Charge indicator with comparator - very sensitive to noise

Question

I need an indicator when the current from my charger is larger than 100mA. I tried the below circuit but this seems very sensitivity to noise. Probably I am getting some ripple in the mV range from the charger.

Is there a more robust, less sensitive way to set this up?

score 2 · Answer 1 · answered May 01 '20 at 15:55

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Yes, there is, add hysteresis to the comparator so the ripple is less likely to trigger the comparator from one state to the next. I used a circuit like this when I had a lot of ripple on the inputs. If your prototyping this, make sure you have good grounds and Vcc to the amp with bypass capacitors

Source: https://www.electronics-tutorials.ws/opamp/op-amp-comparator.html

answered May 01 '20 at 15:55

Voltage Spike

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Thank you. Does it help to place any other capacitors except on VCC? – Reto May 01 '20 at 16:43
If you have a negative rail, or -Vcc then you'll need a bypass capacitor for that also – Voltage Spike May 01 '20 at 16:48

score 2 · Accepted Answer · answered May 01 '20 at 17:05

2

You can try something along these lines:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

R7 and C1 form a low-pass filter with a cutoff frequency of 1.6Hz to filter out load spikes and SMPS noise.

R6 and R9 provide some positive feedback amounting to about 5mA hysteresis. Increase R9 if you want to increase the hysteresis.

R10/C2 balance out any bias current, and can be eliminated if the bias current is small enough.

The op-amp needs to be a low Vos type (your whole signal is ~3mV), high voltage power supply capable and with an input common mode range that includes the positive supply rail. There are some such op-amps available, but only a few.

answered May 01 '20 at 17:05

Spehro Pefhany

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This is what I have: https://www.ti.com/lit/ds/symlink/tlv1805.pdf Should be ok what I see... – Reto May 01 '20 at 18:07
No filter needed at the non-inverting input? – Reto May 01 '20 at 18:13
You don't want a filter there, you want the feedback to take place as fast as possible. – Spehro Pefhany May 01 '20 at 18:14
And what about filtering at the voltage divider R3/R4? – Reto May 01 '20 at 18:16
That comparator has built in hysteresis so you don't need to add it. Unfortunately, it's fixed at 14mV nominal which corresponds to 424mA. Also the offset voltage (error) is +/-6.5mV or about +/-200mA, so it's nowhere near useful. – Spehro Pefhany May 01 '20 at 18:18
Not easy to find... what about this: https://www.mouser.ch/datasheet/2/609/LT1716-1716222.pdf – Reto May 01 '20 at 19:44
Pretty big Vos. Try OPA192. – Spehro Pefhany May 01 '20 at 19:46

analogsystemsrf · Answer 3 · 2020-05-02T15:43:16.723

Add a tiny bit of hysteresis. [ edit: And increase the R_sense to ONE OHM. Plenty of headroom in that 29 volts.]

Insert 10 kΩ below that 100 kΩ resistor. That will have 2.9 V across it.

Now decide how much you want the voltage across the 15 Ω to vary.....1mV, 5mV?

If 1mV, you need 100,000/15 = 6,000:1 more variation, requiring 6 V variation at the top of the 10 kΩ you just added.

Now add a SECOND resistor from comparator output to that 10 kΩ.

score 0 · Answer 4 · answered May 01 '20 at 16:00

0

You can also add low pass filtering to the input of the opamp as in this answer: Using low pass filter for a differential ended receiver system?

answered May 01 '20 at 16:00

Aaron

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Charge indicator with comparator - very sensitive to noise

4 Answers4