Why usually there is no diode across a DC choke filter? - Page 1
Forum section: electronics / beginners

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2024, 12:36:43 pm
content: I don't remember seeing any flyback diode across big DC inductances (when a big series L is used as a DC choke filter), but I'm tempting to add a 1N4007. In my case the L is 1mH and 0.5\$\Omega\$, and the DC can be disconnected suddenly (the connector can be manually unplugged).Is there any reason for not adding a snubber diode, any side effect that I am missing, such that adding a snubber diode across a DC choke filter would be a bad idea?

author: PGPG
author_rank: Frequent Contributor
timestamp: November 11, 2024, 01:33:23 pm
content: Quote from: RoGeorge on November 11, 2024, 12:36:43 pmIn my 12V supply I have 18V transil at input. During Surge pulse (I assume 25A, 50us) transil limits pulse to about 25V. So at choke one side I have for a moment 25V while at the other still 12V. Surge is short enough to not make 12V to rise more than 1..2V (thanks to L and C). So surge is filtered out.Diode across choke will perfectly ruin filter functionality so adding it is certainly very bad idea.I have never met a situation like you describe when current in filter choke can be suddenly disconnected. If someone located choke in one device while capacitor in other so they can be suddenly disconnected than I would say it is very bad design. Instead of adding diode across choke add capacitor on the choke side of disconnection point.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 11, 2024, 10:00:53 pm
content: This one is for a PC amplifier. The audio amp is powered from the PC's power supply, so the DC is already filtered. I've added the choke because lately the CPU background noises become too strong, enters into amplif, and it become audible in the speakers.By adding the 1mH/0.5\$\Omega\$ coil in series with V+ (outside of the PC case, between +12V from the power supply and the +V of the power amplifier), the CPU noises became inaudible, which is good, but the coil is quite big, made on the core of a miniature transformer. I've added a diode anyway, in parallel with the coil, for just in case I forget and unplug the amplifier while powered.

author: zilp
author_rank: Frequent Contributor
timestamp: November 11, 2024, 11:19:50 pm
content: Quote from: RoGeorge on November 11, 2024, 10:00:53 pmCan you explain what bad thing you think would happen without that diode?

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2024, 08:41:51 am
content: When disconnecting a wire, there is some contacts chattering. Because of the coil, that disconnecting chattering can produce spikes of very high voltage. In practice, the spikes can be of many hundreds of volts, or even more, depending of how fast the current turns off (v=L*di/dt).Since choke filters with a diode are rather rare, did a simulation out of curiosity.Circuit 1 is without diode, and circuit 2 is a copy, but with a flyback diode added across the choke, both circuits side by side, to easily compare their behavior. Rload represents the audio amplifier that is powered from PC, and Rhand is presumably the resistance of the hand that grabs the wires during unplugging the power cable of the audio amp.Notice how the voltage spikes without a diode can get very high (Vhand1 is in the megavolt range in this simulation), while the spikes with the freewheeling diode attached (Vhand2) are less than 50V.

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2024, 08:48:14 am
content: i heard bob wildar took a computer to the roof once because he saw MV's in a simulation.i would try it with a oscilloscope to see what your actually dealing withthink about a speaker

author: PGPG
author_rank: Frequent Contributor
timestamp: November 12, 2024, 09:48:39 am
content: Quote from: RoGeorge on November 11, 2024, 10:00:53 pmI was sure that you are writing about disconnecting load from coil being on supply side and not disconnecting coil from supply.In your case to limit spike voltages generated when disconnecting from supply my first idea would be to use unidirectional transil at input but may be I would end with diode like you did.

author: Tim S
author_rank: Contributor
timestamp: November 12, 2024, 10:01:03 am
content: Here's what I do. Reverse polarity protection and disconnect ringing damping. Think buck converter...Tim

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2024, 01:38:35 pm
content: That would work too, and I like the reverse protection role of the diode (though, in my case the amplifier plugs into the PC +12V with a Molex connector, so reversing the polarity by accident is not a concern).LTspice still shows a few hundreds of volts on the diode (with the buck topology, the yellow trace, so I guess one might still get a jolt when disconnecting the power cable, but that would be less likely to happen than without any diode).

