How to safely connect a 12v radio to a 12v leisure battery?

[Jul17pri]

Turning 16 volt to 12 volt reasonable easy, a 7812 chip can do that, but 11.5 to 14.8 volt to 12 volt not so easy.

OK, I'll try a 7812. Maybe 1.5 amps is ample for normal use and it only needs 2.2 amps in exceptional circumstances such as flashing new firmware or re-tuning. Or maybe the speaker wouldn't go as loud at max volume.

A normal diode will have around 0.6 volt drop across it, a red LED around 1.2 volt, a white one around 3 volt, but the problem is 2.2 amp is over what you can drop with cheap devices, so you need some thing like this

basic idea it turns DC to AC transforms it and then changes back to DC and it alters the mark/space ratio to maintain the voltage.

The reason I'm making 12v power supply cables is to avoid using an inverter, which wastes power by turning it into heat and then running a fan to dissipate it. The battery is charged by a solar panel, so needs to last the night. But Battery Guards also avoid damaging the battery by totally discharging it. And there are versions that indicate how much power is left in the battery, which would be useful to know. And not expensive, so maybe worth adding to the system.

 

[davelx]

Turning 16 volt to 12 volt reasonable easy, a 7812 chip can do that, but 11.5 to 14.8 volt to 12 volt not so easy. A normal diode will have around 0.6 volt drop across it, a red LED around 1.2 volt, a white one around 3 volt, but the problem is 2.2 amp is over what you can drop with cheap devices, so you need some thing like this

basic idea it turns DC to AC transforms it and then changes back to DC and it alters the mark/space ratio to maintain the voltage.

That device is not a regulator. It does not change the voltage delivered from the battery, it merely disconnects the load if the battery voltage falls below a level set by the pot.

This is a regulator.

 

[oldbutnotdead]

As someone noted, the 1N4 I originally suggested is a bit under the required spec (been a while since I played with this sort of stuff), the 1n54 whatever referenced can handle more current. Of course if you happen to have a bundle of the smaller ones you could just wire 3 or 4 in parallel...
Anywhere in the cable set is fine, after the fuse is sensible.
Couple of other thoughts- if the battery is a car battery, get a pair of battery connectors- the + and - are different sizes so tricky to get them wrong.
And a big lump of red PVC tape round the + wire is always helpful...

 

[RandomGrinch]

I use a large lead acid battery to power a telescope in the back garden. I've wired a fused car accessory socket to it.
This has the advantage of being able to utilise a huge number of car accessories; including DC/DC converters and a USB power adapter to supply the RPi controller.

 

[ericmark]

That device is not a regulator. It does not change the voltage delivered from the battery, it merely disconnects the load if the battery voltage falls below a level set by the pot.

This is a regulator.

Yes an error on my part, I know I have used a 12 to 230 volt inverter feeding a regulated power supply as it was so expensive to get a 12 to 12 volt inverter, and I could also use the 12 to 230 volt for other things. It seems a waste of power, but I have a power pack 12 Ah with built in inverter at 300 watt which is so handy. Is also a jump start kit, but mainly used to get power without extension leads.

Would be nice if things actually said 11.8 to 14.8 volt rather than 12 volt, as one has no idea of what range the items can take. Had it with G5.3 MR16 bulbs, most simply say 12 volt, some say 10 - 30 volt DC only.

I can see why a fridge has independent power supply dropping 230 volt to 12 volt, this is to get around the EC laws on efficiency, but why a power hungry radio does it I have not a clue, why no build the power supply into the radio? Main reason I would not use a DAB radio is the power they use, if in the home then internet or freeview or satellite means DAB just uses up extra frequencies, and portable they use too much battery, only time we need DAB is for a car radio, and having driven around Wales so often the car radio has to switch to FM it does seem a little pointless with a car.

Aunty Marry rules OK. Well I have even used SSB that really reduces power required, however not really much good for music.

 

[bernardgreen]

I would use a Schottky Diode as this has a lower forward voltage so less of the 12 volt is lost

For example
https://uk.rs-online.com/web/p/schottky-diodes-rectifiers/7743350

Other sources are available

 

[davelx]

The other way to do it is to connect the diode across the supply AFTER the fuse so that with correct polarity it is reversed biased. If the polarity is wrong the diode will conduct and blow the fuse. This avoids any voltage drop, which may be desirable if the 12V battery is not being charged in a vehicle.

