A typically accepted reality about relays and solenoids is that after they’re pushed into the actuated state, solely half as a lot coil voltage and subsequently just one fourth as a lot coil energy, are required to reliably maintain it. Consequently, any solenoid or relay driver that repeatedly applies the complete preliminary actuation voltage to merely maintain is wastefully squandering 4 instances as a lot energy because the job requires.
The only and most cost-effective (partial) answer to this drawback is proven in Determine 1.
Determine 1 Primary driver circuit the place C1 actuates, current-halving R1 sustains, then C1 discharges by way of R1 throughout Toff.
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However as is usually true of “easy and low-cost,” Determine 1’s answer suffers from some prices and issues.
- Whereas R1 efficiently cuts sustaining present by half, it dissipates simply as a lot energy because the coil because it does so. Consequently, complete sustaining energy is ½ slightly than ¼ of actuating energy, so solely half of the theoretical energy financial savings are literally realized.
- When the driving force is turned off, an extended restoration delay have to be imposed previous to the following actuation pulse to permit C1 sufficient time to discharge by way of R1. In any other case, the following actuation pulse could have insufficient amplitude and should fail. This impact is aggravated by the truth that, throughout actuation, C1 costs by way of the parallel mixture of R1 and Rm, however throughout Toff it discharges by way of R1 alone. This makes restoration take twice so long as actuation.
Determine 2 presents a greater performing, albeit much less easy and low-cost, answer that’s the topic of this Design Thought.
Determine 2 Q1 and Q2 cooperate with C to double VL for actuation, Q2 and D2 maintain, then Q3 quickly discharges C by way of R to shortly get better for the following cycle.
Actuation begins with a constructive pulse on the enter, turning Q1 on which drives the underside finish of the coil to -VL and activates Q2 which pulls the highest finish of the coil to +VL. Thus, 2VL seems throughout the coil, insuring dependable actuation. As C charging completes, Schottky diode D2 takes over conduction from Q1. This cuts the sustaining voltage to ½ the actuation worth, and subsequently drops sustaining energy to ¼.
On the finish of the cycle when the incoming sign returns to V0, Q3 activates, initiating a speedy discharge of C by way of D2 and R. In reality, restoration can simply be organized to finish in much less time than the relay or solenoid must drop out. Then no specific inter-cycle delay is critical and restoration time is subsequently successfully zero!
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However what occurs if even doubling the VL logic rail nonetheless doesn’t make sufficient voltage to drive the coil and a better provide rail is required?
Determine 3 addresses that problem with some trickery described in an earlier Design Thought: Driving CMOS totem poles with logic alerts, AC coupling, and grounded gates.
Determine 3 Degree shifting This autumn, R1, and R2 are added to accommodate ++V > VL.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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