I would like to ask for a switch circuit or chip that can be turned on when the output is 5V and turned off when the output is below 3V.

z45217

I would like to ask for a switch circuit or chip that can be turned on when the output is 5V and turned off when the output is below 3V. [Copy link]

 

As the title says, there is a device that outputs 5V2A when running, but also has an output below 3V when in sleep mode, so I want to add a switch circuit behind it

It can realize that when the power supply outputs 5V, the switch is closed to supply power to the load, and it is disconnected when in sleep mode.

Battery powered, so I hope this circuit consumes as little power as possible

Please recommend a suitable chip or circuit. Thank you in advance.

maychang

[I want to add a switch circuit at the back, which can realize that when the power supply outputs 5V, the switch is closed to power the load, and disconnected when in sleep mode]

This should be possible using a bipolar low-power transistor and a P-channel MOS tube.

z45217

maychang posted on 2023-11-14 15:59 [I want to add a switch circuit at the back, which can realize that when the power supply outputs 5V, the switch is closed to power the load, and disconnected when in sleep mode] Use a bipolar low-power triode...

The transistor is turned on at 0.7V, and a resistor is needed to divide the output voltage. 0.7V is not easy to control.

maychang

z45217 Published on 2023-11-14 16:05 The transistor is turned on at 0.7V, and a resistor is used to divide the output voltage. 0.7V is not easy to control

[The transistor is turned on at 0.7V, and a resistor is needed to divide the output voltage. 0.7V is not easy to control.]

Your requirement is that the P-channel MOS tube is turned on when the voltage is 5V (in actual operation, it may need to be adjusted to 4V or 4.5V to turn on), and the MOS tube is turned off when the voltage is lower than 3V (it may need to be adjusted to lower than 3.5V to turn off). Then, connect a Zener diode, a light-emitting diode or an ordinary diode in series with the base of the bipolar transistor and adjust it to meet your requirements.

z45217

maychang posted on 2023-11-14 16:15 [The transistor is turned on at 0.7V, and a resistor is needed to divide the output voltage. 0.7V is not easy to control] Your requirement is to turn on the P-channel MOS tube at 5V voltage (...

In this case, 5 to 6 diodes should be connected in series. Is it okay to add a 3.3V voltage regulator diode to the emitter of the NPN transistor?

maychang

z45217 Published on 2023-11-14 17:21 In this case, 5~6 diodes should be connected in series. Can a 3.3V voltage regulator diode be added to the emitter base of the NPN transistor?

[Is it okay to add a 3.3V Zener diode to the emitter of an NPN transistor?]

This is what I said on the 4th floor: "Connect a voltage regulator tube, a light-emitting tube or an ordinary diode in series with the base of the bipolar transistor."

Note: The leakage current of the voltage regulator and the light-emitting tube must be considered. An appropriate resistor can be connected in parallel to the emitter junction of the bipolar transistor to discharge the leakage current.

z45217

maychang posted on 2023-11-14 17:27 [Is it okay to add a 3.3V Zener diode to the emitter of the NPN transistor?] I said this on the 4th floor: "In series with the base of the bipolar transistor...

Can you show me a picture? I still don't understand.

maychang

z45217 posted on 2023-11-15 09:51 Can you give me a picture? I still don't quite understand

[Can you show me a picture? I still don’t quite understand]

Wait a moment while I draw the electrical schematic.

z45217

maychang posted on 2023-11-15 10:24 [Can you give me a picture? I still don’t quite understand] Wait a moment, and let me draw the electrical schematic.

Thank you, big guy

maychang

z45217 posted on 2023-11-15 09:51 Can you give me a picture? I still don't quite understand

To achieve your requirements, the circuit is roughly as shown above.

maychang

z45217 posted on 2023-11-15 09:51 Can you give me a picture? I still don't quite understand

In the figure, terminal A is connected to your power supply and terminal B is connected to the load.

In the figure, D can be a plurality of ordinary diodes connected in series, or one or more light-emitting tubes connected in series, or a voltage regulator, or even a light-emitting tube and an ordinary diode connected in series. Ordinary diodes and light-emitting tubes use their forward characteristics, while voltage regulators use their reverse characteristics.

For example, the forward voltage of a red LED is about 1.6V, and using two red LEDs is about 3.2V, plus the forward voltage drop of the transistor emitter junction is 0.7V, a total of 3.9V. Then the resistor R1 should flow through a current of, for example, 0.5mA when the voltage across the resistor is 5V-3.9V=1.1V, so R1 should be 2.2 kilo-ohms.

maychang

z45217 posted on 2023-11-15 09:51 Can you give me a picture? I still don't quite understand

The purpose of R2 is to prevent transistor T from turning on when a very small current (leakage current) flows through D. Its value is approximately 2~3 kilo-ohms.

maychang

z45217 posted on 2023-11-15 09:51 Can you give me a picture? I still don't quite understand

R3 can be used in a wide range, generally from 2 kilo-ohms to 10 kilo-ohms. When the value is small, the MOS tube turns on and off faster, and when the value is large, the opposite is true. However, as a power switch, it doesn't matter if it's a little slower.

maychang

z45217 posted on 2023-11-15 09:51 Can you give me a picture? I still don't quite understand

If D uses a red LED and a normal diode in series, the forward voltage drop of D will be 1.6V+0.7V=2.3V. This is lower than two red LEDs and higher than two normal diodes. The forward voltage drop of a green LED is lower than that of two red LEDs and higher than that of a red LED plus a normal diode. Multiple combinations give you multiple choices.

maychang

z45217 posted on 2023-11-15 09:51 Can you give me a picture? I still don't quite understand

If the action threshold of the P-channel MOS tube is required to be accurate, then the transistor T should use a comparator instead, and change the input resistance voltage division to determine the action point. However, the circuit will be complicated and the cost will be higher.

z45217

maychang published on 2023-11-15 11:59 If the action threshold of the P-channel MOS tube is required to be accurate, then the transistor T should use a comparator instead, and change the input resistance voltage divider to determine the action point. ...

Got it, thanks sir.

tagetage

I agree with the picture on the 10th floor, but I would choose a 3V voltage regulator for the light-emitting tube.

maychang

tagetage posted on 2023-11-15 13:58 I agree with the picture on the 10th floor, but for the position of the light-emitting tube, I would choose a voltage regulator tube of about 3V.

[I agree with the picture on the 10th floor, but I would choose a 3V voltage regulator for the light-emitting tube]

The forward nonlinearity of the blue light-emitting tube is stronger than that of the 3V Zener diode.

tagetage

maychang posted on 2023-11-15 14:02 [I agree with the picture on the 10th floor, but I would choose a 3V voltage regulator for the position of the light-emitting tube] The forward nonlinearity of the blue light-emitting tube is higher than that of the 3V...

Thank you for your reply. I haven't compared the volt-ampere curves of the blue LED and the Zener diode carefully, but I chose the Zener diode because it is convenient to change the voltage conduction parameter. Zener diodes of different voltages can be adjusted according to actual needs. If a blue LED is used, this parameter will be fixed, which is not convenient for debugging.

maychang

tagetage posted on 2023-11-15 17:54 Thank you for your reply. I haven't carefully compared the volt-ampere curves of the blue light-emitting diode and the Zener tube, but I chose the Zener tube because it is convenient to change the voltage conduction parameter, ...

[I chose the Zener diode because it is convenient to change the voltage conduction parameter. You can adjust the Zener diodes of different voltages according to actual needs. If you use a blue light-emitting diode, this parameter is fixed.]

That's true.

However, the voltage stabilization of the Zener diode is also very dispersed, roughly the same as the E12 series of resistors.