K
Klaus Kragelund
- Jan 1, 1970
- 0
Klaus said:LTC2862, can take 60V indefinitely, but you need to add clampingKlaus Kragelund wrote:
Klaus Kragelund wrote:
[...]
Another idea is to use the Bourns CDSOT23-SM712, specifically designet
for surge protection for RS485 devices, but add ceramic caps in series
with each line connection so it can tolerate 30V without creating
wonderful smoke, but will be able to clamp surge pulses without
affecting the high speed bus.
Regards
Klaus
Like this:
www.electronicsdesign.dk/tmp/RS485_cap_protection.pdf
But that would not protect against a hard 30VDC applied because an
installer miswired something. It could cause your RS485 chip to go PHUT
unless it has internal protection against this.
The RS485 IC has +/-60V protection rating, so its ok
Which one do you use? The ones on mine (clients's choice) have abs max
ratings of -8V to +12V, and no internal circuitry given.circuit to the VDD node since it will dump current into that node ifit is in transmit mode and is subjected to back fed voltage.
Wow, that sure is the Rolls-Royce of RS485 chips. With a corresponding
price tag
But isn't that bleed-through only an issue if the ground of the LTC2862
has come off? Otherwise this would really be a problem because you'd
just have moved the dissipation from one place to another. I think the
only real protection for 30V continuously is some sort of cut-off,
whether inside or outside a chip. At least a partial one where the
current becomes very small.
When the device is in transmit mode, it has no way of knowing if an appliedvoltage is just a load or a fault. So it has current limit on the outputs and if sourcing into the VDD is detected, the driver is shut off. If 5V is supplied to the device externally, only the current limit is active and thecurrent is steered to the GND or VDD rail.
Regards
Klaus