Hello, This is the Sj20 Buck Boost Converter 20 amps Mppt it looks like that. So this Buck Boost converter is based on the takes the display board off Lt8705. So here's the chip. It's the Linear Technologies Lt8705 Interesting pin Arrangement It's got lots of pins, but sort of some missing from the middle of the top row there.

So this is a complex chip. It pretty much does everything and that means that there's not a huge amount else on this board. Underneath, there are just four mosfets. It's a four mosfet topology.

Buck Boost: Uh, there's a big fat um, current sensing resistor there. and here there's an Op Amp Opa 1612 that is that looks like a regulator over here. there's a little step down Buck Power Supply: It's based on the XL what is it 7015 I Think it is and that appears to generate 5 volts for this fan connector here. And it does seem to me from my tests that a fan May well be necessary.

Single Inductor: Even though this is Buck or boost a couple of big diodes and some capacitors. these are 330 100 volt, 105 degrees C Linear Technology Lt8705 80 volt, VN and V out four switch. but Boost DC DC Controller: So it uses uh, four n-channel mosfets. It has the gate drivers for those and a single inductor which allows a V out above or below the V in it's synchronous rectification so you get very high efficiency.

This works similarly to the LT 3780 LTC 3780 also I think by Linear technology, but this has additional circuitry in it. It has four feedback control loops built in for output voltage, output, current, input voltage, and input current now. I've been waiting for an Lt8705 module um for quite some time and they have been around, but they have tended to be very expensive. and although this is not digitally controlled, it's got three potentiometers.

so it's potentiometer controlled. It does have a digital display which kind of acts as lots of multimeters attached to the input and the output. You can see output voltage, output, current, input, voltage. You can also see on the display the Mppt set point.

Now this Mppt is interesting. It's not maximum PowerPoint tracking. it just targets a certain voltage. So I call it maximum PowerPoint targeting.

But I'll power this up now and we'll take a little look at what we can see on this display, right? First things first: Xt60 to terminal block adapter so that we can feed power in from a power bank. So I've got a power bank down here with a cigarette lighter output so we can get 12 volts or thereabouts at a reasonable current up to about 10 amps. I Think right! let's switch on the power bank and see what appears on the display. Okay, so at the moment it's showing the output voltage that's adjustable.

With this potentiometer, it also shows the output current. There isn't any at the moment because I've not got anything connected. You can also display the input voltage so we've got 12.9 volts coming from the power bank and you can also display this. it's in CV Now it uses the Centigrade or Celsius indicator here, but they call it input constant voltage and this is the voltage you set for Mppt targeting now.

I'm probably not going to do solar panel input in this video, but this buck boost converter is ideally suited to solar panel on the input and a battery on the output right. So I'm going to put this bulb on the output 24 volt brake light bulb for a truck or Lorry Um, Mainly because the power bank keeps shutting its 12 volt output off in the absence of any current drawn. so we just want to draw a bit of current to keep that on, right? Let's turn the power bank back on. Um, I've got this set so that it, oh, that's interesting.

Why did that turn off? Oh, it's gone into current limiting at uh oh, 400 milliamps. Yes, that's not very bright Is it? Yeah. I've got this set to automatically come on. When the power is applied, the output to come on.

you can disable that, press and hold this button and it says out off. So now when I power up the input to this device, let's turn it off, turn it back on again. Um, it will power up the electronics, but it won't switch the output on. Okay, I need to set a reasonable output current now like I say oh, the input's turned off again.

Yeah, that's because I didn't have my bulb actually on. Okay, let's turn the 12 volts of the power bank back on. turn on the lamp. There's a bit of a pulse output there isn't there.

Give it a reasonable current. In fact, let's not limit the current at all. Um, now this is as I say only a monitor. You can only see what's going on.

You can't preset values on it. So we've got output is 18.6 volt. This is a 24 volt bulb and the current is or 800 milliamps. There is a constant current LED down there if it's lit, but it's incredibly dim.

If it's lit, then you know it's in current limiting. So let's go into current limiting. Uh, that's current limiting. There's the Led, the little red one above this red.

LED You can see it's lit, but it's often very dim so we can see output voltage and I can adjust that. Now this is in boost mode of course at the moment because the input voltage is 12.7 Now the output voltage is 19.9 I Can safely take that all the way up to about 24 for this particular bulb. I Can also take that down past the one to one voltage point which is 12 volts and put it into Buck mode so that the output voltage is lower than the input voltage. Now, if you're not using the Mppt function or the input voltage feedback control Loop you want to set it in a Arrangement Like this, you want to set it to a voltage so we'll go to the in CV a voltage below that which is coming in on the input.

so we'll set that down to 11 volts and that way it won't interfere with the output control loops. The voltage control Loop which as I say I can adjust with this part to increase the brightness of the bulb, and the current control Loop which is adjusted with the CC part. Okay, let's switch off the output, remove the bulb, and put on the output. it's that hot here.

