Optical Isolation & Other Counter Transient measures


Thread Starter

Neoh KS

Hi folks:

Am trying to incorporate an RS232 and an RS485 interface into a new product that I have been assigned. These must be built into the product on the same pcb as the rest of the components.

Questions :
1) What kind of opto-isolators are suitable to go along with these ?
2) Is it necessary to have types with Schmidt trigger outputs ?

In one of the commercially available RS232-RS485 convertor, I found also a rectangular boxy component, with the marking "CTC" on it. It is 6 x 19 x 10mm . Other markings are "MSV0505". I wonder if Control Technology Corp has anything to do with it , and what does it do ?

Any comments would be appreciated.

The majority of low cost opto-isolators on the market are fast enough to
work up to the kinds of speeds RS232 gets used at.

They come in single, dual and quad packages so you'll need to pick one from
a range based on how many signals you want to isolate.

Check out HP (I don't have their URL to hand) and look for devices in the
HCPL0600 family. There are thru hole and surface mount devices in the

Dual in line packages can give you up to about 5000v isolation, surface
mount generally doesn't have the same levels of isolation because the
physical distance across the package from the "in" to the "out" is not as

BUT the thing to remember is you need two seperate power supplies to run the
RS232/485 transceivers on either side of the isolation boundary. If you use
the same supplies on either side of the isolation boundary then the
opto-isolators will be useless.

The MSV0505 is a 5v to 5v DC:DC converter which is there to generate an
isolated +5v supply from the "non-isolated" side power supply.

This is then used to power the RS-232 transceiver chip on the "isolated"
side of the isolation boundary.

The point to watch here is what your isolation requirements are.I think the
device you mention is only isolated to about 500v (if that) and hence it
will become the weak link in your isolation.

Check out Newport Components www.newport-comps.com They have a variety of
low cost converters in thru hole and surface mount packages. Have a look at
their NTV series which is surface mount with 3kv isolation.

Otherwise, if this is part of a bigger product you may need an isolated
supply elsewhere, in which case you can get isolated volts there.

Alternatively again, if you don't want the expense of the DCDC converter you
could consider deriving a supply for the isolated side from the RS-232 line
on the isolated side, but there are pro's and con's to this and it isn't
really an option for RS-485.

Whatever you do, remember the isolation boundary must be maintained,
otherwise you're wasting your time.

So when all your fine design work is complete make sure whoever lays out the
printed circuit board understands that you need to maintain maximum
seperation between trackwork and components associated with the two sides of
the isolation boundary.

Hope this helps

Geoff Moore

Straight Forward Solutions Ltd
Maynooth Road, Prosperous, Naas
Co.Kildare, Ireland
+353 (0)45 892739
[email protected]
Dear Neohks , I happen to be the distributor of this MSV-0505 with brand CTC and this is a DC/DC converter with 5V to 5V/200mA 3KVdc isolation .. Are you interested to know more about it ? Could you tell me something about this system ? Awaiting your further , we remain , with best regards , David