you can specify it, using something like the circuit below. (the circuit below is infact for a 'LM338K 1.2 to 30 Volt 5 Amp Regulator', which is way overkill for your needs, but the circuit should work the same) R1 is a 5K Pot (although i'm thinking it will be easier to get hold of a 4.7K...), and R2 is a 240 Ohm 1/4 W Resistor (again, a slightly obscure value, I should think that a similar value should do fine)

Is that helpful?

Tim

P.S. I am right in thinkin that all variable regulators work in the same way, aren't I? Please correct me if i'm wrong...

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Think I'm starting to get out my depth Tim, but thanks very much for your advice all the same. I think I might just go for a fixed regulator to keep things simple, I was thinking of getting the first one on this list from Maplin (i.e. the L7805CP)....

http://www.maplin.co.uk/products/mod...&moduleno=7937

I'm was also going to get two capacitors as Nigel advised just to be on the safe side and have a go at building the circuit originally posted (but with capacitors). 5v might be too much, but if it isn't taking this route looks like the simplest option!...

 

I think your are giving up too easily.

Whats complicated about the circuit diagram? It has two extra components on it - thats all. If you want I can draw do a vero board layout, with pin numbers for all the components, anything you need, just ask, its what we are here for (although ask nigel rather than me, cos he know more... :lol

If you are not sure if you can work with 5v, then you really need a variable regulator. Either check that you can work with 5v, or go with the variable. You won't be on your own, and you'll learn something from it.

Tim

 

Right, that's the info we needed (the actual device itself doesn't really matter).

Perhaps you didn't know you can get 3.3V fixed regulators?. You probably can't get them at Maplin, but RS Components list four different ones - I suspect the LM1086CT-3.3 would be the best one for you, it comes in a TO220 case, like a 7805. They also list one 4V regulator, but that's a surface mount device and only low current.

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Point taken Tim, but my electronics knowledge is so poor that you may regret offering assistance! For starters, just to highlight how embarrisingly cack I am, I don't know what C1, C2, C3 and C4 are in your diagram- I'd hazard a guess at capacitors on the basis of what's been said before, but what size should they be (10uF as you mentioned before??)? I'm also assuming that where the two parallel lines go off the diagram on the left hand side of the page that the top one would be connected to the +'ve terminal of the battery, and the bottom one to the -'ve side of the battery.

Thanks for those details Nigel about other smaller voltage regulators- they may well turn out to be the best option if either a fixed 5v regulator is too much voltage or if, in the process of building Tim's circuit, I get scared and run away :?

I'm extremely grateful for all this (I've probably said that a couple of times by now...)

 

you seem to be missing the point slightly, we are here because we want to help, if we didn't want to help, we wouldn't be here... Anyway, i was just trying to stir up some fighting spirit; spirit of the blitz and all that.

Correct, they are capacitors. Generally you can use whatever you like. I notice nigel only uses two capacitors, one either side of the regulator, both 1uF. Anything up to about 100uF should do the job (I don't think they even need to be the same as each other...). This is true whether you are using the variable regulator, or the fixed one.

Again, correct.

If you are prepared to pay the postage that RS want (I think its a few quid) then it defenatly is the best option (fewer components, won't need any setting up etc), however, You said "go to maplin" so I assumed that it was close to you, and as the postage would probably cost more than the components themselves, it was probably worth getting the variable one form maplin and saving yourself the p&p. Just my opinion.

Tim

 

Ok Tim, I'm going to have a go with a fixed 5v regulator first (just in case it does actually work) but if that dosen't do the business I'm going to dig deep, reach for the stars and go for the variable version because as you said, it doesn't actually look too bad

Just one point, why are there 4 capacitors because, for example, C1 and C2 seem to be doing the same thing in the same place, so can't you just put in a really big one instead of having two? (same goes for C3 and C4). Also what do the capacitors actually do (in suitably simple terms)? What is their function in the circuit?

