Protection diode needed on switched LC tank circuit - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: May 12, 2023, 02:03:09 am
content: As part of a larger system I've set up an LC tank circuit being switched by an incoming 5V to Gnd square wave (SIGNAL_IN) at something pretty close to the LC resonant frequency. I've included series resistors either side of the LC to limit DC currents. I'd like to check though whether I ought to be including a protective diode either pointing up across the LC pair, or up from the bottom end of the lower 100 ohm resistor to the top of the LC pair. With a lone inductive load I'd know I'd need one, but with the cap in parallel and switching it at the resonant frequency I'm no longer sure, does the cap absorb any spikes that a diode would be there to protect against? To my understanding the protection diode would only be serving to protect the collector of the NPN transistor, the 5V supply rail (and anything else on it) should be naturally protected by being on the non-switched side of the inductor, and anything attached to SIGNAL_IN is already protected by the transistor, so long as I prevent anything terrible happening to the collector?ThanksP.S. the intention of this circuit is to create large dI/dt across that inductor, because in the finalised design that inductor is a coil which is being used to induce a signal in other circuitry a fraction of an mm away as part of a resolver system. Protection options which would drastically limit dI/dt wouldn't be a good solution here, but the transistor will ofcourse need protecting I expect (exact inductance of inductor not yet known).

author: jwet
author_rank: Frequent Contributor
timestamp: May 12, 2023, 04:07:51 pm
content: The intention of a resonant LC driven by a current source (collector) is to swing beyond the rails on stored energy. You don't want to snub it like a relay if you want it to swing.Some hints-You have 200 ohms of R in series to limit the charging current on a per cycle basis- no reason to have one above and one below. There is nothing limiting the voltages and currents that can be generated if you get this resonating- its only limited by the Q of the LC- inductor R and Cap parallel R. It could go very high- in both volts and current. I remember a sophomore EE lab where we did something like this- it was an eyen opener as an eighteen year old student. Remember the energy equations- .5Li^ 2 and .5Cv^2- that will be flowing back and forth between the L and C. You pump that thing at resonance- only the finite Q will dissipate energy and keep things from blowing up. You could put a large value R across the LC to peg the Q and give you more control of things. You also want to bypass the high rail to keep that high voltage/current AC out of your supply. Size the transistor Vce and make sure it can take the worst case swing and then double it. Add a zener at the collector of the transistor to limit the max Vce, this could have some resistance in series so it sort of softly clipped the sine of the LC, you could also put dual zeners of opposite polarity with a series R between across the LC to softly limit the amplitude. You also have to figure out how you're going to turn this thing off, that energy has to go somewhere. This is similar to the primary drive of a Tesla coil- you might get something out of looking at those circuits. Have fun and be careful.Good luck.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 12, 2023, 09:25:03 pm
content: By bypassing the high rail do you just mean plenty of decoupling caps between it and Gnd, or do you mean something else? I had thought the use of a series resistor between the top of the LC and the power rail would be providing some protection already, I guess I'll need more.Zener going from Gnd (transistor emitter) to the collector, with a zener voltage a little below the max V_ce the transistor can take? Mostly there to serve as a last ditch attempt to prevent the transistor being destroyed?Is there an equation which would suggest the size of resistor to use in paralell with the LC? Or is this more a trial and error matter? The inductor itself already has a relatiely low DC resistance and will only get lower in the final design.I made an initial test with 10mH, 100pF, 157KHz, my transistor is rated for 45V_ce, I was getting 32V peaks. But I know I'll be using a rather lower inductance coil in the final design, the final design will have a diferent cap value according to the frequency I'll want it to resonate at then, which might well be similar or a few times higher.The thought of even a fraction of 32V getting near the power rail is terrifying, that same power rail in the final design will be running microcontrollers and other stuff which would release the "magic smoke" at >5.6V. And realising that I'd be getting even higher than 32V if the frequency was even closer to perfect resonance (rather than resonance +/- a few KHz) is scarier still.Thank you

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2023, 09:47:14 pm
content: No, nothing needed.But the resistors do cost signal level, and fewer components might be used. If you can say more about your system, signals, layout and other requirements, a better solution may be possible.Tim

author: Infraviolet
author_rank: Super Contributor
timestamp: May 12, 2023, 09:53:28 pm
content: Another thought, what about a series resistor between the top of the L and the bottom of the upper 100R. This would mean that on the upper side of the LC tank there would be some series resistance to dissipate energy each time the resonating current went back and forth between the L and the C, but I'm not sure if the value of such a series resistor might need to be drastically different depending on quite how close to perfect resonance the driving frequency actually got.

