Hi, Newbie with a question

[Nickirons]

Hi,
Just joined the site, my Name is Nick and I'm from London but now
in Switzerland. For a hobby and to make a bit of cash, I restore 2 stroke
motorcycles from the 1980's...
At the moment I am trying to make a small improvement to a bike by
fitting a gear position indicator using an LED 8 segment display. I have
made this and using a matrix of diodes have 7 different earths that light
up my display as I want, and these earths correspond to the gearbox
positions and all is well. What I now have to do is make a transformer to
convert the 12V DC from the battery of the bike to 2.5V DC for my display...
This circuit has been suggested to me and I have all the components
to make it, but before I connect it up to my brand new wiring loom which
cost £250, could any of you see any problems with it, or if it will even work at all
as I don't want to ruin anything on my new loom or battery. I am a mechanical
engineer with 25 years experience, so the bike and its mechanics are no problem,
but my electrical knowledge is very limited. Any help would be greatly appreciated.
Many thanks, Nick.

 

[mab2]

well that would work I think. If you are connecting direct to the battery it should be OK but you might consider adding a low-value resistor on the input just to help protect it from any transients on the B+ line (from ign coil for e.g.)

If you want 1.5A then you should get an LM317T. What current does your display draw? (I'm guessing about 0.2A which should be OK , but bear in mind that there's a minimum current draw of about 5mA if I remember correctly)

If you use a 240Ohm resistor in place of the trim pot you should get a fixed 2.51v out, but the trimmer will give you anything from 1.25v upwards.

If you're worried about damaging your loom then the simple answer it to connect through a fuse ( or even a small light bulb).

 

[tunedwolf]

I am not sure what you are trying to achieve here, but from your description it seems you want to run the LED display directly from this. Are you not using current limiting resistors on your LED display? While it's certainly possible to use the LM317T to drop your 12V to 2.5V and run the LED display directly from it, it's going to get pretty hot as you are converting 9.5V times your current requirement directly into heat. Not to mention that you are complicating your design, in my opinion, un-necessarily.
It would be much better for you to place some current limiting resistors in series, one for each segment on your display and run your display directly from the 12V battery

To calculate your desired resistor value in Ohms, simply take the forward voltage of your LED display, for example 2.5V and subtract that from your supply voltage. In this case 12-2.5 = 9.5V. Now divide that by the desired LED current in Amps. Most general displays lie in the range 15-20mA, but check your datasheet for the display. Let's take 18mA as an example. so 18/1000 = 0.018A and 9.5 / 0.018 = 527 Ohms. The next size up from this in the standard ranges is 560 Ohm, so that's what you would choose. To calculate the size, in terms of Wattage, of resistor, multiply your 9.5V drop by the Current in Amps, so 9.5*0.018 = 0.171 Watts. So a 1/4 Watt resistor is more than adequate in this example.

Hope this helps

 

[Nickirons]

Thanks Mab2, that's very helpful...
I'll get it made up and see what happens. In the mean time if
anyone needs any 2 stroke m/cycle help then just give me a shout.
Thanks again and ATB, Nick.

 

[JimB]

my electrical knowledge is very limited. Any help would be greatly appreciated.

OK, understood.
Can I correct some of your terminology?
LED 8 segment display - more likely a 7 segment display.
What I now have to do is make a transformer to
convert the 12V DC from the battery of the bike to 2.5V DC - Transformers work with AC, not DC. What your circuit shows is a voltage regulator, which does work with DC.

Why do you want 2.5v for the display?
I understand the idea of the 7 segment display and the diode matrix, but have you allowed for current limiting resistors?
An LED is not like an incandescent filament lamp, if you drive it with a fixed voltage you will at some point drive too much current through the LED and kill it. Especially on a motor bike where the temperature can get quite hot.

Actually, you may be better off by just using resistors fron the 12v supply and not using the regulator. Need a bit more info about the 7 seg LEDS and the diode matrix before deciding.

JimB

 

[Nickirons]

This is all becoming a bit complex for me, current limiting resistors, overheating etc.
I'm off to work now but I think when I come back this evening, I'll post up exactly what
I'm trying to achieve and where I've got to so far and data sheet info on the components.
I do appreciate your help with a novice like me and if site admin want to PM me a
PayPal addy I'd be more than willing to make a contribution.
Many thanks, Nick...