Best used with very fast acting semiconductor fuses.

 

[RandomGrinch]

The other way to do it is to connect the diode across the supply AFTER the fuse so that with correct polarity it is reversed biased. If the polarity is wrong the diode will conduct and blow the fuse. This avoids any voltage drop, which may be desirable if the 12V battery is not being charged in a vehicle.

Best used with very fast acting semiconductor fuses.

If you can live with a 1.2V drop, there's no harm using a bridge rectifier!
...and you're halfway to a voltage regulator

 

[Gasguru]

Forget conventional regulator devices..they can be hopelessly inefficient and the input voltage needs to be somewhat higher than the input (known as the dropout voltage...typically 2 V but lower dropouts are available).

Google buck boost converters ....this is the terminolgy used by power supply designers to describe a power supply that can have say a 12V output and perhaps a 5 to 20V input.

As Bernard said a suitably rated Schottky diode will only loose 0.5V or so for polarity protection.
If a diode voltage drop really is too much then a P channel MOSFET can be used (along with a few other components) to give a near zero voltage drop. Google polarity protection etc.

 

[JohnW2]

Awesome! I'll build & test my reverse-polarity-protection with a bulb or volt meter before connecting the radio. As they cost pennies, I'll get some of each type, try a 1N4001 first, and if that blows then try a 1N5401.

Since both only cost pennies, there's little point in getting or 'trying' a 1N4001 - there's no downside of using a IN5401 from the start.

In passing, no-one seems to have mentioned that by using four such diodes as a 'bridge rectifier' (in a 'ring') the radio would still get correct polarity electricity even if the battery were connected the wrong way around. The only 'downside' would be a doubling of the 'voltage drop', but I very much doubt that would be of any consequence for a radio. You would still need a fuse.

Kind Regards, John

 

[mattylad]

If your going to use a diode, then it goes in the + lead.
The diode has a line on it at one end, the other end goes to the voltage source (battery) and the end with the line on goes to the radio.

IF the radio is now not working, are you going to see if that can be repaired before resigning it to the bin?
there are repair workshops popping up all over the country and one may be near you.

 

[JohnW2]

If your going to use a diode, then it goes in the + lead.

I can think of no particular reason why it needs to (in the other lead would work just as well), but I suppose there's something to be said for a 'clear instruction'!

Kind Regards, John

 

[Harry Bloomfield]

I though the radio would just draw the power it needed, but maybe the battery amps exceeded the capacity?

No, what is important is the 12v and the battery being able to supply enough current and for long enough - it should have been fine, but you only suggest it was a 12v radio. Was that a car type radio, or something else? Did it need ac or dc?

Or maybe a combination incorrect polarity and excessive current. i.e. Maybe the radio was protected against incorrect polarity but only up to the recommended 2.2 amps.

A) What do you think went wrong?

Assuming it was 12v dc, likely you got the wrong polarity? Sometimes they are protected by a diode to short the 12v, but that relies on there being a fuse to be in circuit to blow. I would suggest a 3amp, or perhaps 5amp fuse.

 

[Harry Bloomfield]

Bung a diode in series to prevent any current flowing if you get the polarity wrong?

Which will cause some volts drop, the normal way is a diode in parallel with the radio, so it shorts and blows the fuse.

 

[JohnW2]

Which will cause some volts drop, the normal way is a diode in parallel with the radio, so it shorts and blows the fuse.

Yes, that's sometimes done - but the finite time it takes a fuse to blow coupled with the speed at which semiconductors can be killed, means that it is a much less sure way of protecting semiconductor electronics than is 'blocking' reverse polarity with a series diode (or diodes).

Unless this device is much more than 'just a radio', and given the 12V supply, I very much doubt that the sort of voltage drops we're talking about would have any noticeable effect. As I said, even if the path included two Si diodes (maybe about 1.2V VD), using a 'bridge rectifier' approach, I think that would probably be fine for 'a radio'.

I also have to wonder how/why 'just a radio' would use ~26W, although maybe the 2.2A rating of its PSU is far higher than actually needed?

Kind Regards, John