LM324 Power Supply with variable voltage and current

[xavier60]

Response looks normal.

1. 010-011.png
Vout=32.79V
Imax=5.2A
Rload=5.7R,
 should have caused current limiting.

[mike_mike]

should have caused current limiting.

Yes, this is true, the red led was blinking. But on the oscilloscope it appeared as a normal response.

[xavier60]

With tap switching one would not have to worry that much about the heat sink temperature. Still 15 V are a bit optimistic: the older posts suggest 40 and 20 V for the input steps. So the worst case voltage drop is more like a little more than 20 V (like 25 V to have reserve for the drop on relay switching).
There is still the short time peak before the relay actually switches. So the transistors would also have to be good for 40 V for some 10 ms or so.

If the 0.72W/°C above 25°C derating spec also applies to pulse loads, there is still plenty of safe margin.

[xavier60]

With tap switching one would not have to worry that much about the heat sink temperature. Still 15 V are a bit optimistic: the older posts suggest 40 and 20 V for the input steps. So the worst case voltage drop is more like a little more than 20 V (like 25 V to have reserve for the drop on relay switching).
There is still the short time peak before the relay actually switches. So the transistors would also have to be good for 40 V for some 10 ms or so.

If the 0.72W/°C above 25°C derating spec also applies to pulse loads, there is still plenty of safe margin.

Not so certain now. I think I have been misreading the SOA chart.

[xavier60]

The way I see it now, to safely handle 5A at 40V for 10mS, TC should be under 67°C

[mike_mike]

Are you trying to say that I should better use TIP35 ?
Are there any alternatives for the TIP35 ?

I am not sure what temperature would TIP3055 have because the heatsink is limited as dimensions in order to fit on the back of the psu case. The dimensions of the heatsink are h=80mm, L=165mm, fins are 35mm and it has 14 fins. I will mount 2x 80x80x25mm fans on the heatsink and I will power them from a fan controller which will use a sensor to control the speed of the fans.

[Kleinstein]

With tap switching (20/40V input) the TIP3055 should be OK. There are alternatives, but most of them are more expensive unless you get a special offer (hopefully still the correct parts).
If in doubt 1 more transistor may be easier than a larger heat sink or more expensive transistors.  With a fan the heat sink should be large enough.

The SOA is anyway a parameter that is usually not individually tested for the cheaper BJTs (there are premium audio parts that are tested).
On the other side having the SOA from sample testing usually means that most parts are actually better than the specs as they have to add some safty margin for scattering.
So to be on the safe side one should do a worst case (maybe even shift the switching threshold a little to get worse than normal) test before using the supply with any sensitive load.

[xavier60]

Yes, add one more TIP3055 if possible.
This SOA vulnerability could be even worse in my own PSU with only 2 TIP35C. But so far no blown output transistors. The only problem so far has been a blown LM317 while powering an ignition circuit.
A PSU is at its most vulnerable from SOA break down  when the output is shorted while it's already working is a state that cause a high temperature rise of the output transistors.

Also while the high voltage tap is selected.

[xavier60]

4 TIP35 will be much better than the 4 TIP3055.
Are TIP35 difficult to get?
In the mean time, you could try to measure the heatsink temperature. I do it by putting thermal grease down an empty screw hole close to a transistor, then placing a thermocouple probe into the hole.

[mike_mike]

I added one more TIP3055. Now the psu have 5xTIP3055.
I also made some tests with 5x TIP3055, I post the results:
1. 017.png With the probe on R_Shunt, Vout=32.79V, Imax=Iout=5.2A
2. 018.png With the probe on output of the psu, Vout=32.79, Iout=Imax=5.2A
3. 019,020.png With the probe on the output, Vout=32.79V, Imax=5.2A, Rload=6.7R

Edit: the TIP35 are not as cheap as TIP3055, and I have in my workshop more TIP3055 than TIP35, this was the reason that I choose TIP3055.

[xavier60]

That all looks normal.

[Terry Bites]

congratulations. I think you have one of the highest responses to your post ever.
We all love a 324, oh yeah!

