Protect Low impedance (220ohm) Sensor output?? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: March 04, 2017, 07:46:44 pm
content: I have a thermistor sensor output on a device, and that thermistor needs a 220ohm pull up resistor to the ADC reference voltage (5v in this case)So far, so good, but there is a none zero possibility that a mis-wire, or fault, could accidentally impose the system battery voltage (24V nominal, but up to ~30v continuous) on to that thermistor output! The 5v ref supply is clamped with a decent sized TVS to around 6V, so the poor little pull up resistor would have about 24v across it, resulting in it dissipating 2.6 watts! Ouch, which is going to kill it.So possible options to protect that output from continuous connection to Vbatt:1) Use a diode to prevent any reverse current coming into the output. But this brings the highly temperature / current dependent voltage drop of the diode into the system, which will mess up the precision measurement of the thermistor resistance2) Use a PTC fuse. Better, but a typical 50mA PTC, has a non-tripper series resistance of between (according to some datasheets i've looked at) 2.8 and 50 ohms. That again is enough to skew the results, especially if that resistance vary with temperature (which it will, being a PTC device!) and with manufacturing tollerances etc.Both those options can be improved by putting them between the output of a unity gain op amp, with the non-inverting input referenced to down stream of the diode or PTC. But that will take a power rail above the 5v reference voltage, and i haven't got one of those. I could add one of course, but i'd rather not.So, have i missed anything? Is there a better way of achieving this protection? (the ideal case is to have a precisely 220ohm output resistance to the 5v reference rail no matter what protection system is put in place)

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 04, 2017, 08:02:16 pm
content: 1) I hope that the ADC input has a good protection.2) You can use a DMOS device, like BSS139, to protect and limit current on the input? if you can tolerate ~20 Ohm of a temperature dependant resistance in series with 220 Ohm. A larger device, like BSP149, could drop that resistance down to <3 Ohm. As you may see from the attachment, the current in case of an overvoltage on the input, is limited to ~5.5mA and safe for ~50V continuous and 200V instantaneous (<20ms) overload. With BSP149 the maximum overload current should stay under 10mA.CheersAlex

author: David Hess
author_rank: Super Contributor
timestamp: March 04, 2017, 08:22:25 pm
content: One solution and what I would probably do is replace the 220 ohm resistor with a precision current source. Then the diode can be added in series with the output to protect it and its variable voltage drop becomes irrelevant.If the negative compliance voltage of the current source is high, then it will be protected against high negative voltages as well.A faster acting replacement for a PTC resetable fuse is a small incandescent light bulb.

author: max_torque
author_rank: Super Contributor
timestamp: March 05, 2017, 11:50:05 am
content: I can't use a constant current output because the thermistor has a very wide resistance range over it's operating temperature profile, which would make the resulting output voltage a very wide span.I think the NMOS blocking FET option looks good, i'll have a mess around with some circuits and see what happens!Thanks all