Dr Ronald Stiffler SEC technology

[NickZ]

   Gyula:   I seam to be burning out my C1815 transistors bases.  Can you tell me what is the highest voltages that can be used on that  transistor's base? 5v?  When using 12v input, or higher input sources, like my 12v 5w solar panel (20v). I am blowing these small transistors.   And also, is there a benefit to using a higher (or the highest) voltages recommended on the transistor base?
   I have tried that same type of C1815 transistor with all my different value crystals, and the voltages at the collector/emitter is between 10v, and 15v, between the different crystals. With the best highest voltages (15v) when using the 7.2MHz crystal, and not the 13.5MHz crystal, on my oscillator.
   
   Which is the "best" transistor to use when the input is 12v to 30v?
   I will be picking up a couple of those DC to DC convertors in a few days. I just have to make a long trip to get them, as they came from China, but got stuck at post office, instead being shipped to my house.

[Slider2732]

Here's the datasheet Nick:
https://www.mouser.com/ds/2/68/2sc1815-1149989.pdf
Looks like 5V for the Base. A 10K variable pot to the Base should show the input limits...most fun if you have lots of transistors still.
The 'GR' type of C1815 are common on PC monitor CRT chassis, where they haven't used C945's.

Just type in 2S and then a transistor type for datasheets, seems to work well on Startpage.
2SC1815 in this case.

[gyulasun]

Hi Nick,

I wonder how you know you are burning the base of those transistors? (just wish to understand what makes you say that)

If you used a trimmer potmeter (like Itsu did) instead of the fix 100 kOhm base resistor in the oscillator and you adjusted the potmeter to its lowest value which is a short circuit area for the wiper to one of the edges, then the base-emitter diode has much chance to burn out due to the very high base current received dircetly from the 12V (or whatever) supply voltage. This is why it is very advisable to use a 1 k to 10 k resistor in series with the potmeter to avoid the excess base current when the potmeter is turned to a very low value.

You need to understand that the base-emitter junction (in a bipolar transistor) behaves exactly like a normal diode biased in the forward direction.  It is the current fed into the base from a positive voltage source (wrt the emitter and for an NPN type) which should be controlled via a resistor.

A diode as you know start conducting when its anode (in this case it is the base) receives a positive voltage wrt its cathode (here it is the emitter). Both for the diodes and for the transistors base-emitter there is the p-n junction voltage below which the diode cannot conduct: for your Si (silicon) transistor this is around and in between 0.58 to 0.8 V. 
BUT if you connect say a 1.2V battery directly across the base and the emitter (positive to the base and negative to the emitter) without any series current limiting resistor, then you burn out the base-emitter diode in such low power transistor.  (A 2N3055 type has a 4 Amper max base current limitation.)  This is the same as if you connect a diode like 1N4007 directly across the 1.2V battery (also in the forward bias direction of course), then the current can be so high that it would be heat up the diode in no time and it burns out (and during the few seconds it heavily discharges the 1.2V battery). 
So the question really is how much current the base of a transistor is specified for and not what is the highest voltages that can be used, ok? These transistors that the Doc and everyone else uses in such oscillators are called bipolar transistors (and npn types). The field effect transistors (that include MOSFETs of course) are controlled by changing the voltage across their gate and source: they are voltage controlled while the bipolar transistors are said to be current controlled devices, ok?

The 5V base emitter voltage, V_EBo shown in the data sheet Slider linked to means the maximum reverse voltage rating for the base - emitter diode, ok? Nothing to do with the 0.58-0.7V bias (i.e. p-n junction) voltage which developes across the base and emitter when you apply a higher than this bias voltage in the forward direction via a series resistor (to limit the base current).

How can it be known it would mean reverse voltage ??
Notice the letter order in the suffix from left to right: V_EBo (and not V_BEo) where the 5V max limit is given for the emitter and base and the EB order means the reverse direction !

And you can see the BE order in the V_BEsat for instance where the order is base - emitter, this means forward bias direction (the word 'sat' means saturation voltage between the base an emitter in the forward direction. You see the test condition for V_BEsat=1V maximum when I_C=100mA, I_B=10mA.

"Best" transistors to use: Lidmotor listed two.  I listed at least other two types that Itsu kindly ordered and used.

The 'rule of thumb' for such oscillators that have a coil in the collector is to use transistors rated for collector emitter voltages (V_CEo) with at least 3 times higher ratings than the supply voltage applied. The V_CEo for the 2SC1815 is specified as maximum 50 V, so 50/3= 16 V: so the supply voltge should be below 16V to be on the safe side.
For a 30 V supply voltage you wish to use, at least a 100 V rated bipolar transistor should be chosen. Lidmotor uses a BD243C which is a 100 V V_CEo rated type (notice the BD243 type can have A, B and C suffixes that designates exactly the collector emitter voltage ratings, C being the highest for that type).

What a mess is in the semiconductor world you would say...   

Gyula

[Lidmotor]

Nick---I really bothers me that you are having so much trouble.  I guess it is because I can relate to what you are going through. I have been down that road on many projects.  Not the right parts, perhaps a minor error in the build, or simply not really understanding what is happening and why the damn thing won't work.   On a circuit like this crystal oscillator there are not that many things that can go wrong-- if you have parts that are close to the required ones and the circuit is assembled right.  If you are burning up transistors something is basically wrong.  When I run into a situation like this I basically start over at square one.  Get the parts all separated out and make sure each one works and isn't burned out. I usually setup a Joule Thief circuit on a breadboard to test the transistors. Take a look at the circuit diagram and make sure you have that completely right.  Build it up on a breadboard first to see if it runs and to make easy adjustments.  Sometimes I build up a 'dead bug' arrangement rather than solder the parts on a board. It looks ugly but if it works I don't care.
  I know that this all sounds a bit silly to someone like you who has been building for years but perhaps it might help. 

--Lidmotor

[NickZ]

   OK, well now I can see why the 20v 5w solar panel would blow these C1815 transistors. They are OK for voltages of up to 8v or so. When using the 100k base resistors, but only provide about 12v or 13v output at the collector emitter, which is not enough on my oscillator to see some wireless effects.

  I have set up some additional series connected 10k ohm resistors to see which number of them will provide the needed current to the base. That is after going through the 100k base resistor and crystal.
  So if the question is how much current should go to the transistor base, while using let's say 12v input. I don't know the answer, yet.