Today was the day to have a little fun and sit in the plane to configure my panel that I got powered up for the first time a few days ago. Everything is working great - no smoke. It was getting a little chilly, so I thought "well, I should make sure the heated seats work I guess!" I have the heated seats from Classic Aero. I flipped the switch for the pilot side seat, and pop went the fuse. I flipped the switch on the copilot seat, and that fuse blew as well! I'm scratching my head trying to figure out what the common wiring mistake is with them. The heat pads plug into jacks between the seats, then the wiring goes through the relays and switches and up to a fuse block at the panel with 7.5 amp blade fuses (supposedly the seats only draw a little under 6 amps when on high). The fuse block gets power from the power bus, and the grounds for each seat go to my forest of tabs. Did I miss something in how these should be wired up? Since it's both seats, I assume it's a design flaw vs a short in a wire, but I'm not sure what it would be.
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Heated Seats Blowing Fuses
- Thread starter skelrad
- Start date
1. Put a DC clamp on the (+) wire to the seat heater and see what it's actually drawing.
2. Assuming this is a kit, so the wire gauge, + and - is correct, but just in case. Make sure the - wire is the same size as the + wire ** AND ** grounded to the airframe or similar low resistance path to ground (-) at the battery.
2. Assuming this is a kit, so the wire gauge, + and - is correct, but just in case. Make sure the - wire is the same size as the + wire ** AND ** grounded to the airframe or similar low resistance path to ground (-) at the battery.
Just unplug the heat pads from the jacks. Then turn on the switches. If the fuses do not blow, then the aircraft wiring is not at fault.
Resistance type loads have a higher inrush current than other types of loads. Once the heating element warms up, the resistance goes up and the current goes down. Perhaps the the inrush current is greater than 7.5 amps. Once warmed up, the current could drop to the specified 6 amps. I suggest that the fuses be replaced with 10 amp fuses if the wire size is rated for 10 amps or more.
Resistance type loads have a higher inrush current than other types of loads. Once the heating element warms up, the resistance goes up and the current goes down. Perhaps the the inrush current is greater than 7.5 amps. Once warmed up, the current could drop to the specified 6 amps. I suggest that the fuses be replaced with 10 amp fuses if the wire size is rated for 10 amps or more.
A std ATO fuse can run for hours at 110% of capacity. Generally speaking, if a fuse blows the instant you turn on the switch, it is a dead short, drawing at least 200% or more of rated amperage.
I would start with wiring at the relay. It switches between serial and parallel for hi and lo settings and is easy to mis wire and get a short.
I would start with wiring at the relay. It switches between serial and parallel for hi and lo settings and is easy to mis wire and get a short.
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I think you mean inductive loads. Resistive loads, like a light bulb or toaster oven or resistor on a circuit board, have very stable current draws with no inrush. I suspect there are some heating elements that behave as you state, but a seat heater is pretty simple carbon resistor.
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BoydBirchler
Well Known Member
It could be that since the alternator was not running and voltage was below 13.2 you drew more amps at the lower battery resting voltage. Higher the voltage the lower the amps...
Uh, I don’t think so.
I just threw an ohmmeter on the DC power jacks I used for the seat plugs. Both the center pin and outer shell of the jacks are open to ground for some reason (center should be +). I'll have to take a look tomorrow. All 4 jacks have the same issue. The jacks came already wired with pigtails, so I know I didn't accidentally bridge the contacts while soldering or anything, but I must have crossed wires somewhere.
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If youre seat heaters have a hi-low switch, part of what the relays do it change the current flow from series to parallel depending on whether you want hi or low heat. Wiring them up that way is complicated and easy to goof up. As I recall, in that configuration one wire is hot in one position but becomes a ground in the other position.
When I ops tested mine I couldn't figure out what the heck was going on until I drew it out on paper. If it were me, I'd start troubleshooting this by verifying that the relays are wired correctly.
When I ops tested mine I couldn't figure out what the heck was going on until I drew it out on paper. If it were me, I'd start troubleshooting this by verifying that the relays are wired correctly.
Some resistive heaters are designed with a positive temperature coefficient so they do have a larger inrush current, but I agree, a quickly blowing fuse more likely points to a dead short.
