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Protect selfmade PSU sense input from reverse voltage, etc.
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nemail2:

--- Quote from: Kleinstein on July 06, 2019, 03:06:24 pm ---R39 can be quite a bit larger, as the ADC is rather high impedance and the signal is not fast anyway. So some 2.2 K or so should be OK. One could add additional clamping at the base of T10 (e.g. a 3.6-5.1 V zener to ground).
Additional diodes directly at the ADC input may not help very much, as the chip internal "diodes" (usually actually parasitic path to the substrate) often have a relatively low forward voltage.
It could make sense to have a diode directly at the input of the OP to avoid a large negative voltage, just in case.

Depending on the supply current to the ADC, one could consider using a 50 Ohms or similar resistor instead of the ferrite bead - this would limit the current in case of a latch-up and reduce possible damage.

--- End quote ---
Thanks!

I was thinking about increasing R39 as well. Supply current of the ADC when operating is max. 300µA, according to the datasheet.
Regarding the diodes at the ADC input the datasheet says this:

--- Quote ---Keep the absolute voltage of any input within the range shown in Equation 3 to prevent the ESD diodes from turning on.
GND – 0.3 V < V(AINX) < VDD + 0.3 V (3)
If the voltages on the input pins can potentially violate these conditions, use external Schottky diodes and series resistors to limit the input current to safe values (see the Absolute Maximum Ratings table).
--- End quote ---
So I thought, additional diodes would be a good idea. But then those diodes would not protect the opamp inputs and voltage could go there and make damage...

I have a reverse diode on the output of the PSU but that diode gets disconnected if the relais is turned off. So in this case the sense input of the ADC is connected with the load (which might be a fan or a coil) without any reverse biased diode. I guess that is one huge mistake I made. Actually, the relais should disconnect the sense wires from the sense input as well or there should at least be some protection of the sense input like a reverse diode..

So what I'll try (currently building up the sense stage on a breadboard):
- increase R39
- zener at T10 base (actually: what would that do? according to my understanding the npn is current driven so if it conducts or not - and how much - can't be nailed down to a specific voltage if I'm correct?)
- maybe still try some diodes at the ADC inputs, as the datasheet suggests that? or would that be pointless in this context?
- replace L6 with a resistor (or put a resistor in series?)
nemail2:
what about clamping Pin 3 of IC11A to 3.3V? Output voltage of the PSU is 16.384V max. so after the 10:1 voltage divider there never should be more than 1,6384V.
3.3V + Vf of the clamping diodes is something which the ADC inputs should withstand, especially with the raised value R39.

What do you think about that approach?
What about SENSE_GND pin, which is directly connected to the ADC input pin 5? If some motor induces voltage into that pin, the ADC would go the way of the dodo, wouldn't it?

Thanks, as always I'm very grateful for any advice.
Kleinstein:
Clamping before the OP is a little tricky, as small leakage currents matter. It is much easier to do clamping at the output of the OP, before the transistor. The BJT acts as a kind of voltage follower, not as a switch. So the output at the emitter is never higher than the base, normally some 0.5 V lower. So clamping at the base works as well.  At least for a short time the OP is short circuit proof, so that a simple zener to ground is OK here - one could add a series resistor if one wants to avoid a very high supply current for the OP.  With a maximum of some 1.7 V under normal operation the limit could be at 2.7 V already - the OP will compensate for initial leakage.

The OP is already reasonable protected from the 90 K resistor. It is not a perfect protection, but not that bad either. Normally one would use larger resistors here, more like 500 K and possibly a few resistors in series as the voltage per resistor is limited.

The GND side could indeed be a problem too. It depends on the layout. A similar resistor like R39 and cap could be a good idea at least. If not directly connected some clamping could be needed.
floobydust:
I'm not sure why there is an emitter-follower after the op-amp, I don't see it necessary especially if R39 is increased in value as it needs to be.
I don't like running an A/D on 3.3V yet the op-amp ahead is on a 15V rail. You have to be careful which rail comes up first and collapses last, which may be why your fan caused things to blow up.

9.1.3 Input Protection
"The ADS101x are fabricated in a small-geometry, low-voltage process. The analog inputs feature protection diodes to the supply rails. However, the current-handling ability of these diodes is limited, and the ADS101x can be permanently damaged by analog input voltages that exceed approximately 300 mV beyond the rails for extended periods. One way to protect against overvoltage is to place current-limiting resistors on the input lines. The ADS101x analog inputs can withstand continuous currents as large as 10 mA."

The A/D can take 10mA, so R39>1.1k assuming 10mA per channel does not lift up the 3.3V rail.
nemail2:

--- Quote from: Kleinstein on July 07, 2019, 08:40:57 pm ---Clamping before the OP is a little tricky, as small leakage currents matter. It is much easier to do clamping at the output of the OP, before the transistor. The BJT acts as a kind of voltage follower, not as a switch. So the output at the emitter is never higher than the base, normally some 0.5 V lower. So clamping at the base works as well.  At least for a short time the OP is short circuit proof, so that a simple zener to ground is OK here - one could add a series resistor if one wants to avoid a very high supply current for the OP.  With a maximum of some 1.7 V under normal operation the limit could be at 2.7 V already - the OP will compensate for initial leakage.

The OP is already reasonable protected from the 90 K resistor. It is not a perfect protection, but not that bad either. Normally one would use larger resistors here, more like 500 K and possibly a few resistors in series as the voltage per resistor is limited.

The GND side could indeed be a problem too. It depends on the layout. A similar resistor like R39 and cap could be a good idea at least. If not directly connected some clamping could be needed.

--- End quote ---

Thanks! I'm gonna draw that into the schematic and post it here ASAP (already working on it) with my humble plea whether you could check it if i understood it correctly.
So normally the sense input voltage divider would be much higher than 90.9k? I thought that was quite high already.. Again I learned something..


--- Quote from: floobydust on July 07, 2019, 09:00:38 pm ---I'm not sure why there is an emitter-follower after the op-amp, I don't see it necessary especially if R39 is increased in value as it needs to be.
I don't like running an A/D on 3.3V yet the op-amp ahead is on a 15V rail. You have to be careful which rail comes up first and collapses last, which may be why your fan caused things to blow up.

9.1.3 Input Protection
"The ADS101x are fabricated in a small-geometry, low-voltage process. The analog inputs feature protection diodes to the supply rails. However, the current-handling ability of these diodes is limited, and the ADS101x can be permanently damaged by analog input voltages that exceed approximately 300 mV beyond the rails for extended periods. One way to protect against overvoltage is to place current-limiting resistors on the input lines. The ADS101x analog inputs can withstand continuous currents as large as 10 mA."

The A/D can take 10mA, so R39>1.1k assuming 10mA per channel does not lift up the 3.3V rail.

--- End quote ---

Yeah, I'm going to increase that R39. Stupid me didn't really do the math on this one...
The emitter-follower is to raise the working voltage of the opamp a bit. It struggles to output voltages near 0 mV which is what i need for accurate ADC readings at low voltages.
The emitter-follower forces the opamp to output at least the transistors forward drop voltage to get 0mV on the feedback pin.
I agree that 15V on the NPN is not ideal, 3.3V should work as well, right? That would mean that even if the opamp outputs 15V at pin 1, the ADC will see 3.3V at max without anything taking damage - or am I misunderstanding that? Will actually the voltage get from the base through to the emitter and be higher than 3.3V?
I'm sorry, I'm really not a pro yet at transistors - everything self-taught and still the learning curve is very steep.

Thanks!
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