[RichG]

Ok I think, thanks to Zips pictures I have worked out the circuit. Am away from home so please excuse the picture.

Am not certain about the transistor, the marking looks like 6C and that seems to be a BC817, using it's pinout the circuit is unusual but makes sense.

So we have a series protection diode from the +ve, this feeds the collector of the transistor. I am assuming the other diode is a zener, unknown voltage but would guess 4 to 6 volts. The zener is fed through a 1k2 and will hold the base at a constant voltage. The voltage on the emitter of the transistor will be about 0.7V less than the base and is used to drive the two LED's each through a 68 ohm resistor.

So SCX seem to have put in a simple voltage regulator for the LED's. This should mean that the brightness remains constant over a range of voltages, so in analogue mode less variance than a Scalextric car?

So bear with me while I think out loud.

The transistor is rated at 250mW and 500mA. The current rating is fine. Am not sure how many mA each LED will take, but let's guess 50mA so 100mA in total.

So if the emitter voltage is say 5V and we have a 15V supply then the transistor will have 10V across it and is passing 100mA. So power dissipation is 0.1A * 10V which is 1W. Too much.

So perhaps I have assumed the wrong current for the diodes. So looking at it from the other end. The max current for 250mW is at 12.5mA each, 25mA total. Power dissipation now .025A * 10V = 250mW which would put the transistor at it's limit.

So if this is a BC817 it is not suprising that it fails, if there is a suprise it's that it survives even with 14.8 Volts. It could be I am still making a wrong assumption about the LED current. To be safe it would need to be a max of 6mA each.

Also possible that the device is not a BC817. Could someone make a couple of measurements on a good board. With the car at max throttle measure the voltage on the input +ve, then on the three connections of the transistor which is the black square in the middle of the board. Put the negative meter lead on the Negative (RH 0V) input strip.

Rich

 

[zipp]

Hey rich, i have tried to get some info for you.

I am not sure if i have done this right, but i will explain what i have done, and what readings i got.

I set my meter to V 20, and then put the red+ connection to "0" in the pic.

I then put the black- connection to "1" and had full throttle on the car. The meter showed 2.80v.

The same set up for "2" showed 10.51v,
and at point "3", 0.98v

Now the bulb on the left was very bright, but the bulb on the right only just illuminated, even at full throttle.
But i think my board is defective?
Not sure if this is what you wanted Rich? Hope it makes sense to you.

Would be interested to hear from someone with a new, good light board, and the contrasting results.

 

[RichG]

Hi Kev

Thanks for the readings. They look a bit odd. Of course it is possible that I have made false assumptions about what 6C is or could just be that your board is faulty. In particular reading 1 looks wrong, and explains why the LED is very bright, but not why the other one is dim.

Will be easier for me if you could you do them again but this time put the meter negative, Black lead to the other input strip on the RHS.

Then make readings 0 to 3 with the Red lead.

If someone with a good board could make the same readings that would be very helpful.

EDIT... If making measurements could you do 2 more. Please measure the voltage on the outermost connection of each LED.

Rich

 

[zipp]

Hi Rich,

with the black lead on the opposite strip, and the red connector at:
1, we get 12.14v
2= 4.23v
3= 14.08v

Now this makes even a little sense to me.

(i think)

Does this mean that 14.08v is going into the little black thing,
and then the left bulb is getting the 12.14v, and the right bulb only 4.23v?

I take it this little black thingy should be kicking out equal v, at points 1 & 2? and therefore my little black thingy is broken?

I am getting all technical.

 

[RichG]

If the little black thingy is a BC817 then (3) is ok and is close to the voltage on the rails.

(2) also looks ok and will be the voltage of the zener. I had guessed 4 to 6 volts.

However if the base of the transistor (2) is 4.23 Volts then the emmiter (1) should be about 3.5 volts. It looks very high at 12.14 and explains the very bright LED.

So yes if it is a transistor, then it has failed. Also if this is some other device, say a voltage regulator then 12.14 volts is still very high for an LED being fed through a 68 ohm resistor.

Will be interesting to see the other two readings.

Rich

 

[taxi]

No measurements for you Rich, but possibly interesting info. I've just been fiddling about with the DBR9, and applied 3v directly to the legs of the LEDs - both work! So the transistor has failed, disabling the lights, but the actual LEDs are undamaged.

