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If you need more than a design, we’ve got you covered at a fraction of the cost of a machine shop. Magswitch can help you customize a range of components to expand the usability of your tool.
The Zener + pass transistor approach is one that I considered an dropped. Most Zeners need a bit of current to get up into their knee region, where they're stable. After you add the resistor and another SOT NPN it's a larger solution. The LDO is certainly more expensive, but simple and reliable. Or it just could be my prejudices from a long time ago and things have changed now.I assembled one of the second pass 30A prototypes using different FET switches than the first pass -- substituting a Vishay SiSS28DN for the IRLHM620. The Vishay part claimed slightly lower RDSon (1.6mΩ vs. 1.8mΩ @ VGS = 4.5V) and a higher max VDS of 25V instead of 20V. I thought, erroneously, that the higher voltage rating might improve the survivability of input transients when batteries are plugged into the input. The addition of R1 between the input and the LDO solves the problem by eliminating the capacitor (C2) that caused the ringing so a higher voltage rated FET is not required to support 2S-4S LiPo batteries. The switch should dissipate a maximum of 0.6W for an applied current of 40A. Maximum current can exceed 1000A for short durations ( less than 100µs). That should be adequate.
It’s a great magnet-science demo, but you’re probably not going to save money with assemblies like this. It's easier and less expensive to just buy an MJ-95 or MJ-150 and get on with it. The two field pictures show the setup both with and without the steel strike plate of a cabinet door.The twist on this application is that it should remember the state of the switch if the input power is disrupted or disconnected for up to 10 seconds. It would be disastrous if the power glitched during a flight and the switch permanently disconnected all of the electronics controlling the operation of the aircraft. I've decided to pursue four types of switches. One for low current/low voltage applications such as RC gliders without motors. Two with higher current and voltage for motorized RC aircraft. A fourth with an integrated battery eliminator circuit (BEC). Low Current Switch Specs: It is apparent that a 30V rating is not enough to prevent inductive transients damaging the LDO. The second pass, and all the other mag switch circuits, will have a protection resistor to prevent this in the future. After replacing the LDO and the hall-effect device, which was also damaged by the event, I jury-rigged a 330Ω protection resistor in between the LDO and the battery. I plugged/unplugged the battery many times to see if there was any remaining tendency to smoke, but the circuit doesn't have that problem anymore. High Current measurements: With up to 1,100 pounds of hold force, on thin material, important when used in barge and tank building. Productivity and Cost to Deliver
Edit 2020-02-02: BHarbour pointed out my laziness in his comment. The following is a bit of extra work to solidify the design.] Reducing the standby current: The condition of the surface also matters. If it’s painted, the magnet is a tiny bit farther away from the steel, so you get less force. Have a rusty surface? Less force. Maybe your surface has an irregular shape; this might also affect the force. When you rotate the knob, you’re rotating the top diametrically magnetized disc magnet by 180°. Now the magnetic field flows from one magnet, through the steel wall and into the other magnet. A perfect entré into the Magswitch Workholding System. Once you have this tool, get all the other great tool attachments and simply mount them to the base as needed.
Our products are designed to be ‘fail safe’ since there is no need for any power during a lift. With a positive indication of the magnetic circuit, the operator can confidently perform knowing the material is fully attached. Magswitch offers Electric, Pneumatic, or Manual for ‘on-off’ actuation. Performance
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