Sunday, 9 November 2014

Simple R/C Battery Eliminator (Voltage Regulator) using parts salvaged from ATX Power Supply

In the course of building a Tamiya Blitzer Beetle with my son, we realised that the receiver unit couldn't be connected directly to the main 7.2V battery because the receiver has a required input voltage of 4.5V to 6.0V.  I didn't want to add a 4xAA battery pack, so we got to work desiging a voltage regulator. 

We took some current measurements of the receiver and found it could draw up to around 800mA.

Design parameters:
  • Input voltage:     7.2V
  • Output voltage:  4.5V to 6.0V
  • Output current:  1.0A
For an added challenge I thought I'd see if I could build this circuit using parts I had lying around.  

The key to this circuit is the TL431 Programmable Shunt Regulator.  TL431 are very common in ATX power supplies and are essentially an adjustable voltage reference.  Here's the TL431 block diagram:

Now, In order to supply the 800mA current we needed (TL431 is only rated to 100mA) we added an NPN power transistor (KSC5027, also salvaged from the same ATX psu).  Pretty much any NPN transistor will do provided it has sufficient current rating.

Here's the circuit:

'Power Controller' is the 7.2V connection.

R1 and R2 were chosen for a designed output voltage of 5.0V.  

When the input voltage falls below about 5.6V, the output voltage will simply be input voltage less voltage drop across B-E of the transistor.  

At 5.59V input with 1.0A load we get 5.0V output.

At 7.2V input, the circuit provides 5.79V.  Looks like transistor B-E voltage drop is 0.79V at 1.0A load.

Although 10V won't be encountered in the R/C car I thought I'd see how the circuit handled this condition.  Still good, output voltage within the designed range of 4.5V to 6.0V.

And here's the assembled circuit, using 'high rise architecture' method. 
I put the whole thing in some large heatshrink before installing in the car.

Thanks for reading.  Have fun with electronics!

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