# 7805 voltage regulator

Discussion in 'The Projects Forum' started by pashford, Jan 5, 2010.

1. ### pashford Thread Starter New Member

Dec 5, 2009
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0
Hi,
I am newbie and I have been reading a tutorial on the 7805 voltage regulator. I have created a simple circuit to test whether I am grasping the concept or not. The thing that I really don't get is tying the grounds from both sides (9V and 5V) of the circuit together. I have attached a pdf with the circuit diagram (to which I have added a resistor and a led in my breadboard) and two different angled photos of the breadboard. The input voltage is 9V coming from the battery clip. I am just wondering if everything appears correct before I apply power. The capacitor on the input pin of the 7805 is 100 uF and on the output pin it is 10 uF.

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2. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
The regulator schematic and wiring looks OK. However, you may find that you need a small (0.1uF or 100nF) cap on the output of the regulator as well. These regulators can oscillate at high frequency if you don't have that small cap.

Also, looks like you're powering a red LED from the output of the regulator. Looks like you have a 1.2k Ohm (brown, red, red) resistor in series with the LED.
That will probably turn out to be too high of a value of resistance.

Calculating a current limiting resistor for an LED:
Rlimit >= (Vsupply - Vf_LED) / Desired_Current
where:
Vsupply = the voltage supply across your resistor and LED
Vf_LED = the rated forward voltage of your LED at the desired current

Let's say you wanted 15mA current, and your LED was rated at 2v for 15mA current. Your Vsupply is 5v.
Rlimit >= (5v - 2v)/15mA
Rlimit >= 3v/0.015 A
Rlimit >= 200 Ohms. 200 Ohms is a standard value of resistance. If it were not, we'd use the next higher standard value of resistance.
There is a chart of standard resistance values here:
http://www.logwell.com/tech/components/resistor_values.html

3. ### pashford Thread Starter New Member

Dec 5, 2009
7
0
Thank you so much for your help. I switched to a 220 Ohm resistor and connected the battery, but nothing happened. I played around with my multimeter for a while and I think I must have a faulty component somewhere in the circuit. At least now I have a starting point to figure out what is wrong.

4. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Your switch might not be any good.

Try connecting your battery + to the input of the regulator.

5. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
Also make sure the LED is installed in the proper direction. In your circuit, the long lead should be tied to the resistor and short lead to the ground bus.
Edit.... If you have clipped the leads to the same length to better fit your bread board, check for a flat spot on one side near the base. That will go toward the ground in your circuit.