12v to 9v DC questions

[dkoz]

The datasheet for the LM317 which is nearly the same as a 78xx shows a big voltage spike on the output when the input voltage or the load current is changed and the value of the output capacitor is small. The spike is much smaller with a 10uF output capacitor. They recommend a 0.1uF ceramic disc input capacitor.

Now this is more or less what I wanted to hear.

So using a 0.1uF is better than using a 10uF for the output?

So pretty much, I could just stick to the example on the 78xx data sheet and use a 0.33uF cap for the input and a 0.1uF cap for output?

 

[audioguru]

No. The datasheet shows that a 10uF output capacitor reduces the output voltage spike much better than a smaller capacitor.

 

[Nigel Goodwin]

Now this is more or less what I wanted to hear.

So using a 0.1uF is better than using a 10uF for the output?

So pretty much, I could just stick to the example on the 78xx data sheet and use a 0.33uF cap for the input and a 0.1uF cap for output?

You don't seem to be paying any attention to the answers you get?.

IT DOESN'T MATTER - IT'S NOT AT ALL CRITICAL

Of course you could use the values off the datasheet!.

 

[dkoz]

I'm paying attention just fine. I may be a little confused is all.

Anyway, that's your own opinion. You say it's not critical, I say it is.

Maybe you know more than I do but I do know that I need to protect against voltage spikes. And I'm real paranoid about frying my bike and garage door opener so you better bet I'm going to ask repetitive questions.

I'm learning here. There is no need to jump down my throat. It only makes you look like an ass.

 

[dkoz]

No. The datasheet shows that a 10uF output capacitor reduces the output voltage spike much better than a smaller capacitor.

Ahh, that makes more sense. Thanks for your help.

I feel pretty confident now. I just wanted to get this capacitor mess out of the way.

One more question now that I think of it.
If a 10uF capacitor protects better than a .01uF, wouldn't a 100uF cap protect better than a 10uF cap or the bigger I go makes no difference after a certain point?

 

[Nigel Goodwin]

Ahh, that makes more sense. Thanks for your help.

I feel pretty confident now. I just wanted to get this capacitor mess out of the way.

One more question now that I think of it.
If a 10uF capacitor protects better than a .01uF, wouldn't a 100uF cap protect better than a 10uF cap or the bigger I go makes no difference after a certain point?

I think the only 'ass' round here is the person who keeps asking the same question over and over again, and ignoring all the replies!. And if you 'say it is critical', why are you asking questions on here?, when you already know all about it?.

If it'll make you happier though?, I'll make it simpler for you:

1) You MUST include capacitors - I presume you understand this?.

2) It must not be too small.

3) It must not be too big.

Personally I wouldn't go smaller than 0.1uF, or larger than 10uF - although if all I'd got was 22uF, I wouldn't be concerned about it.

You've also misunderstood Audioguru's point about spikes, it wasn't what you think it is. Your requirement is VERY, VERY simple, if you want to use a 7809? (and there's no reason not to, it's a perfectly reasonable choice), any smallish caps (as above) would do perfectly. I would also suggest the comments about not connecting it with the bike running, or while starting it, are completely unfounded. You're making your project far more difficult than it is!.

BTW, I would suggest you don't attack or harrass moderators, as you WILL be banned - consider this your ONLY warning!.

 

[dkoz]

I asked the questions over and over because I was confused and I felt the question was not answered to my understanding.

I'm not being the ass here. I'm new to this so you see the posts above as repetitive questions. I see them as answers heading to a final resolution.

You can flex your MOD muscles all you want. It would not be the first time I was banned from a forum because a Mod got upset for no good apparent reason.

I do appreciate your help though. The technical aspect of your last response is what I wanted to hear, the rest of it just speaks volumes about the type of people running this forum.

Again, I appreciate everyones help greatly. You all, correction, some of you, seem to forget everyone needs to start somewhere. Nobody is born with all knowledge. In the future, if you don't have the patience, then ignore the impulse to come in a discussion and ruin a guys learning experience.

And I'm thinking this may be my last post so take care all!

 

[Hero999]

Well as far as I'm concerned there's no reason why you should treat a moderator with any more respect than any other member here.

However, I can understand why Nigel was annoyed with you, if you're looking for a particulare piece of information then please say so rather than asking the same question over and over.

