Circuit switch on only if voltage is above 3.5V

Other Hardware General Electronics
1

I have bought a DC-DC-step-up / step-down converter.
Before using It i tested its behaviour on low voltages and discovered that the module
goes wild.

Example: outputvoltage adjused to 5V
if the input-voltage drops below 3.5V the outputvoltage goes up to 25V even under some load of an LED.

So now I am looking for a simple additional circuit that would switch off as soon as the voltage drops below 3.6V so that the output voltage does not go haywire.

A small quicent-current of some mA is acceptable.

I guess something with a MOS-FET transistor would be suitable but I have not enough electronic knowledge to develop such a circuit on my own.

Suggestions very much appreciated

2

Most modules have a chip controlling the voltage. Often these chips have an enable pin that could be connect (via resistor or just a PCB connection) to VCC to make it always on. Cutting that connection allows you to control that enable pin in some other way.

3

How much power you want to use at the DC/DC?
What is the full range of input voltage?

4

A bad design.
What other funny things might it do?
If it were me I throw it in the trash.

5

The IC used in your DC-DC converter may have an Under Voltage Lock Out pin that is used to do what you want to do.

Here is an extract from the LM2596 datasheet, that shows you how to add an under voltage lock out circuit to an IC that has an enable pin:

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Have a look at the datasheet for the IC used in your DC-DC converter module.
It may be possible to fit the additional components needed.

I guess something with a MOS-FET transistor would be suitable but I have not enough electronic knowledge to develop such a circuit on my own.

Give us a link to the DC-DC converter, or at least the IC part number, that you have for further assistance.

6

Thank you very much all for answering.

This is the module

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image1920×3634 656 KB

it has a XL6009E1 chip

Now what is really strange is the fact that the datasheet
DC-DC-switch-step-up-step-down-XL6009.PDF (255.7 KB)

states
Wide 5V to 32V Input Voltage Range
Positive or Negative Output Voltage
Programming with a Single Feedback Pin
Current Mode Control Provides Excellent Transient Response

How is it possible that this chip works at 3.6V?
Is below 5V out of specs?

7

That is very possible as the specification is a guaranteed operating range not an absolute rating. This value will change over temperature etc. You will find out of specification operation capability with most semiconductor devices. There are three leaded devices that will give an output when the voltage goes low.

8

Ooh, I have some that do that. They look similar to yours, but not identical (same chip ID but only one inductor on the board. I wouldn't use them in any circuit, ever. I keep them as a reminder, in case I accidentally acquire more, but I've clearly marked them as BAD.

I've gotten some pretty bad things from Aliexpress, but these might be the worst.

My notes say that it doesn't buck (only boosts), and that the output goes to 30v when the input drops to 3.0 to 3.4v.