I don't know how I managed to do this, but I used a Zener instead of a MURS120 for D7, bottom right (that was the closest model I had on hand to the specified diode)
It won't affect normal operation of the simulation, as long as the voltage on the filter cap doesn't exceed 33v.

 

Hi ChrisChemist116,


More...

1. The current sense voltage (CS to CL) is ~0.66V at 25C. Therefore R8 0.33 will set the maximum current to .65/.33=2 Amps maximum.
2. Put 2 0.33 resistors in parallel to get a 1 Amp limit.
3. The constant current in P3 generates an offset voltage to reduce the current limit depending on the value of P3 and the current from Q1.
4. Adjust R5 so the constant current from Q1 is 660uA.
5. I changed D5 LED to one with a lower voltage drop(3V to 1.8V).

I have attached an updated schematic.


Sgtwookie: I wondered what those zeners were for... you must have been sleepy. As you can see, I just deleted them, but 1N4002's in there place wouldn't be a bad idea.

Regards,
Ifixit

 

Hi,




I grabbed the design file that sgtwookie posted and ran a few tests. That design has some problems. Here is a list of defects and recommendations:

1. The transformer output is 30V RMS at 1 Amp. Minus 2V(1V each for two diodes), minus the minimum droup of the ripple voltage at 1 Amp is 10V, minus 2V for series regulator transistor headroom. Therefore the maximum output at 1A is 30V -2-10-2= 16 V.
2. Changing the filter cap to 5600uF reduces the ripple to 3V and allows an output of 23V.
3. Using a transformer with a 38V secondary would allow the 30V output.
4. The current source, Q1, needs 3.5 volts of headroom to work properly, plus the drop across R8 means 4.1 Volts is required above 30V for current limiting to work right whin the output is set above 26V.
5. Change the Vcc voltage to 35V. Caution: Since the Vcc is -4.6V, this will put the 723 near its maximum operating voltage of 40V.
6. Adjust P3 so that it is at minimum resistance, which will allow for 2A current maximum. This will ensure that current limiting is not killing your output during your first power up tests.
7. Adjust the P1 pot so that pin 5(non inv) is approximately 0V wrt ground.

I have more, but it is late... time for bed.

Regards,
Ifixit

First of all thank you for your valuable suggestions, i knew something was wrong with the schematic. I will do it again and see what happens.

I think i have a 5600 uf capacitor somewhere in my workbench, aside from that i dont have a 38V (secondary) transformer, but i do have a 33V one, can i use it?.

I understand the point of reaching 723 to its maximum operating voltage, but i think it will be safe if its below 40V even if it can go up to 39.6V?.
I have adjusted P1 to its minimum but still no changes noticed.

Anyways i will do the whole circuit again.

Hi ChrisChemist116,




More...

1. The current sense voltage (CS to CL) is ~0.66V at 25C. Therefore R8 0.33 will set the maximum current to .65/.33=2 Amps maximum.
2. Put 2 0.33 resistors in parallel to get a 1 Amp limit.
3. The constant current in P3 generates an offset voltage to reduce the current limit depending on the value of P3 and the current from Q1.
4. Adjust R5 so the constant current from Q1 is 660uA.
5. I changed D5 LED to one with a lower voltage drop(3V to 1.8V).

I have attached an updated schematic.


Sgtwookie: I wondered what those zeners were for... you must have been sleepy. As you can see, I just deleted them, but 1N4002's in there place wouldn't be a bad idea.

Regards,
Ifixit

Thanks for adding the schematic along with your suggestions. This response is after taking notice it was posted with your message.

I had the same question about what were those zeners?, as in the original schematic featured 1N4001. I can see in the new schematic they have all gone. Would restore them do any change to the output?

I see a LED has been added to filtering capacitor output and is in series with 33V zener cathode, do its purpose is to act as an ON indicator?, the same goes for D5, is it an ON indicator too?

All i can say it is that these new changes proposed on the schematic i haven't tried yet, because it is some late here. But i will do it tomorrow in the morning maybe it will solve the problem.

Thanks ifixit.

 

Hi,

• The LED in series with the zener is to get 35V, because the LED has a 2V forward drop like a small zener.
• The diodes won't effect the output. I think they are intended to protect the PS output from externally connect power sources.
• D5 acts like a 1.8V zener reference for the current source.
• The changes were not intended to fix your problem, but to greatly improve the design so it can meet the specs.
• With regards to your problem, you likely have something not hooked up right, or a bad component.
• You should be able to find the problem by taking a few measurements with a voltmeter.
• The transformer can be what ever will work for you. Tip: if you use a 32VAC transformer with a 5-6A capability, much more than you need, you may get enough voltage from it at 1Amp.

Regards,
Ifixit