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Voltage Transient suppression in your RV's DC Systems

 

I often read RV Forums, just to see what issues people are having, as well as trends in the marketplace - at least from an owner's perspective. One thing that keeps coming back is the number of repairs of DC systems that seem to occur; especially today's popular strip LEDs and overhead LED puck lights. They seem to all have a high failure rate. I suppose though that some of that is due to the "replace-vs-repair" mentality of RV dealerships1 - especially when the RV is still under warranty, but I am wondering if there isn't something else going on...

Perhaps there may be some issue to surge suppression in regards to DC systems. We all know about AC Surge suppression and the need to protect our RVs from the power grid. We spend hundreds of dollars on fancy Energy Management Systems. But what about our DC power system? Is there a potential problem there that is responsible for wiping out LEDs and other devices?

 

Note:

The opinons expressed here are amateur in nature and for entertainment purposes only. You are solely responsible for any action you may take as the result of reading this text.

 

Time for some research.

Littlefuse,2 among other component manuacturers have published white papers detailing with this very topic. In fact, it seems many other manufacturers are using the Littlefuse research data. In that white paper, it is said that under certain circumstances, over 100V transient spikes can be developed in an automotive system during normal operation (engine starting and stopping, ignition, power windows operating, etc), and in fact, they manufacture an entire line of surge suppression devices that are commonly in use today in the automotive industry.

Now we are getting somewhere...

Next up; ISO standard 7637 - Road vehicles Electrical disturbances from conduction and coupling3 . This is an international standard that (in part 2) provides detailed test procedures that can be done in a lab environment that can replicate the common surges found on 12VDC automotive systems.

 

Some readers have criticized this article, claiming that a battery is a natural surge protector. However, we are dealing with more than simple surges... we also have transient voltages from "spikes", as well as induced power surges from lightning strikes. When I did the research, I could not find any studies that supported the battery itself providing adequate protection.

 

From my research, it became pretty apparent to me that DC powerline transients and surges are real - there is even an international standard about it for pete's sake. These surges can cause damage to electronic circuits, especially the sensitive modern-day stuff (LEDs, automation and remote systems, etc). Now the next question is, is the RV sitting out in your front driveway adequately protected from DC surges.

The first question might be perhaps... we are looking at automotive systems, not RV, right. Well, we are looking at 12VDC systems, regardless of what they are used in. But lets look at a block diagram of a RV, especially when connected to a tow vehicle:

 

 

Admittantly this is a simplistic view of the 12VDC system in your RV. However, as shown hooked to the tow vehicle, the vehicle's 12VDC system is directly connected to the RV (which is how it charges the RV's battery). We can probably safely assume that the vehicle has sufficient surge suppression built-in to prevent it as a source, but what about when you connect or disconnect the RV from the vehicle... can there be a source of surge at that point?

It must be noted that the primary source of damaging surges are inductive loads, primarily during start-stop, but anytime they are active, they can induce damaging currents in the powerline.

And on the RV side, especially when disconnected from the tow vehicle, anytime you operate a slide, cycle the water pump, and other activities, the potential exists that you are generating a damaging surge, especially if the RV has no surge suppression.

Does the converter have surge suppression built-in? Not according to one of the higher quality converter manufacturers. I did confirm this by contacting them, and no - they do not provide any surge suppression circuitry on the DC power bus.

And what about other sources of surges? Take for example the front caps of many modern RVs, all lit up with LED strips. Seems between hearing from other owners as well as each time I visit a RV dealership, there is a unit in there having the strips replaced. Do these strips have a high failure rate?

Is it just possible that driving down the road with dry air flowing over the fiberglass front cap at 60mph, that enough static electricity is being generated to damage the LEDs? I can't prove it without some pretty expensive testing, but neither can the RV industry.

It's my guess this is a probable reason for failure, since these strips are not protected against surges.

NOW HEAR THIS!

I also discovered that many of the RV electronic systems, such as refrigerator, furnace, and water heater control boards, leveling systems, and others already have surge suppressors built into their circuit boards. Did you hear that?

