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Need to vary PSU from 24V to 27V

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cupoftea

Hi,
We want to vary the vout of RSP-1500-27 PSU from 24V to 27V with a pot.
So from page 4 of datasheet, we need a pot which twiddles from approx 1k5 to infinite?
Could you shed any light on this?

Datasheet doesnt offer much guidance from 90% to 100%...might there be a big tolerance also in this region?

RSP-1500-27 PSU datasheet

Vo = mRpot + Voff, Rpot in ohms, m in Volts/Ohm, so at 0 ohms (assuming you have the 27V supply)

so Voff at 0 ohms = .7 x 27 volts = 18.9V

@ 1.8K ohms

(.9)(27) = m1800 + Voff = 1800m + 18.9 = 24.3

So m = (24.3 – 18.9) / 1800 = .003

So we have Vout = .003 x Rpot + 18.9 or Rpot = (Vout – 18.9) /.003

So at 24V Rpot = (24 – 18.9) / .003 = 1700 ohms
So at 27V Rpot = (27 – 18.9) / .003 = 2700 ohms

So pot wants to range from 1700 ohms to 2700 ohms,.

So use a series R of 1700 ohms with 1000 ohms pot.

This is approximate because their graph shows linear behavior, except 100% out of supply is
infinite R which is not linear with the graph.

Regards, Dana.

Datasheet doesnt offer much guidance from 90% to 100%...might there be a big tolerance also in this region?
Rather systematic nonlinearity than tolerance. But you can't estimate the behaviour without assuming a circuit model. An obvious model would be a pot with an internal series resistor put in the voltage divider. Enough points to fit divider ratio, divider impedance and series resistor. Or assume a divider for the reference voltage.

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