Adding a current limit led to my lm723 bench psu

[dr pepper]

Chaps, I have a relic psu on my bench, it has 2 independant lm723 regs in it.
I'd like to stuff an led (one for each reg) on the front panel (and maybe a 555 to trigger a beeper when current limit is activated) so I know if the supply is in CV or CI.

I found a schematic for another lm723 supply here:



The documentation doesnt say what d5 (led) is for, it looks like it might be a current limit indicator, hence my interest, if so how does it work?, I'm more used to microcontroller circuits.

I had an idea for a current limit led before I found this schematic, connect the base of an npn tranny through a resistor to the cl pin, and the emitter to the cs pin, then connect the cathode of the led to the collector, and up to the supply for the '723 through a resistor, basically connecting the tranny that switches the led base emiter accross the current sense resistor, so that what the vdrop > 0.6v the led comes on.

Which should I go for my idea or the attached schematic.

 

[Timescope]

The led D5, resistor R5 and transistor T1 are a constant current source of about 1mA = (Vf - Vbe)/R5 assuming Vf =1.8v, Vbe =0.6v, R5 = 1.2k where Vf is the forward voltage of the led and Vbe is the base - emitter voltage of the transistor T1. The 723 current sense circuit is the base - emitter circuit of an internal transistor and needs 0.6 volts to start limiting the current so the additional voltage drop caused by this current flowing through P3 enables the output current limit to be set to zero ( when 1mA x P3 resistance = 0.6v). You could add another transistor between pins 1 and 10 (check the schematic on the datasheet) and use the collector to drive an led to indicate current limit, the idea you mentioned in your post.

Timescope

 

[MrAl]

Hi,

If you can afford another 0.7v drop i'd go with the additional transistor. Add 100 ohm resistor in series with the base for conditions that could occur and cause the base current to increase higher than the spec for that transistor.
As im sure you know, using the base emitter of a transistor to sense current (an old style used many times in the past) means there is some temperature dependence on the level where the transistor turns 'on'. This is also the case with the 723 as it was never intended to be a super accurate current sensing device just a pretty good voltage regulator. We used the 723 as far back as i can remember, early 1970's perhaps.

 

[dr pepper]

Timescope, thanks for an excellent description, yes I understand that clearly.
The led is the reference that the transistor uses to maintain the led's vf accross the load resistor r5 -vbe of the tranny.
I think what stopped me from grasping that in the first place was the use of the led as a voltage ref, something that I spose was popular in the early 70's, its not something I've seen a lot.

The datasheet shows pin 1 as n/c for the dil version, I think you mean the pinout for the t05 can version, pins 1 and 10 on that are the base/emitter junctions of the inbuilt current limit tranny.
So when the current limit pot is at max resistance the current limit will be at 0ma (well 1ma), and when the pot is at min resistance the imedance of the current sense resistor will take precedence and the current limit will be controlled by the sense resistor only, in this case 2a.

I'll add the mods to my supply, it only needs 3 wires connecting.

I'll put the led part of an opto isolator in series with the limit led on each supply (2 supplies are independant) then connect the phototranny collectors together to the 555 that controls the buzzer, or maybe I'll program a pic to do that yet.

Mr Al, do you mean put the current limit led transistor in series with the current limit transistor within the '723, I spose that would guarrantee that the led come on when the '723 is actually limiting, I might need another tranny to switch the led as the voltage between the current sense terminals and the + rail vary wildy being an adjustable supply.
Yes this supply is right out of the 70's, its been blown up a few times.

 

[Timescope]

Thanks. Yes I was referring to the T05 can version (I still have some of them) . The opto isolator idea is great, once again showing how versatile and useful opto isolators are. Hope all goes well.

Timescope

 

[dr pepper]

Ok then, I have done some mods made possible from your advice.
The current limit works great.
The brightness of the current limit led depended on the o/p voltage which wasnt that great, so I've constructed another current source using another led/pnp trans to supply the anode of the current limit indicator led, the led brightness is now nearly independant of o/p voltage.
I used a 2n3904 trans in parallel with the current limit trans in the 723 to start with, but I found the curremt limit led brightness came on slow, real slow current was 1/10 of what the resistance wanted to pull at the set voltage before the led was fully on, I replaced the '3904 with a bc108 and now its much better but still comes on slow, I need some sort of trigger circuit.

Edit: the situation is somewhat improved now I removed the 30yr old led I was using for indicating current limit and replaced it with a new high brightness one.

 

[MrAl]

Timescope, thanks for an excellent description, yes I understand that clearly.
The led is the reference that the transistor uses to maintain the led's vf accross the load resistor r5 -vbe of the tranny.
I think what stopped me from grasping that in the first place was the use of the led as a voltage ref, something that I spose was popular in the early 70's, its not something I've seen a lot.

