LT1085 constant current. will this work?

Thread Starter

wall_

Joined Sep 11, 2013
44
THIS is where the discussion needed to start.

So let's flesh this out into a reasonable set of basic specifications. THEN we can start discussing ways to design a circuit that meets them.

Let's assume that you already have black box that is an adjustable constant current power supply with a voltage limiter.

Is that REALLY going to do what you want?

It might be fine for powering some LEDs, or laser diodes, or charging batteries (at least certain kinds of batteries), as long as you don't try to do more than one of these at a time.

But that "power external peripherals" raises red flags. What kind of external peripherals are you planning to power with a constant current supply? Even if you have them, you can't power them at the same time that you are doing anything else for which you want a constant current.

So the first thing to consider is whether this thing, if built, is really going to do what you are hoping it will?

If not, then the best route is probably to decide what things it is reasonable to do with it, and take anything else off the table. The alternative is to rework your description of what you want into one that can reasonably serve all these masters. The latter will likely lead to significant complications, both in the specs and in the design. It might be better to take things off the table and plan to use a different, better-suited supply for those things.

Assuming that you still want an adjustable constant-current supply with a voltage limit, what range of currents do you want/need the current output to be adjustable over? What is the maximum output voltage that you want it to be able to output? What is the range of voltages that you want to be able to adjust the voltage limiter to?

Keep in mind that if your max current and your max voltage combine to result in a high power level, that the design gets more complicated. Quick. You might have to settle for a supply that is derated, meaning that the higher current, the less the max output voltage is at that current. Implementing this also complicates the design, unless it is done by either the user being responsible for applying the limitations, or the limitations are imposed by having the unit shut down if a temperature sensor indicates it's getting too hot.

The more power that the unit has to dissipate and/or deliver, the more attractive using a switch-mode supply becomes.

Be careful not to wish for the moon -- the more you want, the harder it's going to be to design and implement and the more it's going to cost. Be driven by an honest assessment of what you NEED, not what you would like.
I know that I can't power multiple different things at once from a single power supply, I just want to build my own with these regulators I have laying around.
I know the CC part is not helpful for powering peripherals, but it can still act as a current limit for the CV part of the supply.
I don't plan on powering high current devices to this supply, and even if I do, I'll keep the input voltage as close as possible to the output voltage since these are linear regulators.
 

Ian0

Joined Aug 7, 2020
9,668
I know that I can't power multiple different things at once from a single power supply,
Yes you can - most peripherals run off the same voltage.
I know it's old, but it is probably worth your while looking at the LM723 datasheet
https://www.ti.com/lit/ds/symlink/lm723.pdf
it shows you how to make a voltage-regulated power supply with a current limit, which is probably the most useful sort.
You can adapt it easily to give a small amount of adjustment to the current limit.
 

Thread Starter

wall_

Joined Sep 11, 2013
44
Should D1 connect to OUT before or after Rsense?
And can I place a capacitor on the ADJ pin on the left regulator? If so, should I also add a protection diode between ADJ and OUT?
1679476822484.png
 

WBahn

Joined Mar 31, 2012
29,978
I know that I can't power multiple different things at once from a single power supply, I just want to build my own with these regulators I have laying around.
I know the CC part is not helpful for powering peripherals, but it can still act as a current limit for the CV part of the supply.
You can power multiple devices from a constant-voltage supply, as long as they all need the same voltage. This is done all the time by placing all of the devices in parallel. Each device pulls the current that it needs and the supply has to provide the total of those currents.

In principle, this works they other way as well -- you could power multiple devices from a constant-current supply, as long as they all need the same current. Here, you would place them all in series and each device would develop the voltage it needs and the supply has to provide the total of those voltages. This is exactly what is happening when you use a constant-current supply to power multiple LEDs in series.

I don't plan on powering high current devices to this supply, and even if I do, I'll keep the input voltage as close as possible to the output voltage since these are linear regulators.
We still don't have any kind of a useful spec to work with.

What is the maximum current that you need this supply to be able to deliver?

What is the maximum voltage that you need this supply to be able to deliver?
 

Thread Starter

wall_

Joined Sep 11, 2013
44
We still don't have any kind of a useful spec to work with.

What is the maximum current that you need this supply to be able to deliver?

What is the maximum voltage that you need this supply to be able to deliver?
I'm happy with the 3A that this regulator is able to provide.
I will only be powering low voltage devices such as high power white LEDs and laser diodes, charging lithium batteries, etc.
As for input, I will only be using 12V from an ATX power supply or lithium batteries in series.

You can power multiple devices from a constant-voltage supply, as long as they all need the same voltage. This is done all the time by placing all of the devices in parallel. Each device pulls the current that it needs and the supply has to provide the total of those currents.
I know I can parallel multiple devices from the same voltage source as long as they all can work at said voltage, or cars would require 100 batteries.
 

Thread Starter

wall_

Joined Sep 11, 2013
44
I've finally gathered all the necessary parts and some spare time for this project.

One thing I had overlooked in the original schematic is that the output voltage will go as high as it can when the wiper loses contact with the track.
Does the PNP transistor in this schematic protect the output when the wiper loses contact with the track? Sorry for the VeroRoute sreenshot as I was already planning the breadboard layout when I added the transistor.LT1085.png
 
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