The Stovebolt Site Forums The Shop Area The Electrical Bay Heater Switch is HOT!!!

I hate to ask, but after about 15 minutes with my heater on in my 53-3100, the switch gets hot enough to burn me. Well not the switch but the backside of it. Anyways I poked my head under the dash as best I could (im norwegian and my square head dont fit so good!) and it doesnt look like any wires are grounding out so I was wondering, is the wiring to small? Do I need to swap to a lower gauge? Or is this normal and I just use it sporadically? Just curious b4 I catch the dash on fire!

Thanks for your time-Jeff

depending on the particular switch you have, it'll have a resistance on it that cuts the voltage down when you have it on less than full speed - heat is how the excess juice is 'thrown away' by that resistor

shouldn't be a problem if everything is in good condition with original gauge wires, and the only thing you might look at is if all the wire connections are good and tight, as loose connections also generate heat, and make surre you have a fuse on the circuit

Bill

rustednuts,

As Bill indicated, these switches seem to get quite hot when you run the fan motor at the slower speeds.

There have been previous posts similar to yours about fried fingers. It sure seems weird to me that Detroit would design it like this.

Maybe someone will shed some light on this subject.

Stuart

easy: I [current draw of motor]=E [12V]/R [the resistance of the motor windings] translates to P=IxE
power in watts [like yer portable electric heater, which is just a resistance wire element], is what makes heat, and that's determined by the current the motor uses to run times the voltage you have - the slower you run the motor, the more current has to be dumped someplace else, which is the resistance unit on the back of [or inside] the switch, and that piece is quite small, hence can get red hot

here\'s the repro switch - note the ceramic backing - here's the back of an original switch - the coil of wire is the "heater element" that gets rid of the extra heat, which is why part of the switch assembly is ceramic - newer style switches have better ways of dissipating the heat

Detroit didn't design it, God did - just the way electricity works!!

Bill

Bill,

Maybe not so "easy". This is my unprofessional take on the situation. Current is a function of the device...you don't normally "change" or "regulate" current, you vary the voltage.

Amperage, or current draw is "work done". The heater switch under the dash has a resistance wire in the back..the rheostat. This rheostat varies VOLTAGE to the motor, not amperage.

The less voltage, the slower the motor goes. I think if you plug the figures into the Ohm's Law pie, it will show that the lower the applied voltage, the smaller the current draw through the heater motor..it's going slower..it's doing less "work", it's drawing less current!

The heat the switch is producing is a function of how much VOLTAGE it is regulating. The voltage that the resitance wire is not allowing through to the windings of the heater motor must be delt with somehow..lets change its state..how about into HEAT. Isn't heat the last state of all matter?

Voltage is what you are GIVING it. amperage is what it is USING.

I think this is how it works, but I only made it to the 7th grade.


Stuart

s'why electricity makes yer head hurt!

rheostats are measured in watts, and are "valves to control current"

you vary the apparent voltage by controlling the current - the voltage in yer portable electric heater isn't varying when the element gets hot, the element is 'dissipating' the current - the motor requires a certain amount of current to turn at a given speed because of it's internal resistance - you can change the speed by reducing the voltage or increaseing the resistance to reduce the current - battery voltage remains constant, so we introduce resistance to decrease current down the line - yer right, current is the work done, ie: heat, volts is potential work, but doesn't travel, current does

the rheostat on the switch varies resistance so the motor can't turn at full speed - haven't dropped the voltage, but increased the resistance - you might look at it that the switch is 'dropping the voltage', because the rest of that pie is E=IxR - it's still the current that creates the heat - same amount of work [heat] in the circuit, but some of it is in heat generated by the motor, some is heat generated by the rheostat, at full speed the motor generates all the heat - that's why the adapters for running 6V things off 12V come mounted on big ceramic blocks, to dissipate the excess 'potential' as current generated heat :confused: so that's why the motor goes round and the switch gets hot

Bill

Bill,


I have no clue what you are trying to say. I thought I understood basic electricity, as I have dabbled in it for many moons. I will stand by what I "think I understand", but I guess I won't share it with anyone else, as it sure isn't the same as the way you are explaining it.