A little bit put on the output. Um, this super capacitor module which has been heavily modified. It has got little LEDs on each protection circuit. It's got some terminal posts.

It's got a nice analog meter which shows that currently it's at about two and a half volts, so let's put that there and we'll charge that up. I'll get some wires hooked up and then we can watch the voltage climb slowly all the way from Two and a Half volts to 16.2 I think is the maximum of this module. There's no problem connecting um, batteries or super capacitors to the output of this thing because you've got the mosfet here. mosfet here, inductor between them and one more set points one way and the other mosfet in terms of the body diodes points the other way.

so you can't get a feed through the unit from output to input. Uh, what will happen when I connect? This is that the voltage on the Super Cap two and a half volts? Um, oh, it's not enough. Actually, it will back feed into these output capacitors and also into the little green LED here which is simply strapped across the output so the green LED on the output can be back fed from a battery or a capacitor on the output. But let's get the input in now and start charging that Super Cap right? I've balanced the Super Cat module on a piece of blue tack switch on the power bank and what we need is an output voltage that leaves as high as the 16.2 Oh, you can see the current output voltage because yes, it backfeeds to the point where the microcontroller measures the output voltage.

Um, we? What was it? What was it set to about? Oh, 20 something? I Think that's probably all right. What I might need to do is bring down the current, but let's just turn it on and the output voltage is climbing up. We've got two amps now that's going to overwhelm the protection circuits on the Super Cap. so I'll bring that down.

Well, I'll leave it where it is for the moment so we can watch the voltage on the Supercom cap module rise up. But the Super Capacitor module has these Um discharge circuits which Is 10 ohms in parallel with 10 ohms. That's 5 ohms and these are 2.7 volt. Let's call it 2.5 So that's 2.5 volts.

5 ohms. That's half an amp. So when these turn on, they can dissipate half an amp of current. So what I want to do is charge this Super Cat module at less than half an amp so that these protection circuits are capable of offsetting all of the incoming current and the power bank shut off once again.

Oh, because I haven't got that switched on. so let's wind the current limit down. So now I'm using the Output Current Feedback Control Loop Not using the output voltage feedback controller because that's set way over the 8 volts of this Super Cat bank. So I'll bring this down to 400 milliamps so that we know that we're not going to overwhelm.

So yeah, 0.4 We're not going to overwhelm these protection circuits. And now we can see the voltage rising up on the Supercap module. You can see it's about nine volts there. it's nine volts indicated here.

We can also look at the input voltage. that's the Power Bank input voltage coming on. Uh, through this Xt60 here so that's 12.9 We can also look at this input constant voltage that's the Mppt set point which is set on this temperature so that feedback control Loop is not in use at the moment. we can look at current on the output which is 400 milliamps.

If you press and hold this button for a few seconds, you can also see it as Watts on the output so that's about four. Watts Yeah, 10 volts, 400 milliamps is 4. Watts You can go back to Amps now and so now I'm going to wait. Uh, I'll tell you what I'll do I'll keep filming and I'll speed this section up.

Wait for this to rise up to 16 and a bit volts and then you should see the blue LEDs coming on on this Supercat module. Foreign. Okay, so I think this is a very versatile module because you can have a solar panel on the input. um, where you can Target the voltage on the solar panel.

So even if you're charging a 24 volt battery, you can set this pot to Target the solar panel voltage at I don't know, 17 18 volts. Whatever gives you maximum power. You can also have a battery on the input and the battery on the output. Or you could have a battery on the input and a load on the output like this light bulb for example or something a bit beefier.

Okay, I seem to set the voltage the voltage limit too low because the power's dropped off to almost nothing. So let's just raise that up a bit and now it's current limiting. You can see the current limiting Parts the little lid there. it's very dim as I say and now the protection circuits on the Super capacitor module are coming on.

and they will offset the full 400 milliamps because they are dumping 500 milliamps or more Actually, because these are 2.7 volts into the Uh two 10 Ohm resistors in parallel. So I'll just run this shot for a bit so we get a nice little light show on these six protection circuit. LEDs right? What I'm going to do now is add into the mix. Uh, this little relay.

