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Automotive/Seat Heater Pad for motorcycle/car


Hi Expert!

I have a seat heater element that my boss is having produced for use with a motorcycle. We have 3 samples from Chinese manufactures.
One measures a resistance of 5.0ohms and draws 2.3-2.4 amps of current using 13.3 Volts.  this seems reasonable  - pad is a wire style resister.  I have a second pad from another manufacturer that is exactly the same save for some difference in the type of wire used.  It measures 19.1 ohms and draws 0.6 amps of current.  This seems out of wack since the two pads are same size/shape and arrangement of wire within the pad.  The only difference is likely the type of wire used. My question is - what possible dangers are there with a resistance this high? 19 ohms seems wrong......
Many Thanks for your expertise !

Thanks much for the kind commentary and rating.  

The thought occurred to me that, if you get into any amount of analysis of products in respect to their ratings, especially heating products such as seat heaters, but also including more generally-applied equipment such as the alternator driving such loads, that getting some basic equipment for tracking temperature and temperature rise over time, along with the voltage and current values could add much to your information!

Finding the "hot spot" of said products via a temperature probe (resistive-thermal-device or RTD, typically a thermistor) and then tracking its changing values over loading and time could give you very good info as to whether the use of such equipment might push the power supply beyond its ratings, or possibly even pose a burn or possibly even a fire hazard.    

Attached herewith is a jpg of a temperature trace I took via an RTD probe I set into the stator winding of the alternator on my son's Mazda 3 - this was done in answer to a suggestion made by a service shop which suggested he might need a new alternator.

Opinion, especially informed opinion is good, but evidence is even better!

The DMM used to acquire this data cost less than $100 dollars, including port connector and its own application software.  Too, the file can be custom-processed in Excel (etc), as I did here.

Another useful tool is an infraRed thermal-imaging camera - these have become a lot cheaper in recent years, Fluke offering a unit which nicely moved good basic units into the few hundred dolar range.  I have a sample image from one of those which I can add, although to a separate reply, as the AE system will take one image at a time, to encourage that these be used only where really useful - I understand and appreciate that concern.     

Again, thanks, and I am glad to deal with thoughtful queries as I can make the time.

Cheers ... EGK

* * *

Interesting, and a good exercise in Ohm's law calculation, being mindful of the fact that Resistance itself increases with temperature!

Looking at the first example:  13.3v / 5.0 ohms => 2.66 A, a calculated value, based on the parameters (physical constant meaning of the word) as given.  

In fact, the current you measured whilst this was operating was in the 2.3 to 2.4 A range. (I would imagine that the reading moved from 2.4 to 2.3 A and not the other way around.)  Calculating dissipated power from 2.4A * 13.3V (I assume a "stiff", ie low-R source, with  negligible voltage drop, therefore a more-or-less constant V) would yield 31.92W, dropping to 2.3A * 13.3V to yield 30.59W ... if you had taken further current and voltage reading while this was in operation, I would expect that the current and power would drop off more, until a Temperature equilibrium was reached.  It would have been good to have been measuring T rise in or on the seat heater too!  (With a logging DMM such a Fluke 289, one could track T (or V or I or whatever over time, and find the point where balance is reached.)

Anyway, sample one, dissipating in the neighbourhood of 30 W and change, strikes me as being in the right rating range for a seat heater.

Sample 2 does seem to follow a different design goal.  13.3V dropped over 19.1 ohms calculates to 22.6 ohms - that is a "cold" (NTP ambient, ie 25C) calculation, of course, and Temp rise of the resistive element will drop the current and the dissipated power.

Power for this one, going by the parameters given again, would yield 13.3V / 19.1 ohms = 0.696 A, about a tenth of an amp over the 0.6A you measured, again with the pad heating up, and its resistance rising with temperature over time.

The power calculation is rather "anemic" in my opinion, however ... 13.3V * 0.6A (runnning rating) would yield 7.98 W, and the "cold" (parametric) calculation - V^2/R - would yield (13.3V)^2 / 19.1 => 9.26W, again higher than the running value, since this is a "cold" calculation.

The second one definitely would draw less power - which is a Good Thing - but also produces less heat - which may not be such a good thing, if what it produces does not meet the requirement.  It could be that the latter heater concentrates its heat in one smaller area, and relies on user tests where people might report positively on effectively they feel the warmth.  I am only hypothesising this, and that because this does strike me as an interesting problem!

