Friday, December 8, 2023


RVelectricity – Just Ask Mike: Can I build a “SoftStartRV”-type controller for cheap?

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) with the subject line – JAM. This week a reader wants to know if he can build a “SoftStartRV”-type controller for cheap.

Dear Mike,

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

Dear 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 FreonTM 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 For more info on Mike’s qualifications as an electrical expert, click here.
Join Mike’s popular and informative Facebook group.
And you don’t want to miss Mike’s webcasts on his YouTube channel.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign


• 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




0 0 votes
Article Rating
Subscribe to comments
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Oldest Most Voted
Inline Feedbacks
View all comments

Luciano Velarde (@guest_129386)
2 years ago

You can build a atomic bomb using YouTube why not a Softstartrv?? WOW and these people vote!!

Sean Foege (@guest_122711)
2 years ago

Hey there, GREAT articles! I have lots of reading to do here!
But I feel like the real question was never answered because you never threw the hard start capacitor amperage draw from the ac source in the same line graph as the ssrv vs stock. I understand it’s not easy to replicate all the greatness of the ssrv, but, from a functional perspective, doesn’t a basic hard start cap do the same thing? If I get a big cap and install it, wouldn’t it reduce the starting strain on my AC power to the point that a 2k generator would start a small AC unit? What about double caps? Quadruple? You could parallel 10 large caps for the price of the ssrv; am I to believe that wouldn’t allow a small generator to start the AC? I have trouble believing that. Seems like you could get 4 together, make a box, and put your main power feed from the generator through it right at the plug and not even need to go on the roof. Or just hard wire them in to the generator itself??

Wolfe (@guest_122236)
2 years ago

CAN you build a working softstart cheaply? Absolutely. It’s pretty old technology, and doesn’t take super-expensive parts in the controller at all. No, it’s not just capacitors — it’s a ramped PWM chopper circuit internally more like an automated light dimmer. Mike’s transmission analogy is appropo as well — stock AC starts in 5th gear, this SSRV lets you push instead of slam up to speed.

WILL you get a schematic? SSRV is advertised 5 times on this page as I write this, so RVT is unlikely to want to hurt an advertiser. But even beyond that, RVT isn’t likely to accept liability for readers playing with line wattages, or responsibility for you blowing up your $1500 AC on their guidance. I similarly built an all-in-one tester than detects energized ground when someone said it was impossible, but I don’t share that schematic because folks can figure it out like I did without making me liable there.

I’d agree with Mike: if you need a schematic, you shouldn’t build one.

Ron Schmitz (@guest_121998)
2 years ago

Last summer in TX had to live in our MH for 3 wks while house remodel. Only have 30a for MH at home so bought 2 SoftStarts with RV Travel discounts. Easy install & great tech support & both a/c’s on 30a.

BILLY Bob Thronton (@guest_121990)
2 years ago

If soft start is so good for the a/c unit, why are they not installed from the factory. A few questions here if I may. 1. Does a hard start hurt the a/c unit. 2. Does the hard start cause a shorter life for the a/c unit.

Can anyone chime in here with factual data, so it’s easier to make a buy decision on thks item.

Wolfe (@guest_122226)
2 years ago

1) Not installed because a) manufacturers squeeze every dime out that they think customers won’t notice and b) many buyers never will notice because they keep their rig plugged in.
2) A “hard start” is a big capacitor for stubborn AC, and is harsher; a “soft start” is for weaker generators and kinder.

Those who refer to the RV as a “camper” are more likely to be offgridders who notice, even then not if you have a 4KW generator. Trying to start your AC on a 2KW generator is the main usage case for the SoftStartRV, and the price makes buying a $700 4KW more economical than a $600 2KW + $300 SSRV if you haven’t already purchased a 2KW and don’t mind the weight of a 4KW.

Last edited 2 years ago by Wolfe
Jesse W Crouse (@guest_121477)
2 years ago

So this guy wants to crap all over all the hard work, time and effort put into the Soft Start for a few dollars in his pocket. The price I paid for my 2 Soft Starts is small in comparison to what they do and the peace of mind I get from buying a properly engineered product with product liability insurance. Anything goes wrong with it guess who’s neck is on the chopping block.

