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Test Circuits Needed

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 01 December, 2007 » Test Circuits Needed « Previous Next »

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scott2734
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Post Number: 236
Registered: 10-2005

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Posted on Wednesday, 21 November, 2007 - 03:32 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I started electronic's engineering school about 2 months back and we are about to get into using function generators and oscilloscopes. I do a lot of my labs at home so that helps me learn more and keep more in my head. I would like some test circuits using both of these instruments if anyone has some. The circuits do not need to be complicated, a few resisters , a coil here and there.If anyone has any links or circuits they have done that are simple that would be great. Thanks.
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tvmender
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Post Number: 124
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Posted on Wednesday, 21 November, 2007 - 10:50 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Scott,

A web search would give results. I would maybe look at oscillator and amplifier circuits first as performing measurements on this type of circuit is quite interesting. 555 based stuff and maybe 741/transistor based etc...

EPE have the Teach In articles which quite often go through simple circuits and measurements.

Is that the type of stuff you are looking for?

Good luck,

- Tvmender
"Nothing unreal exists"
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scott2734
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Posted on Wednesday, 21 November, 2007 - 11:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Yes, simple as possible as im learning the equipment, not the circuit right now. Of course the two do go hand in hand some what.

I looked up some circuits online, i figured there would be thoudsands of them, but only come up with a few that look somewhat promising.
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poplar10
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Post Number: 26
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Posted on Thursday, 22 November, 2007 - 08:35 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Do you mean this sort of thing ? I found this by googling for "How to use an oscilloscope".

http://www.doctronics.co.uk/scope.htm

John
Nothing's impossible, I have found ...
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obiwan
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Post Number: 2035
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Posted on Thursday, 22 November, 2007 - 10:31 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Scott, I'm not sure what you're looking for exactly.

I understand you're trying to learn the equipment.

But does that mean you have a function generator and you're looking for "circuits" that do something, so you can use the scope to check them out, thereby learning the scope at the same time?

Or are you looking for circuits that make like a "function generator" so you can look at it with the scope??

If you'd like a function generator chip, let me know and I'll send you a couple. They're not great, but they're pretty good and simple. (XR2206) I think that one goes up to about a MHz, sine, square, triangle....

A 4046 PLL chip might be good to play with too, you get digital and analog in the same chip. View how you can get a DC analog voltage out of a square wave signal using PWM or FM. Lots to see with a scope.

So if you can clarify a little more what you're looking for, maybe I can help you out some.
Do Not Hit The Fly That Lands On The Tigers Head.
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741
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Post Number: 67
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Posted on Thursday, 22 November, 2007 - 11:13 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Scott,

As I understand your post, you'd like some circuit diagrams so that you can apply test waves from a ready-made generator to various nodes, and observe (and understand) the results at other nodes.

Eg R-L-C type circuits - if that is right, I'm sure the people on this forum will have a lot of good ideas.

I'd start with series RC, then RL, then RLC. See if the phases and amplitudes are as predicted for a 1 Volt amplitude sine-wave. Choose the frequency to suit the component values.

Eg if series R is 1K, then peak RC series current is 1Volt/1k = 1mA, which is reasonable.

If you apply a step pulse (0,1,0) for about 5*RC = 5*1000*1E-6 = 0.005 or greater, C will be about fully charged. When the pulse falls, C will discharge with same time constant.

Maybe use a square wave with period .01 = 100Hz.

Observe the voltage at the RC junction. Also, try differential-mode scope readings across C and across R. Always, Vin = Vc + Vr.

Not so for RLC due to phase differences...

Also, work out a circuit to confirm visually that a capacitor passes only AC. Eg place a 1./5V battery in series with the RC. The amplitudes should be unaffected (except for a blip just as the battery is connected)

Also try square waves with relatively short mark-to-space ratio. That way you should see exponential charge-decay on the RC for example.
There are some interesting tricks whereby you measure the time to decay to (1/e) and thus simply derive the RC time constant.

