Hello I am trying to design a 12.6V power supply using the circuit below. I plan to use this to charge a 12V lithium battery, but I need to add some circuitry that will limit the current inrush especially when the battery is at low voltages.
I think a circuit like this is what I am going for, but I am unable to size the specific resistors and transistors...
Can anyone offer some advice to me
R1 must be small enough to make Tr1 supply the maximum current at maximum output voltage. The maximum current is determined by R2 and a voltage of about 0.7V (diode forward current).
ZD1 determines the lowest output voltage.
Thank you, do you happen to know any equations I can use? I saw the article was posting these equations. But they are related to the "foldback current limited circuit"
I cannot identify foldback current limiting in your circuit. For your circuit I gave you the rules for R1, you have to take into account the minimum voltage difference (input-output) and minimum current amplification of Tr1 for determination of the maximum R1 value.
There isn't foldback limiting in the circuit I posted above. I was saying that the article I linked above goes on to describe the foldback limiting circuit and lists design equations for this circuit. But not design equations for the constant current limiting circuit, which is what I am interested in and posted above.. Hope this made sense.
Is there a way to calculate precisely my max current if I assume this 0.7V forward current and some value for R2?
Ohm's law:
I=U/R ~ 0.7/R2
R=U/I ~ 0.7/Imax
If this is true then max current is independent of R1. I think you have actually already confirmed this but I am trying to understand how this is possible. For example what if I put a 1MOhm resistor for R1 then wouldn't the voltage at the TR1 base approach GND and therefore the transistor would never conduct.? Am I incorrect with my logic here
It was mentioned earlier that:
So is it that I fix my max current using R2 but then R1 affects the voltage drop from input to output. In my application I actually would prefer as little voltage drop as possible from input to output... So should R1 be as miniscule as possible? Therefore this means I should use a wide PCB trace etc to make this as small as possible.
There isn't really a mention of the component circled below in the article I listed, but is this just a simple junction? Not a capacitor or resistor
Schematics should read left to right, input to output. Highest voltage at top, lowest or gnd at bottom, parts oriented to appropriate rail.
At least ghost in the battery and it's params, mAH, C, etc.
Then you should use a PNP for Tr1 and add another transistor to drive it. See high-side switch circuits.
It's a pot for adjustment of the output voltage.
You a proper charger for some types of lithium battery , they can’t be charged off a constant voltage with current limit .
Multi cells also need balancing circuitry to keep the charge of each cell the same .
Have a google for your battery type .
I see now! I have caught an NPN on fire by not putting a resistor on the base. This didn't come to my mind initially but now I realize without a resistor here I think I'd catch this one on fire too!
Also I do see your point about the pnp for Tr1.
I was having trouble understanding the pot adjustment so I slightly simplified the approach
Hunting for parts led me to this circuit:
Now I do understand there is at least one major flaw in this circuit. This is that when the battery voltage hit some max value say 12.4V the supply voltage will still be connected to the battery it needs to open a switch or something when the inlet voltage approaches the battery voltage so I am still off here.
The goal of this circuit is to charge a battery to a max of 12.6v with an upper current limitation of 350mA. I understand this may not be applicable to all batteries etc. But I am just trying to solve this problem with these simplified constraints.
Also I wanted to clarify why I chose the 1250 Ohm resistor for R1, this was assuming a gain of about 50 on this NPN transistor. Such that if 350mA through the collector would require 7mA at the base. Assuming the lower end of the voltage 11V this would mean 11/0.007 = 1570ohm. Then I took away some resistance just for error margin...
Sorry to disappoint you, but the voltage is only the difference between input and output voltage, minus the voltage drop on Tr1 Ube and R2. For a low drop regulator you definitely need a PNP transistor, then you can have 11V on the base resistor.
Not disappointing more like embarrassing haha. I am struggling a little with the proper layout. My best idea seems to be the below circuit. Have experimented with a few other layouts but this seems to make the most sense.
What voltage do you expect to have on the base of Q2?
I would assume 12.6-11V is this incorrect?
No, because Q2 will drop only about 0.6 volts across its base-emitter junction. Also, there is no current limiting resistor for Q1 base current.
Ah yes you are definitely right about the PNP base resistor. I think it would've caused some problems haha. Since I am going for at least 7mA I added R3 at 1.5k ohms. I was trying to understand your comment about the 0.6V though.. Not saying you are wrong or something but where did this number come from?
Adding R3
I was thinking about your comment though about the base and emitter of the NPN being relatively close in voltage so this means that the set-up below is probably what I want then because it will allow the higher voltages to flow to reach the diodes... I marked in red the areas where you mentioned the 0.6V difference..
Now you have no current limiting resistor for Z1 and it will fry.
Let me repeat what I said long ago:
You still have to add that third transistor for proper feedback.
I see so step in the wrong direction. Looking back at the circuit below I see the diodes are protected by the R1 resistor. So it must be incorrect that I am pulling the PNP all the way to ground through the NPN transistor.
If the PNP isn't being pulled to ground though it does mean I have incorectly sized R3 as there is actually just a little voltage drop not the full 12.6V so R3 would need to be smaller.
