Some additional ideas for the SZBK07 DC-DC converter

[@diytricks7234]

Thank you for the video. I have checked the website as well I saw the different graphs there the one with the correlation between Voltage and Resistance. It was kinda a linear function V=0.312R+1.118 So to keep the voltage between 6 and 12 we can simply manage to get a variant resistance from 24.5k(6v)and 37.34k(12v)

[@CuriousScientist]

Glad it was helpful! You might want to rewatch my video, because I got very different values for the resistances.

[@diytricks7234]

@@CuriousScientist The values I have given above are for the R3 which is the trimmer. I was referring to something like this: ibb.co/126nhvs Trimmer will be put in parallel with a series potentiometer and resistor

[@CuriousScientist]

This looks nice. By adding the 6.8k resistor, you set up a constant bottom limit for the voltage. With the onboard trimmer, you can then control the upper limit, and with the 22 k potmeter, you can move within the created range.

[@diytricks7234]

@@CuriousScientist Yes that's the idea, the picture belongs to another buck convertor, I need to figure out the correct resistors for this one

[@diytricks7234]

@@CuriousScientist Around 50k resistor and 100k potentiometer should give the range 25k to 37.5k When trimmer is set to around 50k

[@aboimad5178]

thank you my friend .... i dont have the potentiometer but i use my microcontroller's pwm to control the szbk07 and it works .... what did i lost when i used the pwm ?!

[@CuriousScientist]

You're welcome! I don't exactly know how you use the PWM but I assume that you use a filter circuit and feed the PWM signal through this filter directly into the feedback of the buck controller. In that case, you did not lose anything, but if you remove the potentiometer, the buck controller has no idea about the output voltage, therefore you have to make sure that there's always a control signal before starting the converter. Otherwise I would expect funny or erroneous behavior. But this is just an assumption as I never used the circuit without a potentiometer.

[@aboimad5178]

@@CuriousScientist thank u my friend i set the module on 16v i didnt remove the potentiometer i connected the pwm with a 13k resister to the feed back (same potentiometer pin) added 100uf smd capacitor and 47k to ground it really works fine for me but i am trying to improve it only ..

[@CuriousScientist]

I see! So you are sort of smoothing the PWM signal and feed the resulting voltage in the same way as I do. If it works, use it, but a DAC or a digital potentiometer would work better as they produce pure DC voltage. Check my other videos in this playlist.

[@aboimad5178]

@@CuriousScientist thak u I appreciate

[@nxt-1]

My end goal is to have a set of TEC's driven CC and controlled by a PWM signal from some other board in my system. Before this video I was thinking about setting the min and max voltage/current in software on the STM32 from your previous video. Now I think I might merge and adapt both of your projects, so I use the trimmer to set V_max, the 50k pot to set V_min and use a digital 100k pot connected to an STM32 to do the adjusting. Thanks for the video's, keep up the good work

[@CuriousScientist]

Thank you! In my most recent video I did exactly the same, I just didn't use the digital potentiometer for the middle component. Just make sure that you don't cross 5 V across the potentiometers. I will perfect this circuit and concept and upload a few more videos in the future.

[@michal-pales]

@Curious Scientist Hi. I also have changed trimmer to 50K pot (CV). It works OK for some time but now without any obvious reason pot doesn't change voltage at all. When pot approaches low resistance (near to 0 ohm), it's sparking and smells. I know it's because of high current passing by but why is there any current at all? If no load is connect. May be someting wrong with buck converter itself? Can you please give some advice? Thank you!

[@CuriousScientist]

Hi! If you go to very low resistance values, typically below 10k if the output voltage is max (36 V), then you can potentially meet some issues. These potentiometers are typically rated for maximum 125 mW power which you would cross if you go around 10k at 36V output voltage. To avoid this, first of all, don't replace the potentiometers with different values, keep the 100k, or if you do, always check the values to see if you can potentially damage something. Check the max expected output voltage, and follow Ohm's law. There is high current through the resistor without load, because the CV potentiometer itself is also a load. It goes from the positive output voltage terminal to the buck controller chip. Therefore, there is always some current passed through it, even when you don't put anything on the converter. If you are planning to use a lower value potentiometer, put a 12k resistor with it in series. With this value you will never cross the 125 mW max load (assuming you are using 125 mW rated potmeters), but obviously, this can put some limitations to the use. Also, it is a good idea to buy good quality potentiometers. Some cheap potentiometers are just pure garbage and they burn out for no reason. I had a few failing potmeters as well. Also, there are potentiometers with higher wattage values.

[@michal-pales]

@@CuriousScientist Thank you for your answer! Weird is that I use it with 12V input only. I used it couple of times with output 1.5V - 11.8V without problem. Of course I test it with 2 pcs of 100k with same problem. But as I said, rotating pot doesn't change output V. Maybe some resistor, capacitor or something else on buck has blow up

[@CuriousScientist]

You are welcome! It can also be that the mosfet gave up. I saw some people mentioning that they managed to destroy the mosfets of this circuit.

[@michal-pales]

@@CuriousScientist you're right. mosfets seems dead. much appreciated your help. keep up good work!

[@CuriousScientist]

You are welcome! I hope you can replace the mosfets easily and use the converter again.

[@Paul-rs4gd]

Thank you very much for providing this information. I have used the SZBK07 in a couple of projects, but I did not realise it was so flexible. I want to hack the SZBK07 for a new project so that it can be turned on and off by a Raspberry Pi. The LM25116 datasheet says I just have to pull EN low (below 3.3V). I am guessing the switch on the board is connected from ground to TP3 (on the schematic that you provided), so I could take it out, and replace it with an open collector logic output. I am worried about the voltage at TP3 though - R13 is connected to Vin (which can be very high). I envision the untimely death of a Pi ! I'll do some measurements when I receive a new unit.

[@CuriousScientist]

I haven't checked how the onboard switch works, but if you want to be on the safe side, just replace it with a relay.

[@Paul-rs4gd]

@@CuriousScientist I finally did some voltage measurements on the switch. Vin is 19V. The switch has 3 pins - I am guessing it is SPDT. With the switch powering down the Buck converter, I measure 0V on 2 pins, and 0.6mV on the pin nearest the edge of the board). When I flip the switch, I see 11V on the middle pin, and the pin nearest edge of board, and 0V on the other pin. I find the measurements a little difficult to explain, but my hypothesis is: middle pin of switch(the gang) is connected to TP3; Pin nearest edge is unconnected; Third pin to ground. Here I'm assuming that 0.6mv on the unconnected pin is noise. If I am right, there is no need to remove the switch. An open collector transistor from ground to middle pin could turn the converter off, provided that it is rated for sufficient voltage (Vin to be safe). The transistor and switch would work in parallel. Either one could turn the converter off. I still have to test this idea.

[@ImreLengyel]

@Paul How did you go with with the control using the EN pin? I'm hoping to drive some LED lights dimmed by PWM, so relay switching would not be suitable.

[@lizandrobitang5199]

How can i use this for MPPT of solar?

[@CuriousScientist]

Sorry, but I don't know. I have never worked with solar stuff, so I don't know too much about the components.