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Author Topic: Electrical Faux Pas  (Read 27283 times)

wattsup

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Re: Electrical Faux Pas
« Reply #30 on: December 13, 2007, 08:20:04 PM »
@z_p_e

Sorry to bother you again about transitors but is it possible to explain how the transistor works in more laymen terms. Voltage enters at base but where does it go? I mean in terms of input and output or is there a mixed input/output?

z_p_e

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Re: Electrical Faux Pas
« Reply #31 on: December 14, 2007, 01:40:16 AM »
wattsup.

The BJT transistor is a current-controlled current-source. That means one terminal (the base) receives a current, and the current flowing between the other two terminals (collector to emitter) is controlled by this current.

Do you understand how a diode works? You forward bias it (make it conduct) by applying around 0.6V across it. It is the same for the base of a transistor. The voltage is used only to cause this "diode" to conduct. The more the base conducts, the more current you will have flowing from collector to emitter. But for simplicity, the base is isolated from the emitter and collector. (I am trying to use layman's terms).

So the input voltage to the base does not go anywhere. It is very similar to applying a voltage to a resistor with one end grounded.You are creating a current in that resistor. We do the same in a base, we create a current in it. The return path to ground for the base is through the emitter though, even though you can think of the base as being separate from both the collector and emitter.

Think of the base as a control for the emitter and collector current, that's all. How much current is required on the base to control the collector-emitter current is a function of the gain of the transistor. Don't think of the base an input, it is a control.

An example: if you want 100mA collector current, and your transistor has a gain (HFE) of 100, you will only require 1mA of current in the base.

I hope this hasn't caused more confusion. Let me know if you understand it.

Cheers,
Darren

Evil Roy Slade

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Re: Electrical Faux Pas
« Reply #32 on: December 14, 2007, 03:35:08 AM »
Nicely done ZPE,

I just want to clarify something in case wattsup gets all excited about 1ma turning into 100ma.

There needs to be a power source connected to the collector to provide current. The base current is merely CONTROLLING the collector current as ZPE describes.

An analogy is how you control the water flow out of a tap by turning the knob. The water pressure is created by the reservoir not by the act of turning the tap. If the reservoir level is low then you will only get a trickle of water no matter how much the tap is turned on (so to speak).

So if the power supply connected to the collector is incapable of supplying 100ma then you will only get what it can provide. But it's not 'free' current.

ERS

z_p_e

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Re: Electrical Faux Pas
« Reply #33 on: December 14, 2007, 03:49:28 AM »
Thanks ERS.

It's nice to get some help once in a while.

Yes, wattsup, the source (battery etc) flows only from collector to emitter.

If you wanted to, you could use ERS's analogy and say that sufficient current on the base is equivalent to opening the water tap. The pressure on one end of the water valve is the amount of supply voltage, and the rate of water flow through the water valve is equivalent to the current level from collector to emitter.

wattsup

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Re: Electrical Faux Pas
« Reply #34 on: December 14, 2007, 05:49:52 AM »
@z_p_e and @ERS

Thanks, it's starting to sink in. Finally!!

So let's say that I had a transitor with a gain of 100 HFE;

I put positive of a 12 vdc supply to the collector.
   (I know I would need a good size transistor for this).
I put the positive of a 12 vdc bulb to the emitter.
I ground the bulb to the negative of the power supply.
I send 0.012 volts into the base.
Stop. This is where I am getting concerned.

1) So, will the emitter only give 1.2 volts to the bulb?
2) Now if I put .12 volts on the base, will the emitter put 12 volts to the bulb.

3) Now you said the base goes to the emitter also.
Is the base using the emitter as a ground?

4) What happens if I put the transistor base on the negative side of a circuit, but still send the 12 vdc positive into the collector that is given to the emitter and bulb? Would the base negative cause a short to the emitter positive? I might have done that a few times? Would this blow a transistor? I've already baked a few of them. lol

5) So let's say I do not use the collector at all. I only put 12 volts to the base. Will this 12 volts automatically be sent to the emitter and still light up the bulb? I have noticed this once and thought the transistor was shot. Is this normal?


Evil Roy Slade

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Re: Electrical Faux Pas
« Reply #35 on: December 14, 2007, 07:10:40 AM »
@wattsup

Remember what zpe said in an earlier post. The transistor is a CURRENT controlled current source.

When you apply a voltage to the base the current will flow according to the resistive load on the base and emitter.

This is VERY simplistic but use Ohm's law to calculate the current. Things get complicated if you include threshold voltages, ac voltages, capacitance and inductance etc. Ignore these for now cos it will just make your head hurt.

The current will then be amplified (if it can) by the transistor gain, say 100 times.

That current will then be flowing through the bulb.

Again use Ohm's law to calculate the resulting voltage across the bulb. This voltage CANNOT be greater than the power supply voltage.

All devices have voltage and current ratings on them. If you exceed these ratings expect failures.
Analogy: The human body was not designed for unassisted flight. Therefore, jumping off the top of a skyscraper will result in a distinctly unhealthy outcome.

Learn these laws: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html#c1

Use these laws to calculate expected voltages and currents BEFORE applying power and you will find that more of your devices will live to work another day!

Enjoy!

ERS

wattsup

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Re: Electrical Faux Pas
« Reply #36 on: December 14, 2007, 09:02:32 PM »
Thanks alot ERS for that and also for your other post in probable anticipation of what I was going to ask next, which I did not have to.

