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| Blow oscilloscope measuring coil Back EMF |
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| T3sl4co1l:
The answer is more easily found by modeling the coil and surrounding elements. First of all, we're generally not dealing with coils in the abstract, but flyback / inductive EMF is only a concern where it's an unintended/inevitable side-effect of a component, like a relay or solenoid. Coils, inductors, in general use, we aren't concerned with the EMF because that's entirely the reason we've chosen them. Whether as a resonant coil in a tuning network, or the active element in a flyback converter, the EMF is fully constrained by the circuit it is embedded within. So, assuming you're working with relays or solenoids here. Typically such a coil has an overall L || R characteristic, due to eddy currents in the core. This is exaggerated for solenoids (usually a solid piece of iron), significant for small solenoids and relays (often using pieces of stamped metal, less thickness = less time for field to penetrate the material), and optimized out of real inductors and transformers (using laminated sheets, or materials like ferrite that carry significantly less eddy current). For turn-off flyback purposes, we can assume the core linear, and therefore characterize it at signal amplitudes. The easiest way to characterize a coil is probably, using a sine generator and scope, connect the generator through a resistor to the solenoid, and measure the voltages at both ends of the resistor, with respect to ground. (So, scope probes, generator and solenoid are all common ground, and resistor connects to generator and solenoid, and both scope probes.) Use a calculation like this, https://www.seventransistorlabs.com/Calc/RLC.html#vec and tabulate values over a range of frequencies. Say 10Hz to 100kHz or more, taking a few points per decade, more if you find an interesting "kink" in the response. Likely, you'll need to switch resistors from time to time, or voltage scales; just make note when/where you're doing that. Then plot it: take abs(Z); compare Re(Z) to Im(Z); etc. Most likely you'll see Re(Z) ~ Im(Z) over much of the range (the signature of eddy currents in bulk metal), maybe you'll see a long asymptote where it behaves this way, or in fact behaves inductively (|Z| ~ F). The maximum impedance |Z|, at whatever frequency corresponds to turn-off rate, gives the peak flyback voltage for that device, when turned off at that rate. You might never be able to achieve such rate; for example a 1A solenoid driven by IRF520 will still be loaded by the few 100 pF of the transistor, plus some 10pF and kohms equivalent of whatever probe you connect to it. You would be able to infer, from these data, that something very fast and high performance, like an air spark (switching can occur within some ~ns; fractional ~pF loading is possible), could generate such-and-such voltages. For coils other than electromechanical ones, the answer is much simpler: don't use it that way! The EMF is not something god-given, it's an expression of the coil's response into the surrounding circuit. Simply don't let that voltage appear, and you won't see it! Hence clamping diodes or zeners being a common sight in coil driver circuits. An SMPS fully constrains the inductor voltage at all times (between switching transistors or clamping/rectifying diodes); even the boost/flyback converter (suitable for generating high voltages) can be constrained by, not just device characteristics (you can't go higher than the avalanche rating of transistors/diodes used..), but the control circuit as well. Tim |
| ptluis:
--- Quote from: tautech on August 24, 2023, 09:49:51 pm --- --- Quote from: ptluis on August 24, 2023, 09:05:08 pm ---I always use a flyback diode, but what I'm trying to capture is how high this EMF reach without any kind of protection on the coil. --- End quote --- To what end ? You're dealing with switched magnetics so back EMF will always be present and its level is always relative to the impedance of the return/discharge path. Place appropriate snubbers across it and move on with the rest of your design, whatever that may be. --- End quote --- BTW to avoid opening a new topic, can you tell me what is the best Siglent website to download the latest firmware for Siglent scopes? For example SDS1202X-E on: Siglent.eu latest firmware is V1.3.26 Siglenteu.com latest firmware is V1.3.27 |
| ptluis:
