Update: Adding an output transistor – see below.

So you’d kind of like to know when the battery in your stompbox  is getting a bit flat but you’re not sure how to proceed.

The first problem is that you don’t want your low-battery monitoring circuit to make a significant contribution to battery drain – which rules out parts like the MC34064 “volt sense” circuit because it has a quiescent current of about half a milliamp.

The only part that fits the bill and is readily available (in the UK at least) is the TC54 from Microchip.  The TC54 is available in a number of packages, output-styles and voltage variations but the one for us is the TC54VN4302EZB.  This variant comes in a TO92 package, has a reference voltage of 4.3V and an N-channel open-drain output.

A brand spanky new alkaline PP3/6F22/1604 battery has a voltage of about 9.5V.  We shall (quite arbitrarily) assume that it is dead when the voltage drops to about 7.3V.  But the TC54 has a reference voltage of 4.3V (and they don’t make a variation with a reference voltage of 7.3V) so we have to cheat.

Here is the TC54:

TC54

 

and here is the circuit we are going to implement:

TC54 Schematic

The TC54 works on a supply voltage (derived from the Vin terminal) from 0.7V to 10V so we don’t need to worry about keeping the TC54 alive.

In a nutshell, the TC54 monitors the voltage at the Vin terminal and when this voltage falls below the internal reference voltage the Vout terminal is pulled low (and the LED switches on to indicate that the battery is failing).  If the input voltage rises above Vref + ~200mV (about 4.5V), Vout goes high again (and the LED switches off).

We want to detect a battery voltage of about 7.3V (not 4.3V) so we use R1 and R2 to form a potential divider showing a fixed proportion of the battery voltage to the TC54.  We need to find values for R1 and R2 so that when the battery voltage falls to 7.3V, the TC54 input voltage falls to 4.3V.

The Microchip TC54 datasheet suggests a bleed current (Iq) of about 100uA and also advises that the voltage at the Vout terminal when it is pulled low is about 0.5V.  Now we have enough data to do some calculations.

When the battery is fresh, Vbat = 9.5V so if Iq is 100uA then (by Ohm’s law):

R1 + R2 = 9.5V/100uA = 95kOhm

We want the TC54 to switch at a battery voltage of 7.3V, so:

4.3V/7.3V = 0.59

R1 and R2 form a potential divider with the voltage across R2 being 0.59 x Vbat, so:

R2/(R1+R2) = 0.59

We can now solve for R2 because we know R1 + R2 = 95kOhm

R2/95 = 0.59

R2 = 0.59 x 95

R2 = 56kOhm and R1 = 39kOhm (i.e. 95kOhm – 56kOhm).

Let’s assume we are going to use a low-current LED such as the Kingbright L7104LID (which has Vf of 1.7V and a nominal current of 2mA).  This led will happily shine at 1.5mA forward current (we don’t want to over-tax an already failing battery) so we calculate Rload as follows:

Rload = (Vbat – Vled – Vout)/Iload

Rload = (7.3 – 1.7 – 0.5)V/1.5mA

Rload = 3400Ohm.

So a 3.3kOhm or 3.6kOhm resistor can be used for Rload, giving:

TC54 schematic with values

The reason why Microchip suggest 100uA for Iq is that you can then safely ignore Iss (which is only 1uA) making for an easy calculation.  As we increase the values of R1 and R2 to reduce the quiescent current (Iq), Iss becomes more significant.  For example, if we reduce Iq to about 30uA we can redo the calculations above (ignoring Iss) and find that R1 = 120kOhm and R2 = 180kOhm to give a lo-battery set-point of a nominal 7.16V.  However, the effect of Iss raises the set-point back to 7.3V.  Neat.

So here’s the absolute, final, Stompville-approved, 9V low-battery monitor which pulls only 32uA in its quiescent state and switches on a low-current LED (1.4mA) when the battery voltage drops to 7.3V:

TC54 final schematic

Microchip doesn’t make a 7.3V version of the TC54 means so we need two more resistors than would otherwise be necessary – but the absolute maximum voltage at Vin is 10V – so adding these extra resistors means we can use a wall wart with an output voltage up to about 16.5V before we fry the TC54.  If we used a TC54 with a lower reference voltage, we could increase the maximum wall-wart supply voltage but the 4.3V version is more readily available.  Of course the above could all be built into a hybrid LED.  Our wish is that Microchip make a version of the TC54 with a 7.3V reference, a Vin(max) of (say) 24V and a constant-current sink output of 1.5mA – all built into a 3mm LED so our low-battery solution is reduced to one component.  Dream on. 