author: jwet
author_rank: Frequent Contributor
timestamp: May 13, 2023, 12:30:18 am
content: Responses from jwet-yes- bypassing would be from the high rail. It might make sense to bring Vcc over and put a buik C and bypass there- maybe 100 uF and .01 uF. Then have a 200 ohm resistor and tie it to ground to through a .1 uf. This double bypassed point is your supply for the top of the inductor. The inductor would see a dc resistance of 200 ohms like now and fairly low ac impedance. The collector will swing around +-50 or volts peak to peak sitting on Vcc baseline volts DC.Zener going to ground or pair of zeners across LC to limit max excursion.Equation for R value. The Q of your L sounds high perhaps 100 or more (measure it). Good caps have a Q well over 100 and often 200. Decide what Q you want- this will affect how narrow the resonance and how high the peak voltage will go in resonance. Adding some parallel R across the LC will lower Q- the 200 ohm series R isn't really in the circuit- you just have a high Q LC. A good target for Q might be 100. Q is Xl/R- 10 mH at 157khz is is about j10 kohm. this would be 100 ohms of series R or about a meg of parallel R. Remember that at resonance the L and C reactance cancel each other out and you're left with a real vector that depends on total R.The resonance might be pretty narrow (hi Q). 157 Khz is nominal resonance but could be off a bit due to tolerances- not sure if your seeing peak peak volts.You want to condition that rail a bit (as in first bit) before sharing it.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 13, 2023, 10:20:22 pm
content: Any chance of a quickly drawn diagram of your suggested power rail protections? Words don't seem to work all that well for describing networks of interconnecting parts.I looked up about puting some series resistance in the path between one end of the L and the same end of the C, but as soon as (R/L)^2 gets above about 0.04*(1/LC) you get meaningful changes to the resonant frequency, and not long after you get (R/L)^2=(1/LC), f=0 Hz, and imaginary roots only for R larger still. So it seems even a pretty tiny series resistance there can completely destroy resonance. I'm fairly sure I don't want to destroy the resonance (the dI/dt of the signal it generates would then likely become too weak to be induced in to other conductors in the final application), just ensure it can't leak high voltages to the power rail, or anywhere else, and won't damage the NPN low side transistor.EDIT: Just thinking, this is surely the same problem as they must have when designing wireless charging coils, the whole idea of such things is to work at the resonant frequency of the coil in parallel with a cap, that gives a much more efficient energy transfer than any situations where you try to put a switching signal in to a coil any other way. Are there any examples of those circuits to show how they protect their switching transistors and power rails? The only examples I can immediately see in searches are the very most basic design fundemantal principles DIY sort, so neither show any protection nor any reason why it isn't needed (guess there aren't meant to last long enough for that to matter), but are there some more complete long term usage grade examples shown anyone can think of?ThanksEDIT 2: seems a lot of wirelass charging transmitters actually have their Land C in series rather than parallel. Would that simplify the protection of the transistor below and power rail above?

author: jwet
author_rank: Frequent Contributor
timestamp: May 15, 2023, 07:50:30 pm
content: I started doing this on Sunday morning (mother's day here in the US) and then got distracted. I was trying to send you a simulation file in Falstad that you can play with- have to learn Falstad a bit better- neat tool. Give me a day or two.

author: Infraviolet
author_rank: Super Contributor
timestamp: May 16, 2023, 09:03:55 pm
content: Falstad? Will that run on Linux (or under Wine)?

author: Infraviolet
author_rank: Super Contributor
timestamp: May 18, 2023, 03:30:45 pm
content: Unless you really want to have a go at simulating for your own interest, feel free not to. Thank you very much for trying though.I did some more tests with the physical components, presence of huge voltages seems to be entirely dependent on having a very big inductor in there, a smaller inductor (with the cap adjusted so as to still be resonant) gives much smaller peaks, well within the transistor's rating. Adding a diode after the "lower" resistor pointing to the 5V power rail allowed even further, though not strictly necessary in these circumstances, clamping, and did so with only a small reduction in the amount of near field signal this was able to transfer to a receiving inductor beside it.I think the huge spikes were a lot more about inductor switch-off effects than about resonance, although the cap clearly does still play a value as without it the voltage on the inductor oscillates many times as the transistor switches off, whereas with it there is just a single peak for the inductor's voltage.And with a smaller inductor, which at a guess is more similar in size to the finalised system than the inductor I first tested with was, there aren't any noticable voltage spikes on the "top" end of the LC pair (just below the top resistor). So I'd guess no further means of protecting the 5V rail, however sensitive other parts on it may be, either.Thanks