Yes, I figured there were far more sophisticated heating elements than a simple resistive wire. I have installed a few seat heaters, including cutting them to fit, and can confirm they are nothing exotic. It is just a simple, thin carbon trace transferred on to a cloth pad, with another pad over it, making a sandwich.
This might help?
Heated Seat Wiring Question(s)
I am wiring my plane and got to the fabrication of the switches for the seat heaters yesterday and then got a bit flummoxed. The seats and heaters came from Classic Aero Designs. The wiring diagram is attached and the wiring for each part of the seat (back and bottom) is just two 14 AWG wires...
vansairforce.net
I went through the exercise of figuring out how the automotive seat heaters that you but on line are wired a few months back and wrote a quick article on it. Thought I’d kept more pictures, but apparently not….if you zoom in on the picture, you can see the wiring scheme.
www.kitplanes.com
Paul
Reverse Engineering Heated Seats - KITPLANES
How to they make it “high” and “low” with just a single SPDT switch and SPST relay?
www.kitplanes.com
Paul
I'm still stumped. I took a look a Paul's diagram, and it sure seems like I have everything wired correctly (although I think it's a given I obviously do not!). The wiring harness that was sent was complete already, and the only change I made was to cut off the connectors at the seats and install different jacks so I could easily plug and unplug them. I looked over the harness, and it seems to match up with what Paul shows. I'm still getting a short somewhere though, even though I've traced every wire and can't find one. I did try putting a 10 amp fuse in just to see if it was just a high draw at the initial startup, but that made no difference. Whether on low or high, the fuse blows immediately.
Norman CYYJ
Well Known Member
What color is the inside of the fuse after blows. If just melted too small I'd black and silver you have a short
Good deal -- thanks for the follow up!
I'm printing up a few bushings to isolate the barrel of the DC jack receptacle from the floor pan. I'm curious to understand what's happening though. Here's what I see: the outside of the receptacle is - and the inside pin +. With the seat jack not plugged into that, there is no continuity between the inside and the outside. However, when I plug the seat in, now I get a path from the positive wire coming into the center pin to the outside barrel. When I test the plug on the seat, there is some continuity between the inside and outside barrel, but I assume that's normal (it's not 0 ohms - it has resistance from the heating element). It was on oversight on my part not to insulate the jack from the seat pan regardless. I'm just not understanding why plugging the seat plug into the jack would cause the short like this, since I thought the inside of the plug was isolated from the outside, so the only path for current was through the heating element. Somehow it seems that once I plug it into the jack, current is finding a path of lesser resistance. Insulating the plug seems to fix this, but I'm still confused as to what is actually happening (electrical rookie, but learning...).
Brandon, Do the seats have high and low temperature, or just on and off?
Do the jacks and plugs have 2 terminals or 3 terminals?
https://stompboxelectronics.com/wp-content/uploads/2023/07/StereoAudioConnection-1.webp
Do the jacks and plugs have 2 terminals or 3 terminals?
https://stompboxelectronics.com/wp-content/uploads/2023/07/StereoAudioConnection-1.webp
They have high/low. The jacks I was using are just standard DC jacks with the 2 terminals (barrel jacks, not the audio type connectors you show in the link). I got all of the wiring sorted, and the short seems to be happening because the outer sleeve (-) of the DC receptacle was contacting the floor pan. When I pulled it out so there was no contact with the floor pan, it all worked fine. I'm still puzzled as to why there is no short along that path until the jack is plugged into the receptacle, but moving on. I've decided just to swap out the jacks with molex anyway, since I discovered the amp rating on the jacks is right on the edge of what is acceptable anyway.
If wired according to Paul's diagram, the jacks and plugs need to have 3 terminals, not 2.
Looking at Paul's diagram, Note that the right side of heater 1 is always grounded. So that wire should connect to the grounded terminal of a 3 terminal jack.
The junction of the two heating elements connects to a second jack terminal.
And the right side of heater two connects to the 3d jack terminal.
-
If a 2 terminal jack is used, it will not be possible to switch between high and low heat.
The two seat heaters have two separate plugs - 2 wires going to each. I just chopped off the 2 pin molex for each heating pad that the harness from the manufacturer came with and replaced it with the barrel plug. After all of that, I'm just going to go back to the molex that it came with and deal with the less tidy wire routing into the floor.