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 12, 2024, 02:28:14 pm
content: Quote from: coppercone2 on November 12, 2024, 08:48:14 amYou're probably thinking about Bob Pease throwing a computer off the roof of National Semiconductor, which he did (pictures and video exist). He did it because he hated simulation and how newer engineers would believe simulation results without an idea of what should actually be happening.I was trained that lab data trumps simulation ALWAYS.

author: zilp
author_rank: Frequent Contributor
timestamp: November 12, 2024, 03:14:52 pm
content: Quote from: RoGeorge on November 12, 2024, 08:41:51 amHu? So, the user would be touching the positive lead of the power input inside the device when unpluging the power?! (and presumably ground that's connected through the audio connection or something?) That sounds ... unusual?!

author: Siwastaja
author_rank: Super Contributor
timestamp: November 12, 2024, 03:50:47 pm
content: Quote from: AnalogTodd on November 12, 2024, 02:28:14 pmOf course but this is a stupid oversimplification. Lab data might be 10000000 million billion times more expensive to gather. In real world the choice is not between lab tests and simulation, it is between using simulations and not having the projectat all.

author: SeanB
author_rank: Super Contributor
timestamp: November 12, 2024, 04:32:51 pm
content: Better than a diode is a damping resistor across the coil, which would probably be in the range of 47R or so, around 10 times the DC resistance of the coil. Then the MOV across input and output, for 12v 18V MOV will be fine, to clamp high voltage to something manageable.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 12, 2024, 04:51:08 pm
content: Quote from: zilp on November 12, 2024, 03:14:52 pmNo, not really, Rhand is there to mark the side where I was expecting high voltage when the Molex connector is unplugged.And of course, in practice the high voltage won't be in the MV range, but might not be negligible either. Might be enough to arch and pierce the isolation of the wires inside the coil itself, or it might even arch somewhere outside of the coil.Who doesn't believe it will be high voltage when you cut the current through a coil, try to measure with a DMM the resistance of the primary turns in a disconnected mains transformer. But keep your hands on the primary terminals while removing the DMM probes.

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2024, 10:00:43 pm
content: measure what you build with a scope it should tell you the real numberi want to know what it actually doesspeakers are a big inductor with DC on them sometimes, I guess the cross over and the resistor (thermal) deal with that problemthis could result in some seriously dodgy engineering if you don't know the true numberyou can justify input and output TVS, but a component across a element inside the circuit to protect it from another element.. you really want to know if you need thatpeople seem to just put foil capacitors there for this reasonbut maybe you should use a battery for this test, if it actually can break a power supplyhttps://www.diyaudio.com/community/threads/clarification-on-morgan-jones-choke-input-snubber.405826/So by just working, a normal choke power supply seems to have this problem too, where they use a rectifier with a choke to reduce the capacitor requirement. A switch is sharper turn off.But I think if you CAN use capacitors, its better then a diode, because it adds more filtering and also protects the circuit

author: Sourav
author_rank: Newbie
timestamp: November 13, 2024, 06:33:26 am
content: Comment on the output capacitor: Based on my understanding, when a load is attached, the capacitor charges up to the supply voltage. When the load is removed or switched (such as with an electronic load tester, a processor, or a bunch of blinking power LED), the choke generates a back EMF. Since the capacitor is already charged, wouldn't this back EMF attempt to overcharge it? If the inductor is large enough, couldn’t this degrade the capacitor's lifespan? And even if a higher voltage capacitor is used, wouldn’t it remain charged at this higher voltage?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 13, 2024, 10:32:44 am
content: The back EMF at the rectifier side could be clamped by the diodes of the rectifier. If one wants one could consider an extra diode across the rectifierd DC side to lower to voltage drop (1 instead of 2 diodes), but usually this is not really needed and not helping much. The small capacitor could help with EMI.Yes the back EMF from the inductor would charge the capacitor. Normally the energy in the inductor is however relatively small and the rise in the capacitor voltage not that large. With a significant load the voltage at the capacitor would also be quite a bit lower (e.g. 25% range) than the no load voltage. So there may not even be an overshoot in the voltage, just the energy from the inductor helping to charge the ouput capacitor to the no load voltage. If the voltage overshoots, it would remain high, but there normally should be some discharge path for the capacitor for safety reasons anyway.It depends of cause on the ratio of inductor to capacitor and also the exact time when the load is removed.The main idea of the inductor is usually to improve on the power factor and avoid mains harmonics / short current spikes. It is less about getting away with a smaller capacitor. This is at least today and not very high power.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 13, 2024, 06:25:58 pm
content: Quote from: Siwastaja on November 12, 2024, 03:50:47 pmI think you're misunderstanding me here. Pease hated simulation because newer engineers would believeanythingsimulations told them, they wouldn't apply critical thinking skills on results viewed.As for lab data trumping simulations, it's not a stupid simplification. If you have lab data, then that should always be trusted over simulation results. Simulations are great when trying to do a first cut to see if a project might work, but once you get a board built, lab measurements should be trusted more. If your simulation says a circuit should be stable but the build in the lab isn't, which do you believe?I once had an engineer tell me that because he couldn't get a macro-model for a chip working right that he didn't trust the measurements we were making in the lab.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 14, 2024, 02:00:32 pm
content: I think Bob threw a computer off the roof because you go to jail for throwing idiots of a roof.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 15, 2024, 06:51:47 am
content: If the problem you see is HV 'ringing' then use a tuned series filter, (L,C).X