 

Actual example. OK then. I want to measure my RV and see if I can detect any transients.

I have to state that is somewhat difficult to detect such transients. It basically requires a fast acting digital storage oscilloscope. I do have a Rigol DS1054z 50Mhz storage scope, but I am not sure if it if fast enough... ISO standard 7637 does recommend a 400Mhz scope, which is too expensive for the electronic hobbyist. But still, perhaps I will get lucky and detect something.

So I hooked my scope up and operated all of the slides, awning, water pump, leveling system... and I connected and disconnected the truck with the engine running, and started and stopped the truck engine... nada! No indication of surges at all.

That means the surges either do not exist, or I cannot measure them with my gear. The truck I am not that surprised with as it is well known the contain surge suppression devices. To tell you the truth, I am not all that surprised that the scope didn't detect the slides or levelers in operation either as they are not directly connected to the DC power bus, but rather through their respective controller boards, and those boards may very well have surge suppression devices (remember I did see what looked like suppression devices on the remote control board).

So it basically comes down to the water pump. I have already winterized the RV so I really don't want to run the pump excessively. I just happened to have a spare Sur-Flo pump, new - still in the box, that I can hook up to the 12V DC power system. You can run the pump dry, so I should have no problems in a mock-up consisting of the pump and a on-off switch. If surges exist on the DC power system, I should surely be able to see them near the pump. My oscilloscope has a Pass/Fail function wherein I can provide parameters that if the waveform exceeds the setting, the scope will stop and display the event that caused the problem. This kind of detection capability is essential in determining surge transients as otherwise they will be too quick to see.

And the result:

 

 

Now that is what I call irrefutable proof that turning on motor circuits can introduce transient surges in the DC power system.

So let's analyze the scope output. An oscilloscope is essentially a voltmeter, and plots voltage vs. time. Voltage is represented on the vertical scale, and time is represented on AC coupling. That means that only AC voltages are shown. At the beginning of the event (where the "1" arrow is), represents zero volts on the line. Notice that the signal is slightly above the center line - that represents a bit of some AC noise riding on the DC power line - possibly due to the combiner being a bit (electrically) noisy. At any rate, this is the beginning of the signal capture, and the pump switch is open.

Next you will see a series of large spikes; this corresponds to the switch closing. The switch is actually "bouncing", banging on and off at a rapid rate until it stays closed. This can generate a significant transient spike, and in this case, voltage swings both positive and negative above and below the 12V power system, and in this case the spikes are at 52.8V. And if you add that to the nominal 13.5VDC of the DC power system (remember DC is not being shown), we are seeing at least 60V on the DC line. As you can also see at the right of the screen, the pulses stop.

After some experimentation with the scope settings, I was also able to detect the following signal while switching the pump on and off.

 

 

It is interesting to note that the first scope example approximates ISO 7637-2 TP3B ("Automotive Transients due to Switching"), while the second scope example approximates ISO 7637-2 TP1 ("Automotive transients due to disconnection from inductive loads"). So with my limited equipment, I was able to detect at least two conditions covered by ISO 7637-2.

 

 

How long do you think your LED puck lights will last when each time your water pump cycles on and off, it is banging the LEDs with 60V? And what about those expensive DC circuit boards for the fridge, heater, and furnace. Are those being damaged as well?

I also have to theorize that in addition to what I was able to detect, there are likely other transients I cannot detect due to my limited equipment. I am going with this hypothesis in determining what actions to take to suppress these transients.

 

Actual example 2. Some of my dedicated readers might recall my Cylon Eye project. I was having some difficulty in the LED strips only lasting a month or two. So I added surge suppressors to the LEDs, and after 6 months, no more failures. Near the end of this project, I will show you how to retrofit your Cylon Eye (if you built one) so as to protect it like I did. The success I had with the Cylon Eye, more than anything else led me down this path of researching surge suppression techniques.

 


Video

 

 

References:

1: RV Industry Death Spiral.
2: Suppression of Transients in an Automotive Environment.
3: ISO standard 7637.

 


Last reviewed and/or updated Feb 2, 2018