--- Quote from: T3sl4co1l on August 24, 2023, 10:01:54 pm ---The answer is more easily found by modeling the coil and surrounding elements. First of all, we're generally not dealing with coils in the abstract, but flyback / inductive EMF is only a concern where it's an unintended/inevitable side-effect of a component, like a relay or solenoid. Coils, inductors, in general use, we aren't concerned with the EMF because that's entirely the reason we've chosen them. Whether as a resonant coil in a tuning network, or the active element in a flyback converter, the EMF is fully constrained by the circuit it is embedded within. So, assuming you're working with relays or solenoids here. Typically such a coil has an overall L || R characteristic, due to eddy currents in the core. This is exaggerated for solenoids (usually a solid piece of iron), significant for small solenoids and relays (often using pieces of stamped metal, less thickness = less time for field to penetrate the material), and optimized out of real inductors and transformers (using laminated sheets, or materials like ferrite that carry significantly less eddy current). For turn-off flyback purposes, we can assume the core linear, and therefore characterize it at signal amplitudes. The easiest way to characterize a coil is probably, using a sine generator and scope, connect the generator through a resistor to the solenoid, and measure the voltages at both ends of the resistor, with respect to ground. (So, scope probes, generator and solenoid are all common ground, and resistor connects to generator and solenoid, and both scope probes.) Use a calculation like this, https://www.seventransistorlabs.com/Calc/RLC.html#vec and tabulate values over a range of frequencies. Say 10Hz to 100kHz or more, taking a few points per decade, more if you find an interesting "kink" in the response. Likely, you'll need to switch resistors from time to time, or voltage scales; just make note when/where you're doing that. Then plot it: take abs(Z); compare Re(Z) to Im(Z); etc. Most likely you'll see Re(Z) ~ Im(Z) over much of the range (the signature of eddy currents in bulk metal), maybe you'll see a long asymptote where it behaves this way, or in fact behaves inductively (|Z| ~ F). The maximum impedance |Z|, at whatever frequency corresponds to turn-off rate, gives the peak flyback voltage for that device, when turned off at that rate. You might never be able to achieve such rate; for example a 1A solenoid driven by IRF520 will still be loaded by the few 100 pF of the transistor, plus some 10pF and kohms equivalent of whatever probe you connect to it. You would be able to infer, from these data, that something very fast and high performance, like an air spark (switching can occur within some ~ns; fractional ~pF loading is possible), could generate such-and-such voltages. For coils other than electromechanical ones, the answer is much simpler: don't use it that way! The EMF is not something god-given, it's an expression of the coil's response into the surrounding circuit. Simply don't let that voltage appear, and you won't see it! Hence clamping diodes or zeners being a common sight in coil driver circuits. An SMPS fully constrains the inductor voltage at all times (between switching transistors or clamping/rectifying diodes); even the boost/flyback converter (suitable for generating high voltages) can be constrained by, not just device characteristics (you can't go higher than the avalanche rating of transistors/diodes used..), but the control circuit as well. Tim --- End quote --- Thank you very much for your explanation! What I'm trying to do is to design a portable module to protect the output of power supplies when I connect inductive loads like relays, contactors, electric egr solenoid, etc for testing purposes. When using digital power supplies sometimes the flyback diode isn't enough to stop the PS or the display from shutdown. No problems with analog PS with analog displays. The flyback approach isn't enough and that's one of the reasons I want to see how high back EMF goes. But I won't do it anymore I'm going to save the scope, I already see what's going on :) Like you write "Simply don't let that voltage appear" and that's what I'm going to do. :-+ |
| tautech:
--- Quote from: ptluis on August 24, 2023, 10:12:15 pm ---BTW to avoid opening a new topic, can you tell me what is the best Siglent website to download the latest firmware for Siglent scopes? --- End quote --- I always us the HQ website. --- Quote ---For example SDS1202X-E on: Siglent.eu latest firmware is V1.3.26 Siglenteu.com latest firmware is V1.3.27 --- End quote --- 1.3.27 is the latest and there are some important fixes for SDS1202X-E. https://int.siglent.com/upload_file/zip/firmware/Oscilloscope/SDS1202X-E_V1.3.27_EN.zip |
| ptluis:
--- Quote from: tautech on August 24, 2023, 10:34:32 pm --- --- Quote from: ptluis on August 24, 2023, 10:12:15 pm ---BTW to avoid opening a new topic, can you tell me what is the best Siglent website to download the latest firmware for Siglent scopes? --- End quote --- I always us the HQ website. --- Quote ---For example SDS1202X-E on: Siglent.eu latest firmware is V1.3.26 Siglenteu.com latest firmware is V1.3.27 --- End quote --- 1.3.27 is the latest and there are some important fixes for SDS1202X-E. https://int.siglent.com/upload_file/zip/firmware/Oscilloscope/SDS1202X-E_V1.3.27_EN.zip --- End quote --- Thank you! Link added. Firmware downloaded! :-+ |
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