Linear lab power supply

[JuanGg]

That's right  sorry. I'll draw an schematic anyways.

PD: Merry Christmas!

    Juan

[JuanGg]

I have soldered the perfboard with the raw PSU and the 12 V regulator. I have mounted the two transformers for the auxiliary supplies as well. There are now a total of 5 transformers in the case.

12 V regulator seems to work well and doesn't even get warm drawing 300 mA. It has little noise with no load and under resistive loads, but when connecting one of the 12 V fans, I get what's on the screenshot below. I suppose it's due to the fact of the fan being a brushless motor and drawing current spikes or whatever. I'll try adding some more output capacitance (now i'ts just a 0.1 uF as recomended by the datasheet), but what I've tried so far (10 uF, 100 uF) does not have an effect.
    Juan

[xavier60]

Looks like a combination of things. Those small high speed fans must draw very high current pulses along with the secondary voltage being on the low side for a  regulated  12V rail. Increasing the size of the capacitor after the bridge will help but looks like it will never run both fans well at full speed.

[Kleinstein]

Extra capacitance after the regulator has a limited effect, as ideally the voltage is not changing much. Some of the fans even have quite some capacitance inside. In this case a resistor (e.g. 10 Ohms range) before the fan can help. So the fan would still see voltage drops, but the rest of the circuit would see less.

[JuanGg]

Capacitor after the bridge is 1000 uF.  This 12 V regulator is running one of the fans, as there will be an identical board for the other channel. Fans do not need to run at full speed. I was thinking of using PWM, and this may make it worse. I'll try with the resistor.
I am just worried that this noise can spread to the rest of the PSU, as I am deriving the 8 V and 5 V rail from this 12 V.
I could try to derive all rails from the unregulated supply, to make them more independent. I'd have to desolder the 12 V reg from the rectifier board, and put all regulators on the main board.
    Juan

[xavier60]

Capacitor after the bridge is 1000 uF.  This 12 V regulator is running one of the fans, as there will be an identical board for the other channel. Fans do not need to run at full speed. I was thinking of using PWM, and this may make it worse. I'll try with the resistor.
I am just worried that this noise can spread to the rest of the PSU, as I am deriving the 8 V and 5 V rail from this 12 V.
I could try to derive all rails from the unregulated supply, to make them more independent. I'd have to desolder the 12 V reg from the rectifier board, and put all regulators on the main board.
    Juan

A series resistor should make things better but no matter what you do, expect the 8V regulator to drop out. The 5V regulator might be ok. And they need to be powered directly from the unregulated low voltage rail. Because the 8V rail isn't being used for the reference, some ripple shouldn't cause a big problem until you find a more suitable transformer.
Try adding more capacitance to see if it causes a useful drop in ripple.

[JuanGg]

A series resistor should make things better but no matter what you do, expect the 8V regulator to drop out. The 5V regulator might be ok. And they need to be powered directly from the unregulated low voltage rail. Because the 8V rail isn't being used for the reference, some ripple shouldn't cause a big problem until you find a more suitable transformer.
Try adding more capacitance to see if it causes a useful drop in ripple.

I'll desolder the 12 V regulator from the rectifier board, and run the raw DC to the main board, where I'll regulate it to 12, 8 and 5 V. I'll try adding the series resistor. What do you mean by a more suitable transformer?

    Juan

[xavier60]

A series resistor should make things better but no matter what you do, expect the 8V regulator to drop out. The 5V regulator might be ok. And they need to be powered directly from the unregulated low voltage rail. Because the 8V rail isn't being used for the reference, some ripple shouldn't cause a big problem until you find a more suitable transformer.
Try adding more capacitance to see if it causes a useful drop in ripple.

I'll desolder the 12 V regulator from the rectifier board, and run the raw DC to the main board, where I'll regulate it to 12, 8 and 5 V. I'll try adding the series resistor. What do you mean by a more suitable transformer?

    Juan

Something with higher voltage and maybe higher current. My last power supply project uses a 15Vac 10VA transformer to supply a 12V regulator which powers a 60mm fan, but the fan draws only 220mA at full speed.

[JuanGg]

Something with higher voltage and maybe higher current. My last power supply project uses a 15Vac 10VA transformer to supply a 12V regulator which powers a 60mm fan, but the fan draws only 220mA at full speed.

