**Soft Starters and Inrush Current: A DIY Solution**
As we all know, power supplies are not the only victims of big inrush currents. All kinds of motors come with a similar problem, which is why they're often associated with soft starters. But that's a subject for another video.
**The Problem with Inrush Currents**
Inrush currents can be a significant issue when dealing with appliances and power supplies. Not only do they pose an under-voltage problem, but they also waste quite a bit of power through the input resistor. This is not desirable, to say the least.
**A Big Input Capacitor Can Help**
One solution to these problems is to use a big input capacitor in your power supply, which would require inrush current limiting components. I opened up a power supply that features a large input capacitor and was surprised to find no resistors in series with the input voltage. Instead, I found a green flat disk labeled as a 10 ohm NTC (thermistor).
**NTC Thermistors: A Common Solution**
This thermistor is perfect for inrush current limiting. In its normal, unpowered state, it has a resistance of 10 ohms, which is ideal for preventing big inrush currents. However, as soon as the operational current passes through it, the NTC becomes harder and drops in resistance, resulting in less power losses.
**A Problem with NTC Thermistors**
The only problem with using an NTC thermistor is that it takes a bit of time to cool down after being cut off from power. This means another big inrush current can occur if you turn on the power shortly after turning it off.
**Creating a Solution: The Relay Circuit**
To solve this problem, I decided to stick with the resistor while adding a small circuit to bypass it 1 second after power-up, minimizing any voltage drop. After tinkering with a few components, I came up with a schematic that requires only a relay, two resistors, one capacitor, one diode, and one MOSFET.
**The Relay Circuit Explained**
As soon as the power is supplied, the current for the inverter flows through the resistor because the relay is not activated. The gate capacitor of the MOSFET was discharged through the 100 kilo-ohm resistor. While the inrush current is already decreasing, the 10 microfarad capacitor gets shorted up to 6 volts through a pull-up resistor.
**The Relay Circuit Activates**
We've got a relay coil current of milliamperes 36 millions, which means almost no power losses through the MOSFETs. The time constant of the RC network is around 1 second, which means it takes around 1 second to reach 63.2% of the 6 volts max value.
**The Relay Circuit Turns On**
This should be the time when the MOSFET starts conducting and turns on the relay coil, which then closes the relay contacts, thus bypassing the resistor. We do have a constant power loss of around Harper Watts due to the relay, but that's honestly nothing compared to the battery capacity or previous power losses through the resistor.
**Testing the Relay Circuit**
I tested the circuit meticulously and as you can see, the relay activates with just enough delay. We've got our lovely capacitor charge/discharge curves and the MOSFET wastes pretty much no power.
**Building the Relay Circuit**
After testing this circuit with the inverter and coming to the conclusion that everything works as expected, I started soldering all the components onto a piece of perfboard according to the previously shown schematic. If you're interested in creating a similar circuit, you can find more information about it in the video description.
**Altering the Power Resistor Value**
It's essential to use the smallest value possible for your relay circuit, as if it fails, you would waste the smallest amount of power. For example, my inverter needs 3.3 ohms, while my boost converter wouldn't trigger the charge controller even with a small 1-ohm resistor.
**Soldering the Relay Circuit**
As soon as my circuit was complete, I directly soldered it to the inverters inputs and after closing everything up, bringing the components back to my garage. It seems like everything works fine.
**Conclusion**
Awesome power supplies though are not the only victim of big inrush currents. All kinds of motors come with a similar problem, which is why they're often associated with soft starters. But that's a subject for another video.
If you enjoyed this project and learned a bit about soft starters and the inrush current problematic, don't forget to Like, share, subscribe, and hit the notification below. Stay creative and we'll see you next time.