Stuart

I had a problem with a newer truck and the relays burning out for the air conditioner. The relays got so hot I couldn't touch it. After having a "professional" diagnose the problem as the blower motor, thermostat, switch and charge me $500, after several failures, I finally figured out the the charging system was failing and putting out too much voltage. I switched out my alternator and the problem was solved. Might want to see what kind of voltage your system is putting into your battery under load.

Jeff,

I am Danish. Our heads are small and somewhat flat so we have a better time getting under the dash to look at stuff. My problem is the neck. My neck gets real sore from holding up my head.

Is your switch one of the red ones that clamp to the bottom of your dash? That is the one I had. It has a red plastic front, a ceramic backing with a spiral wire that the electricity goes through. As you turned the front knob the lever went across the spiral wire and the fan moved faster. That thing gets real hot.

So, I put this problem on this website and it was recommended that I buy one of the old switches. I found one on ebay and payed about $20 for it. I think it is a GMC switch. It is two speed and has coils sticking out the back. They are probably for cooling purposes. It works great and I suggest you go that route.

As for the rest of you electronical folks. I did not understand what you were discussing but in my simpleton way I appreciated your help with my problem. Thank you.

I hope this helped.

ok Stuart, one more
- you visualize what's happening by saying the voltage is being reduced, but electrically what happens is the current delivered to the motor is reduced - in practice it's always current regulation, not voltage regulation, as the system is designed so the generator and regulator maintain a constant voltage

- if the system is good the truck supplies 6 or 12V always
- voltage is the potential for work the battery contains
- current is what 'delivers' the work, electrons move from the battery to the motor, from the point of highest voltage to lowest [why the fuel sender is on the ground side]
- resistance is what has to be overcome to produce work - the complete motor circuit has a resistance
- power is work done - heat in an electrical circuit

the power available to the motor must be reduced, and the battery potential can't be reduced, so the current in the circuit must be reduced, which is done by adding resistance to the circuit, which uses up some of the current generated power before it gets to the motor - the result may be quite the same as if you dropped the battery voltage a bit, but the real 'action' is the dissipation of some of the current as heat [power] in the switch - it's the current flow that really generates the work, not the voltage potential

plug some figures in here , keeping in mind that the voltage to the circuit can't vary, and see this simple circuit provided by your very own govt - read the 'bottom line'

and Dennis - the difference in the switches is that the multiple 'coils' on the switch [which are still resistance units], are better able to dissipate the heat, and they were usually covered w/ a sleeve of insulation for that, and to lessen the chance of someone touching them, the basics are still the same

Bill

Stuart:

Allow me to dredge up my 12th Grade Physics...

Keep in mind that the Law of Conservation of Energy applies here, too. Ohm's law doesn't get to have all of the fun.

You have x amount of energy going into the rheostat switch. You have y amount of energy coming out.

Because of the resistence in that rheostat, the value of y is always going to be less than that of x.

Since the law states both sides must be equal (energy cannot be created or destroyed, only modified), you then have z amount of energy tied up in heat production.

So, we have x (incoming energy) = y (outgoing energy) + z (heat producing energy).

The lower the value in y, the greater the value in z, so if you have your heater turned down to low, the switch will get VERY hot.

Hope that this made sense. They're remodeling the bathrooms on this floor and they're playing with torches and getting us all buzzed.

Bill,

I want to believe you..I want you to show me the error of my ways. I used the neat calculator you linked, compliments of Uncle Sam (?). Every time I plug in a lower voltage..lower than 6 volts, the current drops in proportion to it.

You keep saying "the power to the motor must be reduced"...it is. The rheostat drops the voltage, or power available to the motor. With this reduced voltage, current will flow less..the motor turns slower.

If you have a circuit with twelve volts, measured on a meter, and no work is being done..nothing is running, no motor or light, then when you put your amprobe on the circuit, it will read zero. You have 12 volts of PRESSURE, but nothing is flowing..no CURRENT.

