Saving power with relays and solenoids
Up till now I have only spoken about control of voltage, but you can achieve the same ends by controlling current. Now I could bore the pair of you to tears (since everyone else has given up and gone away) and wade through current controllers made from discrete components that I have only thought about in theory, but there is an easy way out. There are some ICs that do this!
The most straightforward is the Constant Current Relay Driver and the dual channel TLE7241, from Infineon. They also have a six channel device, the TLE6288R, as well as another device - the TLE82453 for linear solenoids. At first blush, I have no idea what a linear solenoid is and how it differs from a regular solenoid so I will have to wait for one of you to set me straight (sorry - bad pun!)
Texas Instruments offers the DRV120. Maxim does an 8 channel driver MAX4822-4825. I have also just discovered an IC manufacturer I had never heard of before, iC Haus, who make 3 Power Saving Relay/Solenoid drivers.
Some final notes on this discussion: your choice of relay/solenoid can also have a great effect on power consumption. If you choose latching relays, then the power requirement drops to zero in the steady state. Manufacturers make “sensitive” components that require less current to activate.
You also need to be careful of the ratings on devices. Some are rated for intermittent operation and so may not be candidates for extended activity where power reduction would be desired. Forgive me if I harp on a bit about the necessity for trial and error when there manufacturers do not provide data and then extrapolating the results into production. Beware. And a final caution - an unobvious side-effect is that with power saving methods applied the energy may be insufficient to keep the relay/solenoid activated in high vibration/shock environments.
When I wrote this blog I had started working on a project using the device in Figure 4. In a case of cosmic comeuppance, within 2 months I was working on another project with a solenoid similar to the data in Figure 5. Not only are there critical facts missing in the data (like absolute maximum voltage), Guardian has absolutely NO technical support. My customer says it is a 1A solenoid and maximum activation time of 4 seconds or it overheats.
This time there was no information on how to PWM the drive.
Figure 5: A Guardian Electric solenoid marked as LT8X16-29.7-24VDC. This is a close as I could find on the Guardian web site. (Source: Guardian Electric)
So I looked at what I had written above. I intended to go for the PWM approach and I see that I was really banal in my description - how exactly would you go about determining the initial pulse and the holding PWM as well as the frequency. I recommend that you use a configuration that allows you to easily adjust the parameters - I used a Cypress PSoC5LP development kit that has a single potentiometer on it as well as a bunch of switches and LEDs and a ton of I/O.
I was fortunate in that the solenoid and the attached mechanism is completely visible and so I could see exactly what was going on.
I configured my set-up to provide a pulse to activate the solenoid. I wrote a small program to read the pot’s setting and convert it to the on-time. I started at about 1 second and then scaled back to see where the solenoid stopped pulling in or at least seemed hesitant. It is surprisingly short, sub 100mS. I set the pulse width to 150mS. I then configured the micro to start with the 150mS start pulse and then convert to a PWM. I choose a 2KHz frequency for the PWM signal. I rewrote the program to adjust the PWM setting according to the position of the pot. I then tried activating the solenoid and at each attempt scaled back on the PWM to see where it started dropping out. It turned out the speed of the driver transistor was the limiting factor and it couldn’t keep up at less than 10% and so the limit was effectively 10% PWM. (see Figure 6)
Figure 6: The PWM from the micro is seen on the upper trace and the output from the driver transistor on the lower trace. The PWM is ~8% and you can see how the output trace is effectively more because of the driver response. (Source: Author)
At 2KHz there was an audible whine. I upped the frequency to 4KHz, but once again the driver struggled, so it was back to 2KHz. I don’t think it will matter in the application, but time will tell.
So in summary I got a start pulse of 150mS, a PWM signal at 2KHz and PWM of 10%. The current went down from ~800mA to 76mA. Not bad!
Related Items and References:
DC Relay Coil Power Reduction Options (scroll down to find link)
Proper Coil Drive is Critical to Good Relay and Contactor Performance (scroll down to find link)