Transistor Discussion

[jadaro2600]

For a greater understanding, I though that I would start a thread about transistors.  Post any link to information you might have here about transistors ( just pile it on, even if it's a half baked idea ).  Since one of the primary uses as of late has been for the Joule Thief Circuit, then it seems relevant to ask this / these question(s) here rather than muddy up an already lengthy thread:

And since I know no other location to post this...it lands here.

What happens when the transistor exceeds its switching frequency?  - I assume it begins to heat up?

And, in the case of the Joule Thief Circuit, why would the current rather flow through the Light Emitting Diodes rather than through the Collector Emitter path of the transistor?

Any thoughts?

[Low-Q]

Bandwidth of a transistor is determined by the gain you want. This gain is determined by the input resistance and the negative feedback resistance. The transistors have something called HFE. HFE is the transistor gain without negative feedback - maximum gain. The higher gain is selected, the less the bandwidth. It happens when the switch frequency rate exceeds the transistor bandwidth, the gain goes down automatically. You have in transistors a C component that limits the bandwidth. C is a reactive component, so at high frequencies you'll have low impedance, thus more current flow, but at the same time lower voltage drop. So overall effect should be the same. Moreover, when this transistor to be connected in a circuit containing capacitive or inductive components, the transistor gain can lead to oscillation with the components it is controlling. Oscillation is caused by feedback from the circuit for a certain frequency is in phase with the signal out, and it the gain goes out of control. This may cause the transistor to overheat, and burns up. Therefore it is important to place small capacitors between the appropriate pins on the transistor so near it as possible. And for operation amplifiers, this is important. They tend to have a HFE for several millions.

(Google translate isn't THAT good )

Br.

Vidar

[gyulasun]

What happens when the transistor exceeds its switching frequency?  - I assume it begins to heat up?

And, in the case of the Joule Thief Circuit, why would the current rather flow through the Light Emitting Diodes rather than through the Collector Emitter path of the transistor?

Any thoughts?

Yes, a transistor begins to heat up when driven by higher frequencies it is designed/manufactured for.  The reason is the charge carriers inside are unable to follow the demanded quicker movements and their only possible "answer" is heat development. This happens even if you carefully operate your transistor well within its Safe Operational Area (SOA for short and you can find this in most of the data sheets).  Of course this is a brief explanation but may reveal the main cause.  HOWEVER, I do not think in case of the Joule thief circuit this is the main cause of heating because the switching frequencies involved here are relatively low for most of the transistor types used. I mean the so called transit frequency, f_T referred to in data sheets, it means the maximum frequency where the transistor gain reduces to unity. And if you consider a good rule of thumb of using 5 to 10 times higher transit frequency transistors for amplifying a signal of a given frequency then you realise that in case of JT here the max frequency involved is well under 100 kHz, 10 times of this gives 1 MHz and most of the transistors have got an f_T around or well above 1 MHz.

So I think the reason a transistor gets warm or develop heat in such JF circuits here is mainly due to power dissipation defined by its instantenous collector-emitter voltage and collector current (observing SOA conditions for that transistor type of course in any moment).  You can reduce heat by using heat sinks on the body of a transistor and even in case of plastic and (not metal) 'casings' you may still use a gapped  metal cylinder stuffed with heat conducting compound  and clipped onto the plastic case to cool the body of a transistor.

Regarding the LED and the transistor current: do we speak about the same current that flows in both? Only their effective amplitude is what comparable.  The current flowing through the collector-emitter path is determined mainly by the battery voltage and the coil's impedance (DC resistance and inductive reactance at the switching frequency).  And the current of the LED is determined by the flyback pulse energy which is a function mainly of the self inductance of the coil, the current change through the coil and the rate of change of the current, ok?  Although the flyback current (that comes from the collapsing magnetic field due to the sudden switch-off of the coil current) has got a much higher peak value than the collector current at switch-on,  conventional physics explain it as the two currents' effective value can be the same only in an ideal loss-less component world, otherwise the effective collector current is always a bit higher than that of the flyback current. 
This boils down to the following: if you want to reduce transistor dissipation to a minimum than you have to use types with minimum saturation collector-emitter voltage, once the collector current is a given quantity by the circuit design. There are many special switching type transistors manufactured with low saturation voltage, look for types with such feature, like in Kodak flash lights circuits for instance.

rgds,  Gyula

[jadaro2600]

Thank you all for your replies.  Over the last few days I've been reading about transistors here and there.  I'll be performing a few more experiments with the joule thief as I have it set up.

I'de like to add this link to this thread in any event - this person has quite a few videos about electron tubes and transistors with regard to radios: http://www.youtube.com/user/AllAmericanFiveRadio

[electricme]

@jadaro2600
@Low-Q
@gyulasun

First thanks to jadaro2600 for starting this thread.
I have thought the transisters being used on the Pirates JT thread needed further work or explination.

Secondly thanks to Low-Q and gyulasum for their explinations, which at my current level of understanding, I got drowned with the technical lession lol.
But my ears pricked up at the suggested usage of the Kodak transisters, now I think it makes sence to use them as they have been designed with high voltage switching.

So after sending this off, I will seek to tear apart a Kodak or Fuji or other camera that comes my way to experiment with.

But one last question, for all, which is the best device to use to drive a torid with a bit of current?
Should I use a MOSFET, NPN Transistor (2N3055) or a IGBT

I have a supply of transisters that drive the stepper motors out of Fisher and Paekel washing machines, these are found secured to the side of the water cooling heatsinks. Would these do? I just discovered that F&P have altered their heatsink design, no more water cooling, they are using G4RC10s transisters, hmmmmm.

General Jim