> A resistor for current limiting in serial and a 5V zener diode in parallel
> should do the whole Job of the DC/DC Converter!!

He wanted isolation.

An opto coupler will do the job easily and cheaply IF you have adequate
voltage/current at the receiving end.

A photo coupled FET coupler will do the job with no power per se at the
receive end
and only a small power requirement at the sending end.

If using the existing DC-DC converter the advice from others should work: ie
Place a zener diode (slightly above 24v) across the input plus add a series
input resistor that drops minimal current when the converter is operating.
(Say 100 ohms as a first guess). Maybe a 10 uF electrolytic at the converter
input as well. (Resistor depends on operating current. Spec sheet is not
useful for no load current. At 10 mA the 100r will drop 1 volt. Choose R to
drop no more than a volt or so. The resistor gives the protection zener some
input resistance to act as a voltage divider with. The capacitor slows fast
spikes somewhat while the zener thinks about getting involved.

With the resistor in place as above you could use a zener equal to or only
very slightly higher than the pump supply voltage. This gives you as much
headroom as possible for voltage spikes. The converter is only MEANT to
operate on 26.4v max and has an absolute maximum input voltage of 29v. It
should be quite easy to kill it with a voltage spike.

V ----RRRR------------------ Converter_Input
                          |     |
                         Z   C
                         Z    |
                         Z    |
                          |     |
--------------------------------

R = ? 100 ohm
C = 10 uF say.

Paste into notepad if the above diagram is a mess.



        RM

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