# Help determing errors in circuit schematic

Discussion in 'General Electronics Chat' started by Stonekaters, Aug 5, 2016.

1. ### Stonekaters Thread Starter New Member

Mar 16, 2016
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Hello, I have come up with this circuit and I'd like to know if anyone could have a look at it and tell me what errors I have made. Thank you very much in advance

2. ### bertus Administrator

Apr 5, 2008
15,799
2,385
Hello,

There will be no current in R3 , R4 , VRAPOT and VRBPOT as the shown leds are connected in reverse polarity.
There are no values given for R3 , R4 , VRAPOT and VRBPOT.

Bertus

3. ### Stonekaters Thread Starter New Member

Mar 16, 2016
9
0
Oh sorry, didn't realise I remove the values in there, R3 and R4 are 330ohms to limit LED current, the POT are 10K.
So I will need to swap resistor and LED and inverse the LED?

4. ### AlbertHall Well-Known Member

Jun 4, 2014
2,261
448
You can leave the resistors alone, just reverse the LEDs.th
The LED current has to flow through the pots and as they are 10k (plus 330 Ohms) the LED current may be very low depending on the supply voltage. If the supply is 5V (as I suspect it may be) then this will need different connections.
What are the output voltages from the pots supposed to represent?

5. ### Stonekaters Thread Starter New Member

Mar 16, 2016
9
0
Ok, I'd like the current to flow through the resistor, through the LED and through the POT, could I do that by just inversing the order of the components in the line( from top to bottom: POT, LED, resistor)?
2 displays are meant to connect one in each to show voltage. The voltage supply is meant to be 9V, my main goal is to create two voltage rails with a LED, a switch and a POT (to select voltage) on each rail.

6. ### AlbertHall Well-Known Member

Jun 4, 2014
2,261
448
How much current do you want draw from the voltage rails?
The current available will be very low using 10k pots.

7. ### Stonekaters Thread Starter New Member

Mar 16, 2016
9
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I still need to do a bit of research on that, I'd like to power arduino projects with it so I need to check what sort of current levels I need. I would get more current using 1K POTs, right?

8. ### crutschow Expert

Mar 14, 2008
13,482
3,369
If you told us exactly what the circuit is supposed to do then we could better advise you on whether the circuit is correct or not.

Nothing in the circuit is appropriate for powering any Arduino projects.
For that you likely need a voltage regulator.

9. ### Stonekaters Thread Starter New Member

Mar 16, 2016
9
0
My idea was to create a voltage divider using a POT to be able to change the voltage value and use a voltmeter display to show the voltage selected, then use that voltage to power a breadboard.

10. ### AlbertHall Well-Known Member

Jun 4, 2014
2,261
448
But that voltage will change as soon as you draw any current. Using a 10k pot, and assuming it is set to the middle (half the input voltage), the voltage will fall by 2.5V when you draw just 1mA from that voltage rail.

You really need two adjustable power supplies, perhaps using LM317.

BobaMosfet likes this.
11. ### Alec_t AAC Fanatic!

Sep 17, 2013
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As stated, you need a voltage regulator. The pot can set the voltage that the regulator regulates.

12. ### Stonekaters Thread Starter New Member

Mar 16, 2016
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Ok, so I'll need two LM317 to regulate the voltage selected by the Pot. And, what will happen with the current using the voltage regulator?

13. ### AlbertHall Well-Known Member

Jun 4, 2014
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448
The input of the regulator should be connected via the relay contact to you V+ (i.e. not via the LED. The LED and its 330 Ohm resistor can be connected between the relay contact and GND). If you use the LM317 you can draw up to 1.5A while maintaining the same output voltage. It is worth mentioning that the minimum voltage from the LM317 is 1.25V

14. ### crutschow Expert

Mar 14, 2008
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3,369
The input current to the regulator is essentially the same as the output current.

15. ### Stonekaters Thread Starter New Member

Mar 16, 2016
9
0
I'll have to redo the hole thing, I need to figure out how to wire the regulators..

16. ### crutschow Expert

Mar 14, 2008
13,482
3,369
The regulator data sheet will tell you what you need to know.

17. ### hp1729 Well-Known Member

Nov 23, 2015
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I would suggest putting the LED and 330 ohm resistor in parallel with the pots.

18. ### hp1729 Well-Known Member

Nov 23, 2015
2,090
232

Two alternatives.

19. ### AlbertHall Well-Known Member

Jun 4, 2014
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The pots by themselves, or even with the help of an LM358 (typical short circuit output current 40mA) will struggle to power arduino projects. 40mA is only 4 LEDs.

20. ### Alec_t AAC Fanatic!

Sep 17, 2013
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1,135
The LM358 can't source or sink much current (~10-40mA) so isn't really suitable for powering projects.