If you turn the heater switch all the way to its slow fan speed setting, you reduce the voltage to it minimum, the current flow to its minimum, but the little coils get red hot..ouch!

They don't get red hot from current flow, its flowing its least. You can see this when you plug those figures into the calculator. Its flowing its least because if has the least amount of voltage pushing it. The excess voltage is given off as heat.

A 6 volt fan motor draws 6.3 amps. The windings have .95 ohms of resistance. If you plug these values into Uncle Sam's chart, the only way to change the outcome is to play with the voltage.

Current is a function of voltage, not the other way around. In the little diagram of the batteries and wire, the lamp grew dimmer because there was less current supplied to it..BECAUSE the resistance wire reduced the available voltage.

This is the way I see it. I found a Ohmite rheostat on the shelf, powered it up with 12 volts and stuck my meter on it. As it was turned up, the voltage climbed, as it was turned down, the voltage went down.

If there had been a small motor in the circuit when I did this silly test, its speed would have fluctuated with the rotation of the knob.

The motor wasn't magically wanting more or less CURRENT, it still wanted it full operating current,BUT there wasn't adequate VOLTAGE there to push it..to supply it. Current is something that is USED, it isn't waiting in the wire to go somewhere.

If you have a wire with a million volts in it and everything is turned off, you don't have any current flow..zip, nada!

I think this needs to be put to bed. I am sticking with this theory, flawed or otherwise. Thanks for the great links, and just because I live south of the 49th, doesn't mean I embrace the current (pun) administration.

My dear old father was born on a farm outside of Edmonton in the dead of winter, so I guess I am part "north of the 49th".

Stuart

Stuart, you've proved me right!! [and calculator is Calif govt, not US]
I pointed out you CANNOT change the voltage of the battery, it is what it is
the rheostat DOES NOT 'drop the voltage', it consumes current by increasing resistance - power is a direct function of current flow - no current no power, regardless of how many volts - in the formulas it's resistance that's changed w/ the switch

when you turn the switch to it's minimum the voltage is the same, and the current drained from the battery is the same, but it gets used by the hot switch and doesn't reach the motor - the motor takes 6.3amps to turn full speed, less will slow it down

plug those figures in but DO NOT change the 6v, change the resistance, which is what the switch does, adds resistance - you'll see that for 6v adding resistance lowers the amperage, where it's lowered is at the motor - still the same in the circuit [and from the battery], but some is being 'burned off' in the extra resistance

"Current is a function of voltage, not the other way around. In the little diagram of the batteries and wire, the lamp grew dimmer because there was less current supplied to it..BECAUSE the resistance wire reduced the available voltage."
NO
current is a function of resistance, and the increased resistance of the switch "stops" some of the current flow - the available voltage is the same - 6V, and the battery is still losing the same amount of amps

your error is thinkin that voltage is the variable in the circuit, that voltage does the work, that current is only a function of voltage - as I said before, the current is the movement that produces the work - there is still the same voltage 'push' happening, but some of the same level of current is being lost as heat along the way so it doesn't reach the motor

Brendan puts it in a different equation, same thing: x= the amps that the total resistance draws [motor+switch], y=current that gets to the motor, z=current lost at the switch as heat

think of a battery as a repository of amps [why they rate them that way] not volts - different things on the truck use different amounts of amps - when the amps are all gone the battery is dead, but it's still 6volts, just needs to be refilled! better?

Bill

WOW!?! er... uh?!...

I can certainly see some are more oppinionated then others!

Thanks for all the help. As long as this is normal, Great! I have the old 6volt switch under the dash thats just flips up and down has what I feel to be a ceramic cylinder with a wire coiled around it. Ok It works.

I really didnt think I was going to 'spark' a 'charged' debate. ha ha-I love being a dork!

Thanks again for the help everyone! -Jeff

Jeff,

This is better than politics isn't it. I am not sure if this is normal or not..guess I must be a dork too.


Stuart

Bill,

Why does the blower motor slow down when you turn the little knob. Which value has changed..volts-watts-ohms-amps. Don't say "power" because that isn't one of Mr. Ohm's values.

