Solenoid power supply advices - Page 1
Forum section: electronics / projects

author: zkrx
author_rank: Contributor
timestamp: July 08, 2023, 12:17:24 pm
content: Hi,I have this solenoid (model # 154660-224):https://ch.farnell.com/ledex/154660-224/tubular-solenoid-pull-17w-12-4vdc/dp/3276534https://www.farnell.com/datasheets/2869016.pdfThis is a somewhat beefy 9.3R solenoid that I want to use for recoil simulation in a plastic gun. This means that the solenoid will be enabled for a very short time (in the order of 30ms), in bursts and not continuously.I struggle to give it a decent supply. Obviously, if I want a stronger kick, I need to increase the voltage. But then current increases too, and DC supplies that are able to deliver get big and very expensive. I've had some success with a 40V lab supply at around 6A (give or take, I forgot).I'd like to be able to give it something around 100Vdc, perhaps even a bit more. I want it to be somewhat afordable. Given the fact that current will be drawn for dozens of ms at most, I figured that there should be a way to reduce the PSU's performances.I've thought about batteries, but 100V would mean a lot of cells. I've thought about redressing the 230V mains myself, but I will need too many expensive capacitors. An SMPS is just expensive, and designing one myself sounds like hell (at least from what I've read).Lately, I've started looking into camera flash circuits since this sounds a bit like what I want to do: store enough energy in a cap, step up the voltage and release it to the inductor.Does someone know where I should look? Some advice?Thanks

author: Siwastaja
author_rank: Super Contributor
timestamp: July 08, 2023, 12:34:16 pm
content: First verify 100V gives a stronger action. At some current, iron core saturation starts to eat the relative permeability, basically multiplication factor from current to force, so that each next ampere gives less and less extra force, finally not much anything. Maybe the thing has excess iron in it, maybe not, so test with higher and higher voltages until you see no significant improvement.A simple boost converter with large output capacitor would work, with a further MOSFET which connects the capacitor into the solenoid. Simple boost topology is good for maybe to 1:10 boost ratio, after that you might want some center-tapped or transformer-based topology; camera flash circuits would be good references for simple/cheap design.

author: Ian.M
author_rank: Super Contributor
timestamp: July 08, 2023, 12:44:05 pm
content: Yes, camera flash style circuits are the way to go. Basically a current limited boost or flyback converter charging a big capacitor + some way of discharging it through the coil. For a voltage boost of less than 10x, I agree you can get away with a simple boost converter. Above that, you run into efficiency issues with a simple single inductor boost converter so need a ferrite core transformer in a flyback configuration. This also brings the advantage of there being no DC connection between the supply and the output, so as long as the flyback converter is inactive, the load current will decay to zero allowing you to simply use a low-side SCR to control the coil, triggering it to generate a self-terminating pulse. There a few devils in the details e.g. the need to prevent resonant reversal of the voltage across the (polarised) HV capacitor after firing.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 08, 2023, 02:35:31 pm
content: Directly connecting a capacitor with a MOSFET switch is ideal-ish because the inductance of the coil limits current rise time and you want as fast rise time as possible, so excess voltage initially doesn't matter - it's not the voltage which causes saturation, but current, and current will come later. With C chosen properly, when the iron is reaching deep in the saturation, capacitor would have discharged already somewhat. This is not of course perfectly accurate, but it's kinda correct and very easy to implement.It is easy to experiment in lab, just choose some arbitrary but sane voltage, say 60V, and charge different size of electrolytic capacitors up to that voltage. You will find the point after which adding more capacitance gives diminishing returns.

author: zkrx
author_rank: Contributor
timestamp: July 10, 2023, 08:21:50 pm
content: Hey thank you both for your replies.So I could find an old camera somewhere (junkyard or used market) and repurpose it's flash circuitry. Hacking something like this:I would add a bigger cap (caps?) and replace the flash tube with a MOSFET connected to my solenoid.I wonder how big of a capacitance I will need.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 11, 2023, 07:32:24 am
content: Camera flash circuit is optimized for going from 1.5V to 300V, and for lowest possible cost, it might not be easy to modify and performance when used to charge much larger capacitor to 100V might not be satisfactory, but by all means try it out.Per definition of farad = (A*s)/V, where V is the voltage difference between fully charged and what you consider "empty" (something that does not give enough force in your application). Having an idea of necessary solenoid current (10A? 20A?) and time (20ms? 50ms? 100ms?), you can calculate the required charge A*s. This gives you some ballpark idea for the capacitance.For example, letting the capacitor discharge from 50V to 20V while supplying 10A for 100ms,capacitance = (10A * 0.1s)/30V = 33333uF. Quite large value which underlines the point that you can't afford using a capacitor rated to too much extra (unused) voltage because then the capacitor bank will be even bigger in physical size.

author: tautech
author_rank: Super Contributor
timestamp: July 11, 2023, 07:54:02 am
content: Quote from: zkrx on July 08, 2023, 12:17:24 pmPSU is one thing, properly managing the back EMF is another.If you graduate to using silicon for the switching this is something that you need to solve.

