 0.6V.

Voltage drop isn't a big deal with most sets - especially those with high rails (e.g. two PP9). The loss of output power amounts to an inaudible fraction of a dB in most cases.

Another approach is to use an inverse-parallel diode. This is best done with a series fuse, but in practice most "wall wart" PSUs current-limit fairly harmlessly providing the overload is brief. If using a 12V car power lead, make sure that is fused (they usually are), and reduce the fuse size if necessary.

The two PP9 radios have some form of reverse polarity prevention built in, it would take a very determined person to connect both batteries the wrong way and have the set on. Sets with only one battery can be given a short term wrong polarity zap while fitting a new one (if on).

[G6Tanuki]

Originally Posted by trh01uk

There is a more complex solution which does not require any voltage drop at all, but is not quite as flexible in that you have to get the DC polarity right. It does totally protect against reverse polarity though.

And that is to use a relay and a diode in series with its coil. To save describing it, look here for a schematic. (That's just a random example of many, which I refer to save myself the bother of drawing it!)

That approach was standard practice in the likes of the Pye "Westminster" mobile-radios: they were designed with a floating-chassis (so didn't care whether they were in a positive or negative earth vehicle). The on/off switch on the control-head controlled a relay that switched power to the transceiver itself. A diode made sure the relay didn't pull in if the polarity was wrong.

My modern approach to polarity-protection is to put a heavy-rated diode across the power rails such that if you apply wrong-polarity DC the diode conducts and murders a fuse somewhere.

Stud-mout 50-Amp diodes are cheap.

[emeritus]

I used to have a Philips 12" mains TV that could be run from a 12V DC supply, and it had the "inverse parallel plus fuse" protection for 12V operation. It really needed the full 12V for satisfactory operation.

[kalee20]

I use an inverse parallel diode too, in fact I added one yesterday to the 12V power input of a public address amplifier. There's going to be an inline fuse added, upstream of the diode.

Doing this, there is no diode drop lost from the supply voltage, unlike what happens with series diodes.

I only protect DC equipment which could be damaged, such as transistorised stuff, and valve equipment with electrolytic capacitors across the supply rails. I have a couple of battery portables with non-polarised supply line decoupling capacitors, and I don't intend to do anything here. Reverse polarity does not hurt valves.

[BottleMan]

For those who wish to take the MOSFET route -

--
"Winfield Hill" <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote in
message news:...
> Graham Holloway wrote...
> >
> > I believe it is pretty well standard practice to use an N-channel
> > MOSFET in the return lead of military power supplies (28V input).
> > Drain to supply negative, source to the negative of the PSU and
> > the gate driven by a protected derivative of the positive supply.
> >
> > Kind regards
> >
> > Graham Holloway
> > WPS/Accuphon Electronics
> > (Tel/Fax 0(044)1233 662599)
>
> That's interesting (and well stated). Do you have any links for us?
>
>
> --
> Thanks,
> - Win

Sorry, no links. Aquired knowledge while working for various power supply
companies in the UK. BTW, it's also common to include surge protection
circuitry with the reverse-polarity circuitry as well.

Regards

Graham Holloway