By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM. This week a reader wants to know if he can build a “SoftStartRV”-type controller for cheap.
I know that the SoftStartRV air conditioner controller must do what it says it does. After all, I see all the good product reviews, and really appreciate you diving into it. Even your graphs make it easy for someone like me to understand that it works. But I’ve got to believe there is a cheap way to build one myself. Isn’t it just some capacitors in a box? Will you publish the schematic and parts list so we can build these ourselves for maybe $30 or so? I understand why you may not want to publish this info, but please give us a break and draw out how to build one. Thanks! —Thomas
I get several emails every week very similar to this one. The general idea is that they like the idea of a soft starter for their rooftop air conditioner, believe that it works as advertised, but don’t want to pay the $300 or so for a factory-built SoftStartRV™ controller.
They think there must be just a few parts inside worth maybe $10, and that if I showed them how to build a “SoftStartRV”-type controller from parts they can buy online, then they could save themselves almost $300 for one.
Could I design a “SoftStartRV”-type controller for you?
Of course I could. I’ve designed and built much more complicated circuits and devices over the years. And I do know the basics of how a SoftStartRV controller operates. After all, as part of my study on how induction motors work I built a HRDL (High Rate Data Logger) that samples voltage and current at up to 192,000 times a second with down to 1ppm (part per million) resolution.
But I didn’t build HRDL (pronounced “hurdle”) so I could design a DIY SoftStartRV from a kit. Nope, I built HRDL so I could study how induction motor starting currents actually operate in real time. I kept getting inquiries about hard-start capacitors, and every textbook I studied was lacking in enough detail for me. So that’s why I built a very high-speed data logger that would allow me to see inside of the surge currents of compressor motors. Think of it as an MRI for an electronic circuit.
What did I find out?
While hard-start capacitors are pretty simple devices, basically just an extra starting capacitor with some sort of disconnect relay (either time or voltage sensitive), the SoftStartRV unit is pretty complicated under the hood. Instead of giving the compressor motor a swift kick in the pants for 150 milliseconds (like a traditional starting capacitor), or an even bigger push for over 2 seconds (like a hard-start capacitor), the soft start controller acts like an automatic transmission in your car. That spreads out the angular acceleration of the compressor rotor over 300 milliseconds of time, thereby reducing the starting current surge from 50 amperes for 150ms down to 24 amperes for 300ms.
How does the SoftStartRV controller do this?
As you can see from an expanded view of my HRDL graph, there’s a very high-speed chopper circuit involved, along with a sawtooth wave control circuit that modulates the 60-Hz current of the starter winding and ramps it up. And this process is tuned so that the compressor rotor comes up to speed before the back pressure of the Freon
TM gas compression increases the torque requirements too high.
Dance of the electrons
It’s sort of a delicate ballet of current, torque, inertia and timing. And just like a ballerina makes a pirouette look easy, it certainly ain’t. The same goes for your vehicle. While we may all complain about how much a modern car costs, and what it takes to repair one, a modern vehicle is the culmination of millions of engineering hours and many thousands of experiments.
So can I build a “SoftStartRV”-type controller myself?
Well, I’m estimating that building a SoftStartRV unit yourself would take a few hundred hours of engineering time, along with thousands of dollars in test equipment, along with destroying a few air conditioners in the process. Then you need to design and build a circuit board, source the components, assemble it, then test operation. But even then you wouldn’t have a warranty, or a service desk to help you install it.
So, the answer is NO?
Yes, the answer is no…. This is a complex piece of electronics, and there’s a lot that goes into the design and build of a little box with a seemingly simple function. So while you can buy a hard-start capacitor for less than $20, that’s not the same kind of device at all. A SoftStartRV controller is worth every penny they charge you for one, especially if you factor in the free installation help.
Back to work …
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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• Mike Sokol “Ask the Expert” With Danny Rahner from SoftStartRV – Video: Easy 6-Step, No-Splice Install for SoftStartRV
• Use your RV air conditioner in low-power situations