author: CosteC
author_rank: Regular Contributor
timestamp: July 11, 2023, 08:00:21 am
content: Back EMF can be managed easilly (at expense of heat), still making PSU quickly charging capacitors is not trivial - it will need to have relativly high power. Maybe easier it would be to rise battery voltage and drive it directly? Or change solenoid into more "low voltage" type?Currents will rise, for sure, yet with todays MOSFETs it shall be easy to deal with.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 11, 2023, 08:14:32 am
content: Quote from: tautech on July 11, 2023, 07:54:02 am"Managing"? Why not just say it out loud, "don't forget the freewheeling diode". On the other hand, if the capacitor is fully discharged to the coil and the switch never turned off, even the diode is not necessary.Quote from: CosteC on July 11, 2023, 08:00:21 amThermals are not an issue. The coil would be switched at 30-40V or so, and Vf of a schottky diode is like 2% of this, hence around 2% of coil power. With the single turn-off event, 2% of the inductance-stored energy would be dissipated in the diode, completely negligible.

author: Ian.M
author_rank: Super Contributor
timestamp: July 11, 2023, 11:13:34 am
content: If the coil's DC resistance is low enough, the discharge will be under-damped and the coil will 'ring' with the capacitor, reversing the voltage on it. This is highly undesirable if the capacitor is polarised. At the minimum an additional clamping diode to prevent the capacitor voltage swinging (significantly) negative will be required.

author: inse
author_rank: Super Contributor
timestamp: July 11, 2023, 01:04:42 pm
content: Perhaps the best idea is to find out the right combination of voltage, capacitance and activation time in a lab setup (mockup) for the desired recoil.And then see if this can be integrated into the mobile version considering recharge rate.Which I personally doubt.A bullet has several thousand Joules of energy, if you only want a fraction of the effect, it’s a little bit of math to determine the feasibility.

author: zkrx
author_rank: Contributor
timestamp: July 16, 2023, 05:32:51 pm
content: Thank you all for your input. What you said led me to reconsider my approach.A bit of clarification regarding my application as it was not correctly explained. The kickback is for an IR gun that I'll use to play arcade games (Time Crisis anyone?). I am currently repurposing a nerf gun for that. In this regard, it does not need to be running on battery. It is not a mobile device and I can supply it with mains if I need to.In fact, I think that the easiest route is to use mains to power the solenoid. I tried doing it in a lab setup before, but I didn't have big caps at hand and 200-300uF was just to small to provide enough current. I will order a couple of 1500uF/400V+ caps.I did a couple of calculations to estimate the solenoid's inductance. Here's what I have (please excuse the roughness of the draft). Assuming I did good (I didn't double check), that's a 23.4uH coil:I took a shot at LTSpice (first time!). Here is what I came up with:Since I'm not willing to use a transformer for this if I can avoid it, I will use an optocoupler to drive the MOSFET from an MCU. I will look into having a slow fuse in series with the solenoid just in case the MOSFET remains stuck closed (MCU bug or MOSFET damage). Also with that much capacitance, I might want to look into some inrush current limiting method to avoid tripping the breaker (a simple series resistor, NTC, relay or whatever).Maybe periodically feeding 30A to the coil for ~[30; 100] ms is unwise. I will do some testing with a variac and slower voltages first and monitor the solenoid temperature.

author: zkrx
author_rank: Contributor
timestamp: July 16, 2023, 06:32:03 pm
content: These caps are quite pricey. One 1500uF cap is probably plenty (even too much?) for my application as ripple drops to "only" around 210V. And it is cheap enough (~15$ a piece). Ideally I would salvage one from an old piece of junk, but they seem rare in common appliances.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 16, 2023, 06:56:20 pm
content: D6 serves no purpose except to add extra power loss after MOSFET turn-off, allowing the solenoid to release (if there is a return spring, for example) a little bit faster because the inductance-stored energy is now spent in both the coil plus D6, so it stops creating force quicker after turn-off; this strategy is used in relays to speed up switching to avoid contact arcing. I don't think it's necessary here because it's the turn-ON which is creating most of the jolt; spring return is rather small jolt anyway. Thus, D5 alone will do, and you don't need to think about the power dissipation and survival of D6.Using mains directly sounds like a horrible idea. Then you need to safety isolate all the driving circuits etc. Also you have A LOT of excess power which does not generate extra force but DOES generate a lot of extra heat; P = U^2/R . This quadratic increase in waste heat is the reason why you really don't want to add any more voltage than necessary to produce the maximum torque, somewhere around iron saturation. Do the test I suggested to find maximum voltage which gives you extra force.With the cost and size of those massive filter caps, you would get a small switch mode power supply. Even if the supply would be undersized (for average power), you would get away with smaller capacitors because your efficiency would be so much higher (see previous point about U^2/R).

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 17, 2023, 07:31:23 am
content: Quote from: zkrx on July 10, 2023, 08:21:50 pmIt depends on your step-up converter possibilities. I guess you may go up to 1000 uF or a bit more.1000 uF is 0.001 F, so Tau = RC = 0.001*9 R = 0.009 sec or 9 ms. Tau of 9 ms is not close to your required 30 ms but the capacitor will be feeded from step-up converter at the same time, so it'll help a bit.