Good general electronics forum?

[mushy]

I'm relearning basics, and I'm looking for a good general electronics forum for asking noobish questions. Any suggestions?

[Justin Thomas]

I think the "Music Ectronics" one that you posted your question in is a good one... I'm sure I can speak for many noobs (myself included) when I say we'd all like to benefit from your questions, and we probably have the samexquestions but are too embarrassed to ask. And even the humbler honest pros here will fess up to not knowing everything. Remember - "the only stupid question is the unasked one."

Justin

[tedmich]

I do not have a clue, since the vast majority of us are certified super geniuses!



seriously I read a lot, Navy Neets series, Pete Millet's web site

[tedmich]

course SGM may be the first to answer, gotta wait for a quorum often...

[mushy]

Ok, instead of creating a new thread, here is a noob question: I relearned basic dc series, parallel, series/parallel theory the other day, just playing around with some resistors, doing the calculations, and measuring for comparison. For simplicity sake: for a single resistor that is measuring 545 ohm, across a power supply that is measuring 10.7 Vdc loaded, I'm measuring a current of around 3 mA, where I calculate 19.6 mA. What might be causing the difference, here?

Two semesters of electronics classes, and we never did this stuff... For anyone considering taking classes, check out your school thoroughly before signing up. Had I done a little prying, I could have found out that every single student in the program preceding mine failed their certification exams, that the school's overall graduation rate is lower than 20% (only really graduating business and nursing students), and that the school was having a very hard time finding competent instructors for the electronics tech classes. Believe me, it became obvious that there were major problems after just a few days in classes. I should have dropped right then, spending the money on books and tools instead. /End whatever this is.

[g1]

for a single resistor that is measuring 545 ohm, across a power supply that is measuring 10.7 Vdc loaded, I'm measuring a current of around 3 mA, where I calculate 19.6 mA. What might be causing the difference, here?

Something is not measuring the current right? I'll assume you have measured the resistor and it is 545 ohms.
The meter is in series with the resistor and the probes are in the right jacks for current measurement? Do you have a second meter so you can check voltage across the resistor at the same time you are measuring current?
Also, some meters have fuses for the current ranges.

[mushy]

Something is not measuring the current right? I'll assume you have measured the resistor and it is 545 ohms.
The meter is in series with the resistor and the probes are in the right jacks for current measurement? Do you have a second meter so you can check voltage across the resistor at the same time you are measuring current?
Also, some meters have fuses for the current ranges.

I borrowed another meter, and all is right. Yep, mine is junk...harbor freight (it has also been loaned to everyone and his sister). It's not the fuse.

Back to it. I'm sure I will have plenty more noob stuff to post in here.

[mushy]

Another one: How can ac and dc exist on the same circuit without changing one another?

[tedmich]

I borrowed another meter, and all is right. Yep, mine is junk...harbor freight (it has also been loaned to everyone and his sister). It's not the fuse.

Back to it. I'm sure I will have plenty more noob stuff to post in here.

everybody deals with measurement errors, theoretical stuff is all perfect!

Read all this stuff!
NEETS - Navy Electricity and Electronics Training Series


PS 187 years and Georg Ohm is still on top!

[mushy]

everybody deals with measurement errors, theoretical stuff is all perfect!

Read all this stuff!
NEETS - Navy Electricity and Electronics Training Series

I have read some of that before. I'll give it another look after I finish my current book.

[eschertron]

Another one: How can ac and dc exist on the same circuit without changing one another?

OK, I'm learning too, but I'll take a stab at it.

First a disclaimer: the AC in a guitar amp (for example) can be amplified and sent to the output terminals BECAUSE of the DC, so the assertion that the AC and DC don't change another might be a bit misleading.

In terms of measurement instruments, some are designed to sense whether there's more AC than DC present (or vice versa) and display a reading based on the stronger of the two elements. Other meters require the user to pick. Some may be better at rejecting DC when measuring AC than others, some may be better at averaging AC when measuring DC. A less-than-professional grade meter may be poor at differentiating the two sources and display garbage when both are present.

The distinction between AC and DC for the engineer or technician is an arbitrary one. DC is nothing more than an extremely low frequency AC wave component (zero Hertz, actually!) and can be treated that way mathematically. DC on your AC signal can be called an 'offset', and is common in an amp (remember the disclaimer). Between gain stages it is removed by using AC coupling capacitors. The DC component can be ignored for much amp circuit analysis, but is very important to keep in mind for calculating headroom, power, etc. We talk about the AC and DC like they're two different things, because their origins are usually different (in an amp, at least). But the AC wave gets more powerful from input to speakers as it takes energy provided by the DC.

[mushy]

OK, I'm learning too, but I'll take a stab at it.

First a disclaimer: the AC in a guitar amp (for example) can be amplified and sent to the output terminals BECAUSE of the DC, so the assertion that the AC and DC don't change another might be a bit misleading.

In terms of measurement instruments, some are designed to sense whether there's more AC than DC present (or vice versa) and display a reading based on the stronger of the two elements. Other meters require the user to pick. Some may be better at rejecting DC when measuring AC than others, some may be better at averaging AC when measuring DC. A less-than-professional grade meter may be poor at differentiating the two sources and display garbage when both are present.

The distinction between AC and DC for the engineer or technician is an arbitrary one. DC is nothing more than an extremely low frequency AC wave component (zero Hertz, actually!) and can be treated that way mathematically. DC on your AC signal can be called an 'offset', and is common in an amp (remember the disclaimer). Between gain stages it is removed by using AC coupling capacitors. The DC component can be ignored for much amp circuit analysis, but is very important to keep in mind for calculating headroom, power, etc. We talk about the AC and DC like they're two different things, because their origins are usually different (in an amp, at least). But the AC wave gets more powerful from input to speakers as it takes energy provided by the DC.

So dc creates an offset, moving the ac reference, from say 0V, to whatever the dc voltage is? I.e., the ac waveform is riding the dc line? And a coupling cap moves the ac waveform back to a zero reference. I guess it could be thought of like that, or as ac modulating dc. Are two things happing here? The waveform reference point is moving away from zero, and the waveform is increasing in amplitude, before the coupling cap?

Thanks for the reply, btw. I think it is good to understand these types of things, and I never could get straight answers from instructors on these types of questions. One guy told me that he's a mechanical engineer, so don't ask him.

[Leo_Gnardo]

Another one: How can ac and dc exist on the same circuit without changing one another?

In an amp, how about the signal riding the bias voltage on its way to the output tube grids.

There was an MXR multiFX unit back in the 70s', that "floated" control signals on the 12V DC supply to the pedal, and connected pedal to amp with any quarter-inch cable, nothing special needed. Another type of case of AC superimposed on DC.

[Leo_Gnardo]

So dc creates an offset, moving the ac reference, from say 0V, to whatever the dc voltage is? I.e., the ac waveform is riding the dc line?

Right.

And a coupling cap moves the ac waveform back to a 0 reference.

or whatever DC offset is present at the other side of that cap, have a look at the caps that transmit signal to the grids of output tubes.

One guy told me that he's a mechanical engineer, so don't ask him.

You deserve better instructors. I wonder if he's a dud of a mech e too.

[tedmich]

In an amp, how about the signal riding the bias voltage on its way to the output tube grids.

There was an MXR multiFX unit back in the 70s', that "floated" control signals on the 12V DC supply to the pedal, and connected pedal to amp with any quarter-inch cable, nothing special needed. Another type of case of AC superimposed on DC.

Phantom Power!