GH> RW> !---------------(resistor)-!+ ! GH> RW> ! ! GH> RW> !+-(LED)! The headlight switch provides 12 volts to the junction of the two branches, i.e., the left side of the headlamp and the 220 ohm resistor. If the headlamp filament is good, then you'll have some (undefined) current through the headlamp and some current through the 220 ohm resistor. You can easily calculate the current through the resistor if you know the forward voltage drop through the LED. The resistor-LED current will be I=(12-VLED)/220, where VLED is the LED forward voltage drop, usually between 2 and 3 volts. If VLED was 2.5 volts, then the current would be I=(12-2.5)/220 or 0.043 amps. We weren't given the resistance of the headlamp filament, but we know the current will be I=12/Rlamp, where Rlamp is the steady-state hot filament resistance, usually about 3 ohms. That would yield a filament current I=12/3 or 4 amps. (Three ohms was an estimate.) All of the above was with a good filament, but what happens when the filament burns out or opens? The filament resistance becomes infinite, which means that the filament current falls to zero. How does this affect the resistor-LED current? Practically speaking, it does not affect that current. You still will have 12 volts applied to the left side of the resistor-LED branch, and you still have a current of I=(12-2.5)/220 or 0.043 amps. The bottom line is that there is no difference in the resistor-LED current, no matter whether the lamp filament is good or failed. The current does not change. Now, you might argue that in the real world there is some source resistance in the headlamp switch. That's true. Then you might assert that the filament current would thus affect the voltage applied to the resistor-LED branch. That would be true too. However, the change in supply voltage would be very minimal, and you would hard pressed to detect such a small change in the LED current. Perhaps that's what you mean by "Thevinizing." I'll go ahead and repost the circuit below for your convenience. Let me know if I you have any questions. I'll be more than pleased to answer them. Just don't insult me, please. GH> RW> )--(Headlight Switch)-+ Low GH> RW> | beam GH> RW> +-(Headlamp)-+---------------------!-(Ground GH> RW> ! ! GH> RW> ! 220 ohm ! GH> RW> ! 1/4 watt ! GH> RW> !---------------(resistor)-!+ ! GH> RW> ! ! GH> RW> !+-(LED)! --- QScan/PCB v1.19b / 01-0232 * Origin: IBMNet Connection - Indpls, IN - 317-882-5575 28.8 USR (1:231/1) --------------- FIDO MESSAGE AREA==> TOPIC: 246 AUTOMOTIVE Ref: F3E00014 Date: 03/06/98 From: ROY J. TELLASON Time: 08:06pm \/To: GARY HALL (Read 0 times) Subj: Vehicle Confiscation gary hall wrote in a message to Don Dellmann: gh> On (03 Mar 98) Don Dellmann wrote to gary hall... DD> While tripping merrily through the mail, gary hall was overheard RW> )--(Headlight Switch)-+ Low RW> | beam RW> +-(Headlamp)-+---------------------!-(Ground) RW> ! ! RW> ! 220 ohm ! RW> ! 1/4 watt ! RW> !---------------(resistor)-!+ ! RW> ! ! RW> !+-(LED)! ML> RW> DD> No, as long as the resistance of the LED circuit is high enough, the DD> LED should stay off unless the lamp burns out, then the LED would DD> light. DD> The headlight bulb is of sufficiently low resistance that as long as DD> it is working it is effectively "shorting out" the LED. gh> You got your people mixed up but what you said is exactly what gh> I was saying. A headlamp has a Res of about 1.2 Ohms with the gh> 220 or higher I dont think 220 will do it the current flow gh> through the led will not be enough to illuminate it. Have you *TRIED* to figure it out? Let's do that, now. A typical LED has a forward voltage drop of about 1.8 volts over a fairly wide range of current (yeah, it's going to vary somewhat but not enough to matter here). Assuming that you have a 12.6 volt battery voltage, that leaves 10.8 volts to be dropped across the resistor. At that voltage, a resistor of that value is going to have about 49 mA flowing through it, which is a bit much for most LEDs to deal with. If I wanted to use a resistor to allow an LED to operate in an automotive environment, I'd probably be up around 1k ohms or so rather than 220 ohms. Or maybe a bit lower, but certainly not lower than 470 ohms or so. gh> if the headlamp fails then all current will flow through the gh> resistor and LED. If the headlight fails then the current through the LED will be determined by the battery voltage, the forward voltage drop across the LED, and the value of the resistor. If the headlight *doesn't* fail, the same holds true. gh> The math whiz that put the Ohms law theory to it forgot about gh> Multi source schematics which must be done with a Loop gh> equasion, or Thelvinized. I have no clue here as to what you're talking about, or how it applies to the situation we're looking at here. The circuit illustrated above has, for all practical purposes, a fixed supply voltage with differing degrees of current flow possible. The circuit has two branches, with current flow being different in each branch according to the resistance present as per ohms law. THE CURRENT FLOW IN EITHER BRANCH HAS ABSOLUTELY NO EFFECT WHATSOEVER ON WHAT'S HAPPENING IN THE OTHER BRANCH! If you have any disagreement with any of this feel free to express it and we'll take it from there. If this is getting a little off topic for the echo then we can take it up in ELECTRONICS or any place else you'd care to... email: roy.j.tellason%tanstaaf@frackit.com --- * Origin: TANSTAAFL BBS 717-432-0764 (1:270/615) --------------- FIDO MESSAGE AREA==> TOPIC: 246 AUTOMOTIVE Ref: F3E00015 Date: 03/07/98 From: MARK HOFMANN Time: 09:51am \/To: ALL (Read 0 times) Subj: Monte Carlo SS & Gas mileage. I have been working on trying to get better gas mileage out of my 86' Monte Carlo SS. It has 91k on it currently. I have replaced the carb, and put a rebuild one on it. I have also replaced the O2 sensor. I went from struggling to get 11mpg, to 14mpg. I don't get any better gas mileage highway at all, than city - which brings me to the next point. I am getting an error code on my Vehicle Speed Sensor when the car is in park, and idling. If that is indeed bad, I would think that would have a good deal to do with gas mileage. I also get O2 sensor error codes (lean mixture), which is really strange. The car runs 100% perfectly.. You would never know it is getting these errors, but I would like to correct them. Anyone have any experience in replacing the VSS? Also, my fan clutch never seems to unlock. The fan spins all the time, so matter if it is cold or warm. That needs to get fixed also. Any help is appreciated! Thanks in advance. - Mark Hofmann weather@bcpl.net --- WWIVToss v.1.40 Registered * Origin: Weather Station, Baltimore, MD *USA* 410-882-8887 (1:261/1304.0) --------------- FIDO MESSAGE AREA==> TOPIC: 246 AUTOMOTIVE Ref: F3E00016 Date: 03/07/98 From: MARK LOGSDON Time: 12:52pm \/To: ROY J. TELLASON (Read 0 times) Subj: OHM'S LAW RJ> gh> I was saying. A headlamp has a Res of about 1.2 Ohms with the RJ> gh> 220 or higher I dont think 220 will do it the current flow RJ> gh> through the led will not be enough to illuminate it. RJ> RJ> Have you *TRIED* to figure it out? Let's do that, now. Obviously he hasn't bothered. RJ> A typical LED has a forward voltage drop of about 1.8 volts over a RJ> fairly wide range of current (yeah, it's going to vary somewhat but RJ> not enough to matter here). Assuming that you have a 12.6 volt battery RJ> voltage, that leaves 10.8 I believe I earlier said the voltage would be between 2 and 3 volts, but yes, it really should be between 1.2 and 2.4 volts. Your 1.8 volts is a good nominal value to assume. Though in this case, my using a forward voltage drop of 2.5 rather than 1.8 volts would yield a difference of only 6%. Mea culpa. :) RJ> If the headlight fails then the current through the LED will be RJ> determined by the battery voltage, the forward voltage drop across the RJ> LED, and the value of the resistor. If the headlight *doesn't* fail, RJ> the same holds true. I believe most LED's have a maximum continuous current of about 0.050 amperes or a little higher. The 220 ohm resistor would provide a current near that value. Your 470 ohms is a better choice here. RJ> If the headlight fails then the current through the LED will be RJ> determined by the battery voltage, the forward voltage drop across the RJ> LED, and the value of the resistor. If the headlight *doesn't* fail, RJ> the same holds true. In other words, you are saying that the LED current will be the same whether the lamp filament fails or not. In other words, the circuit is USELESS as drawn. Roy, it's refreshing to see at least one other person besides me has taken the time to analyze the circuit. RJ> gh> The math whiz that put the Ohms law theory to it forgot about RJ> gh> Multi source schematics which must be done with a Loop RJ> gh> equasion, or Thelvinized. RJ> RJ> I have no clue here as to what you're talking about, or how it applies RJ> to the situation we're looking at here. Apparently the definition of "math whiz" today is someone can perform the simple algebra that most of us older folks were taught in the second or third grade. That's no surprise when you see how the public schools today are turning out high-school graduates who can't read or do simple algebra. I suppose today's definition of "reading whiz" would be someone who has mastered the reading of road signs. :( RJ> The circuit illustrated above has, for all practical purposes, a RJ> fixed supply voltage with differing degrees of current flow possible. RJ> The circuit has two branches, with current flow being different in RJ> each branch according to the resistance present as per ohms law. RJ> THE CURRENT FLOW IN EITHER BRANCH HAS ABSOLUTELY NO EFFECT WHATSOEVER RJ> ON WHAT'S HAPPENING IN THE OTHER BRANCH! Bingo! Roy, you did analyze the circuit and have shown the circuit does not work as advertised. --- QScan/PCB v1.19b / 01-0232 * Origin: IBMNet Connection - Indpls, IN - 317-882-5575 28.8 USR (1:231/1) --------------- FIDO MESSAGE AREA==> TOPIC: 246 AUTOMOTIVE Ref: F3E00017 Date: 03/07/98 From: JOHN PUMMILL Time: 02:34pm \/To: RUBEN LOPEZ (Read 0 times) Subj: TCC Lockup RL> Well, it still runs great. I Was driving with my needle RL> buried (85+MPH) just the other day with absoulutley no RL> hesitation suprisingly enough, it didn't waste nearly as RL> much as you would think. BTW it's a 1991 chevy full-size RL> extended cab with the 305 TBI smallblock and the 740 Auto RL> trans. It wastes more than you think. Hesitation is just one symptom that may or may not occur. The damage to the O2 sensor and the strain on the throttle body are more concerns. After a period of time the block learn and integrator will account for some of the variables that are not present. My guess is your block learn is down around 105 or so. --- FMail 0.94 * Origin: Running from the Klan in the Fiero (1:123/30) --------------- FIDO MESSAGE AREA==> TOPIC: 246 AUTOMOTIVE Ref: F3E00018 Date: 03/07/98 From: JOHN PUMMILL Time: 02:35pm \/To: KENNY HENDERSON (Read 0 times) Subj: 1999 Cars KH> How much weight would it save? (seriously) Probably total 45-60 pounds, eliminate some strain on the engine, etc. --- FMail 0.94 * Origin: Running from the Klan in the Fiero (1:123/30)