If by chance you or z_p_e stumble on the Mini TPU thread, I asked for any EEer to possibly give their explanation on how the EM circuit is working and actually "when" it is activating the transistor in that circuit. It is located here;

http://www.overunity.com/index.php/topic,3599.msg63912.html#msg63912

Thanks again. Transistors are getting clearer and I have done some experiments last night without blowing any. I was beginning to feel like the Terminator.

Evil Roy Slade

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Re: Electrical Faux Pas
« Reply #37 on: December 14, 2007, 11:23:12 PM »
@wattsup
You won't like my answer but here it is anyway.

The thread you linked to is: Re: Self Running Micro TPU, with closed loop.

This implies the design goal of the circuit is to run without a power source forever.

Fundamentally this cannot happen. Hence an analysis of the circuit is irrelevant because
it is fundamentally flawed, no matter what combination of bits are put together.

If you choose to use the laws of physics to help design your circuits (e.g Ohm's law) then you
must accept them as being correct. Not some, but ALL of them. Ignoring the Laws won't make them go away.

Keep asking questions, think, learn. 

ERS

EMdevices

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Re: Electrical Faux Pas
« Reply #38 on: December 14, 2007, 11:39:02 PM »
Guys,

I explained already how it works, and if you know basic electronics you will understand.   You can use Ohms law and other laws just fine. The device is super efficient because it recycle it's energy, but the charge in the main capacitor will run down eventualy due to the ever present LOSSES (resistance of the wires etc..) IT IS NOT OVER UNITY !!!!!

wattsup,   the transistor turns on when the voltage at the small cap connected to the base reaches about 0.6 V , or whatever the threshold is for your particular transistor.     The resistor R and capacitor C determine the PERIOD between flickers.    The larger the resistor value "R" and the larger the capacitance value "C" the longer the time between flickers of the LED.   I used big values like 1 M ohm and C = 22 u F , I even got it to flicker once every 15 seconds !!!  and the slower this flickering period, the longer it lasts, since the losses occur when it flickers occur, so less flikers per second less losses per second, so longer running time.   Other improvements can be made as well, and others are trying to squize it to perfection   LOL   :)

EM

Evil Roy Slade

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Re: Electrical Faux Pas
« Reply #39 on: December 14, 2007, 11:43:41 PM »
@EM,

Thank you for the clarification. I did not read the whole thread and I apologise for
drawing an incorrect conclusion of the thread title.

ERS

angryScientist

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Re: Electrical Faux Pas
« Reply #40 on: December 15, 2007, 05:36:48 AM »
Here is a helpful guide. It discuses which capacitor, resistor to use at which frequencies,
board layout for RF, soldering, PC board etching. It's just some useful tips for RF design.

http://home.sandiego.edu/~ekim/otherjunk/rf_proto.pdf


I found it through this site:
http://www.101science.com/rfdesign.htm

It also has links to dozens of Transistor Tutorials.
http://101science.com/tranbasic.htm

wattsup

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Re: Electrical Faux Pas
« Reply #41 on: December 18, 2007, 07:06:31 AM »
@z_p_e, @ERS and all

Thanks for your infos.

OK the transistor is understood (cooked only one more), thanks again.

Now I know I can pulse the base with a resistor and capacitor. Is there an easy way I can calculate the resistors and caps I would need to let's say take 10 transistors and pulse each base at it's own time?

Also, instead of resistors, I think if I used 10 pots and instead of using a capacitor, what if I used a variable capacitor (VC), what type of values should I look for?

Would a pot and a VC together enable me to adjust these base on times, on the fly.

That would be a coup for me.

z_p_e

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Re: Electrical Faux Pas
« Reply #42 on: December 18, 2007, 01:53:48 PM »
wattsup,

If you are referring to EM's circuit, the voltage through the resistor/capacitor on the base is more of a "ramp" than a pulse.

Maybe you should explain what you want to do with these 10 transistors. Do they all switch ON at the same time, or will each have their own timing input?

In regards to variable resistors (pots) and caps, sure you can use these to vary the timing on the fly. Keep in mind however that you will likely not find a variable cap value much higher than a few hundred picofarads or so, definitely not in the uF's. Normally, you would not need to vary both anyway, just use a pot. If you really need to vary the cap in addition to the resistor, then use a rotary switch and switch in various ranges of cap values. That's how it is normally done.

wattsup

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Re: Electrical Faux Pas
« Reply #43 on: December 18, 2007, 03:10:18 PM »
@z_p_e

The 10 transistors would be pulsed at different times or in succession, either on or off. I am saying 10 but it could be 4 or 5 transistors. I have coils and devices here that have been crying for pulsing and I am up to that point.

OK, so what you say about the pots and VC's is that their range of action is very tight, meaning I would need to have a good idea about the actual final values and gets pots that are close to that range to then fine tune it. That's a bummer, because the ranges are so wide to start with.

For the resistors, is it possible to use a decade box and add a pot in series. Then I could just switch resistors on the box and use the pot to fine tune at each resistance, or do I need a different pot for each resistance in the decade box.

For the capacitors, what if I found an old radio tuner. Does this have more range. Or can I find a switchable small capacitor bank and put a VC in series, like for the resistors, to better fine tune at each fixed cap value.

wattsup

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Re: Electrical Faux Pas
« Reply #44 on: January 19, 2008, 05:39:44 PM »
@all

Since z_p_e is no longer around, thought I'd put this here.

One thing that has bugged me for a long time now was how to skin magnet wire. When you have 20 of these to do using sand paper, it starts getting messy, your fingers ache and it takes forever.

So, I just found this video attached below.

If anyone has a better idea, I'm all ears, uh eyes, uh fingers, uh whatever. lol