12v - 7v = 5v but how ?

[CPD]

I want to use a bluetooth adapter on board but it requires a 5v dc input for which it provides a 240 plug in transformer which could be pluigged into an inverter whioch I dont want to do. Are there any other options ? Thanks, as usual.

 

[duncan]

find an old car adapter to take your 12v down to 5v and get busy soldering the connections..........can't remember one of hand but I have seen 5v on one of the kids old power leads

alternative get an adjustable output unit (maplins?) and set to 5v - you may even get the right plug with it!

 

[smeaks]

maplins do a 12v cigar type plug with a multi output switch and set of plugs allowing variable voltages and jack plugs to be utilised. They dont cost alot... but cant remember how much. I used one to rig a charger for my VHF via the mains adapted input.

 

[mikefleetwood]

Does the Bluetooth adaptor need precicely 5V? - if so, RS, Maplin, etc do cheap small DC-DC converters for 12V to 5V. Most small electronic devices have internal regulation and are quite happy with anything from 6-12V, but you will need to check what it says first.

 

[alan]

Use an in-car adapter, available from any car shop (Halfords) or Maplins. Cost about 10 squids. Make sure it has the current capability that you require. (More current = more cost). Just bought one the other day, from local electronics shop, for 1 amp output and cost me about 17 euros.

Alan.

 

[CPD]

Thanks for your help chaps - sorted. /forums/images/graemlins/smile.gif

 

[tangofour]

I have a tube full of switching 5v power supply devices - 8 to 30 volt input, a precise, current limited ( short circuit proof ) 5 volt out at 2 Amps. Around 90% efficiency. Just one external capacitor required - 100uF ( small component )

£8 each!

 

[pappaecho]

I used a Traco chip from Rs Components which converts 12 volts to a stabilised 5 volts, which provides two outputs from one chip. One output for the 5 volts GPS system and the second for the Mp3 player.... Its in a Jag, but would work equally well in a boat

 

[William_H]

Just for the sake of waffling on. You need a regulated power supply for this device because your boat voltage will vary from 11.5 when battery is low to 14 volts when battery is being charged.
You could use a lot of silicon diodes to subtracy 7 volts (about 10 in series) but this would leave you with a voltage varying between 4.5 volts and 7 volts.
A regulator chip has a transistor in line between the 12 v supply and the output where 5 volts is desired. The output voltage is sampled by an amplifier and compared to a voltage reference, which then moves the transistor towards more conduction or less ensuring that regardless (within limits) of the current used at 5volts or the input voltage the output voltage remains 5 volts exactly.

A chip with almost all the components costs about 3 oz dollars or 1 and bit squid. However it is probably easier to buy a regulator set up in a box.
An alternative style of regulator actually switches the regulator transistor of an on at 100K times per second and the ratio of on to off is varied to produce the 5 volts. This tye is more complex and can produce radio interference.

The thing about the simple regulator first described is that the transistor in the regulator wastes (dissipates) the drop in voltage in heat. So if your device uses 5 watts at 5 volts (1 amp) then the regulator will have to dissipate the 7 volts at 1 amp or 7 watts of heat (or up to 9watts) So the regulator chip will have a power dissipation limit.
The switching type are more efficient in that by storing the energy in an inductor the output current can be a lot more than the input current (like a transformer) with less heating so often a higher power capacity.
But I am sure for your Blue tooth adapter power is not a concern.
I hope this waffle gives you an idea of wehat is happening. olewill