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andy55:
Any Triac or SCR device after being triggered will remain on until the current thru the device drops below a holding current which is relatively small compared with the maximum current rating of the device.
In an AC circuit this happens once every half cycle.
In a normal DC circuit this never happens until you use some other means to force the current to less than the holding current.
To switch a DC circuit you need to use a mechanical contact, a transistor, FET, or if you use an SCR or Triac, then you need some other means to force the current low such as a capacitor and another SCR.
Any time you switch an inductive load you need something to limit the peak voltage from the inductive kick. Usually for your type of load you would put a reverse biased diode across the inductive load unless drop-out time was critical, then an RC snubber would be used.
If you search far enough you can probably find an optically isolated, or some other kind of isolation, SSR based on an FET.
How often is this brake operated? What is wrong with a P&B KUP relay? For 35 years we have used the KUP to switch large clutches ( 120 VDC at 1 A ) in automotive pinion preload adjust machines. Here the clutch is switched on and off several times every minute 24 hrs per day. Typical life about 6 months. If you used another relay to periodically change the direction of current flow thru the controlling relay then the contact life could be considerably extended. The type of failure we encounter is a unidirectional metal transfer due to the DC that produces a conical metal mound on one contact and a mating hole on the other. Ultimately the contacts weld together. We use the KUP 11D15 which has silver cadmium oxide contacts that are better for inductive load circuits at 120 VAC, but not good for low voltage, 5 v and below.
Another possibility is to use the SCR to turn on the brake, then short the SCR with a mechanical relay contact, remove the SCR trigger, and open the mechanical contact when you want to turn off the brake. Turn on with the SCR eliminates problems from mechanical bounce of closing the mechanical contact.
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