Help with small changes in plan

[ElectroElvis]

Hello evryone, I have one favour to ask you, I got a pencil-written schematics in Slovakian language which i havbe no clue about it and he asked if it is possible to make it work on 14V input?

Which parts he needs to change to work properly? To be honest i am very confused at looking at this schematis especially at positive rail with no negative one :/

Thank you all

[dannyf]

I would start with the capacitors, check the diodes' transistors' Vr and Vceo.

The plan seems to suggest that those leds are 3.5v@20ma. If so, the output is about 3.5+0.7(p-n junction) + 0.3v (bat48) - 0.7v (p-n junction), just shy of 4v.

If you are brave, just hook it to 14v - it was designed for 12v and 12v isn't that much different from 14v.

Two (potential) issues with the plan:

1) there is a lot of current going through the npn transistor, at least 40ma, without any load attached. that means a power dissipation of (12-4)v*40ma = 0.3w - a lot to dissipate for those tiny transistors (200c/w thermal resistance at least).

2) the topology has very little gain so its line regulation / psrr aren't great.

[Rufus]

Hello evryone, I have one favour to ask you, I got a pencil-written schematics in Slovakian language which i havbe no clue about it and he asked if it is possible to make it work on 14V input?

Well we don't have a clue either, so some idea what it is supposed to do would be helpful.

It looks like some sort of voltage comparing indicator where one set of LEDs or the other lights when the voltage difference between the two inputs is more than +/- about 1.5v.

So it doesn't have a 12 or 14v input,  it has a maximum difference between inputs and with the 10v rated capacitors seeing the difference less a diode drop it wasn't designed for 12v difference anyway.

[ElectroElvis]

Schematics is for small railroad lights inside train that would be smallest smd size possible, this schematics is made for 12V, friend who gave me this needs to work on 14v due to his digital decoder system which works at 14v.

[kizzap]

How dodgy can you go with the design? If you have a little space, just add 3 diodes to the voltage input, (maybe 4 if you want to play real safe). each diode drops ~0.7V, so 3 would drop 2.1V, which would just get you under the 12V Max the schematic hints to.

The obvious better option would be to redesign the circuit to work at the higher voltage.

-kizzap

I think I have the circuit figured out. It looks like it might be for a set of headlight/tail-lights, that swap colours when the locomotive switches directions. OP can you confirm this?

If my suspicions are correct, my Diode option won't work.

[ivan747]

How dodgy can you go with the design? If you have a little space, just add 3 diodes to the voltage input, (maybe 4 if you want to play real safe). each diode drops ~0.7V, so 3 would drop 2.1V, which would just get you under the 12V Max the schematic hints to.

The obvious better option would be to redesign the circuit to work at the higher voltage.

-kizzap

Or better yet, use two zener diodes.

[kizzap]

Yep, looks like a polarity detection circuit to change LEDs depending on the direction of the train. Translating the text at the caps gives the "forward" and "backward" options.

The lighter coloured LEDs are the "front" yellow LEDs, and the darker LEDs are the "rear" red ones.

I'm 100% certain now that the centre Line is our conventional "ground", and that the input at the top swings between negative and positive.

I'm not certain but I believe that the transistor circuit limits the voltage out the LEDs to 2.3 volts maximum, and the electro is there to filter the PWM signal for the speed of the train so that the LEDs don't flicker or dim.

-kizzap

[Lurch]

Unless there is more to this project then converting from 12 to 14V isn't going to work as the track voltage doesn't reverse, it is just a constant 14VDC at the same polarity, so the LED's will always be lit for one direction. If this unit is going to be connected to a decoders outputs then you could ditch most of the switching and just power the 2 rails from 2 outputs and either swap the resistors or add a couple of diodes to each bank to drop the rail voltage to 12V. You could also meet in the middle maybe, depending on size and gauge of the track as DCC components should work down to 12V quite happily.

Also see http://www.brian-lambert.co.uk/DCC.htm#Decoder%20controller%20lighting for some more info. (nothing to do with me, just a link).

[dodad_guy]

Weird circuit!  Center buss is the common.  If the top input is positive, then the top section lights.  If the input is negative, then the bottom lights.  If the input is AC, then both sections light.  I don't understand how the maximum input voltage could be 12 volts with filter capacitors rated at 10 volts. 

Use a LM7808 after the top diode and use a LM7908 after the bottom diode to limit the voltages going to the filter capacitors.  The regulators will reference to the middle common line.

[kizzap]

Unless there is more to this project then converting from 12 to 14V isn't going to work as the track voltage doesn't reverse, it is just a constant 14VDC at the same polarity, so the LED's will always be lit for one direction. If this unit is going to be connected to a decoders outputs then you could ditch most of the switching and just power the 2 rails from 2 outputs and either swap the resistors or add a couple of diodes to each bank to drop the rail voltage to 12V. You could also meet in the middle maybe, depending on size and gauge of the track as DCC components should work down to 12V quite happily.

Also see http://www.brian-lambert.co.uk/DCC.htm#Decoder%20controller%20lighting for some more info. (nothing to do with me, just a link).

Great link, explains things a little more, and it still makes sense, you can simply connect this circuit in parallel with the motor, to go backwards, the motor still needs to be fed a reverse voltage. Not knowing the output voltage to the motor, I could believe that it is still fed to the maximum 12V...

Weird circuit!  Center buss is the common.  If the top input is positive, then the top section lights.  If the input is negative, then the bottom lights.  If the input is AC, then both sections light.  I don't understand how the maximum input voltage could be 12 volts with filter capacitors rated at 10 volts. 

Use a LM7808 after the top diode and use a LM7908 after the bottom diode to limit the voltages going to the filter capacitors.  The regulators will reference to the middle common line.

Yeah the voltage of the caps is strange (assuming it is 10V). Using voltage regulators could be problematic with the reverse voltages.

TL;DR: I say make the circuit as is, and wire it in parallel with the motor.

-kizzap

[Lurch]

Not knowing the output voltage to the motor, I could believe that it is still fed to the maximum 12V...

Should be. I would assume this is not what the OP has in mind though with wanting to convert to 14V. Worth checking the loading of the outputs though before just attaching this across the motor wires as a decent sized DC motor and a stack of LED's might upset some decoders.

[dodad_guy]

Putting the voltage regulators after the diodes will prevent polarity problems.