author: vk6zgo
author_rank: Super Contributor
timestamp: November 15, 2024, 09:19:51 am
content: Quote from: Tim S on November 12, 2024, 10:01:03 amExactly.The diode needs to be across the DC supply.Putting it in parallel will adversely affect the function of the inductor.A diode in "reverse parallel" across a relay coil is OK , because in that case, inductance is simply a by product of using a relay, but in the OP's case it is the reason for the inductor's presence.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 15, 2024, 09:37:31 am
content: Quote from: Siwastaja on November 12, 2024, 03:50:47 pmWhen there were no simulations, people still made things, especially choke input filters.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 15, 2024, 09:45:05 am
content: Quote from: Sourav on November 13, 2024, 06:33:26 amThe back emf is a transient, so effectively an ac waveform.Capacitors, once charged, will still conduct for ac signals.That is how interstage coupling capacitors work.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 15, 2024, 09:52:09 am
content: Quote from: AnalogTodd on November 13, 2024, 06:25:58 pmOver the years, I have seen a number of people in the Beginner's section puzzled because their "Tuned Radio Frequency" (TRF) radio using an Op Amp works perfectly in simulation, but is an oscillator when a real one is built.

author: MrAl
author_rank: Super Contributor
timestamp: November 15, 2024, 10:27:09 am
content: Quote from: RoGeorge on November 11, 2024, 12:36:43 pmHi,The short answer is that in a regular DC power supply with rectifier diodes, you simply do not need an extra diode.The inductor is never disconnected from the circuit so it always finds a path to discharge. For the typical DC power supply the inductor discharges through the rectifier diodes if the power is turned off.As long as you do not disconnect the inductor itself, it will always be OK. That's the way most simple DC power supplies work.Also, it's not only when the power is disconnected that this happens, but also the way the inductor works whenever the input voltage goes low. That's the intended function actually, to keep current flowing when the AC cycle gets to a low point in the phase.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 15, 2024, 02:37:11 pm
content: Quote from: vk6zgo on November 15, 2024, 09:52:09 amThat is exactly why I was trained to trust lab data. Sure, the simulation says everything should work perfectly. If you were to try going to production with that, you're going to lose a lot of money. Also a lot of the reason that simulation must be looked at with skepticism as well; experience in the lab should give an engineer an understanding of what to expect for circuit behavior.