Btw, a quite big difference between "cold" parameter-based calculations and dynamic calculations using measured load values for operating circuits when one applies these to typical incandescent (ancient technology now!) lamps - using an ohmmeter or DMM to measure filament resistance will give a cold value that typically is about a TENTH of the resitance value calculated from the measured operating voltage and current.

Would the higher resistance unit be more dangerous?  On the surface, it might not appear that it should be.  However, IF that higher resistance and the power it dissipates is concentrated over a much SMALLER area with the high-resistance element than it is with the lower-resistance element (the latter being more spread out), then it "might" be found that high resistance unit could be dnagerous IF it is concentrating its heat over too small an area, causing dangerous temperatures rise in that small area.

I would carry on doing what you are doing, but would add use of a temperature probe, so as to monitor how the warmth is distributed over area, and to determine if there might be any potentially dangerous "Hot Spots" where the temperature may reach an unsafe level, especially if the flow of heat is overly confined, especially when one is sitting on the seat.

I hope that I have given you some ideas on this.  Thanks for a well formulated and presented quetion, and a good question as well!

Regards ... EGK  


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Ernest (Ernie) Kenward


The challenges I most enjoy are thoughtful technical questions of a trouble-shooting nature in both electrical, power electronic and mechanical systems, mainly automotive but also machine control and small-machine PLC applications. Please note, however, that I am NOT a walking shop manual! I DO, however, make it a point to have those manuals and other service literature for those vehicles I DO own, and highly recommend that anyone serious about maintenance or modification of their vehicles do the same; MOST of your answers WILL be found there. For that matter, I do NOT go out of my way to acquire shop manuals for any vehicle I do NOT own! That being the case, any general query to me along the lines of "What is the meaning of this code read from the ECU of my 2015 XYZ?" or "Where is the fuse for the windshield washer pump found?" (try your car's electrical distribution panel for a start!) will not go far. What I do offer is a pretty good collection of literature, insights and hands-on experience with 1950s to 1980's Ford products (plus a developing database of information and practice with the Mercedes diesel cars), along with an engineering perspective and the ability to design and implement custom control, electrical and mechanical subsystems for vehicles. For that reason, I am happy to make my thoughts and efforts available to those who are of like mind and/or are seriously making a point of learning about their vehicles. Use the Opportunity to Learn!


A key skill in my work and hobby pursuits both is STRATEGIC TROUBLESHOOTING. I am a senior instructor in Electrical Engineering Technology at a leading Canadian polytechnic, my areas being Electrical Power and Industrial Control, electrical and electronics design and manufacturing, and AutoCAD and related CAD/CAE software - plus equipment problem-solving and new equipment design and prototyping. Hobby-wise, I have 30-plus years of experience in auto restoration, mostly in electrical and mechanical systems. Ongoing projects include a 1959 Edsel Corsair, my 1978 Ford E250 class-B motorhome conversion, and the care and upkeep of my Mercedes 300CD. My vehicles become engineering test beds for electrical and mechanical upgrades as ideas present themselves. This includes the design and production of circuit boards to restore or enhance features for which no OEM replacement parts are obtainable, or where better specifications or reliability can be had via newer concepts. Regarding the E250 RV conversion, I designed and continue to revise a custom power distribution system, managed by a Programmable Controller (PLC); this has made most revisions as easy as uploading new firmware as I develop it. The "mini" PLC is a powerful device for custom automotive control systems. One good example (there are many) would be the Moeller "Easy Relay"; these offer a wealth of control, monitoring and variable-and-status display options for such projects. A good example project which has worked well is that one for my RV noted above, which has been on the job - revised in firmware only - for a decade now. It is a load management and charging control system to avoid the sulfation-induced early failure that often befalls deep-cycle batteries used in RV power applications. The battery installed in 2003 lasted long enough to more tnan pay for the PLC that contributed to its longer life ... and the PLC will be there for the next battery as well!

IEEE - senior member ... past WCC Student Activities; SME - senior member ... past chair, greater Vancouver chapter chair 318; Edsel Owners' Club - have served in various capacities on chapter executive during seventies; have been Power and Driveline resource on the Edsel Owners' Club "E-team" for more than a decade.

Graduate of UBC

Awards and Honors
Certificates of appreciation from IEEE and SME for work in student and chapter activities

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