Mike Sokol (@guest_121523)
2 years ago
Reply to  Jesse W Crouse

Well yeah… the only time I design one-off electronic devices is when no commercial products are available to perform the needed function. Hence HRDL, which combines the function of a data storage scope with a long-term data logger. I have both of these measurement tools already, but neither would let me see enough detail. So while HRDL itself is a pretty simple device, it would be very expensive to replicate outside of my lab. I just happened to have the thousands of dollars worth of audio capture devices, along with all sorts of load banks and calibration equipment to build one. Closest commercial device to do the job was $80,000, plus it still wouldn’t do the slow-motion playback that HRDL is capable of. But I would never build a SSRV on my own for all the reasons I articulated in my article.

Wolfe (@guest_122254)
2 years ago
Reply to  Jesse W Crouse

I don’t think it’s “crapping” on SSRV at all to want to build your own. Many folks have more technical knowledge than money, so the desire is natural. It MAY be unwise to duplicate complicated/even dangerous engineering if you lack serious knowledge, but you DO have the right to take risks.

There is also a difference between parallel development and patent infringement — I had a good guess how SSRV worked from the stated functionality, and never took one apart. The softstart tech is so old in industrial settings that I can’t believe anything important is patented if the PTO is doing their job re: “obviousness.” What could be novel/protected is any auto-tuning if the SSRV unit even does that (I suspect there are other learning softstarts, so don’t trust that — afterall, there are other legal “SSRV” competitors like MicroAir and the $150 European one).

Paul Lusby (@guest_121447)
2 years ago

As the proud owner of a couple SoftStarts, I can assure anyone interested that what the cost of the units is justified on the basis of customer service alone. My first install was over the Christmas/New Year holiday. I uploaded a question via the contact form on December 31, and received an IN PERSON customer service call to assist the next morning, on New Year’s day.
I was so impressed I have recommended the company and product to anyone interested, and just helped a friend try to figure out what he needed last weekend. I uploaded the model number and a couple questions on Saturday evening, and received a personal support call bright and early Sunday EASTER MORNING!
The customer service from the SoftStart folks are just too good for words. Worth every penny.

Snayte (@guest_121382)
2 years ago


Do you think the air conditioner manufacturers might implement a soft start type circuit built in to their units from the factory at some point? Seems like it would be a wise thing to do and a good selling point.

Last edited 2 years ago by Snayte
Mike Sokol (@guest_121461)
2 years ago
Reply to  Snayte

I’m currently installing a mini-split air conditioner in my living room, and it has a DC compressor motor under PWM (Pulse Width Modulation) control. So there’s no starting surge current with this type of air conditioner. I’ve heard a few rumors that some high-end RVs may have that kind of compressor technology, but I haven’t seen one yet.

Tommy Molnar (@guest_121376)
2 years ago

I had a SoftStart installed last year. It works exactly as advertised and I love it. My old dinky Champion 2000 watter runs our single a/c with ease. I even sold my second Champion that I bought to parallel and run the a/c.Then I sold my new HF Predator 3500 genset because of this. So now I’m thinking of running a quick experiment using my (also new) Lithium Ion batteries and pure sine wave inverter. I know that my nominal 200 Ah’s of battery power can’t replace shore power for true cooling, I just want to see if it will work. That’s later today. What do you think, Mike?  😉 

Mike Sokol (@guest_121462)
2 years ago
Reply to  Tommy Molnar

Yes, it should work to get the compressor started…. I’ve done this dozens of times in my shop. But how much run time you have is the big question.

Ray (@guest_121354)
2 years ago

I appreciate the article and thought there was more to the SoftStart than its appearance. I still have a question however. Provided they are never turned on simultaneously, can one SoftStart be installed so as to be available to assist 2 ACs?

Tom (@guest_121329)
2 years ago

Hold my beer, watch this. ZAP!

Glen Cowgill (@guest_121311)
2 years ago

“You don’t pay me for what I do, you pay me for what I know” applies here and you didn’t even delve into the Patent laws. Good answer Mike.

Bob P (@guest_121316)
2 years ago
Reply to  Glen Cowgill


Sign up for the

RVtravel Newsletter

Sign up and receive 3 FREE RV Checklists: Set-Up, Take-Down and Packing List.