Okay, so today we're going to be talking about the Improved Hail and Current Pump. It's a Op-amp circuit with five resistors and a voltage reference which can be used to generate a constant current for a few milliamps tens of millions, which can be used to drive an LED or a sensor or anything that requires a small amount of current, which doesn't require particularly high efficiency. So let's get started here. We have a difference amplifier.

A difference amplifier is probably the fundamental building block of a Halland current pump. and you'll notice that this is a unity gain difference amplifier. That means that the input and output have a gain of 1. they multiplied by 1.

There's no scaling, and there's a few different things that are interesting about a different sound player. The first is that it doesn't matter if one of these terminals is ground. In fact, it doesn't matter what they are at all. You'll notice that the output voltage isn't changing.

That's because the output is dependent on only the voltage at this node subtracted from the voltage at this node. It doesn't depend on the the values reference to this ground at all. It's just a subtraction of these two. And in this case, it's 1.

Minus 0 is 1. And in this case, oh, it's 0 minus minus 1, which is 1. Again, and there you go, you have it at the output. So that's the fundamental building block.

and if you add a resistor to load the Op-amp just a load resistor. It doesn't actually do anything to the gain. It doesn't do anything to the grounds, and It really doesn't affect the circuit at all. So that is probably the first step to building a Halland current pump.

first. I'm gonna remove this ground because it really doesn't matter. it doesn't do anything for the circuit. And I guess the next thing to notice is that if you have 1 volt on the output one k here, then you're always going to have one milliamp going through it and that that can be used.

That can be a useful trait in understanding a Halland current pump. So if we made this one own which I'm going to do now, you'll notice that the output hasn't changed at all and the the gain hasn't changed at all. There's an interesting property of this circuit, and it's that the whole circuit is basically referenced. To this point, it's the only ground in the circuit, and that means you can make it a virtual ground, which is frequently created with just a unity gain Op-amp So let's create a buffer and we're going to buffer ground.

It's kind of useless, but I will get to this and I'm just going to move the load all the way up here. I'm still going to connect it to ground and then I'm going to connect that buffer up here. Just move it down a little bit and move that up. So this is all fine.

now. we've got this virtual ground here and we can also connect it here. We can connect the virtual ground to the ground of here and it doesn't actually matter what this voltage is, the whole circuit will shift with it because every element every node in the circuit is referenced to this point here. the virtual ground where the absolute ground is over here.

So what we're going to do is apply a voltage source here and see what happens. So if I put one volt here, the whole circuit has shifted up. That's 2 volts. That's 2 volts.

Okay, so with that, we can say that this is a true virtual ground. We can use this as a virtual ground without really worrying too much and this output is still actually 1 when you compare it to the virtual ground which is here. So let's do that. Let's probe the Op-amp output with respect to the virtual ground and I'll just use one of these probes over here just like this one and you'll see it is one volt as it always should have been.

So if we change the offset of the whole circuit of two volts now, it's outputting three volts. but the virtual ground is now 2 volts and 3 minus 2 is still 1 volt. So what we're able to do with that is create a constant voltage over this resistance and a constant voltage over resistor is a constant current. So now what we can do is something like this.

We can put a another resistor here and no matter what we change this to, it will still have one milliamp going through it. where the path through the resistor is down here and I just make more obviously labeled circuit. There you go Where the path is down here for the current and the reason it's constant is only because you've got a constant voltage across this resistor. Now a unity gain buffer has a few properties to it.

It has the property that the input is very high impedance, a very high impedance when compared to these values here. and the other property is that the output is very low impedance as in it has it has a very low output resistance and that is actually what this has. This 1 ohm is very low resistance, So maybe without too much error, we can simply remove that buffer entirely and place it like this. Now this has a problem.

You notice that a lot of the current is flowing in from the non-inverting side of the Op-amp and that is creating error. See, we've actually got one-and-a-half amps going one and a half million. It's going through that output resistor. Now, how do we deal with that? Well, the real issue was the impedance.

We we want that that terminal here to be as high impedance as possible. so all we have to do is increase the resistors for that section of the circuit. And now almost no current is traveling through this section of the circuit because it's behaving very high impedance as in very high resistance. That means that you've got a very low current if the voltage isn't enormous.

So you can say that the vast majority of the current is flowing through this resistor here through here and you only lose a very small fraction through this wire. In this case. it's five micro amps which is just nothing. and this is the Helmet Current Pump.

It's a very simple circuit. The sources of error are of course the current going through this section. Feedback Loop the input offset of the Op-amp You with high impedance feedback resistors. You you get input current.

You get this current bias and that will create a sort of offset which can become a problem. and you can also get some stability issues with the circuit and sometimes you'll want to do something like this. but often this isn't the case and often you can just get away with just doing some old resistors. And yeah, all right, this is the Elinchrom pump.

Hope you've learnt something from this video. if you liked it, Leave a thumbs in the thingy and I will see you next time. Bye you.