I am still trying to see it your way, so keep it simple..I'm a dork.


Stuart

Jeff, not a debate, but a learning session about a subject that baffles nearly everyone - including me! I was required to learn the concepts enuff to fool the USAF into letting me fix their weapons systems, but not enuff to be a teacher we're all dorks together when it comes to electrons!

Stuart - the switch is a variable resistance, so it's ohms that you change by turning it - the motor is also a resistance, which is arranged magneticly to use some of the power to turn, but some is lost to heat, which is why electric motors get hot

power is measured in watts, and watts are 'generated' by amps moving thru resistance - increase the resistance and you reduce the current to the motor, same voltage at the battery pushing the same amps to the circuit, less heat in the motor, more heat in the switch

the amps [current] thru any resistance [like the heating element in an electric heater] create heat by 'burning' the amount of power supplied by the amount of volts

I think the problem is thinking in terms of volts - the generator doesn't generate volts, but amps, which 'refill' the battery, or supply circuits directly - everything is 6 v or 12, like the house is always 110 - the variable lighting controllers in your house are just like the heater switch, they reduce the amps [current] so the 100w bulb [a resistance] can only produce 50w, but the extra watts have to go someplace, which is into heat from the controller, another resistance

clearer?

Bill

Bill,

So the switch is a variable resistor. When you adjust it, what value are you tweeking. Are you "resisting" voltage or are you "resisting" current flow?

If you are "resisting" voltage, then a meter on the load side would read less as you turned the knob up, right.

If you are "resisting" current flow, a ammeter on the load side would read less as you turned the knob up, but the voltage would still be the same, right.

Thanks for indulging me.

Stuart

you're resisting current flow, so yes, an ammeter on the motor side of the switch will read lower than one on the battery side - and if you use a voltmeter to check from the output side of the switch to ground, you'll still see full voltage

Bill

PMFJI,

Let me throw another concept out since you guys are really close to getting this stuff sorted out.

One of Ohm's buddy's, Kirchoff developed a law used in circuit analysis that says the sum of all the voltages around a circuit must always equal zero'.

In the heater motor circuit that means that if the battery is supplying +6V then the fan motor must have -6 volts across it's terminals. If we have a resistor in the circuit then we must have some combination of voltages between the resistor and the fan motor that equals -6V so that the sum still remains zero.

What happens with the resistor is that you can control the voltage dropped across it by changing it's resistance (our buddy Ohm's law). i.e. if you could make the resistance infinite (open wire) then all of the voltage in the circuit would be 'dropped' acrossed the terminals of the resistor.

If you reduced the resistance to zero (short circuit) then NO voltage would be dropped across the resistor and the fan would 'drop' all of the available 6 volts from the battery.

With the resistor at some mid-point, then some fraction of the available voltage will be dropped across the resistor and the remaining will be dropped across the fan motor. When you switch a varible resistor, you are changing the resistance of the device which changes the voltage drop across it.

Bottom line is that all of the available voltage from the battery MUST be dropped by the rest of the stuff in the circuit (i.e. resistor + fan motor) in order to obey Kirchoff's law.

Bill, I'm a veteran of USAF electronics schools to but my work was so secret I didn't even know what I was doing.

Yer darn tootin that thing gets hot.The good news is youl only grab it once!Been there done that...

I'm not sure I should stick my nose in here, but that is completely opposite of what really happens.

The current in any given circuit, with a given total resistance, is constant throughout the circuit (for resistances in series, not parallel). The electrons going in = the electrons going out. An ammeter placed anywhere in the circuit will read the same.

