How do I get 2.4V

chasroberts

chasroberts

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Hi all

I have one of those small fm transmitters to play music through the boat's sounds system from the laptop. It runs on 2 x AAA batteries @ 1.2V each.

I currently have rechargeables but it's a pain having to keep recharging them as they don't seem to last that long. Anybody got any thoughts on how to get a 5 to 1 step down transformer to reduce 12V to 2.4V or can I use one of those little multi adapters set to 3V off the mains? Will 3V be too much for the transmitter?

Any thoughts or advice much appreciated.

Chas
 
Yes it should run Ok on 3 volts as jimmy says.
However another option is to use a small resistor to charge the nicads from 12 volt ships battery. Charging while using the batts.
The nicads are likely 400mAH rated. The max charge rate then is 40 milliamps for long term connection. and 14 hr recharge.
So 12 volts less the 3 volts of the batteries is 9volts to be dropped at 40ma needs a resistor of R=E/I is 225 ohms which will dissipate (get hot)P= ExI 9x .04 is .36 watts.
So you need a 1 watt ressitor in the range 270 ohms to 430 ohms.
The higher resistance will charge more slowly but be less harmfull to the Nicad batteries. But if the little transmitter draws a lot of current you may need a resitor on the lower side. Resitors are so cheap you should get from RS or Maplins a range and use a lower resitor if you find the battery goes flat after long usage despite the charge current. PS don't operate without the batteries.
The ships Neg goes to the battery negative and the ships positive goes through the resistor to the battery positive. You should check howeve rwwith an ohmeter to ensure the battery negative coonnects to the input screen (ground shield) of the wire just in case battery negative is not connected to system earth. If it is not you will have to ahve a transformer as you suggested on AC. good luck olewill
 
Not exactly the answer to your question, but someone might be interested:
maybe like you I was sick of being brained by flying CD cases, not to mention the room they took up, so went the MP3 player route instead -- iPod with FM charger/transmitter sending sound to the radio/CD player. Works brilliantly, takes up no room, and a lot less power-hungry than a laptop.
 
Thanks for that guys. William. Many thanks but you lost me after "However another option is...."

Thought about the i-pod but I have a 50gb external usb powered hard drive which has got about 15gb of our music on it along with all our photos from the last couple of years and it's still not half full. Will have shore power available for a while as I'm staying here in Gib for now and just looking for something to avoid either a stack of rechargeables always on charge. I'll try the 3V thingy for now and see how we get on.

Once again, many thanks to all for your input.

Chas
 
It may work brilliantly, however it is illegal - the penalty is a fine and confiscation of all RF equipment.

As for the Resistor idea - how does that handle power spikes? You really need regulated voltage.
 
If Ofcom keeps its finger pulled out,these items will be legal to use by the middle of December this year.I've stuck a label on mine prohibiting use until legislation is through!
 
No, they won't. The ones Ofcom are planning to legalise are some 20 dB+ down on the ones currently on sale.
 
right.. and who the hell is going to know? I can see the need for a legal restriction on broadcasting, but broadcasting 7metres...they ll need to be on your boat to notice!!
 
If you want to do a proper job, you can buy from RS Components a Traco chip which reduces 9-18 volts to 2.5 stabilised.
I have recently fitted a similar Traco device to my car so that the "Illegal FM device" ( which runs at 12 volts), and the MP3 player which runs at 5.0 volts can both be run from the 12 volt battery.. and it will work on the boat via the ciggie lighter
Had similar on last car for the 5 volt GPS system.
Traco chip is about the size of a postage stamp and is good for 500 ma, so more than enough grunt for a low power FM transmittery thingy. There are specific models of chip to reduce both 12 and 24 volt power down to various voltages.
I will post the site address and page in a separate posting
 
Hello simon the concept of the resitor is to continually charge with a small current the nicad batteries from the ships 12v supply. The batteries themselves provide the regulation. olewill
 
As he is in Gibraltar it is highly unlikely that any regulations apply. In the UK, every shop is selling the Belkin version which I have, including Maplin, Halfords, Currys, and there is a move to make then legal, as like the CB craze there are so many around it will be impossible to police - hence make then legal.
They work rather well as it happens!
 
Having extolled the virtues of the Traco chip I now find they have discontinued the 2.5 v version. Perhaps the 5 volt version can be stepped down to 2.5 - 3.0 volts - perhaps some forumite can advise?
 
with many new models (even cheaper ones) there is now an aux socket on the front.......I run my ipod sitting in the 12v socket at the helm and via cable to the unit. sorted.

another of those SWMBO happy moments..........well once I had uploaded her CD collection anyway.
 
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