Come up with a brilliant solution .......

On Fri, 23 Sep 2005 07:37:54 GMT, quietguy put finger to keyboard and composed:

Maybe I'm missing something, but wouldn't your transistor be on whenever the lights are on?

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

On Fri, 23 Sep 2005 05:10:34 +0200, " Geocacher" put finger to keyboard and composed:

How continuous? Does the computer test the lamp immediately after switching it on and then ignore the current reading until next time? Or does it keep testing the current during the entire time the lamp is on?

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

On Fri, 23 Sep 2005 09:14:30 GMT, Ross Herbert put finger to keyboard and composed:

I had considered the solution proposed by Craig Hart but refrained from suggesting it because the requirement was for a "simple" method. Anyway, if this level of complexity is allowed, then one could easily satisfy the safety requirement by placing a high side current sensor within the trailer itself.

PNP transistor E C +V o--|---\___/-----> to RC coupled, dummy load switching transistor | | | R | | |--R--|------> to LED lamp array

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

On Thu, 22 Sep 2005 14:02:39 +0200, " Geocacher" put finger to keyboard and composed:

Maybe the idea requires a different approach. Just how smart is the computer? Can you fool it into believing that the trailer is not there? Will the computer continue to test the trailer in this case? Is there a "trailer present" sense wire in the harness to the trailer? If not, then how does the computer distinguish between the case where the trailer is not present and the unlikely case where *all* lamps have failed? Does the computer understand the difference between no current draw and insufficient current draw?

If the trailer isn't there, will the lamps still work, ie does lamp voltage appear at the connector? Does the computer sense the brake switch and then supply power to a lamp, or can the brake switch control the lamp independently of the computer? What happens if the computer fails? Do the lights still operate? This may be a silly question, but do the lights behind the cabin still operate when the trailer is attached? If not, then this would indicate that the lights are under computer control and that the trailer's presence is being sensed.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Since the computer sends a pulse to check the lights this would have to be higher than the normal 'alight' voltage of the taillights etc. So you would set the zener going to the base of the transistor a little higher than the 'alight' voltage - eg the tranister only turns on when the pulse is sent

David

Franc Zabkar wrote:

How do you know this? And why do you think it has to be so?

Ken

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All these suggestions are a load of rubbish... The ONLY way to fix this problem is have the manufacturer fix the computer to correctly handle modern (LED) lamps.

If he does not want to comply then don't buy his product.

If it is a design rule requirement to do the test then he has to comply or the OP has to stay with the original candles (incandecescent lamps) that have to be tested because they fail often on trailers.

Franc (and Ken), both suggestions will work, BUT.... the "fudge" components would have to be integrated as part of the LED lamp assembly in order to conform to the relevant standards, I would imagine. Not only that, if the LED lamp did go open circuit the "fudge" components must not allow the computer to test the dummy load and think that the LED lamp was still ok. As I see it,any sort of dummy load would still allow a successful lamp test when the LED lamp itself is open circuit. It would have to be a "fail safe" arrangement to serve the intended purpose of the computer lamp test. I can't see how it can be done using these methods while still providing a meaningful test result.

Considering the reliability of incandescant bulbs I would stick to using these in the trailer lamps. This way the automatic lamp test will provide a meaningful test result as intended so that faulty lamps can be replaced. After all, it is not as though a few extra watts is going to seriously affect the alternator loading or battery capacity on a truck.

Ross Herbert

I entirely agree......

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I agree - if the truck manufacturer isn't going to do a fix for their firmware to accommodate LED lamps (and it probably isn't trivial, as you'd have to somehow 'know' which lamps were LED and which incandescent) then any fix is either potentially dangerous or self-defeating (ie. the globe in parallel sorta kinda defeats the purpose of having an LED pack in the first place).

Cheers.

Ken

How about

npn C E +V o---------------|---\___/-----> to to LED array | | | / E | | --| pnp |--R1-| | \ C | R2 | | | RD C1 C2 | | | | | o----------------|

RD emulates the incandescent-lamp load. Select R2 & C2 time-constant to satisfy the duration constaint of the 'lamp-test' and then to disconnect that power-hungry load.

Select R1 & C1 time-constant to eliminate the 'light-test' pulses from flashing the leds at night and dusk.

Seems that you'll need to determine two things ;

1. what the pulse-duration of the lamp-test actually is. (I suspect it'll be milliseconds if the pulses are visible during daylight) and
2. what the 'lamp-ok' current threshold is. (10 mA, 100 mA, ???? ) {A cold incandescent has a much lower resistance than a hot operating bulb.}

If one needs to verify that the led-strings are ok then that will complicate things a lot because ;

1. the leds have such a fast response that the eye will detect the test-pulses at night/dusk, and
2. a led in a series-string could be short OR open.

put finger to

Whats your point? If there was a normal globe in cct, it would do the same thing.

On Sun, 25 Sep 2005 00:27:15 +1000, "two bob" put finger to keyboard and composed:

My interpretation is that the transistor is DC coupled. Therefore, if it is on whenever a pulse is present, then it would also be on whenever the lights are on, so why use a transistor at all? The proposal would only make sense if the base drive were AC coupled.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

I don't know 'this' - just guessing. And since the computer measures current draw, it seems unlikely it is an RF or Audio pulse, but rather a DC pulse. And to superimpose a DC pulse onto a 24v DC line, it seemed logical that it would have to be higher than 24v in order to be detected.

But of course I am just 'supposing' - is that a problem for you?

David

Ken Taylor wrote:

I was suggesting DC coupling, but thru a zener so the transistor only conducts when the zener v is exceeded, by the pulse.

David

PS d>

It's not a problem to me, it's just plain wrong. It's not the way lamp current sensing is done.

Ken

"quietguy" wrote in message news: snipped-for-privacy@REMOVE-TO-REPLYconfidential-counselling.com...

Easy, measure the voltage drop across the led array, if it exceeds that of the led(s), no load is switched on. If it is as expected, switch on the load for half a second.

V+ -----+ | RES | +-------Vsense_led | LED | LED | GND

If the two leds have a foward voltage of 1.2V each, then Vsense_led should be 2.4v. If the device fails, Vsense_led will be the same as v+, providing that the input impedance of the measuring curcuit is reasonble high.

I would also be cautious about the dirty signal that will probably be found on the power line. This whole circuit will no doubt need a good dose of filtering.

After playing with a few circuits on paper, my conclusion is that you will need 1 opamp or comparator with 2 transitors. It may be possible to get away with one transistor.

Dare I say it, a 555 in monostable mode may even proved an elegant solution.

All these parts can be had in smd packages now, so adding it to an existing circuit should not consume much real estate.

Might have to download LT's spice and have a play.

Is it possible to grab a trace of the pulse on a cro? Are you using off the shelf inicator/brake led assemblies or manufacturing your own. Can you provide a link to the manufacturer of the assemblies if you do not construct them yourself