Noobie Questions, of a keen learner

Thread Starter

MarkSimms

Joined Apr 6, 2016
54
I'm new to electronics and the forum so Hi all, I hope to be a useful member. I have questions for you high I.Q fellas here . please excuse my noob problems, I'm trying real hard.

A resistor voltage divider can step down a voltage for say 12v DC to 6v DC, but what resister values do I use? the current for the load say a 1Watt 6v bulb seems to generate a much higher current in the voltage divider series resistors than that of the bulb load circuit.

do resistors generate heat when reducing current, what I'm asking is am I draining a lot of power from the battery with resistors or are they just mostly dropping voltage?

how do coils work in lay-mans terms, here is what I know so far, they don't restrict DC, but oppose AC at higher frequencies, also they build an EMF field, I'm not bothered about formulas at the moment I'm looking at how they do what they do, what puzzles me is if a coil is in parallel with a capacitor what does the resonating achieve? does the DC just go straight through the coil?

I'm looking at a crystal AM radio set and the capacitor and coil resonate back and forth, the antenna acts as a capacitor , and the diode demodulates, but why is the coil actually needed? the antenna resonates with the radio waves, what use is the magnetic field in the coil? would the variable capacitor no work with the antenna without the coil? I can sort of think of the resonating as tuning to a frequency, but I cant see why ?
 

Thread Starter

MarkSimms

Joined Apr 6, 2016
54
I'm thinking of buying an osillascope, but was wondering how sensitive they are, eg should you want to see the signwave of the AVC mains supply is that possible? whats the most important property of osillascopes when buying one?
 

hp1729

Joined Nov 23, 2015
2,304
I'm new to electronics and the forum so Hi all, I hope to be a useful member. I have questions for you high I.Q fellas here . please excuse my noob problems, I'm trying real hard.

A resistor voltage divider can step down a voltage for say 12v DC to 6v DC, but what resister values do I use? the current for the load say a 1Watt 6v bulb seems to generate a much higher current in the voltage divider series resistors than that of the bulb load circuit.

do resistors generate heat when reducing current, what I'm asking is am I draining a lot of power from the battery with resistors or are they just mostly dropping voltage?

how do coils work in lay-mans terms, here is what I know so far, they don't restrict DC, but oppose AC at higher frequencies, also they build an EMF field, I'm not bothered about formulas at the moment I'm looking at how they do what they do, what puzzles me is if a coil is in parallel with a capacitor what does the resonating achieve? does the DC just go straight through the coil?

I'm looking at a crystal AM radio set and the capacitor and coil resonate back and forth, the antenna acts as a capacitor , and the diode demodulates, but why is the coil actually needed? the antenna resonates with the radio waves, what use is the magnetic field in the coil? would the variable capacitor no work with the antenna without the coil? I can sort of think of the resonating as tuning to a frequency, but I cant see why ?
Yes, the resistors will waste a lot of energy as heat.

Regarding resonant LC circuits we are talking AC, not DC.

How inductors work in a DC circuit ... when you apply the voltage the magnetic field starts to expand. As it expands it the magnetic field crosses other windings inducing a voltage in the opposite direction as the applied voltage. Eventually the expanding magnetic field stops and all you have is the low resistance of the wire. So inductors oppose a change in current. That's it in 100 words or less.

Re: crystal radio
Yes the circuit would work without the tuned circuit. You would pick up the strongest signal(s) and demodulate them.
 

dl324

Joined Mar 30, 2015
9,323
Welcome to AAC!
A resistor voltage divider can step down a voltage for say 12v DC to 6v DC, but what resister values do I use? the current for the load say a 1Watt 6v bulb seems to generate a much higher current in the voltage divider series resistors than that of the bulb load circuit.
A divider isn't appropriate for this situation. The rule of thumb is you want the current in the divider to be 10X the load current. The current for the bulb will be 1/6A and 1.67A in a divider would waste a lot of power. You want to use some sort of voltage regulator.
do resistors generate heat when reducing current, what I'm asking is am I draining a lot of power from the battery with resistors or are they just mostly dropping voltage?
Power dissipation in resistors is P=IV. It produces heat. They drop voltage, but that's a side effect of them often performing a more useful function.

On a tablet, too painful to discuss inductors...
 

dl324

Joined Mar 30, 2015
9,323
I'm thinking of buying an osillascope, but was wondering how sensitive they are, eg should you want to see the signwave of the AVC mains supply is that possible? whats the most important property of osillascopes when buying one?
Electronics requires attention to detail, so start by spelling words correctly. It's oscilloscope and sine wave. Does AVC mean VAC?

A scope shouldn't be your first purchase. Get a power supply, DVM, etc before a scope.

All scopes should have sufficient vertical range to measure signals of interest. For cases where it doesn't, you can use amplifiers or attenuators to precondition signals.

Be aware that the chassis of a scope should be connected to earth ground and the ground lead is connected to the chassis. This limits how you can probe mains voltage. Unless you know the polarity of mains in a circuit, you should only ground your probe on earth ground and probe hot or neutral. Transformerless mains connected circuits are forbidden from discussion at AAC.

Bandwidth is the most important parameter.
 

Thread Starter

MarkSimms

Joined Apr 6, 2016
54
dl Thankyou . I'm looking into audio AC circuits so a scope is needed(thanks for the Bandwidth tip), I have a cheap DVM
 

dl324

Joined Mar 30, 2015
9,323
I'm looking into audio AC circuits so a scope is needed(thanks for the Bandwidth tip), I have a cheap DVM
I like Tektronix scopes from the 70's-90's. They can be found for less than a dime on the dollar and maintenance manuals are readily available.

Beware of inexpensive DSO's (Digital Storage Oscilloscopes). The old adage of you get what you pay for holds true for them. You need to study the specs in detail to make sure you don't buy a crappy one.

For audio frequencies, you could look into applications that use PC sound cards. Personally I wouldn't use one, but a crappy scope is probably better than none.
 

MrChips

Joined Oct 2, 2009
19,732
@MarkSimms
Welcome to the AAC forums Mark.
So you are new to the forum. Here are some forum etiquette.

Start off with a title that is meaningful to readers. Here is an example: "Do resistors waste power?"

Stick with one question at a time. You don't get extra mileage by loading six questions into one post.

Finally, "you high I.Q. fellas here" is a terrible come-on. Same as "all you smart folks on AAC".
You don't get a gal's attention with a line like that. Curry favouring don't work on technical sites like AAC.
 

ramancini8

Joined Jul 18, 2012
473
Just use a series resistor; regulators, etc. are not needed for lamp ballast. 6V across the bulb leaves 6V across the ballast resistor. The current through the series string is 1W/6V = 1/6 Amp. R = E/I, so R = 6/(1/6) = 36 ohms. QED
 

#12

Joined Nov 30, 2010
18,167
Stick with one question at a time. You don't get extra mileage by loading six questions into one post.
Gotta put a +1 on MrChips for that advice. Keep it simple. One question at a time. The more questions you load in, the more you weed out people that can answer one or two of the questions, but not all 6 of them. The conversation gets really messy with 5 or 6 lines of thought getting intertwined. Half the time, you don't know which question the respondent is answering, and if somebody disagrees with an answer, the conversation gets completely crazy, immediately.
 
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