Olblue is right about the voltage drops across each resistance. The voltage after the resistance will be lower than the voltage before the resistance, by the amount of the drop across the resistance. The total drop in the circuit = battery voltage.

so I'm wrong!? where were you guys yesterday?coulda saved me all that strain - nearly had'em understanding amps are more important than volts - Mr Volta notwithstanding - I did just slip the meter bit off my cuff

so w/ the switch at 'full speed' the motor supplied w/ 6v has 6 amps to make 36 watts [with a built in 1 ohm], and at 50% the motor only has 3 volts, but still has 6 amps so only makes 18 watts [still w/ 1 ohm?], and the switch uses 3 volts to make the other 18 watts w/ the same 6 amps .... but what we're changing is the ohms, doesn't that double the original resistance?

coulda swore I'd never measure less than 6 volts motor to ground, or that adding the switch resistance into the circuit wouldn't double the total circuit resistance .. try it Stuart

OlBlu - Keesler? they never let me know what I was doing either, just said DO IT!

Bill

Bill,

I don't know Kirchoff from Adolph Hitler, but I am sure he knew his sh*t. I am a "gotta see" kind of dummy.

I will explain what I did, and then abandon this thread! I have a variable power supply I built for my shop. 25 amps 0-120 volt AC/DC.

I took my little Grainger heater fan motor, hooked it up and read DC voltage and amperage (it's 12 Volt) to get a baseline.

I then stuck a Ohmite rheostat in series with
the motor to simulate the heater fan control unit. As the knob was turned, the meter indicated, as I expected, a decrease in voltage and the motor would slow.

Doing this same thing, but reading current draw, the reading decreased also..as expected.

I "read" the wrapped wire of the Ohmite rheostat with a IR temp gun as it sat there,the motor running at a reduced speed. The wire temperature climbed as the rheostat did its electrical thang!

This thread has been another indicator for me that lurking is sometimes better than flapping.


Stuart

The gotcha in using Ohm's law in all this is that the motor is not a resistor.

Long story short, when a motor runs, it generates a 'back voltage' that opposes the voltage applied to it. A motor will accelerate only until the 'back voltage' equals the voltage applied to it.

If you measure the voltage and amps through the motor at one voltage and calculate the resistance you'll come up with an equivalent 'resistance'. If you then change the voltage and calculate a second resistance, you'll come up with a different equivalent resistance.

In engineer speak, the 'equivalent resistance' is non-linear as the voltage is varied. In plain English, every time you change the voltage applied to the motor, the equivalent resistance will also change.

Bill, not Keesler, they kept me at Lackland AFB in the secret building without windows.

If anyone is curious, I blame my knowledge of things electrical on an incident when I was about 10 or 11. Growing up on a farm, one of my chores in the summer was to take a scythe and cut the grass under the mile or so of electric fence lest the fence short out on all the grass and the cows get out.

Being none to interested in the job at hand, one day I begin to wonder if electricity could run upstream faster than water would run downstream. It was only a matter of time before I applied the only stream at hand to the fence and got my answer.

BOY HOWDY DOES IT EVER GO UPSTREAM FASTER!!!!

OlBlue:

Two things.

1) Do you have to kill us now, or did you not divulge enough for the men in black to knock on my door?

2) I never had to pee on an electric fence. All I had to do was watch the calves get their first zap and I stayed well enough away from it.

Brendan,

1) It's all on the web now!

Scroll down for a picture of the secret building.

2) Maybe I should have paid more attention to what the calfs were doing...

olblue, by 'equivalent resistance' are you referring to inductive reactance (Xl), since you're referring to a motor's impedence (Z)?
tony

old_blue,

The physics of what goes on inside even a small motor is reasonably impressive. The voltage applied, speed, torque being produced and coil resistance are all involved in determining the exact current through a DC motor at any given set of conditions. The term 'equivalent resistance' is a simplification that I used to try and keep the answer in plain English.

Xl and impedance are much more important in the world of AC motors. Even the mathmaticians had to come up with imaginary numbers to figure out that stuff!

rustednuts,

What have you done. This thread is like some kind of Sci-Fi creature that won't die! :p


Stuart

Hello people. This is Albert Einstein. I have come back from the past to straighten you all out on a few things...

