Let's Make Robots!

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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"after which my eyes hurt and i was knackered"

Yeah, that's the problem when you go from a microscope to the soldering bench.  Check out this thread:

http://letsmakerobots.com/node/32191

or this guy('s profile pic...)

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

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

5 to 24v seems about right.  Remember for Cap ratings the voltage is the upper limit that it can handle.  As far as the capacitance value goes for smoothing out spikes the value isn't usually as important as it would be for timing applications-essentially all it's doing in this case is cutting the tops off of the "spikes" in a sharp-toothed signal wave.  Sometimes pF values are enough to do this satisfactorily.  From the sound of it, you should be able to get away with whatever you have available in that range.

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.

However if the data sheet doesn't help, normally .1 to 10 μF is fine.  I don't know that particular variant of the chip, but usually if it requires more specific values it will say so (The Babblebot for example requires a very specific stack to function.)  The one between Vin and Gnd is usually only necessary if the chip is a long way from the battery and the one between Vout and Gnd is to smooth out spikes for the μC.

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.