Using an an LM7905 to regulate the low side, have I got it right?

[bigal_scorpio]

Hi to all,

I am trying to make a circuit that runs from 12v so I obviously need a regulator. Up to now I have always used the 7805 and had no problems but this time I have to run 8 wires to separate LEDs controlled through a ULN2803 from a PIC16F628A.

The thing is that I can only get 8 core wire and I need all 8 for the LEDs so I was wondering if I could control the low side and feed the ICs and LEDs directly from 12v as in my drawing but I realised that I am not certain how the 7905 works?

Is my drawing wrong and if so can anyone put me right about how to do this?

The project is for a car so I can pick up the 12v or Gnd almost anywhere, I just need to keep the wire down to 8 core if possible as I have loads of it and not a scrap of anything above that.

Al

PS. please excuse my usual crappy drawing.

 

[Roff]

You're close. See the attachment, and notice the changes.
Just be aware that your LEDs/resistors/drivers only have 5V across them, so select the resistors accordingly. Also, the 7905 has to carry the LED current. Can your 7905 supply that much current/power? What are the LED specs?

 

[bigal_scorpio]

Hi Roff,

I see what you have changed. So I wasn't far off then mate, just totally the wrong way round! hehe

As to the LEDs the are 3.6v drop and though they can carry 350mA I was only planning on 240mA (5.8ohm resistors) and was hoping as the LEDs will all have a variable duty cycle that the 7905 could cope. All 8 on full duty would be almost 2A and the 7905 states max as 1.5Aso if they were all on at 75% duty then it would be about all I could expect. I am prepared to give the 7905 a good heatsink though which may help.

Any thoughts on that setup mate?

Al

 

[Roff]

Hi Roff,

I see what you have changed. So I wasn't far off then mate, just totally the wrong way round! hehe

As to the LEDs the are 3.6v drop and though they can carry 350mA I was only planning on 240mA (5.8ohm resistors) and was hoping as the LEDs will all have a variable duty cycle that the 7905 could cope. All 8 on full duty would be almost 2A and the 7905 states max as 1.5Aso if they were all on at 75% duty then it would be about all I could expect. I am prepared to give the 7905 a good heatsink though which may help.

Any thoughts on that setup mate?

Al

I think you have ignored the fact that Vce(sat) of ULN2803 is 1V typical, 1.3V max @ Ic=200mA. IMHO, you don't have enough supply voltage for 3.6V LEDs unless you use MOSFETs as switches.
Also, your 7905 output current will probably be limited by power dissipation. The part is pretty much bulletproof, and will limit output current if it starts to overheat.
(12-5)V*1.5A is 10.5W. Your 12V is going to be more like 13.6V (or more) if the engine is running. That's gonna require a serious heat sink.

 

[Roff]

I don't understand why you want to do this. What's wrong with the obvious way of doing this? Why do you think you need more than 8 wires to go to the LEDs?

 

[bigal_scorpio]

Hi Ron,

The way I see it I need 8 lines to the LEDs and then another to supply them with the power, 9 in all. If you know of another way then I am all in favour of it. I am more than happy to use any method that will work. Suggestions are very welcome.

Al

 

[Roff]

Hi Ron,

The way I see it I need 8 lines to the LEDs and then another to supply them with the power, 9 in all. If you know of another way then I am all in favour of it. I am more than happy to use any method that will work. Suggestions are very welcome.

Al

Here is how I would do it.

 

[bigal_scorpio]

Hi Ron,

Sorted mate! That was my original idea but I didn't have any 3w resistors so I tought why not drop all the voltage.

It does seem much harder to drop than use resistors in the long run though so I have just been on fleabay and ordered 25 of em. And they are in the UK, may be able to build by wednesday eh!

Thanks, Al

 

[Roff]

Hi Ron,

Sorted mate! That was my original idea but I didn't have any 3w resistors so I tought why not drop all the voltage.

It does seem much harder to drop than use resistors in the long run though so I have just been on fleabay and ordered 25 of em. And they are in the UK, may be able to build by wednesday eh!

Thanks, Al

You have to drop the power somewhere. You could use something like a 7808, which would reduce the size of the resistors, but then you would have 6 or 7 watts dissipated in the regulator. You might wind up needing 2 or 3 of them, maybe more.

 

[WTP Pepper]

Isn't the 7905 a -ve voltage regulator?

 

[tvtech]

Post deleted. Missed it completely.

Good luck guys,
tvtech

 

[Roff]

Isn't the 7905 a -ve voltage regulator?

You are correct.