If the resistance stay the same and they voltage goes down the amps go up. that the way it works the lower the voltage the higher the amps.

not quite, super250, but hold that thought so you remember that 6v requires heavier gauge wires than 12v
Stuart, sci-fi can be fun!!
I been sidetracked, but pruning trees leaves time to think, that's fun too, even if it's a puzzle

what the original question concerned, was the heat created by increasing the resistance [why does the switch get hot], so tho I did mention lowered voltage, in fact the voltage [push] available to the circuit hasn't changed, but we're 'burning' some of the current in the resistor - it may be that in the theory this can be explained as the voltage [potential work] being split between the 2 items, but it's the current that's 'working'
is it the voltage or current that burns your finger

somebody's law: "The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit." - the way I understand that is if we introduce the added resistance of the switch to the circuit, we reduce the amount of current to the fan [actually 'burn some'], because we have an unchanging voltage applied to the circuit - a lower voltage at the motor may be a theoretical reason the motor turns slower, but in terms of the circuit, especially the heat in the switch [the question], it's the current that's important - the amount of current supplied by the battery is partly used in the 'heat work' [joules, to bring in another dead guy] in the switch, leaving enough current to create half as much 'motional work' [torque] in the motor [and it's non linear resistance!]

I think Kirchoff's thing includes equality of currents to and from a point, as well as equality of potential differences - if my switch starts getting hotter than usual do I check to see if my battery is 14V?

this just reminds me that I was successful as a tech, dealing w/ the practical workings of electronics, but I packed it in because of the explanations of theory, maybe I should just ignore the electrical questions!

That is why Great grandpa Oehm put an e in our last name!!!! He was tired of all the resistance he was getting from Georg.

Let me jabber on a little about the motor and see if I can explain it's workings a little more.

If you take just the motor (without a fan for the sake of discussusion) and the battery in a circuit, when you connect the motor, it will accelerate to a particular speed and then stay at that speed. The amps will be at some fairly low level without a load on the motor. It is the 'back voltage' generated by the motor turning that determines the speed at which the motor runs and the amount of current flowing in the system. The 'back voltage' is directly related to how fast the motor is turning. If you put on your gloves and squeeze the end of the spinning shaft, two things will happen because of the increased load on the motor. 1.) The motor speed will slow down and 2.) the 'back voltage' will drop. Now with less 'back voltage' , the current through the motor will increase and the torque being produced on the shaft will increase. DC motors are designed to have a nearly linear relationship between torque output and current draw with as small a drop in speed as possible. i.e. more torque out = more current in.

Kirchoff has a second law which says that the current entering a device must equal the current leaving a device. With our simple heater circuit, that means that the current in every part of the circuit is alays equal.

In your example Bill, with the a resistor in the circuit with the fan motor, and we haven't changed the setting of the variable resistor or changed the battery voltage, and the resistor is now running much hotter than it did before, then we must have more current in the circuit. Given that the resistance hasn't changed and the battery voltage hasn't changed, then only thing left that could increase the current is that something is slowing down the motor and causing it to work harder. i.e. the bushings have gone dry, there is a string wound around the shaft, or maybe debris is jammed up against the fan wheel.

This will increase the current in the motor which causes more current to flow in the entire circuit including the resistor which now gets hotter.

I'll quit there since I'm sure I've already told you more than I know.

On a more practical note, is there any way to fix the ceramic coating on those little resistor coils? Every one I've seen has been crumbled.

Bill,

If you are taking a vote on your last sentence, put me down for a real big AYE!


Stuart

NAY! This is good stuff!

(Especially if you've spent your whole life trying to make sense of electricity and it has always inspired the answer, "Because I said so!" Which is why I can do the basics but often I have no idea why the fix works.)

Kimberly,

This isn't good stuff when you are perplexed with a electrical problem and the answer you get is not only written in a language you don't understand, it makes little sense.

If you hopped a plane to Senegal, and had to converse with the natives there, how long do you think it would take them to catch onto the fact you couldn't speak their language?

Once they caught on, would you keep trying to speak their language and BS them into thinking you could..over and over!

Immersion certainly isn't the BEST way to "teach" the language is it?


Stuart

it's called immersion - best way to learn a language if you wanna learn

Bill