Author Topic: transistors  (Read 5122 times)

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Offline queennikki1972Topic starter

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transistors
« on: August 14, 2019, 02:28:33 pm »
Nothing I build with the small black silicone based transistors like the BC547 seem to work. I need a book or something to understand them better. I google the direction of the pin out for emitter base and collector, but none of them seem to work. Are they more heat senstive than say capictors and resistors..? For Reference I purchased bulk from china if that matters. I have "Getting Started in Electronics by Forrest Mims" and I plan on buying "The Art of Electronics" as well.


Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Offline kosine

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Re: transistors
« Reply #1 on: August 14, 2019, 03:08:14 pm »
As well as books, I'd suggest having a play with LTspice. You can very quickly rig up some simple BC547 circuits and see how they respond.

I wouldn't waste time building and measuring circuits on a breadboard to begin with. Learn using spice, test the results later on.

It may also help if you go and learn about Gummel plots: http://www.eng.auburn.edu/~niuguof/multisimdev/_images/grapher_gummel.png

It's basically two logarithmic current plots. The lower one is the base current, the upper is the collector-emitter current. Pick any base voltage and the plot shows the corresponding currents. The collector-emitter current will flow through anything connected to the collector or emitter (such as resistors, LEDs etc.). From that you can then figure out the collector and emitter voltages.

Try some simple LTSpice simulations and you should see that the voltage across the transistor is whatever it needs to be to "take up the slack", so to speak.

 

Offline dcbrown73

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Re: transistors
« Reply #2 on: August 14, 2019, 04:11:00 pm »
Here is a quick tutorial on the BC547 that explains how it works and example circuit.

Good luck!
Why exactly do people feel I should have read their post before I responded?  As if that was necessary for me to get my point across.
 

Offline magic

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Re: transistors
« Reply #3 on: August 14, 2019, 05:27:37 pm »
Freewheeling diode antiparallel with Q1 :wtf:
 

Offline mikerj

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Re: transistors
« Reply #4 on: August 14, 2019, 05:40:35 pm »
Semiconductors in general tend to be more sensitive to heat damage than passive components, but you have to try fairly hard to damage a bipolar transistor in a leaded package like a BC547.  That said you shouldn't need to apply the iron for more than a few seconds per lead to make a solder joint.

Chinese components are often suspect, there is a vast market in counterfeit parts that either work poorly or not at all but they tend to target the more expensive parts.  Jellybean transistors are so cheap to manufacture there would be little profit in counterfeiting them, but that's not to say it doesn't happen.

What circuit have you been trying to construct, do you have a schematic?
 

Offline MarkF

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Re: transistors
« Reply #5 on: August 14, 2019, 06:08:07 pm »
This my help (NPN Amplifier) :  https://www.electronics-tutorials.ws/transistor/tran_2.html

Pinout:
 

Online ledtester

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Re: transistors
« Reply #6 on: August 14, 2019, 06:13:14 pm »
What circuits don't work for you?

Note that the metronome and light flashers circuits on page 104 of Getting Started in Electronics require an NPN and a PNP transistor.
 

Offline queennikki1972Topic starter

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Re: transistors
« Reply #7 on: August 14, 2019, 11:53:02 pm »
I will try to the circuits again in Getting Started in Electronics..

Thank you everyone..
Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Online Circlotron

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Re: transistors
« Reply #8 on: August 15, 2019, 12:51:05 am »
Freewheeling diode antiparallel with Q1 :wtf:
Yeah...  :palm:
But if the inductive motor load is in the emitter circuit it doesn't need any clamp diode for over voltage protection. When the transistor turns off, the motor will try and pull the emitter below earth (base) potential, partially turning the transistor back on and clamping the emitter voltage to 0.6V or so below base voltage. Does dissipate a bit in the transistor though, so a reverse (preferably schottky) diode across the motor is the proper way to do it.
 

Offline Audioguru again

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Re: transistors
« Reply #9 on: August 15, 2019, 01:28:01 am »
Nothing I build with the small black silicone based transistors like the BC547 seem to work. For Reference I purchased bulk from china if that matters.
Parts from China are frequently counterfeit, fake or defective. Buy name-brand parts from a local genuine electronic parts distributor like Digikey. 
 

Online ledtester

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Re: transistors
« Reply #10 on: August 15, 2019, 01:42:03 am »
Here's how to perform a "sanity check" on your transistors with the diode test function of your multimeter:

https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07

One of the "$20 component testers" (which you can get now for ~ $10) is also a convenient tool to have. Search ebay/aliexpress for "component tester".

Even the much poo-pooed "hfe transistor tester" port found on many cheap multimeters is useful:

811197-0

When the transistor is good and plugged in correctly you should get a number >50 for a small signal transistor like the BC547.


