Protecting 5V circuit analog output from high voltage shorts - Page 1
Forum section: electronics / projects

author: aiq25
author_rank: Regular Contributor
timestamp: October 24, 2017, 02:36:44 pm
content: Hello,I'm designing a circuit that that has a 5V VIN and has an analog output (it's a simple sensor). There is a possibility that the VIN could be 15V for a relatively long time, so I came up with a overvoltage protection circuitry as well as reverse polarity protection.There is also a change the output of the sensor could accidently be shorted to 15V (there is a second 15V supply on board). How can I protect the output of the circuit? The output is buffered using an op-amp. I'm worried if the op-amp VCC is 5V but VOUT is shorted to 15V then the op-amp will get damaged.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 24, 2017, 02:56:31 pm
content: One way (both on input and output) is to use a MOV (which will 'eat' excess voltage) for short transients. If you expect the problem to last longer than a few us, use PTC's to protect the MOVs. Keep in mind that designing with PTCs is a nightmare most of the time...

author: aiq25
author_rank: Regular Contributor
timestamp: October 24, 2017, 03:32:54 pm
content: Thanks.I'm thinking the overvoltage could last few seconds actually (or possibly more). That's why for the input protection I used a transistor based design to clamp the VCC of the circuit (VCC is VIN after the reverse polarity protection).I was thinking of using a MOV but I don't think it will work because the transients could last so long.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: October 24, 2017, 03:52:15 pm
content: How much current do you need to be delivered from the output? Would a simple resistor work, which would limit the current to a safe value (i.e 1K will limit the current at <15mA when 15V is connected to the output) ?CheersAlex

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2017, 04:32:20 pm
content: Quote from: aiq25 on October 24, 2017, 02:36:44 pmAdd a current limiting resistor (or depletion FET current regulator or incandescent bulb, see below) in series with the operational amplifier output and take feedback through another series resistor from the output. Add shunt protection diodes as necessary. Now the output and feedback paths are protected. Note however that this will limit the maximum output current at the output voltage extremes. If this is a problem, then the output voltage range of the operational amplifier will need to be increased or a different design used like a transconductance output with feedback.A couple ofdepletion mode MOSFETscould protect the output up to 100s of volts.Also, small high voltage incandescent bulbs work better than series resistors in this application. In the event of overload, they operate like resetable PTC (positive temperature coefficient) fuses but much faster.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 24, 2017, 07:14:43 pm
content: Quote from: aiq25 on October 24, 2017, 03:32:54 pmHence the PTC.

author: BravoV
author_rank: Super Contributor
timestamp: October 24, 2017, 07:20:25 pm
content: Quote from: Ice-Tea on October 24, 2017, 02:56:31 pmWhy is that ? Mind share an example ?

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 24, 2017, 07:50:13 pm
content: There are very large spreads/tolerances. If you take into account high and low temperatures it's often very hard balancing things out... On the one hand, it's not allowed to trip under any conditions below a certain current (the current it needs to 'hold') on the other hand it must trip under all conditions above a certain current (and within a certain time) or it will either be worth nothing as a protection or the (power) supply won't be able to trip it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2017, 02:03:45 am
content: A single diode.Assuming no other constraints (you didn't specify), this seems to satisfy the criteria.Tim

author: technix
author_rank: Super Contributor
timestamp: October 25, 2017, 05:18:01 am
content: If you are okay with higher input impedance, you can use reverse biased Schottky diode clamp and a current limit resistor. The diodes clamp the chip input to -0.3V to Vdd+0.3V range, and the resistor prevents the diodes from frying under prolonged exposure to out-of-range voltages.

author: aiq25
author_rank: Regular Contributor
timestamp: October 25, 2017, 01:54:42 pm
content: Quote from: T3sl4co1l on October 25, 2017, 02:03:45 amThanks. I went with a two diode approach. One inline with the VCC of the op-amp and the other as a clamp from the output of the op-amp to VCC pin of the op-amp (after the inline diode). This requires the use of an op-amp with a higher operating voltage since basically this configuration changes the operating voltage to the overvoltage on the output pin. Also would require Schottky diodes so that the op-amp is still within spec.