Introduction: Adding a Current Limit Feature to a Buck/Boost Converter

Picture of Adding a Current Limit Feature to a Buck/Boost Converter

In this project we will have a closer look at a common buck/boost converter and create a small, additional circuit that adds a current limit feature to it. With it, the buck/boost converter can be used just like a variable lab bench power supply. Let's get started!

Step 1: Watch the Video!

The video gives you all information you need to recreate the circuit. In the next steps though, I will present you some additional information.

Step 2: Order Your Components!

Picture of Order Your Components!

Here you can find a parts list with example seller:

Amazon.com:

1x Buck/Boost Converter: http://amzn.to/2yh7zIW

1x LF33 Voltage Regulator: http://amzn.to/2xqkjim

1x 10nF Capacitor: http://amzn.to/2xJjZfG

1x 10µF Capacitor: http://amzn.to/2wyLbIQ

1x 0.1Ω Current shunt: http://amzn.to/2fsKACV

2x 3.3kΩ, 2x 100kΩ Resistor: http://amzn.to/2jCWbUL

1x MCP602 OpAmp: http://amzn.to/2yirlUy

1x 10kΩ Trimmer: http://amzn.to/2wyvJwu

1x UF4007 Diode: http://amzn.to/2fsKJpX

1x Voltage/Current Meter: http://amzn.to/2xddOiH

Ebay:

1x Buck/Boost Converter: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x LF33 Voltage Regulator: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x 10nF Capacitor: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x 10µF Capacitor: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x 0.1Ω Current shunt: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

2x 3.3kΩ, 2x 100kΩ Resistor: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x MCP602 OpAmp: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x 10kΩ Trimmer: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x UF4007 Diode: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

1x Voltage/Current Meter: http://rover.ebay.com/rover/1/711-53200-19255-0/1?...

Amazon.de:

1x Buck/Boost Converter: http://amzn.to/2w1WgTz

1x LF33 Voltage Regulator: http://amzn.to/2w1f2dF

1x 10nF Capacitor: http://amzn.to/2xIowip

1x 10µF Capacitor: http://amzn.to/2habIql

1x 0.1Ω Current shunt: http://amzn.to/2haAOWd

2x 3.3kΩ, 2x 100kΩ Resistor: http://amzn.to/2w2jSaI

1x MCP602 OpAmp: http://amzn.to/2x61uBr

1x 10kΩ Trimmer: http://amzn.to/2x9NmVc

1x UF4007 Diode: http://amzn.to/2w2vzxU

1x Voltage/Current Meter: http://amzn.to/2xJ2YCt

Step 3: Create the Circuit!

Picture of Create the Circuit!

Here you can find the schematic and pictures of my completed circuit. Use it as a reference to create your own.

A rather tricky part is the current path on the output side of the converter. If you want to hook up the current shunt and the V/I meter then your wiring should be like this: Out+ -->Load+ -->Load- --> Red Wire I Meter --> Black Wire I Meter --> Current Shunt 1 --> Current Shunt 2 --> Out-

Step 4: Success!

Picture of Success!

You did it! You just added a current limit feature to your Buck/Boost converter!


Feel free to check out my YouTube channel for more awesome projects:

http://www.youtube.com/user/greatscottlab

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Comments

vgregorio (author)2017-10-15

Is it possible to replace the MCP602 with an LM358? thank you!

DIY Circuits (author)2017-09-17

Out of curiosity, is there an LM equivalent of this op amp, and if not a complete equivalent then at least something that would do the job?

The ubiquitous LM358 is used in most Chinese modules. Cheap as dirt too.

Polymorph (author)PirateKittyK2017-09-24

The ones I have with LM358 current limiting use low side shunt resistance. Which means you absolutely cannot use more than one in a circuit, as you cannot tie the grounds together.

I'm not sure an LM358 will work as intended in this with high side shunt resistance. It is certainly worth a try, but keep in mind that it is a very old op amp. Although it can work in a rather low voltage circuit, the output is limited to 1.5V below V+. So 3.3V is rather low.

PirateKittyK (author)Polymorph2017-09-24

Of course it works. Its configured as a differential amplifier.

Just because a part has been around for many years, doesn't mean its bad!

Max Vout = Vsupply - 1.5V

Min Vout= 20mV

its not a problem with +5 Volts supply. Its not a rail-to-rail op amp.

Polymorph (author)PirateKittyK2017-09-25

I didn't say it won't work, I said it might not work.

The circuit as configured is running from 3.3V. The differential amp has a gain of about 30. At 1A, that is 0.1V x 30 = 3V. Right there, the output of IC1a cannot go above about 1.8V.

The output of IC1b is meant to drive the sense line of the buck regulator high, so the regulator will lower the output thinking it is too high. But with a max of 1.8V output and a 0.6V drop across D1, it is only about 1.2V maximum. Possibly worse, as fast recovery diodes sometimes drop a bit more voltage than the garden variety silicon rectifier diodes. Many regulators use 1.25V as the reference voltage and so the Sense line is at 1.25V already.

If you raise the Op Amp supply voltage to 5V, it might work. And replace D1 with a shottky diode.

But it isn't as simple as just looking at the specs and saying it will work. You need to consider operation of the circuit. It certainly could work with changes to the circuit, both have about the same offset voltage and Vcc requirements. It is the output range where the LM358 has an issue with this circuit.

The MCP602 isn't really that expensive. The eBay link the author gives is only $4 for 5 with shipping, about 80 cents each.

Correction: I see a problem with this circuit as designed, for either IC. The common mode input range for the MCP602 is V+ - 1.2V, and for the LM358 is V+ - 1.5V. So the Op Amp really needs a supply voltage higher than 12V, or the circuit redesigned. Right now the common mode voltage with 12V on the current shunt is 12 x 100k/(100k + 3.3k) = 11.6V

PirateKittyK (author)Polymorph2017-09-25

I have a similar design in my linear power supply. I use a 0R01 instead of 0R1, a 1K instead of 3K3 and run the LM358 from +5. Works for me. Then again, I am not using this buck/boost module.

I always use 3A schottky's everywhere reverse leakage is not a problem. I have about a hundred left!

He has a schottky symbol in his schematic, but uses a fast diode part.

Its not the cost, LM358 is available everywhere. Microchip parts are hard to get in developing countries and shipping costs are prohibitive. I think that would be a problem for some.

Polymorph (author)PirateKittyK2017-09-25

Exactly, you used different values to make it work.

I have several DROK modules with current limiting built-in, but they use low side current monitoring with an LM358 on board. Fine if using only one, but a pain if not as the current limiting won't work when using common grounds. I thought I might try altering them a bit to make it high side.

Cliffystones (author)2017-09-19

I have to of these exact buck-boost modules. I have used one to keep the LED spotlights on my workbench at optimal brightness when the solar drops below 11.5v after a long day, and it works fabulous.

But I believe I believe there is a simpler add-on circuit that should do the job. Here is a link to the schematic and the article. These are used to set a constant current source on a vacuum tube's plate supply. Hi haven't tried them with the Drok, but see no reason they shouldn't work.

http://www.diyaudioprojects.com/Technical/Current-Regulator/

Polymorph (author)Cliffystones2017-09-24

Those circuits drop out a lot of voltage. And rather than acting by turning down the switching regulator, they drop out the input voltage, causing the buck regulator to work harder and then essentially malfunction.

par64guy (author)2017-09-19

Very nice!

kavish laxkar (author)2017-09-17

Help full.:)

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