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Architectural question regarding motherboard solenoids

By Lokalaskurar
Apr 8, 2011
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  1. Before 2006, if you looked close to the CPU on common AT and ATX-boards, you could almost always find a solenoid or three. But how come these have almost vanished from motherboards over the years?

    I know that the Asus Striker still uses solenoids, for instance.

    The common electrolyte-capacitor disappeared as solid-state capacitors where more up-to-date, but does this mean that there are currently more modern ways of alternating local magnetic field resistance in motherboards, other than using solenoids?

    Just being curious :cool:
     
  2. Tmagic650

    Tmagic650 TS Ambassador Posts: 17,244   +234

    Are you confusing Solenoids with relays? Cost-cutting is another driving force for these changes in motherboard manufacturing
     
  3. captaincranky

    captaincranky TechSpot Addict Posts: 11,691   +1,879

    Motherboards had relays on them....? Wow, missed that one.

    The only relays I know for certain are still prevalent, are the speaker protection relays in audio amplifiers, and perhaps in the power button circuits of some old CRT monitors and televisions.

    (And Tmagic, give the guy a break, we both know that a "relay" is a "solenoid", but with a magnetically activated switch attached).
     
  4. Tmagic650

    Tmagic650 TS Ambassador Posts: 17,244   +234

    Yes, both relays and solenoids can mechanically isolate a low power source from a high power source, or one circuit from another
     
  5. captaincranky

    captaincranky TechSpot Addict Posts: 11,691   +1,879

    Actually, I sort of forgot car horn and heater fan relays. The purpose there is to prevent the actual on-off switch from pitting due to high current and inductive arcing.

    Obviously easier to put in a new horn relay, than tear the steering column apart to change the horn switch. I've always appreciated that.


    So, on a motherboard, do they isolate the case power button?
     
  6. Tmagic650

    Tmagic650 TS Ambassador Posts: 17,244   +234

    "So, on a motherboard, do they isolate the case power button?"...

    No, I'm trying to remember... maybe isolate power supply outputs to the motherboard voltage and current circuits
     
  7. Lokalaskurar

    Lokalaskurar TS Enthusiast Topic Starter Posts: 544

    That does not sound too far-fetched if you ask me. The motherboard's solenoids do have copper cable of a size similar to that of the input-males inside the 24-pin power socket.

    Hm... I've seen in some cases (many cases, come to think of it) that the on/off button's cables are twined. Is this possibly done to prevent (ok... reduce) EM interference? Must say that I've never heard about inductive arcing before :eek: . Is it related to the sudden voltage-spike which occurs when a circuit is broken? (aka. Lenz's law?)
     
  8. captaincranky

    captaincranky TechSpot Addict Posts: 11,691   +1,879

    You're actually trying to access several different phenomena and come up with a "unified field theory" here.

    A lot of the EMI generated by a computer relies on the case itself for its suppression. The RF fields created by onboard oscillators (clocks especially), pass though the metal of the case. As they do, they are shunted to ground by, well the ground terminal of the wall socket.

    The wires of say, the power switch being twisted together has little effect on reducing EMI. This is because the >>case<<power switch operates on pure DC. DC doesn't create EMI, which is after all, alternating current. However, open wires can act as an antenna for AC fields that might be present. Hence, some manufacturers supply a magnetic donut to route the case leads through as a suppressor. For actual more complete suppression of EMI coming into or going out of a circuit, a grounded, shielded connector is used.

    An inductive load is called "impedance", and unlike a pure resistive load, varies by frequency. This is why a loud speaker's "impedance" is a "nominal X ohms", because the actual resistive effect varies by frequency.

    Any inductor, be it loudspeaker, RF choke, solenoid, or relay has at it's basis, a coil of wire. A coil of wire is basically half of a transformer. Transformers step voltage up or down, by virtue of the simple mechanical ratio of turns from the primary circuit . (1 turn primary @ 10 volts would equal 40 volts @ 4 turns in the secondary. Transformers only work in an AC circuit!. But, they also respond to DC if it's a transient spike. That's why, when the circuit is first powered up, the voltage increases.

