circuit analysis and calculating hysteresis attached circuit - Page 1
Forum section: electronics / beginners

author: loveelectronics2024
author_rank: Contributor
timestamp: January 06, 2025, 07:59:58 pm
content: The circuit attached is used as an on/off switch to turn 60 ohms led on when v2 is below around 0.6V or below and to turn led 60 ohms off when v2 is above 0.6V. I have been trying for four months to do circuit analysis and to calculate hysteresis but I finally I admit I need help. Part of the problem is I am not sure where to start for circuit analysis and Q1 collector tied to Q2 base and resistor r5 also tied to Q3 collector is confusing me. Can someone please calculate hysteresis for the below circuit and go through the circuit analysis?

author: ledtester
author_rank: Super Contributor
timestamp: January 08, 2025, 05:11:23 am
content: It would be very helpful for the rest of us if you would include your .asc file with your post.Quote from: loveelectronics2024 on January 06, 2025, 07:59:58 pmI would let LTSpice give you the hysteresis values setting Vin to be an increasing then decreasing ramp function and plotting the current through the LED. The points where the vertical segments of the LED current curve intersect the Vin ramps tell you the hysteresis points.In the example below the values are 0.64V to go from on to off and about 0.52V to go from off to on.Note that this is with pretty strong feedback from the 1K resistor. When R5 is set to 10K the hysteresis points are only about 20mV apart.See the hysteresis-circuit-2.asc attachment for the LTspice file.

author: MrAl
author_rank: Super Contributor
timestamp: January 08, 2025, 05:35:41 am
content: Quote from: loveelectronics2024 on January 06, 2025, 07:59:58 pmHello there,To start, you would only be able to get an estimate especially from a 'hand' calculation of this circuit. The estimate you get this way would be considered good enough though in most cases. Part of the reason for this is that the transistor behavior depends on the model used, and when doing it by hand we usually use a knocked down version of a spice model. That would be a limited Gummel Poon model, and even that makes it hard to do so we even go as far as to knock that down a little too into a pseudo Gummel Poon. This is a model that replaces each transistor with a current controlled current source (CCCS) and a fixed voltage diode. You can allow the voltage of the diode to vary though so you can estimate the effect that would have on the accuracy of the calculation.All this is still considered useful and good enough for many applications.This model would be made from the CCCS and the fixed voltage diode, and perhaps a base resistor that represents the internal resistance. The base resistor also helps to calculate everything.The CCCS output goes from collector to emitter, and for the NPN it would point down. The input to the CCCS is the base emitter, and that also goes through the aforementioned diode. The base resistor of course goes in series with the base.The attachment shows the model. If you want to use a different model that's up to you though.I can show you how to form the expressions but I have a feeling you already know how to do that.If you use a simulator you can measure the hysteresis that way too without doing any calculations. It's still an estimate though.