[mike_mike]

@xavier60 , are the protection diodes on the output and on the power transistors enough if they are only 3A diodes ? (1N5408) ?
Should I change them to 6A diodes ? P600 or 6A6 diodes ?

[xavier60]

The diode across the output transistors has to be able to charge the reservoir capacitors if a large battery is correctly connected to the PSU while not being powered by the mains. 1N5408 is specified to handle 200A for 8.3mS which should be more than enough unless your reservoir capacitors are very large.
What are you expecting the diode across the output to protect against?

[mike_mike]

What are you expecting the diode across the output to protect against?

For example if I power a motor, to protect against back emf.

I removed the old 1N5408 and I put 6A6 (at the output) and P600k (on the power transistors).
I made a few tests:
1. 036.png - with the probe on the output of the psu, Vout=32.8V, I=5.2A=Imax
2. 039.png - with the probe on the R_Shunt, Vout=32.8V, I=5.2A=Imax
3. 041, 043 - with the probe on the output of the psu, Vout=32.8V, Imax=5.2A, Rload=6.7R

[xavier60]

What are you expecting the diode across the output to protect against?

For example if I power a motor, to protect against back emf.

Personally, I don't think that motor back EMF can damage a typical PSU. In any case, putting an 1N5408 across the output won't hurt anything unless a large battery is connected the wrong way to the PSU. The diode will pass a high current, causing the output leads to likely melt.
Current sourcing PSUs can actually briefly tolerate reverse polarity applied to the output. I have purposely connected a 12V lead acid battery the wrong way to my PSU for 1 second. It survived ok. Although it's very stressful for the output transistors and output capacitor.
I think the main reason PSUs have the diode across the output is in case PSUs are connected in series. If there is an overload, the diode will prevent more than a small amount of reverse polarity being applied to the weaker PSU.

[mike_mike]

Is necessary testing again the psu if changing the diodes ? from 3A to 6A diodes ?
Or, should I leave the 3A diodes into the circuit ?

[xavier60]

Is there any point in testing again the psu if changing the diodes ?

No, the diodes dont affect stability. What sort motors do you test?
The 3A diodes will be fine.

[mike_mike]

For now, I will test only 12V and 24V fans (1-2W power).
But I don't know what types of motor I will test in the future.

Edit: Probably I will power some pcb drill motors which are bigger than the fan motors.

[mike_mike]

I will power some pcb drill motors

For example I have a 12V/45W pcb drill and I will power it from this psu.

[mike_mike]

Is there any difference in using 0.22 /5W resistors connected in E of the TIP3055 transisors instead of 0.1  /5W resistors ?

[xavier60]

Higher resistance improves current sharing. No problem so long as the dissipation is reasonable and the resistors are all the same value as each other.

[mike_mike]

Hello @xavier60, the power supply is working OK, but I wanted to make the green led to be more bright, so I modified R30 and R31 from 2.2k to 1k each. This means that the red led will also be more bright, but this is not a problem.
Do I need to remake all the tests because of this modification ?

edit: I measured the voltage across R30 and R31 and it looks like: V(R30) = 5.247V and the current through R30 is 0.0052A=5.2mA (this happens when red led is on and green led is off) and V(R31)=2.911V and the current through R31 is 0.0029A=2.9mA (this happens when red led is off and green led is on).

[xavier60]

Hello @xavier60, the power supply is working OK, but I wanted to make the green led to be more bright, so I modified R30 and R31 from 2.2k to 1k each. This means that the red led will also be more bright, but this is not a problem.
Do I need to remake all the tests because of this modification ?

The CV and CC opamps will be sinking more current. You will need to calculate how close it is the maximum rating.
Repost the schematic for now.  It would have been better to find brighter LEDs.

Edit: You also need to include the current through the pull-up resistor.

[mike_mike]

Schematic attached.
I could find brighter leds.

I am still searching for the schematic which contains the driver (D45H11) and power transistors (5xTIP35CW in this particular schematic).

edit: I will use some brighter LED's and I will replace R30 and R31 by 2k2. The 2k2 value is the one used until now. Please find below the link to the modification of the value of R30 and R31:
https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3728311/#msg3728311