Hi,
some time ago I've built a power supply based on 3x LM317HVT IC's (circuit in attachment) and now I'd like to add current limiting to it. I was looking for a circuit that would limit the output current to 3A (max 3.5A) but I couldn't find anything that would do that at any given voltage (1.25V - 52V). Can anyone help me with building such circuit?
Thanks in advance

 

Welcome to AAC!

What do you want current limit to do? Constant voltage at maximum allowed current? Lower voltage to not exceed max current?

do that at any given voltage (1.25V - 52V)

Where is 52V coming from?

 

I'm using a transformer with 44V AC secondary that gives me 52V DC at the output. I would like to limit the current so it never exceeds 3A - 3.5A.

 

Hi Vr6, You would need to put in a current sense resistor after the output from the 317 ICs. To minimise power loss in the resistor you could then use something like an INA180, which then gives you a voltage proportional to the current taken. If you simply want to switch off the power when you take too much current, you can then simply use a comparator to increase the voltage on the adjust pin of the LM317 (through a diode for example) - you will need to add some delay circuit into it so it doesnt simply oscillate of and on however.
If you want to drop the voltage until you are the maximum allowed current then you will need to take the signal, use a differential amplifier (to give you a signal >0V when your current is exceeded) and then feed this into the adjust pin (this will have to be done with a summing amplifier, so it there is 0V from the current protection, the voltage regulation is as normal.) - Not having a negative going power supply will prevent the difference amplifier from "interfering" until there is an over-current situation.

There is also a circuit in the data sheet for the LM317:

 

I'm using a transformer with 44V AC secondary that gives me 52V DC at the output.

That is a lot of voltage to drop over the regulators.
What output volts are you after? Are you using it with low o/p volts a lot of the time?
A switch mode supply would be a much better way to go.
When you short the output, or run low volts out at full current, there will be over 180Watts dissipated by the regulators.

 

That is a lot of voltage to drop over the regulators.
What output volts are you after?

The range I'm after is 3V to 36V.

 

Thats a lot of power to dissipate for 3V @ 3.5A! (170W!)
Most supplies usually have relays inside which actually switch between transformer taps to change the full scale range & prevent having to cope with such large amounts of heat dissipation.

 

Thats a lot of power to dissipate for 3V @ 3.5A! (170W!)
Most supplies usually have relays inside which actually switch between transformer taps to change the full scale range & prevent having to cope with such large amounts of heat dissipation.

Transformer I'm using has only two taps 26V and 44V.

 

Ah you could then put in an additional feature to switch to the 26V tap, when your voltage drops to this level, though its a bit more complex to implement.

 

.. that said, the most efficient and simple is to actually use a switching DC-DC regulator.

 

Thanks for suggestions. Maybe I will pass on 3A and limit the current to just 1A.

 

Linear regulators are really not used as much as they used to, because of the problem with dissipating the heat. Something like this:
http://www.ti.com/lit/ds/symlink/tps54560.pdf
For your application is very much more compact (though there are many different ICs around). You also have the benefit of being able to draw more current at lower voltages.

 

I'm using a transformer with 44V AC secondary that gives me 52V DC at the output. I would like to limit the current so it never exceeds 3A - 3.5A.

Okay.

You said the schematic was for your power supply, but it isn't. It stated 24V for the input, which I interpreted as 24VDC. I forgot that you stated you were using the high voltage regulator as soon as I looked at the schematic and saw LM317T.

Personally, I'd've used a power transistor instead of the other two voltage regulators.

As noted, a linear regulator isn't appropriate for your requirements.

 

Okay.

You said the schematic was for your power supply, but it isn't. It stated 24V for the input, which I interpreted as 24VDC. I forgot that you stated you were using the high voltage regulator as soon as I looked at the schematic and saw LM317T.

I used that schematic and changed 317T's to HVT's as I said in first post.

 

Thanks for suggestions. Maybe I will pass on 3A and limit the current to just 1A.

You'll be dissipating 3W in the load and more than 15 times that in the regulators.

A switching regulator would still be more appropriate.

 

I used that schematic and changed 317T's to HVT's as I said in first post.

In other words:

What else might be different?

 

If you don't need a constant-current type limiter, which can dissipate a lot of power, you could use a "hiccup" limiter which rapidly shuts the current on and off to do the limit, which dissipates very little power.