Seat heater elements are 2 wire, not 3. It is a simple resistor. I think you are mis reading the diagram.
slight thread hijack. i have lemke seats with heaters. the control is off, 1-5. i get static on the radio when i turn it on. the static is the highest at 1 ( least hot) and almost none at 5. i have the ground wired directly to the airplane ground block. any suggestions for trouble shooting ?
If your setup has a knob that controls heat from a range of 1-5, it most like is using PWM and those a known to create noise issues when cheaper poorly designed modules are used. Usually no way around it other then designing a better pwm circuit. Should be able to cut that out and replace with switch and relay wired according to pauls diagram. These are very common and can find sample wiring diagrams on the web. That will leave you with only a high and low setting though, but will be noise free.
For seats, to make them easily removable you might consider using a Powerpole connector. The bulkhead or cable connectors from that family can make this a neat job.
They are easily removed, keyed for polarity, come in various current capacities etc.
You don't need a specialized crimper to do a handful, standard crimpers and soldering work fine with them.
PowerPole
Anderson Power Powerpole and SB Connectors | Powerwerx
Browse Anderson Powerpole and SB connectors from 15A to 350A, including housings, contacts, and kits for PP15–PP45, PP75, SB50, SB120, SB175, and SB350. Ideal for battery, vehicle, amateur radio, and industrial DC power.
powerwerx.com
This housing size can take 15/30/45 Amp pins.
Anderson Powerpole Connector Series PP15-45 | Powerwerx
Shop Anderson Powerpole PP15, PP30, and PP45 connectors, contacts, housings, bonded pairs, panel-mount parts, tools, and assortments for clean DC power builds.
powerwerx.com
There are also panel or bulkhead connectors from the or other vendors.
There are a ton of vendors who make premade cables as well.
Powerpole® Cables - West Mountain Radio
Powerpole® cables are high current, low voltage DC power cables. Powerpole® connectors allow for minimal contact resistance at high current and easy polarity identification.
www.westmountainradio.com
I did not misread Paul's diagram.
Notice that there is a junction dot on the left side of the two heaters. That is wire 1.
The right side of each heater has a junction dot. Those are wires 2 & 3.
Evidently the OP did not wire according to Paul's diagram, but instead used a total of 4 wires, 2 for each heating element.
It is difficult to troubleshoot from a long distance without having a schematic diagram and trying to interpret text messages.
Sorry, but it all depends on how you sketch - if I confused you, I’m sorry…that’s my way of sketching circuits. the heater pads all terminate in a simple two-wire connector. All junctions are outside of them. the switch connector is three-wire, and of course the socket for the relay has more.
Paul
The schematic is accurate, except that the plugs and sockets are missing. I assumed incorrectly that the junction of the two heating elements was internal to seat heater. Regardless, it is still a mystery as to why the fuse blew only when the heat seats were plugged in.
Nope. The resistance of many resistive loads such as incandescent bulbs and heat strips is temperature-dependent. The resistance at low temperature can be an order of magnitude lower than the steady-state operating resistance resulting in a current inrush at startup. The resistance of "simple carbon resistors" is also temperature dependent, but it's a more complex function.
I agree with jdabney. Capacitors resist any change of voltage. Inductors resist any change of current. When voltage is first applied to an inductor, there is no inrush current. Just the opposite. Current is slow to build up. When voltage is removed from an inductive circuit, the inductor tries to keep the current flowing.
So, help me understand. Recently worked on building a battery backup and generator support for my sump pump; a simple inductive motor. Specifications from manufacturer state: Nominal or steady state current of ~9 amps, starting current ~27 amps. I can't seem to resolve that with your statement.
From the internet:
Motor counter electromotive force (EMF), or back EMF, is a voltage generated by a motor's spinning armature that opposes the motor's supply voltage. It is proportional to the motor's rotational speed and is what limits the current drawn by the motor as it speeds up. Without back EMF, a motor would draw excessive current and could be damaged, especially at startup when the speed is zero and back EMF is also zero.
Motor counter electromotive force (EMF), or back EMF, is a voltage generated by a motor's spinning armature that opposes the motor's supply voltage. It is proportional to the motor's rotational speed and is what limits the current drawn by the motor as it speeds up. Without back EMF, a motor would draw excessive current and could be damaged, especially at startup when the speed is zero and back EMF is also zero.
Thanks, but You didn’t explain how your premise that inductors do not experience surge current relates to my inductive motor that has a surge current 3 times greater than nominal current.
