CRO experts please help

Thread Starter

Pratik_S

Joined Apr 14, 2015
49
For the stated application (motor driver) fT and interelectrode capacitance are not important characteristics --- Ic (maximum collector current), hfe or 'beta' (forward transadmitance [essentionaly current gain]) and maximum interelectrode 'voltages' will be the significant considerations...

Best regards
HP
Agreed to that. But did you check the waveform i have attached? My question is why are those unwanted oscillations generated only when i use a MPSA42 transistor, while there are no oscillations when i use BC547 . Why is this so?
 
Agreed to that. But did you check the waveform i have attached? My question is why are those unwanted oscillations generated only when i use a MPSA42 transistor, while there are no oscillations when i use BC547 . Why is this so?
Such ringing is often upshot of parasitic reactance -- Such being the case, the MPSA42 circuit, for whatever reason, exhibits higher 'q' than the BC547 circuit --- Good design practice, however, will externally 'damp' (i.e. 'snub') the circuit...

Best regards
HP
 

Thread Starter

Pratik_S

Joined Apr 14, 2015
49
Such ringing is often upshot of parasitic reactance -- Such being the case, the MPSA42 circuit, for whatever reason, exhibits higher 'q' than the BC547 circuit --- Good design practice, however, will externally 'damp' (i.e. 'snub') the circuit...

Best regards
HP
Can you please elaborate " the MPSA42 circuit, for whatever reason, exhibits higher 'q' than the BC547 circuit" . Thanks.
 
Can you please elaborate " the MPSA42 circuit, for whatever reason, exhibits higher 'q' than the BC547 circuit" . Thanks.
Q=quality factor -- in this context reactance to resistance ratio... High 'Q' circuits, being, as they are, narrow band filters, necessarily 'accommodate' resonances -- an undesirable characteristic in your application...

Best regards
HP
 

Bordodynov

Joined May 20, 2015
3,177
Transistors have different time of saturation.The more this time, the less hesitation.Take slow bipolar transistor with high gain.You can enable the Schottky Diode transistor in parallel.
Bordodynov.
 

MikeML

Joined Oct 2, 2009
5,444
Isn't the diode preventing the overvoltage in this case? Like in the typical double-pulse testing?
...
What if high dV/dt triggers the transistor again when switching off resulting in an oscillation? In that case the transistor should still be not destroyed.
Pratik has done the test with the snubber diode in place, and sees the expected voltage at the collector of the NPN where the voltage is clamped one diode drop above the supply voltage.

He is trying to demonstrate what happens when he leaves out the snubber diode. The relay is inductive, and has a ~100mA current established in it at the instant that the NPN switching transistor turns off. As the transistor turns off, the current keeps flowing into the collector-base, collector-emitter junction of the transistor. There is a few pF of junction capacitance, and a few more pF of stray wiring capacitance. There is some internal capacitance shunting the relay coil (turn-to-turn). All of these add together to form an equivalent capacitance, which when combined with the coil inductance creates a series RLC circuit.

Here is a simulation to demonstrate. I am taking a guess at some of the capacitances and resistances. It will be up to Pratik to make measurements to confirm the values.

C1 represents the junction capacitance of the (turned off) NPN. C2 is stray wiring capacitance. The Parallel Capacitance is the effective capacitance of the turns in the relay coil (WAG). The Series Resistance establishes the initial coil current. The Parallel Resistance eventually damps the coil current.

The switch, S1 is initially closed, and opens at time = 20us, simulating the NPN which was formerly on, and turns off. Look at V(c), which is effectively the collector of the NPN. With out any other mechanism to limit the voltage at that node, it shoots upwards toward 3kV at 25us. Obviously, that is going to do bad things to the collector junction of the NPN, which has a breakdown voltage of ~300V.

It is up to Pratik to decide what he is studying. Inductive flyback, or breakdown behavior in some transistor????

218.gif
 

Thread Starter

Pratik_S

Joined Apr 14, 2015
49
Pratik has done the test with the snubber diode in place, and sees the expected voltage at the collector of the NPN where the voltage is clamped one diode drop above the supply voltage.

He is trying to demonstrate what happens when he leaves out the snubber diode. The relay is inductive, and has a ~100mA current established in it at the instant that the NPN switching transistor turns off. As the transistor turns off, the current keeps flowing into the collector-base, collector-emitter junction of the transistor. There is a few pF of junction capacitance, and a few more pF of stray wiring capacitance. There is some internal capacitance shunting the relay coil (turn-to-turn). All of these add together to form an equivalent capacitance, which when combined with the coil inductance creates a series RLC circuit.

Here is a simulation to demonstrate. I am taking a guess at some of the capacitances and resistances. It will be up to Pratik to make measurements to confirm the values.

C1 represents the junction capacitance of the (turned off) NPN. C2 is stray wiring capacitance. The Parallel Capacitance is the effective capacitance of the turns in the relay coil (WAG). The Series Resistance establishes the initial coil current. The Parallel Resistance eventually damps the coil current.

The switch, S1 is initially closed, and opens at time = 20us, simulating the NPN which was formerly on, and turns off. Look at V(c), which is effectively the collector of the NPN. With out any other mechanism to limit the voltage at that node, it shoots upwards toward 3kV at 25us. Obviously, that is going to do bad things to the collector junction of the NPN, which has a breakdown voltage of ~300V.

It is up to Pratik to decide what he is studying. Inductive flyback, or breakdown behavior in some transistor????

View attachment 86131
Thanks a lot Mike for taking out your time and describing it so properly. I was initially studying the inductive flyback but while experimenting came across this phenomena of different transistors so started digging into it.
 
Please do not start new threads if you already have an existing thread on that topic. It leads to chaos and confusion.
Indeed @Pratik_S --- Speaking for myself, the quality of my replies would have been greatly augmented via apprehension of 'the big picture', as it were... Another point; Disparity of subject matter and thread title is 'off putting' to many -- CIP a thread title which appears to request assistance with test equipment but, instead, focuses upon switching circuity, leaves one with the impression of a rambling, ill-conceived inquiry...

With genuinely constructive intent
HP
 
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