 

Well, i'm not an expert (obviously) but someone explained it to me as this: their function is to 'smooth' the power. when there is more voltage/current than usual, they will 'absorb it', and when there is less, they will discharge, making the the supply 'smoother'. In simple terms.

You probably don't even need a 'really big one', just one normal sized one would do fine. If you were running it from the mains, it would be different. Just one 10uF on each side of the regulator should do fine. Maybe the diag will help (i know it says 3.3v, but ignore that)

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Things are starting to look quite promising, I like the look of that new diagram a lot. I presume the chap which I've circled in the diagram (below) is the 5K Pot- my understanding is that this is basically a variable resistor, is this right and is there anything particular I should know about them?

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I'm afraid not!.

The diagram is for a fixed regulator, the symbol you circled is a 'ground' symbol - also the diagram should have a 'dot' just above it, to show the horizontal and vertical lines join.

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Ok, now I feel stupid. I'm just going to print myself out a list of circuit diagram symbols so that doesn't happen again...

..right, I'm back in the game. I've taken the original circuit diagram (for a variable output current) and just crossed out what you suggested might be unnecessary capacitors and circled the 'Pot' (hopefully I've got it right this time). Is what I am left with (i.e. without the 2 extra capacitors) a suitable circuit to use if I want to adjust the output voltage? and back to my point pre-getting-the-circuit-symbol-wrong, is there anything particular I should know about 'Pots'?

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lol, sorry, my fault.

as you had said you were going to go for a fixed regulator, thats what the diag was for, I just though it might tidy up all the bits and bobs that have been brought up the thread. I should have pointed that out.

Not sure why the dot didn't appear (I really need to get a different circuit drawing package, Tina is rubbish...)

take a peek at http://www.kpsec.freeuk.com/symbol.htm, it has a lot of stuff on it, but it is useful reference

Yes.

There isn't much to know about pots, except, which legs are which! lol, on the diag, the part with the arrow is the middle leg, it doesn't matter which way round you connect the other two (one of them is unconnected anyway). But hang on, if you are going for the fixed regulator, you don't need the pot, thats only in the circuit for the variable regulator...??

Nigel, what would we do without you...?

Tim.

P.S. where abouts in Wales are you? just out of interest.

 

Yeh Tim, sorry it's all got a bit confused with the two circuit options. I'm going to try it with a fixed regulator first (just because it's simpler), but if that doesn't work then I'm going to go with the variable version and a Pot so I thought I'd get the low down on that as well whilst this is all in discussion. Right, I think that's been the most constructive day I've had at work for a long while..wish me luck as I attempt the different options over the weekend- I'll keep you posted with developments on Monday,
cheers, Paul
p.s Cardiff, but only pretending to be Welsh

 

Tried the circuit with a fixed regulator and it worked!! thanks everyone, definitely couldn't have done it without all your help. No doubt I'll be back on the forum again with a new batch of amateur questions!
Paul :lol: :lol: :lol:

 

Not sure if my 'advisors' about this topic are still out there, or will read this thread again, but things haven't worked out quite as I thought so I need some advice again if that's ok...

Whilst the 5v voltage regulator was working ok in terms of producing the required voltage with a normal 9v battery, when I tried it with a bigger 8.4 volt radio-controlled-car-type battery the output from the voltage regulator was actually higher which meant it didn't work the device I am building. Anyway my conclusion was that one of those 3 volt voltage regulators might be just the ticket for my needs (even though the posting from RS is a bit on the steep side) so I ordered some and tried wiring one up in exactly the same way as the digram given before (see diagram at bottom of this post) with two 10 uF capacitors, but didn't get 3volt output, in fact didn't really get any output at all, all a bit weird. Is there something I'm missing here, or does this voltage regulator require a different wiring set-up? the voltage regulator I'm talking about can be found at the RS website, and has the stock number 349-4543

(it's the third one down just in case it's not highlighted red- it's the LM1086CT-3·3. This was the one I was previously advised might work for me)






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