How to set SPX variable output value?

SPX17, SPX78 and SPX79 are available with variable output by adding a resistor here:

SPX78 variable SPX78 variable SPX17 Rset SMD
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 use 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. See the chart below for values.

Calculate Rset in kΩ as

Typical values are

Vout Rset in Ω Nearest
1% value (Ohms)
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.


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