Variable SPX
SPX17, SPX78 and SPX79 are available with variable output by adding a resistor here:
When shipped, variable SPX has Vout=30V, so add a resistor to set the correct voltage before you connect it and power it on!
The only difference between variable and fixed Vout is if we add the extra resistor or if you add it. We add a thin film 0603 SMD, you can add same or a ⅛W resistor of your technology choice. The additional Rset resistor is in parallel with a 10k resistor so Vout change is not linear with Rset change. You can also add a 4th pin and put a fixed resistor or potentiometer on the main PCB. See the chart below for values.
Calculate Rset in kΩ as
Typical values are
| Vout | Rset in Ω | Nearest 1% value (Ohms) |
Nominal Vout |
| 5 | 0 | 0 | 5 |
| 6.3 | 548.52 | 549 | 6.3 |
| 7 | 869.57 | 866 | 6.99 |
| 8 | 1363.64 | 1370 | 8.01 |
| 9 | 1904.76 | 1910 | 9.01 |
| 10 | 2500 | 2490 | 9.98 |
| 12 | 3888.89 | 3900 | 12.01 |
| 12.6 | 4367.82 | 4320 | 12.54 |
| 15 | 6666.67 | 6650 | 14.98 |
| 16 | 7857.14 | 7870 | 16.01 |
| 18 | 10833.33 | 10700 | 17.92 |
| 20 | 15000 | 15000 | 20 |
| 24 | 31666.67 | 31600 | 23.99 |
| 30 | none | none | 30 |
You can use a potentiometer to set Vout. A 50k trimmer will adjust from 25.8V to 5V, a 100k trimmer will adjust from 27.7V to 5V. The trimmer is in parallel with a 10k resistor so Vout change is not linear with resistance change.