    Another current inrush phenomenon is caused by capacitors in a circuit. Capacitors are intended to store electricity. So, when a device with caps in the primary circuit is turned, the >> current<< flow is very high until the capacitors have fully charged.

    I don't know the math on this, or who "Lenz" is, but that's a bit of the principles in operation that I'm familiar with.
     
  9. Lokalaskurar

    Lokalaskurar TS Enthusiast Topic Starter Posts: 544

    I wish my physics teacher would've told me this... very interesting to read, truly.

    " The wires of say, the power switch being twisted together has little effect on reducing EMI. This is because the >>case<<power switch operates on pure DC. "

    Sorry, I was a bit stupid, I did not consider that the PSU-output is DC.

    _ _ _
    -------

    " A magnetic doughnut "
    This is the ferrite ring, correct?

    " Transformers only work in an AC circuit! "
    Amen to that, I've read Nikola Tesla's autobiography.

    " But, they also respond to DC if it's a transient spike. That's why, when the circuit is first powered up, the voltage increases. "
    Oh, I did actually not know that...

    " So, when a device with caps in the primary circuit is turned, the >> current << flow is very high until the capacitors have fully charged. "
    This is because the capacitor limits the voltage until it's discharged, right?


    Now, about Lenz's law: "An induced current is always in such a direction as to oppose the motion or change causing it." Quote of Wikipedia really, it's a bit tricky to explain it otherwise.

    But one famous effect of this law is that phenomenon when a circuit is broken, the voltage spikes tremendously because it's trying to stay "circuited". So for an example, a Tesla coil needs a voltage safety gap, otherwise Lenz's law would make sure to burn out the entire transformer when the device is shut off because the voltage spikes.
     
  10. captaincranky

    captaincranky TechSpot Addict Posts: 11,691   +1,879

    Well yes. However in hindsight, I've never actually checked to see if it was indeed magnetic. That was silly of me, but we are talking about the same thing


    What's required is that the electricity "cuts" across the coil, inducing voltage in the secondary. With AC, the flow goes positive, returns to neutral, then goes negative. Each cycle fulfills the requirement. DC goes from zero to full positive, and in doing so it does "cut" across the field, resulting in a single spike.

    You're not even close on this one.

    Capacitors store electricity quantitatively! That is to say they store "amperes". But it is stored at the prevailing "pressure", which is voltage. A much better explanation is that they stabilize the voltage of the circuit

    The classic use of large electrolytic capacitors, is in the final output stage of all audio power amplifiers. Normally, the output stage of an amplifier is directly connected to the B+ supply, which comes directly off the PSU. The caps perform 2 functions. They reduce "AC ripple" in the DC side of the PSU, (The "rectifiers" in the PSU convert AC input to DC, but it is actually still in the form of a half sine wave, or can also be with the negative AC pulse inverted to positive. In this case both the "DC" pulses are half AC sine waves). And returning to topic, the caps second function is to prevent amplifier loading from dropping the B+ voltage. As the voltage drops, caused by perhaps, somebody blasting a power chord on the guitar, the capacitor discharges to maintain voltage equilibrium in the circuit, When the peak passes the caps recharge from the PSU.

    When the amp is first turned, the caps are empty, and the current inrush is formidable. Since the output transistors are wired directly to the PSU, the excess startup current would fry them. Hence, the "speaker protection relay", which separates them from the circuit until it is at operating equilibrium. The current inrush is so massive in this type circuit, it technically is almost a short. At the very least, the output transistors would perceive it to be a short.

    In its most basic form a capacitor is two plates in close proximity. Because opposite charges attract, (just like opposite magnetic poles). the negative potential "holds" the positive charge on the positive plate. The size of the plates and their proximity, define the quantity of electricity that can be stored.A capacitor will pass AC, because the charge potential reverses, the positive plate becomes the negative plate, over and over. Given this template, it should be easy to figure out why the capacitor will block DC and pass AC in the same circuit.


    This is because the field collapses and "cuts" across the inductor. The DC drop is acting as an AC swing toward negative. (At least that explanation is close enough to satisfy me).
     

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