Anyway I don't think anyone here has explained why the capacitor is required. The voltage regulato keeps the output voltage the same by turning the output transistor more on or off according to the current being draw. At high currents the output voltage will start to fall so in order to stop this the error amplifier driving the transistor turns it more on. At low currents the output voltage will start to rise so to os in order to stop this the error amplifier driving the transistor turns it more off.

Here's where the capacitor is required. The problem is that the above process all takes time: if the load suddenly increases at such a rate that the amplifier and output transistor can't keep up the output voltage will fall until the amplifier catches up and brings it back up to the normal level, if the load current suddenly falls at such a rate that the error amplifier and output transistor can't keep up the output voltage will rise until the error amplifier and output transistor catches up. Adding a capacitor helps solves this probelm, if the output voltage rises sharply the capacitor will charge up more, absorbing the voltage spike and if the output voltage suddenly falls it will discharge slightly powering the load during the voltage troff. If the capacitor is too small it won't be big enough to absorb the spikes and supply power during the troffs. Now you would think that the larger the capacitor is the better, to a large extent this is true but large capacitors aren't good at high frequencies as they have a higher parasitic inductance (equivalent to an inductor in series with the capacitor) and as inductors block sudden changes in current it stops it from doing its job effectively. There can also stability issues associated with large capacitors but you need to read up on ampfier stability before you'll understand them and this post has been long enough.

The term for the load suddenly changing is a "load transient" therefore adding a capacitor improves the transient response of the regulator.

 

[dkoz]

Great info Hero.

Now I understand why they are needed and now I can make better judgment on what value caps to use.

Also, I don't see any repetitive questions by myself either.

I asked for help choosing the right value and voltage capacitor twice, in the beginning, and nobody responded.
I asked again, Nigel said there was no "correct" value. I didn't think that was a good enough response so I asked a few more questions until I received a definitive answer.

Now, if I knew the actual purpose of these caps in the first place. Nigel's response would have made complete and total sense and I would have made a decision of what I could have used.

You all need to remember you're dealing with an electronics "dummy" here.
If it's not spelt out, I'm not going to understand it right away.
I can solder together a circuit just fine. Any monkey can do that. But the actual inner-workings of the circuits are a mystery to me and this is where we all seem to be having trouble. You all assume I know these basic things but don't.

 

[dkoz]

Anyway, bickering aside, I've learned allot and I'll be building my voltage regulator soon.

Hopefully other beginners will find this thread helpful as the current existing ones never seemed to cover the capacitors role in detail.

Thanks to everyone who helped, even Nigel

Cheers!

 

[udylevy]

Capacitors choice

The 0.01uF is more in for noise filtering, the 10uF is more for supplying current in the initial dt when your circuit pulls alot of current.
It's common practice to put a large capacitor AND a small one, so I would say use both.
Anyway the size of the capacitor is dependent on the current draw from the circuit, but since the capacitors you mentioned are cheap and availble, I would say-don't bother thinking about it too much, just stick'em there both and be done with it.

 

[udylevy]

unfounded???

I think the only 'ass' round here is the person who keeps asking the same question over and over again, and ignoring all the replies!. And if you 'say it is critical', why are you asking questions on here?, when you already know all about it?.

If it'll make you happier though?, I'll make it simpler for you:

1) You MUST include capacitors - I presume you understand this?.

2) It must not be too small.

3) It must not be too big.

Personally I wouldn't go smaller than 0.1uF, or larger than 10uF - although if all I'd got was 22uF, I wouldn't be concerned about it.

You've also misunderstood Audioguru's point about spikes, it wasn't what you think it is. Your requirement is VERY, VERY simple, if you want to use a 7809? (and there's no reason not to, it's a perfectly reasonable choice), any smallish caps (as above) would do perfectly. I would also suggest the comments about not connecting it with the bike running, or while starting it, are completely unfounded. You're making your project far more difficult than it is!.

BTW, I would suggest you don't attack or harrass moderators, as you WILL be banned - consider this your ONLY warning!.

See the following link: **broken link removed**

 

[palesha]

U can even make 9V voltage regulator by using 7805, 7806, 7808 etc also. try to find what is available. Just u hv to add two resistors more. Use it as voltage divider circuit at the output of regulator IC. From the centre of two resistors connect to the negative terminal ( normally center in +ve regulator) of IC.