lm723 current limit.

[yafch]

hi,
i,ve attached herewith a diagram of adjustable power supply using lm723 regulator and transistors. the voltages of the circuit are the same as i need. but the current limit is described by author of the circuit as from 4.2A-to-18A. i want to modify it for a current limit range from 100ma-to-5A only.what changes should be made? i think i understand the transistors part of the circuit but can,t figure out the resistor values and power, or other ciritical changes to be made, if any. so please give me an advice. i am posting the link to the web page also.the ciruit in question on the web page is "C. Regulator part with variable current limitation (4.2 - 18 A)" thanks to all.Stefan Spnnare: Power supply

 

[Boncuk]

I strongly recommend to look for a better circuit. That one appears to have been made a century ago.

If you want to use it to heat your room it will suffice.

Boncuk

 

[Arkham00]

Current calc

Here is an excel sheet with a diagram and some calculation that show current limits. Just change values of the resistors and find different limits. But pay attention that this circuit, as told by Bonkuk, dissipates a lot of power in current-limit conditions.

 

[Hero999]

User a centre tapped transformer and a tap changer to switch to the centre tap when the output voltage is lower.

Here's an example.
https://www.electro-tech-online.com/threads/changer-reduces-lm317-power-dissipation.87778/

 

[yafch]

oh,sorry........ one more thing important i forgot to mention in my first post is transformer. the transformer that i want to use is center taped 18v-0v-18v @ 5A max. so the circuit will have to regulate the current from 100mA-to-5A, and voltage will be regulated from 2v-to-max available rectified voltage.

during regulation of voltage or current heat is always generated. even simple regulator LM317 get hot a lot at max load. and without a large heatsink cannot be used.
if the circuit in discussion is very old, then please please give me a link to some good design that could be made at home.i must have current regulation. i have seen a lot of designs with LM723 on internet.they all lare based on same basic design with different regulations of voltage and current. thanks a million.

 

[JimB]

so the circuit will have to regulate the current from 100mA-to-5A,

Using a 723, current limiting takes place when the voltage between pins 2 and 3 is 0.63 volts (or there abouts).
In this circuit, that voltage is developed across the 0.15ohm resistor.
At 4.2amp the voltage across the resistor is 0.63 volts.
At 18 amp the voltage across the resistor is 2.7 volts.

The current limit control, when set to one end of its travel, connects the resistor directly to pins 2 and 3. The current limit is 4.2 amp.
When the control is set to the other end of its travel, the voltage from the 0.15ohm resistor is divided from 2.7v to 0.66 volts. The current limit is 18 amp.

To have a current limit of 100mA, you will need to change the 0.15 ohm resistor to 6.2ohm.
However when 5 amp is flowing through this resistor, the voltage across it will be 31 volts, and the power dissipated will be 155watts, a bit excessive!

Using this circuit configuration, you current limits are not practical without switching the current sense resistor.

and voltage will be regulated from 2v-to-max available rectified voltage.

The output voltage cannot be adjusted to the maximum rectified voltage.
There must be some voltage across the 2N3055 pass transistor, so the maximum output will be a few volts less.

JimB

 

[Hero999]

Use a 10R variable resistor in series with the 0R15 for the sense resistor, just make sure it can handle the current.

You're using a centre tapped transformer so it's fairly easy to add tap switching circuit similar to the one posted above.

 

[Boncuk]

Hi yafch,

you might want to take a look at this circuit. It uses Philips BUK116-50L MosFets which are not manufactured anymore. I have contacted NXP-Semiconductors to find out a replacement type.

However you can use any MosFet transistor with the same power characteristics. The BUK116-50L has an internal control circuit which switches it off at any abnormal condition indicating the status with a flag output.

The continuous power output of a BUK116-50L is 50A.

Regards

Boncuk

 

[yafch]

i am really thankfull to all of you for your kind replies. from all of the above discussion i,ve decided to give up this design. so much heat dissipated is not fairly good enough.

Agian much thanks to boncuk for providing another design. i am in habbit of jumping in complex designs rether than simple ones. And about the mosfet, i dont have it and their is no hope for me to find it. so i can go with other simple MOSFET,(i have a bunch of nice mosfets salvaged from different equipments). hope not to make any changes in schematic other than leaving the part of circuit for special functions of BUK116-50L.
thanks agian.

 

[Boncuk]

You can use any other N-Channel MosFet (with the same voltage and current rating) and omit the circuitry for the protection circuit, which are just 2 pins, the power supply and the flag output.

Boncuk

 

[yafch]

hi Boncuk.

some thing funny happened here to me. i was lucky to find from local electronic store a couple of BUK116-50ls not new ones but salvaged from some old circuit.they are in excelent condition. i can buy these.

the other thing is TLC271P,they are not available nor its replacement. i mean the programmable OPAMP models are not available to me.any suggestions? what about common opamp like TL071/TL081/OPA series/?they don,t have bias select pin.is it important?
thanks a million.