Saving power with relays and solenoids

June 19, 2017

antedeluvian-June 19, 2017

There are several factors in taking a particular design approach, and they are not always independent. Some of the considerations when designing drivers for inductive loads are size, cost, switching speed, reliability, power consumption and heat. Inductive loads like relays or solenoids are different from simple resistive or capacitive loads in that they need a certain power to energize the load, but once energized the applied power can be reduced and the load will remained activated. An added benefit of lower current is that it takes less time for the load to de-activate.

I did discuss one aspect of switching inductive loads in my blog “Back EMF and snubber” and I urge you to keep the discussions of snubbers in the back of your mind as you follow my meanderings. No matter whether you opt to cut power to the inductive load or not, there will always be back-EMF.

Not all inductive load data sheets provide sufficient information to apply the following techniques. In fact in my browsing for this blog, almost none do. However the Schrack RT2 relay series provides a fine example. As you can see there is an immense difference between the voltage to pull and the relay and what is required to keep it activated.

Figure 1: RTE24024 highlighted. Despite this detailed information, nowhere in the data sheet does it mention absolute maximum coil voltage. Nobody’s perfect! (Source: TE Connectivity)

The lack of information means that almost any design you do is going to start out as “suck it and see”. And once you actually determine the approach and values you are going to use, allow for variations due to production spread, voltage variation and temperature/humidity changes. If at all possible I will always suggest utilizing a product where the parameters are provided.

Controlling the inductive load requires adjusting the voltage across the coil or modifying the current. If you are going to roll your own here are some approaches. The figures are conceptual - there must be a current limiting resistor in the base of the transistor for a BJT, and there is no indication of the electronic control of the changeover contact in A.

Figure 2. Basic approaches to directly adjust supply voltage to coil.  (Source: Author)

If you can control the supply it is possible to use the approach shown in Figure 2A. If you look at the design ideas listed below you will see that this is the most common approach. One, in fact, uses a second set of contacts to feed back the contact closure to toggle this change. Often they use rectified and smoothed power supplies rather than regulated ones. However variations in mains voltage and transformer manufacture means that you have to allow for wide tolerance which might be OK for a small volume production run (Reference 3 is for a telescope at the Lowell Observatory - I suspect there is only one) but I would use very large margins with large scale production. It is also an inconvenient approach if you have multiple independent coils.

Figure 2B shows a very common technique. When the coil is activated via the transistor Q2 the capacitor C2 acts as a short and the full voltage is applied to the load. C2 charges based on the circuit’s time constant (C2 value and coil’s resistance) slowly dropping the voltage across the coil. The voltage will stabilize at the voltage determined by the resistive divider formed by the coil’s internal resistor and R2. Paul Rako recalls Bob Pease’s thoughts to discuss this approach extensively (reference 5). Although simple, there are some disadvantages. Generally the capacitor tends to have a large value and between the components’ tolerance and temperature characteristics the actual time can have quite a wide distribution. Also at steady state there is power dissipating through R2. That resistor may need to be quite large and may get hot.

If you have the luxury of extra outputs from your micro, then Figure 2C is a modification to Figure 2B that will save the need for a space wasting and uncertain timing capacitor at the expense of another transistor (Q3A). To activate the load turn on both Q3A and Q3B. Once the activation is complete (based on time or confirmation by closed contacts) Q3B can be de-activated.

Nowadays the proposed solution to any problem that need the transform from the digital to the analog domain is to mutter a magic incantation. Chant it with me now: - “P-W-M”! By varying the percentage that a switching waveform is on, we can adjust the average voltage and hence the average power. No additional resistors to dissipate heat and no capacitors to waste space or muck up timing.

Figure 3: A rather uninformative wiring diagram for the connection of a PWM drive. (Source: Author)

Although it would be possible to create a separate PWM signal and gate it with an initiation signal thus requiring two pins to control a load, it would be naïve not to presume that a modern microcontroller has the ability to implement a PWM from 0-100% on a single pin. But it would be nice if the inductive device manufacturer provided details on what would constitute reliable operation. I am not sure how widespread this is, but some manufacturers like ASCO do produce devices specifically to this end as you can see in Figure 4. Guaranteed performance - at long last!

Figure 4: Extract from ASCO HV427246 Gas Shutoff Valve data sheet. (Source: ASCO)

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