This voltage controlled relay unit. I'll put the lamp on here and uh, what we'll do is when the Super Cap gets up to a certain voltage, we'll turn the relay on. Thus, turning the lamp on. This lamp draws about an amp.

I Think at this sort of voltage so it will offset the half amp or the 400 milliamps coming into the caps and drain them down and the relay will just turn on and off and we'll have a flashing lamp. So I want the voltage controlled relay module in those two terminals and then I want the Super Cap. I've taken the Super cap off because I don't want it shorting out and they're being a loud bang. So I want the Super Cat also across the output.

Get in there. Um, Okay, so the relay module is in parallel with the Super Cap. Let's put the Super Cat back on. now.

there's gonna be a little spark I Think here because there's 16 volts on this and the output capacitors are now down to naught volts Really? So let's just see what happens. Yeah, a little spark. Oh, that voltage has triggered this thing. Oh, we can't see that because uh, can't see the display.

I'll get it set up so we can see it. This has turned off again because the power bank is turned off because it's there's no current being drawn. Okay, there we are. That's all working.

So 400 milliamps Uh Current limit on this Sj20 power supply. Charging the Super capacitor when that gets up to 16 volts. So 15.8 15.9 Uh, when that gets up to 16 volts, Relay turns on, turns this lamp on that puts about a one amp load on the Super cap, so its voltage then falls down. The lower voltage on this is set to 15 and a half.

I Think uh, that's the point where the relay drops out, allows the Super Cap to charge back up at this 400 milliamps and the cycle repeats. Now you should see that the voltage is on these two devices are the same. So 15.8 Yeah, 15.9 Now, this has actually got two decimal places, whereas the relay module has only got one decimal place. But yeah.

so charging the Super Cap to a voltage, discharging it back because of the lamp, down to a lower voltage and oscillating. Uh, yeah. Okay, so now you can see the voltages a little bit better. You can see the needle on the Super Cat module.

The blue LEDs on the Super Cat module won't come on now because at 16 volts which is below the point where they come on, this light bulb comes on and then the voltage on the super cap comes back down. But I'll just run this for a few minutes now at high speed so you can watch the bulb come on and off. And Flash although you can't see the bulb very well, so let me turn that around like that. But I want to be able to see the voltage on there and the bulb, that'll do so, that's the Um ZK Sj20 buck boost power supply.

Um now 400 milliamps is not a high current. In fact, this thing's cold. The heatsink's cold. Um I've run this up to 10 amps and it started to get quite warm.

and one thing I noticed particularly was the output capacitors were getting quite hot. The input capacitors were getting warm. Now possibly this is because the series past mosfets are down here and the top of the mosfet is sitting on the underside of this. PCB So I'm not sure that the positioning of these capacitors is Optimum The other thing is, there's a heatsink here, stuck along the underside of where the chip is and the chip also says in the data sheet that it has to be soldered down.

There's an Underbelly pad on it and it has to be soldered down. to the PCB So presumably the chip gets quite warm. I haven't checked that yet. There is this five volt fan.

Port There are four mounting points here so you could put standoffs there to raise this thing up. and I've purchased a couple of uh, 50 millimeter fans I'm going to experiment with putting those uh on this unit to keep it cool when I was running it at 10 amps. I had a oh, just a standalone fan blowing air on this and that did seem to work. So in another video I'm just trying to be careful not to short things.

um I will do solar panel on the input and the use of this Mppt pot. Now just to mention here that if you want to get this Mppt function to work or at least to get it to Target a certain voltage. so one of my solar panels I know is 17 volts. you have to make sure that neither of the output um, control loops is being used so you have to make sure that the CV part is set high enough.

Say you're charging a battery, You set the CV part higher than the voltage of the battery and you set the CC part high enough that it doesn't current limit on the output and that's fine because your current limiting on the input. Of course, because the solar panel has a maximum current Beyond to which it can't go So by virtue of how solar panels work, your current limiting on the input. So as long as you're not using the feed the out output feedback loops, the Mppt pot thing will work. You might want to use the CV pot so that you don't overcharge the battery, but then of course, when the battery is fully charged, you don't want maximum power from your solar panel.

You want that to back off so that you don't overcharge the battery. But I'll do more on MPP and Mppt and Solar using a solar panel on the input because the other issue is what happens at night this whole unit Powers down and then you get the very slow rise of voltage and current on the input at the beginning of the day. Does this thing boot reliably and of course that I haven't yet tested. But uh yeah, that's the first look at the ZK Sj20 buck boost using the Lt87705.

Uh yeah, just another buck boost. Really cheerio.