At 04:00 PM 2/27/2004 -0800, you wrote:
>Hi all,
>
>I have a project where a vacuum pump outputs a 22V signal indicating
>whether it is working or not.  If it malfunctions (0V) I need to stop the
>process.  The signal goes into a datalogger which is controlling everything
>(I wasn't allowed to use a PIC to set this up).  The datalogger needs a 5V
>signal, but I would like the pump to be electrically isolated because it's
>very expensive and a huge pain to fix/replace.  The way I decided to do
>this (one-time project) was use a 24V-5V DC/DC converter (DCP022405P).  I
>have a 2.2k resistor that goes from the converter's output to the digital
>input (it only tolerates very low current) and also a 500 ohm resistor that
>goes from the output to a LED for monitoring the status.  The inputs of the
>DC/DC converter come straight from the vacuum pump (the pump signal is
>current limited to about 50 mA).
>
>This worked great for about a month until someone unplugged the connections
>to move the setup around.  Then it wouldn't work -- somehow the chip just
>died.  I popped in another chip and it worked fine until the power went out
>last night.  When the power came back on the chip was fried again, no
>unplugging of anything.  I figured maybe some ESD the first time, but now
>I'm not sure, maybe the pump output goes screwy at times (it really
>shouldn't though).  Is there something I should be putting in between the
>pump output and the DC/DC converter input to protect it?  Some kind of
>resistor or RC combo?

The absolute maximum input voltage isn't very high- only 29VDC- really
designed for regulated inputs. Maybe a series resistor and a capacitor
in parallel with a TVS across the converter. Say 220uF/35V across the DC-DC
converter power with something like a 1.5KE27A in parallel and  20 ohms 1W
in series.
It won't guarantee the voltage will stay within the abs. max. rating
(there isn't enough margin for that) but it will likely be good enough.
If you want to be really sure, put about a 5V zener in series as so:

                             5V 1W Zener

                In  ___       /
                 o-|___|--+---|<---+------o +V DC-DC
                 20 ohms  |  /     |
                1W        - /     ---+ 220uF/35V
                    TVS / ^       ---
                          |        |
                         ===      ===
                         GND      GND


You'll be getting lower voltage out of the output though, so you
may have to fiddle something on the output.


>PS I thought about using an optoisolator and a 5V VCC, but the main power
>for everything is 12V.  So I would need a voltage regulator or zener to
>goto 5V for this IC.  Plus I had never used a DC/DC converter so I figured
>this was a good opportunity to play with one.

Usually dead parts mean there's an opportunity to learn something.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics