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Jun 25, 2017, 01:55 AM
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Mastech short circuit switches

The day had finally come to install short circuiting switches on the Mastech. Many circuits were fried because its current limiting was set too high. It was too much trouble to unplug the circuit & plug in a short circuiting wire to set it.

Despite looking simple from outside, the front panel is packed on the inside. There was only 1 place to squeeze the switches in. The LEDs had to be sandwiched between the switches & the plastic. At this time, another desire emerged to have switches for turning off the individual outputs without turning off the manes, but this would have been a much more incremental benefit than the short circuit switches.

With it all installed twice, the switches pointed at the current dial when they were shorting & pointed at the voltage dial when they were open. This was the most intuitive orientation. They worked very briefly, at low voltages & all current up to 5A. When the voltage was set to 30V, the switches quickly failed closed.

The switches open by a spring force which the human has no control over. The human can merely press the spring to make contact, but the spring must break contact by its own force. The spring had arc welded to the contact. It could be broken apart & reassembled, for the most part. Despite being rated for 6A 120V, the switches still failed at 30V 5A.

The best the internet can come up with is 120V AC ratings being equivalent to 14V DC, because in AC there is a zero crossing after 8ms to break the arc while in DC, the arc can continue. While the power supply limits current when quiescent, there is a very high inrush current when short circuiting it. The Chinese must have encountered the same problem, said "f*ck it" in Chinese & left out the short circuit switches.
Last edited by Jack Crossfire; Jun 25, 2017 at 02:06 AM.
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