The datasheet shows pin 1 as n/c for the dil version, I think you mean the pinout for the t05 can version, pins 1 and 10 on that are the base/emitter junctions of the inbuilt current limit tranny.
So when the current limit pot is at max resistance the current limit will be at 0ma (well 1ma), and when the pot is at min resistance the imedance of the current sense resistor will take precedence and the current limit will be controlled by the sense resistor only, in this case 2a.

I'll add the mods to my supply, it only needs 3 wires connecting.

I'll put the led part of an opto isolator in series with the limit led on each supply (2 supplies are independant) then connect the phototranny collectors together to the 555 that controls the buzzer, or maybe I'll program a pic to do that yet.

Mr Al, do you mean put the current limit led transistor in series with the current limit transistor within the '723, I spose that would guarrantee that the led come on when the '723 is actually limiting, I might need another tranny to switch the led as the voltage between the current sense terminals and the + rail vary wildy being an adjustable supply.
Yes this supply is right out of the 70's, its been blown up a few times.

Hi,

Well if you use an NPN transistor for the LED for example and you connect the emitter to ground, you should not see any variation in LED brightness with output voltage setting, or at least not too much. The LED would be powered from the input power, assuming that does not vary wildly, but if it does then perhaps a zener or something. This kind of current sensing is done in the ground line and may not be what you want.
Alternately, use a PNP transistor from the +Vcc power input supply sensing with the base and LED to collector, and LED resistor from LED to ground.
Not shown in the diagrams is the 100 ohm base protection resistor.

If you got it work already then i guess this is moot

quick diagram:

Using NPN:

+Vout                        +VccInput
|                               |
LOAD                            R1
|                               |
+----o base (load ground)      LED
|                               |
Rs                              o collector
|
+----o emitter                
|                             
|
+--true ground (ground for input power too)


Using PNP:



+Vout       
|
+---o emitter PNP
|
Rs                      o collector PNP
|                       |
+---o base PNP          R1
|                       |
LOAD                    LED
|                       |
+---o ground            +----o ground

 

[dr pepper]

Yeah I see what your saying there, I assumed that I had to sense the high side of the load as all the datasheets and examples of the '723 do this, but looking at the datasheet the current limit trans within the chip isnt commited, only the collector is tied to the base of the internal pass trans.
I could ground the internal limit trans emitter and put the sense resistor in the low side of the load, then as you pointed out I could do away with the current source for the led.
I'll try that now and report back.

 

[dr pepper]

Thanks Mr Al, your objectivity has improved my design.
I put the current sense resistor in the low side, from the - of the load to true ground, connected the sense resistor to the current limit trans within the chip, and paralled this with the bc107 limit led trans, the collector of the bc107 goes through a resistor and led to the supply as you suggested.
Also I connected the voltage feedback ground to the opposite side to true ground on the sense resistor, meaning that the 723 will regulate out the vdrop caused by the sense resistor.
I've lost all the parts for the led constant current circuit and now the led reaches full brightness when the current limit circuit has only reduced the o/p by 40mV, excellent.

View attachment 723 supply.pdf

 

[dr pepper]

Well I thought I could improve on the previous circuit, so I have included an op amp for current limit.

The current limit on the '723 is disabled, theres a sense resistor before the +o/p, this is amplified by 1/2 a lf357 acting as a differential amp, the gain is such that full current o/p is 5v (meaning I dont need to worry about a few mv offset, and this can be used to provide an input for a pic), this then goes to a comparator comparing the amplified current voltage with vref on the '723 using the other 1/2 of the '358, when the current exceeds the preset the comprator shuts down the '723 by shorting the comp pin to ground through a 2k2 resistor (as per data sheet remote shut off).
Seems to work really well, and to proper improve the limit led operation the led comes off a 555 triggered by the comparator.

 

[MrAl]

Hi again,

Maybe you can post a circuit drawing so we can take a look?

One thing which you may or may not be aware of is that an op amp can seriously slow down the response to an over current condition. That's why a transistor BE is used so often. The transistor is so fast you cant get any faster, so it protects the circuit remarkably well. An op amp can take 10us or more to respond, so you may want to look into that. You could test it too for various over load conditions like 10 percent, 20 percent, 100 percent, etc., just to see how long it takes to response for each level. It should work faster for the higher levels but still might be too long for comfort. If you post the circuit we would be able to make some estimations.

 

[MrAl]

Cam on ban da post bai, chuc mot tuan moi vui ve va hanh phuc.
Dien dan chung ta se ngay mot manh me hon.

Rough translation:
"Thanks for the post. Wishing you a good week and good luck with the forum as we continue to learn more."

Hi,

You're welcome, and same to you, but i think you should post in English on this forum as most readers here read English.

