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Low Voltage Detector Circuit Calculations
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MSD:
Hi,

I'm designing a circuit where a micro-controller requires the detection of a low voltage situation so that it may store any unsaved data to its internal eeprom before the power goes out. The micro controller has a capacitor that it controls going into its vcc to keep it powered for a few milliseconds after a low voltage situation is detected.

I've found a scamatic and description of a circuit that can detect the low voltage situation at:
https://it.emcelettronica.com/tecniche-hardware-per-la-gestione-del-brown-out-reset-sui-microcontrollori-di-fascia-medio-bassa

I'm using the first variation of the circuit

The reset is going into a pin with an interrupt not directly to the microcontroller reset pin.

According to the website the equation used to calculate the threshold is given by:
Vt = (R1 + R2) (0.4 / R2)

I require a threshold at 4.7v. which gives me 12.9M ohm for R2 when R1 is 10M ohm. I've built the circuit but its dropping out at 5.15v
I've used 1% resistors everywhere the transistors are 2n3904 and 2n3906. I suspect that the issue has to do with the 0.3v hysteresis or the constant of 0.4 in the calculation may be different for the transistors used.

How do I change the hysteresis range and what should the constant be for 2n304/2n3906 transistors?

Thanks




Kasper:
Have you tried using smaller resistors? 1.29M and 1M. or 129K and 100K?

When you use 10M, other high impedance things can become significant for example connecting your meter can affect the circuit.
MSD:
The website suggested using 10M ohm (high values) for R1 so no I havent tried that. however with regards to the meter disrupting the readings, i doubt thats it. I'm reading the voltage directly of the power supply with with 50mV increments while using an led connected throught a transister and current limiting resistor of 100K ohm to detect on an off thresholds. I will try lower values and check back. does the constant in the equation change for 2n3904/2n3906 transistors? also what would the equation be for calculating hysteresis?

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T3sl4co1l:
What is your overall power architecture like?

A typical scenario is: higher voltage input (say 12V), 3.3V logic supply.  You can use a series diode to prevent the 12V supply from discharging through the source when it gets turned off.  That leaves your device on its own as voltage drops.

Say you need 5ms hold-up time to write EEPROM or whatever and verify shutdown.  If your load draws say 10mA, and the regulator drops out at 4V, then you have 12 - 4 = 8V of working room.  If it's a linear regulator, then the 10mA is drawn from the input capacitor, and I = C * dV/dt holds.  Solving for C, we need 6uF minimum.  If it's a switching regulator, more energy from the capacitor is used, and the value can be smaller (in practice, it may be enough just from the input noise filter alone).

Calculator:
https://www.seventransistorlabs.com/Calc/PSHoldUp.html

If you need more current, or more time, you need proportionally more capacitance.  It can get intensive, quite quickly!

As for detecting shutdown -- why not simply wire a voltage divider from +12V to an ADC input?  Or a comparator, if you have a logic rather than analog input handy.  That way, you detect power failing, long before VCC itself is affected.

If you don't have a high voltage input, your options are much more limited.  It may be worth using a wide-input switching regulator (SEPIC, say) to give you that much needed breathing room.  If you're doing it by VCC voltage alone, then you have perhaps a fraction of a volt (say 3.6V nominal, 2.7V shutdown: only 0.9V operating range!) in which to detect the panic condition, and do something about it.  Yes, you can brute-force it with ever larger capacitors, but it's usually the case that you can find other excuses to "do it right". :-+

Tim
David Hess:
The circuit relies on the switching Vbe threshold of T1 which is never going to be very accurate between devices or with temperature.  T1 should either be replaced by a differential pair or driven by a differential pair.  As shown T1 is prone to RF oscillation.

T2 and T3 form an SCR with access to its internal nodes.
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