Creating Controlled Inductive Voltage Spikes - Page 1
Forum section: electronics / beginners

author: TechJunkie97
author_rank: Contributor
timestamp: November 03, 2018, 05:48:36 am
content: Hi, I wanted to create inductive voltage spikes out of curiosity. I want the spike to be of reasonable amplitude as I do not want to damage my Analog Discovery Oscilloscope. I am using the principles of the circuit shown in "Figure 1.PNG". The circuit works as follows:STEP 1:The switch 1 is closed and switch 2 is opened. There is almost no dc resistance of the inductor so 5V/50ohm= 100mA current passes through the first half of the circuit.STEP 2:Then switch 1 is opened and switch 2 is closed instantaneously. Inductor wants to keep 100mA current flowing through it. This will induce a voltage across the resistor R2. Since the current flowing is 100mA, the voltage across the resistor R2 will be (R2)*I=500*.1=50V theoretically. (Resistor R2 (500 ohm) is 10 times higher than R1(50 ohm). so for the same current, the voltage drop will be 10 times) The energy stored in inductor will be gone in a couple of microseconds. Practically I expect a voltage spike across R2 higher than the input voltage (more than 5V and less than the theoretical spike of 50V due to losses) for at least 1 microsecond.MY CIRCUIT:I want the switches to open and close instantaneously so I created a more practical circuit shown in figure "2.PNG". I replaced the switches with BJT transistors (Q1 and Q3). The bases of these transistors are driven by a NOT gate made by transistor Q2. The input of the NOT gate is shared by Q1 base and output of NOT gate goes to Q3 base. This way only one Q1 or Q3 is ON and I can "open and close instantaneously" by pressing the push button.MY PROBLEM:When Q1 is ON and Q2 is off, I expect 5V/50ohm=100mA current flowing through R4 and the inductor. There would be some voltage drop across the collector-emitter junction of Q1. But, when I measured voltages, most of the voltage drops across the collector-emitter junction of Q1. This causes a huge decrease in the expected current. As a result, the voltage spikes in STEP 2 are less than 2V even when R7 in 2.FIG is infinite.What can I do reduce to the voltage drop across the transistor Q1 to the minimum? and is there is anything you want to say about the circuit?Thanks!

author: den
author_rank: Contributor
timestamp: November 03, 2018, 09:11:04 am
content: Hi jaskiratubhi,I will not even try to comment your circuit, not feeling expert enough. I'll just say, that I think "simultaneously switching) it probably not a good idea and is very hard (if not impossible to realize).But the first circuit is actually basic physics, and this spike can be easily realized. Just throw out the right part of the circuit, and put a large resistor in parallel to you coil. This way when the switch is opened, coil will discharge through this resistor. You can vary it's value to alter the spike.You can even do it without the parallel resistor, but only if you can tell what the spike will be with open circuit. This is the case with automotive injectors, for example. The spike is clearly seen when injector is switched off and the spike is under 100 V normally.Den

author: Kevin.D
author_rank: Frequent Contributor
timestamp: November 03, 2018, 02:43:56 pm
content: Think about the polarity of the resultant output voltage across load resistor R2. You can replace switch 2with a diode (cathode pointing to inductor) and you have an simple inverting smps topology (remove R1 also), these can step the input voltage both up or down (but input voltage polarity is inverted), going further and replacing the inductor with a transformer primary (input) and secondary winding (output) and you have a simple flyback topolgy , so the inverting topology is also a flyback topology.Your lower schematic has a few problems but to point out a main one think about what hapens whenthe collector and emitter of those bjt's are pulled negative with respect to gnd by the top end of the inductor (the inductor voltage flips in order to keep the current flowing in the same direction).Regards

author: MasterT
author_rank: Frequent Contributor
timestamp: November 03, 2018, 03:02:21 pm
content: Quote from: jaskiratubhi on November 03, 2018, 05:48:36 amYou can replace Q1 by PNP transistor, it's called "high side switch" - may worth to google for details.