12 volt question

L

lilianroyle

Well-Known Member
Joined
16 May 2001
Messages
309
Location
london
Visit site
at home I have various DC devices which use different voltages (3v 9v etc) and use different AC to DC adaptors. On the boat, if for example I want to run a portable DVD player which uses 9v, can I run it direct from the domestic battery with a fuse, at the moment I tend to run these things through an inverter which seems like a waste of time?energy Can I connect any DC device to a 12v battery without problem?
Thanks
 
The simple answer is no.

You can sometimes get away with putting a resistor in series but that will get hot because effectively its wasting the energy you dont use in the device you are trying to power.

Not a good idea really

More sophisticated is to buy a voltage regulator which I guess Maplin and others supply but not sure thats cost effective

Have fun
 
There are plenty of devices to drop 12V nominal to the various voltages used by these things. Most auto accesory places sell them as does Maplins and many others.
 
Just to be pedantic the common regulator for this service is a series regulator which is a transistor set up as an automatically adjustable resistor. So the transistor wastes the same power by getting hot that a resistor would waste.
The regulator can however adjust to cope with variable current drain from the device and also cope with the variable input voltage. ie 14 volts on charge down to 11 volts when battery is near flat.
But the current drain of the device at say 9v eg .5 amp will result in a current drain of .5 amps at 12v.

A different kind of regulator is the switching type which effectively turns the DC into AC to feed a transformer. This kind can be very efficient for larger loads. And it can have the ability to provide 12v even up to 18v for a computer from an input voltage of 11 to 14.
This type can for a current drain of 1 amp at 6 volts result in a current drain at 12v of .5 amp except that they tend to be only 80% efficient so expect a drain of .6 amp.

Anyway back to the original poster. If your 9v volt device is not too fussy and you only use it on a battery which is not being charged. You could feed it with a string of silicon diodes each dropping .7 volt so 5 diodes should result in a voltage at the device of 3.5 less than the battery voltage. ie 8.5 to 9.5 volts for battery voltage of 12 to 13volts.
All these ideas are better than running an inverter. olewill
 
Hi Ol' Will.
Great description..

rather than use linear devices.. or devices with high collector to emitter current wastage.. 'on resistance'.. use fets.. far more economical... if you PWM the output and stabilise the voltage (simple sense circuit) you have all the voltages you need.. below about 11volts... thats the trouble really.. 12or 13.8 v devicesexpectthat.. then you are into step up transformation.. but even that can be harnessed, use a computer supply of a few amps, then apply your regulators to the output.. Bipolar are soooooooooo old now.. ya ol gimmer lol.... /forums/images/graemlins/smile.gif
 
Please first look at the devices you wish to use. Some have a range of 12 - 80 volts!!! A lot are rated at 13.8 volts, but will comfortably run at 15V which is more than a correctly set up alternator on full blast will generate. try Googling your product with the max input voltage question. If you still need 9V stabilised, then a series regulator chip 7809 (they come in various current-handling sizes) will cost about 3/6d... (they are numbered 78xx where xx is the voltage output, but you need up to 2 volts more in than you get out.)Maplin or RScomponents will supply
 
Hey Joe yes you are right it was the PWM style of regulator I was trying to describe. Much more efficient for larger loads and essential to get more out than you put in. olewill
PS what does lol mean?
 
Hi Will, 'laugh out loud' or 'laughing a lot'


I keep planning on making such a unit offering 3, 5, 6 9 volt outputs.. but never got around to it...
With the great quantity of fets available now, and the cheap price, it is becoming defacto. there are also many cheap step down regs that do it all for ya.. offering a preset level (user selectable) with pwm output.. some even include output fets for a couple of amps..or add another fet to increase current.. that is probably the way to. the advantage is also better voltage regulation and over current protection.

Joe.
 
You must log in or register to reply here.

Share this page

Top