im trying to run a 9v pc speaker from my car battery to use with my pda as a gps speaker, i spotted a maplin multi voltage adapter for a car which is £10 but i cant help thinking theres a cheaper way when all i want is a 3v drop (yes im a cheapskate), itd need to supply 9v @ 1amp ish any ideas?
monolithic regulators are your friend! what you want is a 7809 3-pin regulator and a capacitor. A quick glance at the LM78XX datasheet assures me that "The LM78XX series is available in an aluminium TO-3 package which will allow over 1.0A load current if adequate heat sinking is provided". Most semi companies produce their own version of the 78XX (and 79XX) series, so you shouldn't have any trouble finding them. They do require that the input voltage be about 2.5V higher than the output, so you shouldn't have a problem there. Irritatingly, not all manufacturers make the 7809 (7805, 7812 and 7815 are the standard values), but you can get them - I know, because I got some in South Africa a few years back Fairchild make a 9V version , which is good up to 1A.
Damn - I never heard of a 7809. Crazy! I was going to suggest placing f 1n4001 dioded in series - but I like your solution better.
actually, these days, they come in a whole range of voltages - the Fairchild one I linked to comes in 5V, 6V, 8V, 9V, 10V, 12V, 15V, and 24V. If all else fails, you can always use a 317 (same thing as the fixed regulators above, only you tell it exactly what voltage to produce) to get 9V - those come in a 3A version, IIRC.
7809 is the dog's doo-dahs but it's maybe easier to find a 317T, and it only needs a couple of resistors adding to set a 9v 1.5A output. Or you can use a common 7805 with a 3v9 zener diode between the ground pin and ground (0v) to lift the output to 8.9v
The 7809 is a linear regulator, which would generate a fair amount of heat, maybe try a switching regulator: http://www.maxim-ic.com/quick_view2.cfm?qv_pk=1276&ln=
cpemma the 317t sounds easiest i can pick one up from maplin, what resistors would i need and how would i wire them? sorry i know its a basic question, im happy doing electronics but im useless at calculations and understanding spec sheets. ta
Formula for 9v output is 9 = 1.25(1+r2/r1) which spins round to r2/r1 = (9-1.25)/1.25 = 6.2. So for r1 = 240R, r2 = 1500R =1k5. You don't need the variable pot VR1, connect R2 between 317 pin#1 and ground, R1 as shown.
Slight correction, the final derived formula should read: R2/R1 = (9-1.25)/1.25 OR R2/R1 = (9/1.25) - 1 The result is the same, as R2 comes out to be 1488 ohms, which is as close as makes no difference to a 1K5 Resistor. Note that R2 is 1k5, not 1k as in the diagram (diagram reuse rules!)... sorry, the teacher in me couldn't resist... Error corrected, I'll bring you an apple...
I might have to do this again for a friend, what would the layout be for the 7805? the other one worked out ok but i dont know how to use breadboards so i end up soldering all the components together and it looks a bit messy , anything simpler would be welcome anyway itd be nice to compare the two ways
I've shown a few options here, the zener one is like this: The 1N4001 diode across the regulator is an optional safety device and the resistor R1 isn't really needed with a 7805, the zener cathode going straight to pin #2. The zener current is only about 2mA, so the lowest rating (0.5W?) is OK.
ok that looks simpler, if theres no resistor then theres no connection between output and pin2 at all then? sorry for being a muppet btw
Correct. You can build these up on a strip of 2A screw-terminal connector block as a ghetto method. Point to remember, the 317T tab is connected to the output pin, so don't let it touch any grounded metal or sparks will fly. The 7805 tab is connected to ground pin, so if it touches ground, the output will be the basic 5v, not 5v+Vzener.
ok last couple of qs, what does n stand for in terms of caps? i found a 100n by name but the 330 seems a bit evasive on the maplin site(needs to be from here, its next door) is it shorthand for nf? . You wouldnt believe i did electronics on my physics A-level mind you that was 6 years ago ps i like the screw terminal idea ::edit seems like in this case n = nf but not nesesarily always, its sort of a universal unit that can be used in place of a decimal point, am i right?
nF is nano farads. The 330nF capacitor doesn't have to be exactly 330nF. 100nF, 220nF or 470nF will all be fine, infact, since you're running this from a battery, the capacitor isn't strictly necessary.
Maplin are a bit wild on capacitor units, some types are given in nF and others of exactly the same size in uF or even pF. 10nF = 0.01uF = 10,000pF 100nF = 0.1uF 470nF = 0.47uF Sizes aren't critical, pick ones with nice long leads
ive ended up with poly film caps, do these have a polarity at all? also on that diagram the pin numbering seems reversed compared to the data sheets ive found is this ok?
