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Question Audio capacitor for motor surge current?

Discussion in 'DIY Motion Simulator Building Q&A / FAQ' started by Map63Vette, Mar 10, 2020.

  1. Map63Vette

    Map63Vette Member

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    So I got to thinking, I used to have a big audio capacitor sitting around that I got with a car I purchased years ago. I don't remember if I still have it or managed to sell it, but would something like that be a potentially useful addition to a motion rig? I'm talking something like this: https://www.sonicelectronix.com/cat_i201_1-1-9-farad.html. It seems like it would be able to handle surge currents better if your power supply was on the edge, but didn't know if it would cause more issues than it might help. I was already planning to oversize any power supply I get to try to avoid any current spike issues, but startup current is always going to be a hard peak, so it seemed like something like this could maybe help with that.
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    Last edited: Mar 11, 2020
  2. Ads Master

    Ads Master

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  3. Sam MAHENDRA

    Sam MAHENDRA mahentheman

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    A capacitor when not fully charged is like a dead short circuit. So, when you turn on the power supply, it may trip the power supply via the power supply's short circuit protection circuit. Once the capacitor is partially charged, this may not happen and it will be good to go. Yes, the charged capacitor will provide the short term (we are talking of milli secs) demand, but a battery instead of a capacitor will do a better job.
    In my motion sim with Arduino, I am using the SMC3 sketch with soft-start feature to overcome the motor start-up inrush current.
  4. OZHEAT

    OZHEAT Active Member

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    Also watch out that the audio cap doesn't exceed it's Max voltage, I think it is about 18v. The back EMF of the 12v motors can easily exceed 18v.
    A cap on the supply line will also still have voltage after turning off the power supply, a farrad cap would probably take a while to bleed of to a safe level.
    As Sam said the biggest issue would be inrush current unless you use a current limiting resistor to initially charge the cap.
  5. Map63Vette

    Map63Vette Member

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    Yeah, that initial hookup was the biggest question mark I had. I think it might have some of that built in as there is a small PCB on top that also has a voltage readout, but I've never really played with it. I did find it in the basement the other day though, so I do have it if I wanted to do some testing.
  6. Sam MAHENDRA

    Sam MAHENDRA mahentheman

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    In another application, I used a car head light globe (12 V 100 W) instead of a resistor to limit the inrush current; then I bypassed the globe by a 2 sec delay timer circuit pulling in a 12V 200Amp Heavy Duty Relay bought from ebay (around A$10.00). You can also manually turn the relay on instead of a timer circuit.

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  7. Gadget999

    Gadget999 Active Member

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    do you use batteries as well as the transformer ?

    a car battery will act as a capacitor and buffer the batteries
  8. OZHEAT

    OZHEAT Active Member

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    @Gadget999
    The biggest problem with using a battery is that it has very limited current charging capacity.
    Depending on state of charge(battery's resting voltage) it will only accept so many amps and anything above that will result in boiling the electrolyte(hydrogen production).
    Also realise that a car battery although called a 12v battery but @ 12v it is considered flat and is happy at ~13.2v - 14.7v
    SLA and gel type batteries are even worse in charging amp rate as once you boil the electrolyte it is dead.

    A capacitor will accept pretty much any amps you put to it and the voltage will rise to the source voltage unless you exceed the max voltage of the capacitor, which will result in the capacitor exploding.
  9. Sam MAHENDRA

    Sam MAHENDRA mahentheman

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    Lead acid batteries has an internal resistance, typically 0.3 Ω. This internal resistance limits the current that the battery can supply. Also, it should be noted that if a high surge current is required a car battery is the one to go, compared with a deep cycle, SLA battery.
  10. Map63Vette

    Map63Vette Member

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    I was mostly asking because I just had a capacitor laying around. I haven't built my sim rig yet, just planning right now. I have motors and got some hall sensors and JRKs to play with, but no structure pieces at the moment. I have it all done up in CAD and plans for how to cut and weld my tubing, but haven't had the spare money to play with yet with other stuff going on. I still need to get some real power supplies at some point. I used an old ATX one from a computer for just some initial JRK testing, but it won't even handle a few quick back and forths on the motor without shutting down to protect itself. I have my eye on some beefy LED style ones, but similar to the tubing I just haven't pulled the trigger yet.

    I hadn't really thought about the back current from the motors. I'm still not really sure what I think about that. It's not like they free-wheel that long due to the large gear ratios we deal with, so I'm not really sure if they back feed as much as people tend to think. Then again I've never put a scope on one or anything to really see. I guess you would get the current rush from the field collapsing when you take power away, but I don't think that's as bad as back driving one. Could be wrong though, I'm not an EE by any means, I just play with simple electronics on the side. There does appear to be something already in my motors as I was taking one apart to ground isolate it the other day. Don't know if it's a capacitor for filtering and handling spikes or a diode or something to help with back EMF.
  11. Sieben

    Sieben Active Member

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    Only caps won't might help that much actually. I burned 18v60a tvs suppressor even with several on parallel. With a bunch of caps and left over on purpose the fly-back diodes for testing, with 12volts psu they just were teared appart, while made a steptest for a ffb wheel with wheelcheck with MY1025motor. But with f30u60dn diodes that i use, all works well, with all the same 18volts tvs. And I also have cap on 4 farad. So there are no voltage spikes then with diodes) well just for information.
    Last edited: Apr 7, 2020
  12. czgui

    czgui Member

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    If it is likened to a water channel, the current is equivalent to the size of the water flow, the level of the water channel is equivalent to the voltage level, and the capacitance is equivalent to the reservoir. If the water flow is too fast, the liquid level will be reduced, and the reservoir is large enough to ensure that the size of the water flow changes and affects the liquid level. Equivalent to voltage stability.
  13. Sieben

    Sieben Active Member

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    Supply voltage stability in this case will give higher amps of the PSU. It's total power. Without it there will be voltage sag, whatever caps you will have there.
  14. czgui

    czgui Member

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    More power will solve all problems.
  15. Sieben

    Sieben Active Member

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    It doesn't solve the problem of back EMF, spikes etc. It depends on other things. The driver ability to handle that motors, & a motor itself.
  16. MarcoMade

    MarcoMade Active Member

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    I use car capacitors in my station,
    0.5F for the steering wheel
    and 0.35F for movement.
    the only thing to do is to have an intelligent charging system for the cold boot of the capacitors.
    a charging resistor with a large relay that activates after example 20 seconds when the cap is charged. allowing you to take advantage of the plus of fast energy without crashing the power supply.
    I use Brax inellicap capacitors with charging circuit, very comfortable but expensive
    edit:: 12v only
  17. Map63Vette

    Map63Vette Member

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    Yeah, I think charging current would be my biggest thing to figure out first if I wanted to try them. Also not really sure on the back EMF side of things, but will have to play that one by ear. Pretty much all of my projects are on hold for the next few weeks/months though, so not sure when I'll get around to playing with the setup again.
  18. MarcoMade

    MarcoMade Active Member

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    the large capacitors work in an exemplary way. I state that out of the power supply I also have fast recovery diodes for the EMF.
    attention to the capacitors of 5 or more farad, they are only capacitors in series and therefore their LOW ESR goes to fry. I recommend good capacitors. As far as I know, Brax uses mini-capacitors in parallel to have ultra-speed current.
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  19. Sieben

    Sieben Active Member

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