Power Supply Monitoring Circuit

[steve001]

Afternoon All,

Following another post on here that needed to know a low power supply voltage and trigger an event.
This got me thinking .....

I came up with this circuit - please see attached

The incoming supply DC voltage 7 - 9 V DC is monitored and if low the comparator output goes low, this could then be connected to an Interrupt pin on the micro and used to trigger a "Event"
The 3 x 470 uF Capacitors hold up the supply hopefully long enough to the "Event" to take place

This is only a rough and ready test circuit R1 & R2 possibly need looking at at the output went low when the DC input voltage was at around 4.9 V DC

Hope somebody finds this useful

Steve

[steve001]

Noticed last night if anybody wanting to have a go with this i have put the voltage reference in the wrong place
I would normally place it after the voltage regulator

The circuit works on a bread board as described above, R1 & R2 may need adjusting experience shows that voltage dividers on breadboards tend to give incorrect results due to the connections resistance.

I plan to have a further look at this circuit later and draw it up properly.

Steve

Edit : Change the 2K2 resistor on the TL431C to 390 Ohms when powered from the 5 Volt Rail
The Resistors i used are MF25 Series From Farnell

[steve001]

Afternoon All,

Done a few tests with various capacitors using A PIC16C710 monitoring a DC voltage and turns on or off some transistors to drive a relay (@12V) nothing too spectacular. This current was measured at 3.9mA with a fluke meter. = 0.0195 W.

so using an electronic load in constant Power mode set at 0.02W and a timer circuit to start and stop on a low voltage detect, and disconect the incoming supply. Supplying the various capacitors at varying voltages (Pre Regulator)

1 x 1000uf

Test 1 @ 7.5 Volts

0.32 seconds

Test 2 @ 9 Volts

0.02 W 0.41 seconds

Test 3 @ 12 Volts

0.02 W 0.60 seconds

Test 4 @ 15 Volts

0.02 W 0.87 seconds

====================================

2 x 1000uf

Test 5 @ 7.5 Volts

0.36 seconds

Test 6 @ 9 Volts

0.55 seconds

Test 7 @ 12 Volts

0.87 seconds

Test 8 @ 15 Volts

1.27 seconds

====================================

3 x 1000uf

Test 9 @ 7.5 Volts

0.50 seconds

Test 10 @ 9 Volts

0.69 seconds

Test 11 @ 12 Volts

1.16 seconds

Test 12 @ 15 Volts

1.66 seconds

====================================

4 x 1000uf

Test 13 @ 7.5 Volts

0.64 seconds

Test 14 @ 9 Volts

0.92 seconds

Test 15 @ 12 Volts

1.49 seconds

Test 16 @ 15 Volts

2.10 seconds

====================================

1 x 10,000 uf

Test 17 @7.5 Volts

11.69 seconds

Test 18 @ 9 Volts

20.46 seconds

Test 19 @12 Volts

33.56 seconds

Test 20 @ 15 Volts

51.33 seconds

Steve

[viktor_au]

Hello Steve
Pls have a look at the schematics I changed.
What do you think?

[steve001]

Hi Viktor

What do you want it to do / measure ?

The circuit works by comparing 2 x 2.5 volt signals. Vref from the voltage reference and the voltage divider network.

If the non-inverting (+) input is greater than the inverting (-) input the output will be high.
If the inverting input (-) is greater than the non-inverting (+), the output will be low.

I originally made a mistake and put the TL431 before the voltage regulator, this should be after the regulator ideally.

:edit

In normal operation the output is high.

Its not desirable to have circuits energising on faults.

Steve

[chipfryer27]

Hi Viktor

Welcome to the Dark Side (v10 forum colour scheme versus v8).

Regards

[viktor_au]

Hello Steve, ChipFryer
Re:The circuit works by comparing 2 x 2.5 volt signals. Vref from the voltage reference and the voltage divider network.

[steve001]

Morning Viktor,

The circuit arrangement is a common arrangement that i used to employ on some battery monitoring boards for deep discharge protection.
This voltage is any where from 48 to 254 V DC this is where the voltage divider came from.

I just used the same input method for ease as i took the supply voltage to 15 Volts (see above tables) and altered the voltage divider to suit.

Steve

Edit:

just replied quickly previously, and didn't give it much thought.

Using a voltage divider allows you to better control the operating point of the comparator by setting the divider network to suit your voltage.