author: Sourav
author_rank: Newbie
timestamp: November 15, 2024, 09:42:41 pm
content: I might be skewing the discussion a bit, but you can't rely on the diodes in a full-wave bridge rectifier in the scenario I'm describing, where the load opens instead of the power supply or the inductor.For example, consider an audio amplifier driving a powerful woofer. When the woofer produces a strong beat, it might draw, say, 10A. When the beat stops, the current draw can drop to nearly 0A. The amplifier chip, however, remains connected to the +VCC and GND. In this case, when the woofer stops, the magnetic field of the inductor in series with the power supply and the amplifier collapses. This collapsing field attempts to push 10A through whatever path it can find, regardless of the voltage.I've tested this scenario myself using an electronic load tester with a fast-switching power MOSFET. Without any additional components, my oscilloscope shows the current and voltage waveforms are mostly stable but with minor dips. When I added a 330 µH inductor (wound on a 30mm Sendust toroid core to prevent saturation) in series with the power supply and the electronic load tester, the results were striking. The inductor produced over 50V at 0.5A and over 100V at 1A. But the connected bench power supply was supplying 5VDC (and CC set to max 5A).Adding a flyback diode in anti-parallel with the inductor resolved the issue, but the diode heated up rapidly due to the repetitive collapse of the back EMF. Using a 470 µF capacitor instead showed much better results: the power supply's voltage and current waveforms became more stable with significantly reduced noise. However, with an insufficient capacitor, such as a 1 µF capacitor, the situation worsened compared to using just the inductor, resulting in significant ringing. A 22 µF capacitor improved stability but heated up quickly, though not as much as the load itself.In conclusion, it seems prudent to include a flyback diode, though if the diode becomes too hot, a sufficiently large capacitor might suffice. To be safe, adding a TVS diode might also be a good idea.For reference, I found a similar discussion here:https://electronics.stackexchange.com/questions/541365/inductor-capacitor-filter-load-disconnected/541368#541368

author: 5U4GB
author_rank: Super Contributor
timestamp: November 16, 2024, 09:33:04 am
content: Quote from: AnalogTodd on November 12, 2024, 02:28:14 pmSpecifically:

author: MrAl
author_rank: Super Contributor
timestamp: November 16, 2024, 12:57:42 pm
content: Quote from: Sourav on November 15, 2024, 09:42:41 pmHi,In the case where the load is suddenly disconnected, the output voltage will shoot up. How much it shoots up depends exactly on the inductor value and the filter capacitor value. With a larger capacitor, it will not shoot up as much as with a very small capacitor. It will shoot up though.This is always a problem with inductor input filters because the inductor has a tendency to try to keep the current flowing, and it's the same current as just before the load is disconnected. If the current was 10 amps, then the current at least for a short time, is still 10 amps. This means the capacitor charges up to a higher and higher voltage until the energy in the inductor dissipates. It can actually oscillate too.The thing is, you can't put a diode across the inductor because the inductor must be able to work in the same way that causes the output to shoot up. In other words, if you put a diode across the inductor, the circuit will not work right as the inductor will not be able to supply energy for some parts of the cycle where it needs to supply that energy.This is always a problem, and probably the only way to deal with it is either to make sure the load is securely connected at all times (example: LED driver) or you use a clamp circuit of some kind, which for low power may be a zener. You have to be careful using a zener though probably not that good of an idea. A clamp circuit would do it, but at larger power levels it's going to have to be hefty.You can add capacitance, but that just limits the max surge voltage, which will still go up and almost always by too much.When we have a DC buss, we can often pump energy back into the buss. But this kind of power supply would be the buss, so we can't do thatVoltage feedback helps, but there will still be a time when the output shoots up. That's typical of boost converters for example.You might be able to get away with one of those surge protector devices (transient suppressors), but they do not have an accurate voltage set point. Only a clamp circuit can provide that.

