Supply generation using PWMOUT

[xstamp]

Analog circuitry and sensors often need to operate from a voltage that is greater than the PICAXE supply. I have found a free-running PWM output to be a useful means of generating such voltages. For example, I needed about 9 volts to operate an AD 590 temperature -to- current sensor from a 5 volt PICAXE supply and was able to obtain this using two 10uF capacitors and two Schottky diodes, connected in the standard voltage doubler configuration and driven from a 50:50 PWM output.

This arrangement has no problem providing 9.5 volts (drop is due to series the diode) at 2mA. The frequency of operation does not seem to be critical, with 10 KHz giving about 2mV of ripple. Increasing the load to 4mA reduced the supply to 8.7 volts and increased the ripple to about 15mV. Fortunately, modern sensors and op-amps have current requirements that are much less than this.

It is also possible to generate negative voltages using similar techniques.

 

[lbenson]

I searched the web for "voltage doubler" and got, among others, a "Greinacher voltage doubler circuit". Substituting your values and the 50:50 PWM output, is this the circuit you used?
<code><pre><font size=2 face='Courier'>
pinx -----o--&gt;|----o----o +9.5V @ 2mA
50:50PWM | | +8.7V @ 4mA
| --- 10uF
| ---
| |
0V -------)--------o
| |
| --- 10uF
| ---
| |
'--|&lt;----o----o 0V
</font></pre></code>

 

[xstamp]

No, I used the configurstion shown in the following link

www.reconnsworld.com/forum/read.php?15,7,340

Just replace the 555 drive with a PICAXE PWM output and reduce the capacitors by at least a factor of ten (and make sure you get the polarity correct as shown). The diodes could be small BAT51 types.

555 application notes are probably a good source of charge pump circuits for doublers and negative supply generation.

 

[BeanieBots]

It can also be used to generate a negative rail, also very useful with many analog/op-amp circuits.

 

[lbenson]

OK, is this the effective circuit?

voltage doubler circuit: &quot;Supply generation using PWMOUT&quot;
<code><pre><font size=2 face='Courier'>
GND------o------------.
| |
--- 22uF |
---+ |
| |
+5V------o-----. |
| |
V --- 10uF
+ - ---+
|| | |
pinx -----||---o--&gt;|--o--- +9.5V @ 2mA
50:50PWM || +8.7V @ 4mA
10uF
</font></pre></code>
diodes: ex. 1N4002

 

[xstamp]

Yes, and thanks for going to the trouble of producing an ASCII drawing. I am still holding out for this forum to support schematics and complete projects, as promised some months ago!

The 20uF decoupling capacitor across the rails is not an integral part of the doubler circuit but probably should be there for the PICAXE anyway.

Only problem with your choice of diode is that it drops more voltage than a Schottky type would.

One last thing about this form of supply generation, its very easily turned
onandoffable under software control.