Update – Maximum drain-source voltage and adding an output transistor

You might be thinking of using a Microchip MCP111 in lieu of the TC54VN (and using similar math to calculate the values of R1 and R2), but the output stage of the MCP111 is less robust than the TC54VN. In particular, the maximum output voltage of the Vout pin should not be held higher than Vdd for any significant time. This effectively prevents you implementing the resistor divider trick to adjust the set-point, unless you use an additional transistor on the output to isolate the MCP111 from the supply voltage.

Also, for the TC54VN we should point out that, according to the Absolute Maximum Ratings on the Microchip datasheet, the output voltage (open-drain) acceptable range is (Vss – 0.3V) to 12V

This suggests that the Vds(max) of the output transistor is limited to 12V, so we shouldn’t implement this design for battery voltages higher than about 10V without adding an extra output transistor with suitable Vceo (or BVdss).

You can use any general purpose small-signal p-channel MOSFET or PNP output transistor with a high-enough breakdown voltage. Note that the bipolar (PNP) option is much cheaper (for through-hole at least) but requires an extra resistor.

Note that the schematics below show a 9V battery supply but the additional transistor should be used when the supply voltage is higher than about 10V nominal.

172SV

A wide selection of surface-mount MOSFET devices are available. For through-hole, use a ZVP3306A or similar. The design was prototyped with a ZVP2106A (more expensive than ‘3306).

173SV

Use a 2N5401 or BC557 (or whatever) for the transistor version. svfavicon.png

 

8 Responses to Low-battery monitor with TC54 voltage detector

  1. AndrewM says:

    Hi,
    I changed the resistor values to operate at 12V, with a 10V low battery indicator.
    I tried this circuit and have 2 comments:
    1. The LED glows dimly when the battery is at 12V. The LEd has 1.52V across it.
    2. The low battery function only works when the circuit is reset i.e. by removing and reconnecting the battery. If you gradually lower the voltage the circuit doesnt trigger at 10V, unless you actually disconnect and reconnect the battery. I could get around this by using a pushbutton switch to test for low battery but this kind of defeates the constant monitoring function of the circuit.
    Any ideas how to resolve these issues?
    Regards and thanks for the great write up.
    Andrew

  2. SmudgerD says:

    Hmm. That doesn’t sound right. Can you advise the exact part number of the TC54 and the values of R1 and R2 you are using?
    SmudgerD

  3. AndrewM says:

    Hi,
    Sure thing.
    R1=226K, R2=180k
    TC54VC4302EZB – Mouser 579-TC54VC4302EZB
    LED is Mouser 606-5111F1LC 2mA.
    Thanks
    Andrew

  4. SmudgerD says:

    OK, your resistor values look good.
    I should think the problem arises because you are using a TC54VC rather than TC54VN part. The VC part has a complimentary output and the VN has an open drain output. It’s the VN part you want. Note that I have added an update above about the maximum voltage the output transistor will switch. With your 12V supply, you may need to add an additional transistor rather than switch to the VN part.

  5. AndrewM says:

    You’ve been busy! Thanks for the update and added instructions. Will try the VN version and see how how it goes and add try the additional transistor if required. Thanks

  6. AndrewM says:

    Hi, finally got it working with the ‘old’ circuit and the VN component. Didn’t need the additional components. Am running approx. 11.5V max. Thanks for your help.

  7. mishra says:

    Hope you doing well !!!

    I have got your reference by Mr. Ramesh from Bangalore office.

    I want to use TC54VN3002ECB713 as low voltage detector. I have 3.7V/1400mAh single cell li-ion battery.I want to detect low threshold voltage at 3V to avoid deep discharge of battery (EOD).My aim is to work instrument satisfactorily unless it detect the 3V (i.e low threshold voltage) and then the load has to be disconnect by using switch (FET).

    Could you support me how to implement this circuit.I need complete schematic with mosfet (rated 4A). How do i connect a mosfet (low side/high side switching) with device and could you recommend me mosfet part number.When a battery voltage more than 3V, the same voltage should appear at load and when the battery voltage is less than 3V, the switch disconnect the load.

    • SmudgerD says:

      Connect TC54 Vin and Vss across battery. Connect load+ to battery+. Connect load- to drain of N-channel MOSFET. Connect source of MOSFET to battery-. Connect resistor (say 47k) between battery+ and gate of MOSFET. Connect resistor (say 100 Ohms) between TC54 Vout and gate of MOSFET. There are too many MOSFETs on the market to recommend any particular one.

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