Tuesday, 12 August 2014

Voltage Regulator – Breadboard

Originally, I intended to have two separate projects for voltage regulation (9 to 5V and another planned for 9 to 3.3V), but it turns out that there is very little difference between a circuit for 5V and 3.3V power regulation. The only practical difference is that, if you use an LM7805 transistor you will end up with 5V, or if you use aUA78M33C, that you will end up with 3.3V.

I bought the LM7805 from eBay, and the UA78M33C were purchased from element14. I am fairly happy with the 3.3V, however, it cost more for postage than it did for the transistors (more than $12 AUD while the transistors only cost $8 for 10 of them). Nevertheless, I have them and I’m OK with that.

I used the example breadboard for a power regulator that comes with the Fritzing application, you can go to their website if you want to see the layout (download it, it’s free).


There are 2 x 100nF capacitors, the Voltage Regulator and some jumpers, this is a very straight forward circuit, all that is happening is that voltage goes in through input (1) and comes out through output voltage (3). The capacitors are there to smooth the flow. I inadvertently used 100µF rather than 100nF, resulting in a lower output voltage (you can see that later). The circuit only ran for a couple of minutes so I wasn’t able to observe the longer term effects of using the wrong capacitance on the V-Reg.


I connected the circuit to a 9V battery and measured the output using my multimeter set to 20V.

Here is the multimeter when the circuit is not powered.

Multimeter - No Power

And here we are with the circuit powered.

Multimeter - Powered - LM7805

The voltage fluctuated between 4.96 and 4.97V, I expect that the variation and the <5V result was a consequence of the wrong capacitors.

When the power is disconnected, the delivered voltage bleeds out (as it is stored in the two capacitors). After 3 seconds, the output is 0.09V.

Multimeter - Powering Down - LM7805

I replaced the LM7805 with another from the same batch and got a slightly different result.

Multimeter - Powered - LM7805 - 02

But still in the range.

Then, I replaced the LM7805 with the UA78M33C, powered it up and measured the output again.

Multimeter - Powered - UA78M33C

When I build the circuit in anger, I’ll be using the correct capacitor (tee hee). Overall, this was a very simple experiment and it provides me with the circuit that I’ll need to power the ATTiny84 and ATTiny85 circuits that I’ll be building. The goal is to have a ready regulator circuit that will feed the three ATTiny8x circuits further up the food chain.

Well … actually … I retested with 100nF (0.1µF) and the results were the same. I guess the variation may be in the quality of the capacitor? Either way, I calculate that to be within approximately 1% of the target value (actually 0.6 – 0.8% for the 5V and 0.909% for the 3.3V), hopefully that will be sufficient.

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