One leg of one LED was loose, so that 'failure' was probably down to poor assembly rather than component failure. (Might be worth checking yours, Kev?)

So what do I need to make my lights work properly again?

cheers,

Rob

 

[RichG]

Very interesting Rob. Will be good if they are all like that, trouble is I have a feeling that if the "device" goes short circuit it will take out the LED's.

Need some measurements from a good board, but when I get back home Kev will mail me a car, or perhaps this is an excuse to buy an SCX car

and I will sort it out properly. You will have to wait a little while for a solution.

Device obviously needs replacing, but need to do some maths and fully understand why it is failing. May just need a beefier device or a change to the circuit.

Rich

 

[911]

I sure hope a solution can be found as I won't be even running my SCX cars until I can be confident they won't go up in smoke as previous ones have. RichG if you fancy an SCX car go to Hobbycraft, 30% off and on most other slot goodies as well. I hope you or someone finds something.

 

[zipp]

QUOTE Will be interesting to see the other two readings.Rich, with the black probe on the neg side, touching the outer legs with the red pen,
LH led= 3.95v
RH led= 5.45v

and with the red probe on the pos strip, touching the outer legs of the led's with the black probe,
LH led= 11.24v and smoke from the LH 680 thingy.

(resistor)?
RH led= 9.67v

Rob,
I tried the same thing, by putting direct current to the led's. Unfortunately, (as i am an electronic moron) i put 13.8v across one...POOFF

So led replaced and connections good.

Hey 911,
Do you have a meter to do the readings Rich is requesting? I am pretty sure my circuit is already compromised, so if you have a good board, you maybe able to provide some essential info to those that understand it.

 

[RichG]

Ok well this needs confirming by measurement on a good board, but here's some maths.

A typical High brightness LED takes 20mA.

We have 2 so 40 mA.

Taking even a relatively low voltage on the Rails, say 12.7V, to make the maths easy.

So if the circuit is right then 0.7V is dropped across the input diode leaving 12V at the collector.

The voltage at the emitter will be 4.23V - 0.7V = 3.5V (4.23V is from kev's measurement. This part of the circuit is probably fine and was about what I was expecting)

So the transistor will have 12V- 3.5 V across it & 40 mA flowing through.

So power dissipation will be 8.5 * .04 = 340mW
Of course on a Scaley track with say 16V this will increase to 12.5 *0.04 = 500mW

Transistor is rated ar 250mW. So this is where I started from. What I don't understand is why they work at all on any track.

It seems so far out that I feel I am doing something wrong.

Or SCX have screwed up big time.

So will reserve final judgement until I get a car to measure.

Rich

 

[col_de_turini]

Hello RichG and all,
I have just measured the volts on a Delta lighting board (fully working).

The voltages on the 3 pins of the "device" are: 11.4 (feed), 4.4 (Output to LED resistors) and 4.9 (top of Zener). The LEDs have 3.2V across them. This is all from a supply voltage of 12.2 volts.
So its a fairly simplistic pwer supply stabiliser. Depending on how badly the device has blown the LEDs may have got a whole lot more voltage than they could tolerate. The LEDs can be tested in circuit (open circuit one way and about 1.5 Kohms the other way.
The circuit is as per RichG's diagram btw.
A Skoda Octavia has a simple series drop resistor type circuit. LED's don't seem so bright but that will mean a longer life so maybe the solution is to fit a "ballast" resistor straight to the LEDs (if they are still OK). I guess I should have measured the impedance of the Skoda board :-(
The Delta lights stay brighter longer and so the more sophisticated circuit does have a worthwhile effect in that the lights are on for lower throttle level whereas the Skoda ones are really only effective at higher throttle levels.
(if you remember which bulbs "stay brighter longer" then your slot racing cars probably have pin guides)

 

[RichG]

Thanks for that Col. Well the measurements seem to tally. Also you have confirmed that in practice this is a voltage regulator and has the benefit of giving full output on the lights at small throttle openings.

Unfortunately it seems SCX has got the maths wrong, for the dissipation in the BC817. To be honest I would expect them all to fail.

And the higher the voltage on the rails and the longer your straights, the quicker they will fail.

Solution will be to use a beefier device or do as Col suggests and replace with a simple series resistor.

