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| Protect selfmade PSU sense input from reverse voltage, etc. |
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| nemail2:
Hi there again! Some may have noticed my work on my self made Lab PSU: https://github.com/mamama1/LabPSU_Darlington (online a bit outdated version) I was wondering how I'd protect the differential input of my ADC which I implemented for sense wire terminals on my lab PSU. I imagine, some motor back EMF or reverse voltage might fry the opamp and/or the ADC, in the worst case. attached you'll find screenshots of the relevant part of the board + schematic. How is this normally done? just a diode in reverse + something like diode clamping? please keep in mind that this is a fairly precise PSU with millivolts and milliamps so introducing measurement errors in the 1mA or 1mV range or even higher by the protection circuitry isn't really an option... thanks! edit: i have updated the github version so now all the latest source files are online, including complete pdf schematic and board preview png. edit2: just to get an idea of how it actually looks like, i have attached two photos of the currently built up (previous revision) of this PSU where I have bodged in a ADS1115 ADC on a breadboard. it actually works already quite well but I'm kinda optimizing and redesigning the crap out of it. partially to get better precision and partially just for kicks. |
| floobydust:
R14 limits currents into the OPA2197 op-amp which has input protection, so I don't see a protection issue with voltage sense. You should try the other common trouble scenario, where power is off or setpoint low V but a battery is connected to the output. Example 12V battery and power outage, or you're on but set at 2V, or power is off. Backfeed through D3/D2/D1, and into IC2A, sometimes this damages the driver op-amp. Where does PROT connector go? |
| nemail2:
--- Quote from: floobydust on June 11, 2019, 01:26:08 am ---R14 limits currents into the OPA2197 op-amp which has input protection, so I don't see a protection issue with voltage sense. You should try the other common trouble scenario, where power is off or setpoint low V but a battery is connected to the output. Example 12V battery and power outage, or you're on but set at 2V, or power is off. Backfeed through D3/D2/D1, and into IC2A, sometimes this damages the driver op-amp. Where does PROT connector go? --- End quote --- Hi thanks for you reply and sorry for the late response, I was on vacation + a bit sick as well, unfortunately. Regarding the topic: What about reverse polarity into the opamp/respectively into the ADC? the ADC will see positive voltage where GND should be and the opamp would see GND where a positive voltage should be... I have updated the design, the negative sense terminal isn't connected to AGND anymore but it is directly connected to one of the differential input pins of the ADC (which was connected to AGND previously as well). regarding the other scenarios: power off is no issue because the output relais would be open without voltage. a low setpoint but connected battery with higher voltage would be an issue, though. How'd I protect that PSU from this scenario? Where to start? I_PROT is a jumper which allows me to basically disable CC mode completely. When I disconnect that jumper, IC2B never gets to kick in to limit the current. Just for debugging and measuring, that jumper is normally ALWAYS closed. |
| cur8xgo:
it don't matter what happens on isense (polarity reversed or not) because it aint going to be hurting anything through 90000 ohms especially not the opamp input protection diodes |
| nemail2:
i have slightly changed the circuit. what if one applies like 12V directly to the ADC input pin (SENSE_GND header) and GND to the 90.9k Opamp input (SENSE header)? That would be @ reverse polarity of the sense terminals. Would the 90.9k resistor still protect the ADC input (pin 5, A0-)? The ADC can only tolerate up to it's VCC at it's input pins... |
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