 

Offline magic

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Re: transistors
« Reply #11 on: August 15, 2019, 05:09:56 am »
But if the inductive motor load is in the emitter circuit it doesn't need any clamp diode for over voltage protection. When the transistor turns off, the motor will try and pull the emitter below earth (base) potential, partially turning the transistor back on and clamping the emitter voltage to 0.6V or so below base voltage.
Yeah, the 10k resistor to ground saves the day when emitter falls below Ic/β*10kΩ, a few V negative. Actually, it could be a clever way to quickly brake the motor by applying maximum reverse voltage the transistor can withstand if 10k is replaced with higher resistance or simply a 30V zener. What's the pulsed SOA of BC547? :D
 

Offline Jwillis

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Re: transistors
« Reply #12 on: August 15, 2019, 05:24:12 am »
Transistors functionality is dependent on application.What are you using it for ? What voltages are applied and how much current is pass through the transistor. For example your BC547 has a maximum voltage rating of 50V and a maximum current that can pass through  ("Drawn by the load ")  is 100mA.
The Hfe (AKA beta) is the gain of the transistor .Many factors can alter the gain but for simplification yours has a "Gain" of 110 to 800. Which is to say that the current passing from collector to emitter is 110 to 800 times the current at the base. Sometimes there will be a letter following the BC547 like "A" ,"B" or "C" This is the hfe classification .This means for example that a BC547A will have a hfe "Gain" of between 110 ~ 220.
So in "Common Collector" mode  if  1 mA is applied to the base then the transistor will pass current across the collector  to the emitter between 110ma - 220 ma which would ,of course, exceed its maximum current capability.As you can see it doesn't take much current at the base to drive current across the collector and emitter. So it won't take much to burn out a transistor quickly.You also need to remember that the current at the emitter is the current at the collector Plus the current at the base . An easy solution is to place a resistor at the base that limits the current to the base.So as in Ohms law Voltage divided by the Resistance equals  the current that will pass across the resistor to the base of the transistor.
You have to consider that current requires a Load.Load "draws" current.And the transistor can also be considered a load at the base.So by using a limiting resistor, the base will only "draw" as much as the resistor will allow.So if your supply is 10 volts and you place a resistor of say 100K ohms at the base then there will be 0.1 mA drawn across the resistor to the base. So 0.1mA times a "gain" of 110 equals 11mA from the collector. And 0.1mA from the base plus 11mA from the collector equals 11.1 ma at the emitter. No matter how much current is available from your power supply that transistor will only pass 11.1mA.

Other modes include Common Base and Common Emitter which have different functions .

Playing with transistors can be very interesting. Even blowing them up is educational because you can see limitations and what can happen when limitations are exceeded .Failure can be as informative as success.Just wear eye protection.

Without getting in a conflict about components from Ebay .I know from experience that they're just fine and have always matched design parameters.The only time they fail is when I've done something wrong .Digikey is a good source as well ,but the shipping charges is what puts me off.Maybe it's better in the States I don't know.

If your interested ,as ledtester mentioned,there are inexpensive transistor testers from Ebay  available that are good starter tools for the novice .They will tell you what pins are what and the "gain" .Plus they will test other components such as diodes ,capacitors and resistors .Very handy tools.

 

Online Circlotron

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Re: transistors
« Reply #13 on: August 15, 2019, 05:57:26 am »
for simplification yours has a "Gain" of 110 to 800. Which is to say that the current passing from collector to emitter is 110 to 800 times the current at the base.
Not trying to be pedantic, but with a gain of 110-800 a *change* of base current causes a *change* of collector current that is 110-800 times as great as the base current change. Or was that allowed for in your simplification?
 

Offline queennikki1972Topic starter

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Re: transistors
« Reply #14 on: August 15, 2019, 12:36:39 pm »
So I tried the flashing light circuit on page 104 of Getting started in electronics. Unfortunately the only 100k variable resistor i had was used pull from old stuff and was a little flaky. It did flash properly for a minute so it must have been ok. I will order more parts and keep doing the same circuit until i figure out what im doing wrong. I will also try more expensive parts from digikey as well.
I do have one of the component testers as well. And the parts test ok before using.
Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Offline Jwillis

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Re: transistors
« Reply #15 on: August 15, 2019, 02:14:22 pm »
for simplification yours has a "Gain" of 110 to 800. Which is to say that the current passing from collector to emitter is 110 to 800 times the current at the base.
Not trying to be pedantic, but with a gain of 110-800 a *change* of base current causes a *change* of collector current that is 110-800 times as great as the base current change. Or was that allowed for in your simplification?

Your absolutely correct.Any knowledge is good knowledge.Thanks for pointing that out.

So I tried the flashing light circuit on page 104 of Getting started in electronics. Unfortunately the only 100k variable resistor i had was used pull from old stuff and was a little flaky. It did flash properly for a minute so it must have been ok. I will order more parts and keep doing the same circuit until i figure out what im doing wrong. I will also try more expensive parts from digikey as well.
I do have one of the component testers as well. And the parts test ok before using.

