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I have disassembled an old hard disk and I making the voice coil move by applying a voltage between the terminals of the coil. I plan to use this to raise or lower a flag attached to the actuator arm.

The problem is that if I use a voltage of 5V the actuator can't generate enough torque to lift the flag. If I give it 12V, then it works. The current across the coil is around 600mA.

However at 12V when the actuator is powered the coil will start to heat very quickly, enough to be painful to touch. I have tried to keep it powered up for around half an hour and I didn't see any damage, just a lot of energy being converted into heat.

However I am worried that on the long run the heat might damage the coil. In my application the coil might remain powered for around 3 to 5 hours, in an environment that can get to 40°C ambient temperature.

Am I being paranoid or is there actually a risk of melting the the coils or otherwise damaging them ?

To clarify, after reading some of the answers below: I appreciate the fact that I am using the device well beyond its designed operation parameters for a function it was never intended for. What I would like to know is if this can cause enough stress to destroy it, or if there are no other likely side effects beyond the extra heat that is generated. For example I know that a DC motor that is operated at a too high RPM can destroy itself in a various ways. But I have no idea about a voice coil actuator.

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  • \$\begingroup\$ IME 100C is very painfull to touch, but no problem for most electronics (much less for a coil). \$\endgroup\$ – Wouter van Ooijen Jun 22 '12 at 21:49
  • \$\begingroup\$ @WoutervanOoijen thanks, this the kind of answer I am looking for. When you say "much less for a coil" what do you mean exactly ? That it would require much more power to actually destroy the coil ? \$\endgroup\$ – feralgeometry Jun 22 '12 at 22:37
  • \$\begingroup\$ Power transistors and the like often have a maximum junction temperature well above 100C. My gut feeling says that copper and enamel can withstand even higher temperatures (en.wikipedia.org/wiki/Enameled_wire states that 'other' insulators are only good to 105C, implying that enamel can withstand higher temperatures). Are there other materials in the construction that can get hot? As a stopgap measure, you might add a small fan. \$\endgroup\$ – Wouter van Ooijen Jun 23 '12 at 7:22
  • \$\begingroup\$ @WoutervanOoijen I left the actuator mounted inside the original hard disk case. It's made of aluminium I think and it's acting somehow as an heatsink. The coil is connected to a TIP120 (or TIP122) that I am heatsinking already. Other than that it's all wood. I ran an experiment yesterday and left it powered on for around 3 hours. It didn't show signs of damage, which seems to confirm your theory. \$\endgroup\$ – feralgeometry Jun 23 '12 at 8:30
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Consider adding the counterweight, so there will be less torque needed to do the motion and hold the position. You are overloading the voice coil 10000%. The original torque of voice coil at zero velocity is just few grams to maintain equilibrium with drift of air suspended tiny heads.

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The root cause of your problem is that you are trying to use something very differently than what it was designed for. Old disk voice coils where intended for moving the heads in and out quickly. The motion was in small bursts with little power spent keeping the head at a fixed track. There was also a lot of air flow all around.

These things were not designed to put out significant force for long periods of time, which is probably why it is overheating when you try.

Either make the flag position bi-stable somehow so that the voice coil only needs to be activated to move it between the positions, or find another way to do this. A geared down motor sounds appropriate. With a worm gear drive the system won't be reversible, so the flag should stay wherever you last put it when you power down the motor.

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  • \$\begingroup\$ An RC-servo might do the job too, but it needs a somewhat more complicated controller circuit. \$\endgroup\$ – jippie Jun 22 '12 at 22:01
  • \$\begingroup\$ Thanks for your answer Olin and for the alternatives you suggest. I understand that I am using the device to do something it's not designed for. My original question however was more along the lines of "is there any risk of actually destroying it by using it this way, or the extra stress I'm subjecting it to isn't likely enough to cause permanent damage ?". I'm updating the original question to reflect this. \$\endgroup\$ – feralgeometry Jun 22 '12 at 22:28
  • \$\begingroup\$ @UgoRiboni - You're using a device in a way that was never intended, or tested for. About the only way you are going to reliably determine if you can abuse it in the manner you want is to actually do so, and see if it fails. Ideally, you would get a number of them, and test them until failure, take the average, and volia, you have an life expectancy. Realistically, there is really no way we can provide a definitive answer. \$\endgroup\$ – Connor Wolf Jun 25 '12 at 10:18
  • \$\begingroup\$ @FakeName Your are correct. I actually did test with two of them and they survived a 3+ hours stretch of uninterrupted power lifting the flag, so I'm starting to be optimistic here. \$\endgroup\$ – feralgeometry Jun 25 '12 at 13:43
  • \$\begingroup\$ @UgoRiboni - You should be aware that there may be long-term effects you're not seeing yet (That's why I said test them to their failure point, not just for a few hours). For example, the heat may break down the epoxy that holds the actuator coil together. It could take a long time to fail, but it would still fail earlier then under normal operating conditions. Or the heat could damage the magnets. IIRC, neodymium magnets are sensitive to heat, and I don't know exactly how hot your apparatus is getting. \$\endgroup\$ – Connor Wolf Jun 25 '12 at 21:47
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Seems like you are willing to misuse this part, but think about the consequences: what's the next failure in the system once the voice coil enamel fails short? Is your PSU current limited, or regulated in some way? What will happen when the regulator fails short? Are you going to be exceeding the ratings on the bridge rectifier? How about the mains transformer secondary windings?

If this is an unattended installation of some sort get a fuse on the voice coil circuit so at least you fail-safe, otherwise you might be razing a small wood-fuelled electrical fire rather than a flag.

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  • \$\begingroup\$ Very good advice. The coil is switched by a TIP122 transistor, and when that fails the next step is an adjustable laptop power supply. I think these have internal fuses, but i'm gonna check that. \$\endgroup\$ – feralgeometry Jun 25 '12 at 14:54

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