author: Sourav
author_rank: Newbie
timestamp: November 16, 2024, 03:49:09 pm
content: Quote from: MrAl on November 16, 2024, 12:57:42 pmYeah, that’s absolutely true. In my case, adding an inductor, a flyback diode (antiparallel to the inductor), and a 470 µF capacitor actually smoothed the voltage and current waveforms, even for a 12A switching load. The inductor can also filter out noise. While it makes the circuit slightly less efficient and causes a small voltage drop, the overall operation becomes more stable.The previous IN4007 diode got quite warm,possiblybecause the inductor was limiting the current change required by the load tester, and the diode was conducting without enough time for reverse recovery. A FR207 diode heated up slightly, reaching around 35–40°C at 10A, which is mostly acceptable. Adding a capacitor reduced the heating of both the diode and the inductor. The capacitor also protected against high voltage spikes, improving overall efficiency.So far, I’ve tested this setup with a linear bench power supply and a DC battery. But I think it might not perform well with 50Hz or 60Hz transformers and a full-wave bridge rectifier, as the diode would cancel out the inductor’s EMF, preventing it from maintaining a smooth curve.Still, I think it’s a good idea to use a Pi filter in this case, perhaps with a larger capacitor before the choke to filter most ripples out. Then, combining the inductor, a flyback diode, and an output bulk capacitor can help ensure smoother operation and prevent voltage spikes!In the image, to smooth out the 50Hz input where ΔV = 15% (1.8V), a 5555 µF capacitor is suggested [(1 / (100 * 1.) * 1_000_000]. A 4700 µF capacitor placed before L1 ensures that L1 doesn’t follow the ripple and that the voltage before the inductor is mostly DC without any significant ripple. The inductor used here is of an arbitrary value, chosen to further smooth out voltage ripple and reduce power supply noise. The output bulk capacitor can then eliminate any remaining noise, as I verified using my load tester, resulting in an overall straight voltage line.

author: coppercone2
author_rank: Super Contributor
timestamp: November 16, 2024, 06:10:38 pm
content: can you please try it with a 12V battery and record an oscilloscope picture??

author: Sourav
author_rank: Newbie
timestamp: November 16, 2024, 08:45:57 pm
content: I have 12V battery, but all the scope images I posted ran from 21V battery pack (5xLi-Ion in series),I think the voltage was 19V or so on load

author: coppercone2
author_rank: Super Contributor
timestamp: November 16, 2024, 10:07:08 pm
content: oh i did not even see your postI got a 0.5mH inductor and a 2000uF capacitor. i am thinking about trying to measure this

author: coppercone2
author_rank: Super Contributor
timestamp: November 17, 2024, 12:58:22 am
content: ok well I am glad it did not make 20 megavolts , I think your measurement is accurate to what I would expect from a modest inductor.

author: Sourav
author_rank: Newbie
timestamp: November 17, 2024, 01:40:05 pm
content: LOL, glad it's ok (although I'd be more glad if someone said it's not ok)

author: MrAl
author_rank: Super Contributor
timestamp: November 17, 2024, 02:45:00 pm
content: Quote from: Sourav on November 16, 2024, 03:49:09 pmHi,That diode prevents the left side of the inductor (in your drawing) from going much more negative than the output, which has to have a profound effect on the circuit operation because of the lost filtering action. That means the capacitors could be doing all the filtering work. Would be good to check in a simulator though.Did you simulate this yet?

author: RoGeorge
author_rank: Super Contributor
timestamp: November 17, 2024, 03:17:51 pm
content: The choke filter in my case was added to stop the SMPS noises to enter from the desktop 12V into the power amp. The audio amp is powered from the desktop's +12V. Without the choke, there are audible CPU (modem-like) noises entering through the 12V wires. With the choke filter in series on +12V, no audible noises.The diode doesn't make the choke ineffective as a noise filter. I've left he diode connected in parallel with the 1mH coil, because in practice, with or without the diode, the choke coil stops the noises just the same. So I've left the diode there, in parallel with the 1mH coil. All working well so far.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 18, 2024, 08:15:31 am
content: In case anybody wonders like me, but how come that in practice it still works as a choke with the diode in parallel with the choke coil, it works because the amplitude of the noise is very small. Though, I didn't thought about that when I've added the diode.I was just lucky the noise was too small to open the diode.

author: MrAl
author_rank: Super Contributor
timestamp: November 18, 2024, 10:07:04 am
content: Quote from: RoGeorge on November 18, 2024, 08:15:31 amHi,If the noise is small, then you don't need an inductor as large as 1mH. Normally even a 2uH inductor would work, but does depend on the frequency.However, if you think it is working the way you like it to work, then don't worry about it

author: RoGeorge
author_rank: Super Contributor
timestamp: November 18, 2024, 10:23:15 am
content: The value just happened to be 1mH.That choke was a power filter leftover from a former car radio. It already has had wires and a nice plastic box in which the small transformer-like E+I core was sitting in series with V+. Only the coil and a 470uF to GND (after the coil, on the radio side) was inside the filter box, no diode. The universal tester measured 1.16mH/0.6homs (for the coil alone).