 

[911]

Yes I have the two new boards but my meter hasn't. Blown transistor apparently

. Sorry. I'll help with pics and stuff if I can though.

 

[RichG]

So doing the same maths on Col's actual figures. Each LED is taking 17mA which gives power dissipated in the transistor of 247mW.

So right on the limit, and that with the relatively low supply voltage of 12.4 Volts.
In practice for reliable operation I would rather see a max of 150mW.

When the transistor fails. If it fails open circuit then the LED's will just stop working. If it fails short circuit then the LED's will go very bright but not last very long.

Have we confirmed what voltage a standard SCX set runs at?

So do all SCX cars have this style of light board or is it just some cars? Ooh and if I was to buy one, just in the interest of research, give me a couple of suggestions.

Actually make that a pair, can't have a single car running round the track.

Rich

 

[zipp]

Rich,

QUOTE Have we confirmed what voltage a standard SCX set runs at?I think it's 14v, but it would be nice if someone with an SCX set could confirm.

QUOTE So do all SCX cars have this style of light board or is it just some cars?Not sure, but a lot of folks with similar problems.

Is the purpose of the weak component, to provide a constant brightness in the led's, irrespective of the throttle position?

QUOTE Solution will be to use a beefier device or do as Col suggests and replace with a simple series resistor.Would be good to see some pix of the resistor fix.
If you could use the pic of the board i have posted, and indicate where the resistor should go, and what value, please, that would prove useful.

And i suppose the best fix would be to find a compatible upgrade for the transistor. Any takers?

I will take the Audi A4 and a Aston.

scale's choice

 

[RichG]

The black device is an SOT23 case style BC817-40 (At least until proven otherwise) It's base is held at 4.9V by the zener diode. The emitter will track the base voltage but is 0.5V less (Col's measurements). This voltage is used to drive the LED's via 68 ohm resistors.

So what this means is as you press the throttle the brightness will increase but instead of full brightness being reached at full throttle it will get there at about 6 Volts, and then stay at that brightness.

Above 6 Volts power is dissipated in the transistor, and the rest as they say is history.

My suggested short term mod for those wanting to try it is to replace the two 68 ohm resistors with 120 ohm, or to be very safe or if you run greater than 15V 150 ohm. This will reduce the LED brightness, mabe too much, but will retain the function as described.

Another alternative would be to put in a beefier transistor. Will look into that. Final option is to remove the transistor from the circuit and replace with a resistor connected between points 1 & 3. Something like 680 ohms would be about right, but you would need to experiment.

Please treat all of the above as experimental. I would not normally make recommendations without testing first.

So yes the Audi & the Aston look nice.

Also like the look of the 360

But does anyone know how you know which cars have this type of light board?

Rich

 

[col_de_turini]

Hello RichG and all,
Just about any NPN transistor will do and could be soldered "on top" of the existing device (unless it has gone short circuit) as a preventative move (ie will work in parallel with the existing transistor). Soldering person will have to be neat to do this work though !! In fact I would think that doing any soldering work on this board will be beyond most people. Fitting a series resistor (1K to 680 ohms) in the main feed to the board will mean disconnecting the copper strip or cutting a bit out of it but that would be an easier task.

There is probably plenty of room to fit a "can" type transistor (I plan to use a 2N222) in most cars, certainly there is in the Delta.

 

[RichG]

QUOTE (col_de_turini @ 11 Aug 2006, 12:52) <{POST_SNAPBACK}>Just about any NPN transistor will do

Hmm I think thats how SCX chose theirs.

However 2N222 at 650mW dissipation, probably fine.

Don't forget you have to derate at 5mW/ degC above 25 degrees. Will propose some alternatives when I get a mo.

 

[911]

Looks like were getting some where. Thanks RichG, your a credit to the forum

.

 

[Steve A]

Recently bought both Scaley and SCX Aston DBR9 s to conduct my own trials. Both look great on track but like so many others front LEDs failed totally after short time on SCX car

. If it is indeed the transistor thingy that has blown, how can I prove the LEDs still work? Will it be obvious if the have gone, ie do they melt or go black?
As for recomondations the DBR9 is a great model and had planned to buy the CS6R Corvette to go with it. Maybe Carrera should get my cash instead as I hear great things about their car. What must be said is that the Scalextric car continues to run like clockwork (only faster!).

Rich G did you purchase SCX or have you thought better of it?