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LCD screen power supply

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elsey.jack

New Member
Hello everyone,

I am using an LCD screen to display the temperature. I have a PICAXE (i.e., a PIC) microcontroller talking with the LCD screen. The prototype that I built on a breadboard worked perfectly.

I assembled the final version on protoboard last night, but now I am getting strange behavior from the microcontroller! When the cables to the LCD are connected the microcontroller won't interface with the computer and seems to reset itself erratically when powered up. (Nothing is displayed on the LCD screen either.) The microcontroller works fine if the cables to the LCD screen aren't connected.

I double checked all my wiring and soldering. Everything seems to be connected to where it is supposed to be.

The things that changed between the breadboard prototype and the protoboard version are as follows.

1. Switched 7805 voltage regulators. I didn't want to use the one I had in the prototype so I got another one from Radioshack. It seems to function correctly, supplying 4.97 V.

2. Switched power supply from a AC-DC 5V adapter to 6 AA batteries. In the breadboard prototype I had two capacitors to smooth out the output voltage from the 7805 regulator. I had a 100 uF capacitor on the input pin and a 10 uF capacitor on the output pin. These capacitors aren't present in the protoboard version.

Could it be the lack of capacitors? Strangely, the circuit began to work when I took a 10 uF capacitor and held it between the input pin and ground of the 7805 regulator.

Thanks,
Jack
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Could it be the lack of capacitors? Strangely, the circuit began to work when I took a 10 uF capacitor and held it between the input pin and ground of the 7805 regulator.

Yes, the two capacitors round a 7805 are absolutely essential, if you leave them out you WILL have problems, perhaps not immediately, but certainly in the future (depending on supply, load, temperature etc.).

So nothing 'strange' you left out a vital component, and it worked when you added it.
 

Electronworks

New Member
The input capacitor is essential, especially if you are going to change from one 7805 to another.

I had the same problem - 7805 giving out 6V. Put a 47uF on the front end and all was hunky dory. Looking at the Vin signal, there was a lot of ac, even though I was running from a bench power supply. 47uF removed this.

Also, time for dumb question of the week: what was the output of your 5V power supply.....?

If it was only 5V then the 7805 will not have had enough drop across it to regulate. If it was >7V then it would have been OK.

6 batteries will generate your >7V

:D
 

elsey.jack

New Member
The input capacitor is essential, especially if you are going to change from one 7805 to another.

I had the same problem - 7805 giving out 6V. Put a 47uF on the front end and all was hunky dory. Looking at the Vin signal, there was a lot of ac, even though I was running from a bench power supply. 47uF removed this.

Also, time for dumb question of the week: what was the output of your 5V power supply.....?

If it was only 5V then the 7805 will not have had enough drop across it to regulate. If it was >7V then it would have been OK.

6 batteries will generate your >7V

:D

My problem occurred after I switched from the 5V power supply to batteries. I thought I only needed the capacitors to keep the output voltage smooth, but I guess I was wrong. I'll try installing some capacitors to see if that fixes my problem.
 
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