Swap 9V battery for molex power connector?

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Lizard1977

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#116387 27-Apr-2013 18:03

I've just finished constructing my first ever circuit board - it's one of the Jaycar "Short Circuit series, a "Knight Rider" LED scanner. It runs off a 9V battery, but I was wondering whether it could be powered by molex, so I could run it from my computer? I know Molex supplies 12V, rather than 9V, but I'm unsure whether the extra volts would damage the circuit...

RunningMan

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#806431 27-Apr-2013 18:20

On some of those basic circuits there can often be a pretty wide tolerance. Do the instructions specify an acceptable voltage range? Can you post a circuit diagram?

mattRSK

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#806432 27-Apr-2013 18:20

Without seeing the circuit design I would say you will have issues with blowing LEDs at a higher voltage.

mattRSK

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#806435 27-Apr-2013 18:26

Is it this circuit? http://www.555-timer-circuits.com/knight-rider.html

RunningMan

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#806441 27-Apr-2013 18:55

mattRSK: Is it this circuit? http://www.555-timer-circuits.com/knight-rider.html

I presume it's this http://www.jaycar.co.nz/productView.asp?ID=kj8236&keywords=knight&form=KEYWORD which doesn't seem to have a 555 from the photo...

Lizard1977

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#806468 27-Apr-2013 19:40

RunningMan:
mattRSK: Is it this circuit? http://www.555-timer-circuits.com/knight-rider.html

I presume it's this http://www.jaycar.co.nz/productView.asp?ID=kj8236&keywords=knight&form=KEYWORD which doesn't seem to have a 555 from the photo...

That's the one. Nothing in the instructions that I can see bout the voltage tolerances.

hellonearthisman

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#806501 27-Apr-2013 20:51

If you tap into the 5v line on the Molex it may go, but on dimm.

Or you could make your own voltage divider. tools/voltage_divider

Vin 12v
VOut 9v

R1 = 100ohm
R2 = 300ohm

+12v ----- [ 100ohm ] ---- 9v out X ---- [ 300ohm ] ---- X Ground

To use the 9v your connect to 9v out X and Ground X. :)

https://www.sparkfun.com/tutorials/207

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chevrolux

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#806712 28-Apr-2013 14:04

Can you post a circuit schematic? It probably already has a voltage divider in it that can just be changed to suit 12V.

sleemanj

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#806716 28-Apr-2013 14:21

Simplest solution without knowing the characteristics of your circuit is to just use a 7809 regulator running off the 12v line to supply the 9v you are replacing.

Even from Jaycar a 7809 is only a couple of dollars.
http://www.jaycar.co.nz/productView.asp?ID=ZV1509&form=CAT2&SUBCATID=974#1

TO-220 packaged 7809 from Jaycar, looking at the device from the front, middle leg to ground, left leg to the 12v, and the right leg gives you the 9v you want.

MOLEX Yellow -> 7809 Input
MOLEX Black (either one) -> 7809 Gnd (middle leg)
7809 Output -> redwire of the old 9v connector
MOLEX Black (the other one) -> blackwire of the old 9v connector

---
James Sleeman
I sell lots of stuff for electronic enthusiasts...

RunningMan

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#806748 28-Apr-2013 15:53

Or a 3.3v zener in line with the supply.

Niel

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#806950 28-Apr-2013 22:25

What is the part number of the 3 pin part at the one end of the board? Is it an LM7805 voltage regulator for the TTL ICs? And the LEDs are likely driven to 0V by open collectors so have a series resistor to +9V. What is the LED resistor value? LEDs do fine with up to 30mA (20-25mA is better) so if the LED resistors are over about 350 Ohm then you will be fine, otherwise you will have to change them maybe 470 Ohm of 510 Ohm.

You can never have enough Volvos!