If your could provide that circuit diagram then someone here good walk you through it and mark the pins to  where they go .Even a photo of a sketch is a great help.Some of us don't have that book.
 Electronics is not an exact science. All components will have a tolerance and as for resistors they can be as much as 5% out for ones with a gold  band and 10% for silver band. Even the most common ones from Ebay Have a 1% tolerance .This means that the higher the rated Resistance the more deviation your going to get. For example a 100k 5% resistor can be as high as 5000 ohms out and a 100k 1% resistor can be as high as 1000 ohms out .This is not a problem with simple circuits .There are precision components available but are far to expensive and unnecessary unless you plan on launching a probe to some distant planet.Even Transistors are never perfect.
Remember, there is no such thing as a stupid question.It's only stupid when you stop asking questions and think you know it all.
 

Offline queennikki1972Topic starter

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Re: transistors
« Reply #16 on: August 15, 2019, 03:05:34 pm »
For those needing it here the page and its the bottom right circuit.
Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Offline queennikki1972Topic starter

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Re: transistors
« Reply #17 on: August 15, 2019, 03:10:22 pm »
I assume you could use a red led with this circuit? Even though it calls for some odd ball #222 bulb
Siglent SDS 1202X-E - Heathkit IO-4105 - Dr. Meter 0-30v 5a power supply - 862d+ combo unit - Weller WLC100 - Kunkin KL283 DC load. Not much gear yet.
 

Online ledtester

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Re: transistors
« Reply #18 on: August 15, 2019, 03:27:28 pm »
I assume you could use a red led with this circuit? Even though it calls for some odd ball #222 bulb

Sure - as always, though, put some sort of current limiting resistor in line.


 

Offline Audioguru again

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Re: transistors
« Reply #19 on: August 15, 2019, 04:12:41 pm »
In this thread, people have been talking about hFE and beta being over 100 times for a BC547, but the hFE and beta are only used when the transistor is a linear amplifier that always has lots of C to E voltage and not when it is used as a saturated switch. The flashing light circuit uses both transistors as saturated switches and the datasheet for most little transistors shows that the base current should be 1/10th to 1/20th of the collector current.
 

Online ledtester

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Re: transistors
« Reply #20 on: August 15, 2019, 04:51:42 pm »
... Even though it calls for some odd ball #222 bulb

You have to bear in mind that the book was written in the '80s and the circuit was probably developed by Forrest much early than that as he has been writing about electronics since the early '70s. At that time LEDs were much more expensive, less available and much less efficient -- i.e. not as bright. Indicator lamps were implemented with incandescent or neon bulbs. In this case the circuit is designed to put out a short bright flash, and I imagine he recommends a particular bulb (which has a certain voltage and current rating) to achieve that visual effect.
 

Offline floobydust

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Re: transistors
« Reply #21 on: August 15, 2019, 05:52:58 pm »
So I tried the flashing light circuit on page 104 of Getting started in electronics.

Just a note, there are two resistors missing in Forrest Mim's blinky circuit- it can destroy the transistors when you crank the potentiometer to lowest resistance.
There is no resistor to limit base current to Q1 when the potentiometer is cranked, and this also can damage Q2 as there is no base resistor to limit its base current either when Q1 is full on.

I would add two resistors, like 470R. One in series with the potentiometer that goes to +V, and another in series with the base of the PNP. In the old days, a 222 light bulb acted as a neat flasher lamp when overvoltaged. It is a 2.25V LAMP (two AA batteries) and on 6V or 9V is very bright, for a little while. It only was rated 5hrs lifetime.
« Last Edit: August 15, 2019, 05:55:15 pm by floobydust »
 

Offline Zero999

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Re: transistors
« Reply #22 on: August 15, 2019, 09:41:16 pm »
Some things to note.

If you think a potentiometer is bad and don't have a replacement, simply substitute it for a fixed resistor of half the value.

If the circuit calls for an incandescent lamp, but you want to use an LED. Add a resistor in parallel, as well as the usual LED and series resistor. The circuit might be designed to work properly with the low resistance of an incandescent lamp and might not reliably work with an LED. In this case it will work, but the LED might not turn fully off, especially if it's a high efficiency green, white or blue one.

Finally please do a little image processing on photographed schematics. It makes them clearer, as well as saving bandwidth.
 

Offline Jwillis

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Re: transistors
« Reply #23 on: August 15, 2019, 10:40:56 pm »
I tried your flasher circuit and all it did was cook the transistors. So I took the liberty of redesigning the circuit to one that I tested and indeed works and it won't cook the transistor.I added notes but feel free to make any changes you like for different effects.I was to lazy to diagnose the problem with the first circuit.
 

Online ledtester

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Re: transistors
« Reply #24 on: August 15, 2019, 11:31:18 pm »
For clearer images a PDF of the book is available at:

http://www.wbrc.in/ham/getting-started-in-electronics-forrest-m-mims-iii.pdf
 


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