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ok ok i'm building a voltage reg...erm help?

hi peeps

ive decided to build the voltage regulator board but im clueless about the voltages

ill be using a TS 7805 CZ reg , a larger capacitor in front and a smaller one after ... but how do i decide on the capacitor size?

i want a steady 5v supply for my machine and i have a choice of 6 rechargeable AA batteries (7.2v) or 8 AA's (9.6v) ... i'd prefer to use the 7.2v as i could swap it later for some battery packs i have (when i find a way to charge em) but it just makes the 2.0v vdrop... whereas the 9.6v version makes the 2/3 vdrop some peeps say is better...

any idea's?

oh and could you explain how you came to the conclusion as i'm more used to biological fluid logic rather than maths based logic.

thanks peeps

dom

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go with the rechargeables. They have more current capacity that alkalines all day long. Have you read the datasheet? It should list the suggested capacitors in a default configuration.

i did look at the sheet but it gave the same capacitors for all thier regs (from 5 to 24v) which struck me as odd - there must be some kind of specification ...right? (sheet said 0.33uf then 0.1uf)

when you say rechargeables you mean the AA's right (the packs are also rechargable when i finally build some way of connecting them to a mains adaptor) - i was wondering if 7.2v or 9.6v really made much of a difference.

maybe im misswording - this may take some time ;)

dom

For some reason I thought you were considering using either rechargeables or alkalines. The extra voltage will be wasted. Go for the 7.2. Even lipos come in 7.2v flavors.

acording to the guys

the first capacator can be pretty fluid (from 0.3uf - 10uf sounds ok) - like a bucket with a hole in it, the size of the bucket aint that important

the 7.2v seems better than the 9.6v as there will be less energy wasted as heat

i do love the open source brain

thanks guys

dom

My first ever soldering experience (its a damn site harder than i realised - mines seriously messy)

looking thru my collection of capacitors i realised that after all that fussing i didnt have the right ones anyway so i reverted to my fluid biology logic and picked ones that looked nice.

i then proceded to solder two other pcb's which i had built (a wire to pin convertor and a alternative DIP attatcher) after which my eyes hurt and i was knackered (english term : too tired to care anymore)... so i'll take a multimeter to them then test em out in the next few days and tell you if it melts a hole in my desk.

dom

You could try a Low Drop Out (LDO) voltage regulator, which will give you a little more headroom before the 7.2V battery drops below where the chip can operate. 

Try this post for some more info.

it would appear my steady hands have served me well again

one thing i did notice tho... the steady output takes a sec or two to be truely steady (i think cos i used a larger electrolytic capacitor than i initially aimed for) - im hoping this doesnt mess up the picaxe when i eventually link them in...we shall see (i hear the words "zener diode" floating around in the ether - i think i'm getting the hang of this electronics thingy)

turns out i missunderstood how a zener worked - i thought it only let current thru after it reached a certain voltage... turns out that is true but i didnt realise it also kept hold of the dropout voltage too - ie a 6v one confronted with 7.2v would only let 1.2v thru

eeesh  ... and i made the discovery after i bought the damn things - there a LOT of really bad youtube videos and descriptions around.

any ideas what i CAN use to hold off the current from the volt reg till the capacator has fully charged?

dom

 

You can still make use of a Zener diode by using it as the trigger to a complementary latch, along the lines of this example.

Your circuit doesn't need to use quite as many parts as that one, but the basic idea is that the latch will be turned on via the Zener (replacing the N/O switch), and stay on until the power supply is disconnected, or at least drops to a very low voltage.
If you decide to make your own version of this circuit there will need to be a few modifications but we can help you with that. One option is to replace the transistor that will be driving the voltage regulator with a MOSFET or even a relay, to allow it to handle the current required.

so i use the zener to trigger a transistor which will then allow current to the volt reg.

problem is that the current fluctuates while the cap charges..which then causes dips in the volt reg output - maybe some form of timed pause would make more sense (as a zenner based system would allow output every time it fluctuates above the zener's trigger) as after about 5secs the whole thing seems to sort itself out.

basically i just dont want output till the caps fully charged

one question - how long does an electrolytic capacitor hold onto its charge (cos it might not happen again - this was just my first attempt) or is it drained by the rest of the circuit once the power source is removed

ahhh - its all one big learning (crying?) experience

dom