 

[dr pepper]

I had come up with 2 variations since the last drawing, one using op amps as mentioned, as yes the response wasnt that great, and the other version I've done is one that uses 2 lm723's, one controls voltage and the other controls current limit, the current limit controls the volatge 723 by using its inbuilt current limit trans as per schem, the only thing I'm not sure about is the current limit '723 'see's' 1v of common mode between its opamp input pins, this is because it sits there withn its output grounded untill the current sense input goes above the current set point, at which point the current limit '723 takes control.

Fisrt op amp version schem:

View attachment 723 supply lf358.pdf

Second dual '723 schem (with negative supplyt to get the output to go to ground):

View attachment 723 supply 2 x lm723.pdf

Edit: I had an unfortunate incident where the pass trans went short base collector as it got too hot, this resulted in removing the lid of the '723, blowing the 2k2 resistor feedeing the -4v7 zener and vaporizing the latter zener, so now there is a 3k3 in series with the non inverting i/p of the current limit '723 with a 3v3 zener to ground, so the i/p cant go over 8v with ref to -4v7, as it didnt like +35v

 

[dr pepper]

Ok then I had problems with noise on all designs so far, having an op amp in open loop or high gain causes noise issues, should have know that.
So to get around this I borrowed an idea I saw elsewhere, instead of the current limit op amp operating as a comparator, configure it as a integrator, so the crossover from off to on is gradual without the usual flutter around the switching point caused by noise.

Heres my circuit, this is pretty much final, constructive criticism welcome.
The only issue I have with this is at the moment the '358 dual op amp is operating from the main supply rail at +35v, 3v over its max, so when I can get a packet of 16v zeners I'm going to tap off the main rail and provide +32v for the op amp and the '723, this will restrict the max op/p to +29.5v, but that'll do.
The 1n4148 in series with the opto is to protect it from 25v reverse bias:

View attachment Latest 723 psu.pdf

 

[MrAl]

Hi,

Just a note, you have to be a little careful here because making an op amp into an integrator means it takes more time to respond. That might be ok in a current limit circuit, but for fast overcurrent protection you may want to consider a comparator with hysteresis but so much hysteresis that it acts more like a foldback limiter rather than just a limiter. This way you have your current limit setting and you have your overcurrent trip setting.
Good power supplies have a current limiter and also a short circuit protection circuit that trips when the max current of the supply is exceeded.

 

[dr pepper]

Yes understood, I've tried shorting the supply from the circuit at various settings, nothing bad happens, I think that the transformer only being rated at 1.5a, and the fact there is a 0.47r resistor in series with the negative means that a current high enough to blow the tip147 is unlikely in the event of a short on the o/p, the itegrator catches up in a few ms.
I was thinking an still might implement the built in current limiting of the '723, set at a bit higher than the inetgrator circuit max.
If it were a 20a supply then I'd be paying a lot more attention.
I'm pleased that this circuit produces a very clean o/p even when current limiting, better than my or works lab psu's do, the '723 might be a dinasour but it works well.

 

[dr pepper]

Well during the holidays I have built some more of the supply, I now need to do a duplicate for the other side.
Of course I ended up putting a pic on it, to display volts, current, watts and heatsink temp, it also controls the fan via the temp sensor ds18b20, which is not connected in the pic, hence 127 degrees.





I ordered some pnp trannys instead of npn for the pass device by mistake so I just reconfigured the o/p of the '723 to drive a negative voltage for the base.

I intend to also put a fixed 5v o/p on this supply, using a lm2576 with adjustable current limit.

 

[dr pepper]

Just to make a late update to this thread, using pnp output transistors is a bad idea, even though the datasheet for the LM723 suggests it.
The problem is theres a nasty spike on the o/p that goes to the full dc bus voltage at power on, I think what happens is that the o/p transistor gets switched on by its bias resistor before the reg circuit has time to remove the drive from it.
NPN o/p trannys dont do this.

 

[Mr T]

Hi Mr Al, Dr Peper and Timescope

I have followed your discussion and found it very helpful as I had exactly the same question re current limit indicator led posted by Dr Peper. I need only a very basic regulator and want to use the LM723. I like the original basic circuit with the current sense resistor on the low side of the load. I prefer this method as my LCD meter is disinged to pick up the current for measurement across this sense resistor on the low side of the load.

I have a 60VA trans former (20V RMS @3A). How can I increase the current. The circuit of Dr Peper works for me except that it limits the current at about 1 A. I wold like to try and get about 2,5A at 20V regulated Vout. I have changed pots and even now have the sense resistor = 0R22. Can you help. I(limit) = 0,6/0.22 = 2,7A. What do I do wrong.

I do not want to employ the op amp design that Dr Peper has also posted for certain reasons. I am fine with the simple design with the BC547 connected to the CL pin etc.

 

[MrAl]

Hello,

Try an emitter resistor for the BC547 like 10 ohms or so.

If that doesnt work it could be that the output of the 723 can not drive the 3055 enough so you'd have to add another transistor.