I'd suggest a combination of sketching/calculating results and maybe the freeware SwitcherCAD as useful adjuncts to experiments.

Of course, the final test is always the experiment.
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obiwan
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Post Number: 2039
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Posted on Friday, 23 November, 2007 - 12:22 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

That's what I'm not sure about, if he's looking for circuits like that or making something that creates those function generator wave forms.

I'm guessing you're right 741.


In doing those circuits, I'd be sure to look at the current also, so you can see the relation ship to voltage current in those various circuits.

A current probe would be best. Lacking that, I'd isolate the supply no matter what. Then build a simple "current probe" from an opamp, and insert a small resistance in line where you want to measure the current, and place the input to the opamp across that resistor and then measure it's output with the other probe.
Do Not Hit The Fly That Lands On The Tigers Head.
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scott2734
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Post Number: 241
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Posted on Friday, 23 November, 2007 - 03:05 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Yes 741, that is what im looking for. Iv got a oscilloscope and im looking around for a cheap function generator. Like you said, the circuit im wanting will be in the middle of the function generator and oscilloscope. I want to apply differant waves, shapes and magnitudes from the generator through the test circuit and out to the oscilloscope. I can change the frequency of the generator to get different outputs magnitudes.I didnt make it clear enough in my original post.

I have seen such a circuit a while back on the net that had a couple of resistors, an op amp, and a coil. The circuit was adjustable. You could set your function generator to a specific frequency, and then set the circuit to a certain point, you should be able through math calculate the output magnitude. It was a very good learning tool, but i cant seem to find it now.But, any circuit would help me out, no matter how simple it is.

poplar10, that is a very good learning source, thanks!

Anyone know a good source of used function generators? Im looking on ebay too, but some of them look like they come from the local toy factory .I thought about building one from the chip that obiwan suggested, but me working full time, going to school full time and life, i dont have time to do that.
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dingbat
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Post Number: 6
Registered: 11-2007

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Posted on Tuesday, 27 November, 2007 - 03:35 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

For a beginner it's not easy - (first, you don't say what equipment you have ), also some of your background would help....
Ask at School, they know what's comming next and don't try to race ahead of the class. Use the 'scope to explore the limits of the circuit...and here rely upon the school, or they may suggest an electronics kit ( 30 projects for you to build )...e.g. from Maplin. That way if you damage anything it can be replaced.
You appear to be obsessed with "function generators" - do you have a specific idea in mind? -
Heck,what's wrong with a simple tone generator? ( like the popular 555 ) - If your scope is more than one channel you can watch both the charging and output. However, I'm not sure that you need such gear to learn the fundamentals, there is plenty you can do with only a multimeter ( Do you have one?). When you understand the errors that introduces and its limitation, that's the time to get fancy gear. For most work an Oscilloscope is unnecessary IMHO.
...The 2206 is obsolete and never was good enough. A great pity there is no modern alternative, e.g. for a sweep-generator I have in mind. Generating sine-waves is not easy although a Wein Bridge does a v.good job as an introduction.
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obiwan
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Post Number: 2046
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Posted on Tuesday, 27 November, 2007 - 10:38 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The 8083 (?) is still available, and the MAX-038 as well.

They don't manufacture them any longer, but they're still out there. As I remember, Futurlec had some.

I offered the chip simply because I had some and could give them away, not that they were great, but it'd be a good starting point for somebody that has nothing.

But yeah, it's a shame they don't have any replacements out there for those chips. I guess everything is supposed to be done digitally now, and that would pretty much exclude hobby types for that sort of application.

Scott, I thought I posted here about some circuits, I guess it got lost. I'll try to draw some up in Proteus when I have time, should be enough to keep you busy for a while. But they're really just simple circuits, replacing the L/C/R values with different frequencies.
Do Not Hit The Fly That Lands On The Tigers Head.

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