My fan draws about 100 mA at full speed. I'll change my 500 mA transformer if I come across a suitable one.

Parts I ordered just arrived. They sent 470 uF caps instead 47 uF... but the rest seems fine. I suppose it's time to start doing the perfboard layout.

    Juan

[JuanGg]

Another thing is mounting power devices to the heatsink. Would it be ok to have the TIP35 mounted directly with no thermal washers and leave everything else without heatsink/bent and soldered to the board?
    Juan

[xavier60]

Another thing is mounting power devices to the heatsink. Would it be ok to have the TIP35 mounted directly with no thermal washers and leave everything else without heatsink/bent and soldered to the board?
    Juan

It's ok to mount the TIP35C with no washer. The dissipation of Q3 and Q9  will vary a lot with operating conditions and difficult to calculate with certainty. mainly with Q3 because the gain of the TIP35C is unknown. They would be ok for most conditions for testing but I would not permanently leave them un heat sunk. Q3 won't need a washer.
If you leave Q9 un heat sunk, you might have to decrease the preload current.

[xavier60]

Something that needs to be aware of, some CPU fans control their own speed according to temperate sensed in the hub. If the fan is always drawing in fresh air, it should stay at low speed. All of my stock Intel P4 fans have this behavior.

[JuanGg]

   It's ok to mount the TIP35C with no washer. The dissipation of Q3 and Q9  will vary a lot with operating conditions and difficult to calculate with certainty. mainly with Q3 because the gain of the TIP35C is unknown. They would be ok for most conditions for testing but I would not permanently leave them un heat sunk. Q3 won't need a washer.
If you leave Q9 un heat sunk, you might have to decrease the preload current. 

I have some to-220 insulating pads. I suppose they could be used on the To-126 this transistors come in. I'll heatsink everything.

   Something that needs to be aware of, some CPU fans control their own speed according to temperate sensed in the hub. If the fan is always drawing in fresh air, it should stay at low speed. All of my stock Intel P4 fans have this behavior. 

I'll check for that.

Instead of laying the perfboard on paper I thought I'd have a go at designing a PCB, even if it's just for component placement. I am using Autodesk Eagle, just because it can be synced to Fusion 360, so I can put PCBs on my mechanical assemblies.  For now, I have just input the schematic.

Another thing, can I use any schottky diode, (say IN5817). Instead of the Bat 85 which was only available on Sot-323, and I'll prefer soldering an axial diode on a perfboard. I can use the sot-323 if there's no other option.
    Juan

[Kleinstein]

The diodes for joining the control signals are not that critical. So 1N5817 should be Ok. Using normal 1N4148 might effect the minimal voltage - it depends one details of the rest of the plan.

[xavier60]

D3 can be left out for now until the CV loop is tested. Ill test the circuit with a 1A schottky diode here, it's likely to alter the allowed current overshoot.  R26 might have to be changed.
I use machined IC socket pins where I expect to have to later change a component value.

[JuanGg]

I have designed a PCB from the schematic. It's far from finished, I just wanted to have a go at it. It's the first PCB I lay out (except a couple rather simple hand made ones) so don't expect much. Its probably a non optimal layout, but I may have an idea on where components could be placed.

D3 can be left out for now until the CV loop is tested. Ill test the circuit with a 1A schottky diode here, it's likely to alter the allowed current overshoot.  R26 might have to be changed.
I use machined IC socket pins where I expect to have to later change a component value.

Ok, thank you. I'll prototype the CV loop on a perfboard.

    Juan

[ArthurDent]

On the fans putting garbage on the +12VDC going to the rest of the circuitry, just add a second 7812 and associated caps to only run the fans. It is cheap and will effectively isolate the fan noise from the rest the stuff.

[JuanGg]

On the fans putting garbage on the +12VDC going to the rest of the circuitry, just add a second 7812 and associated caps to only run the fans. It is cheap and will effectively isolate the fan noise from the rest the stuff.

The 12 V reg on the board is going to power just the fan, 5 V reg will do the displays (maybe arduino as well) and the 8 V the analog circuitry.

    Juan

[xavier60]

I have designed a PCB from the schematic. It's far from finished, I just wanted to have a go at it. It's the first PCB I lay out (except a couple rather simple hand made ones) so don't expect much. Its probably a non optimal layout, but I may have an idea on where components could be placed.

D3 can be left out for now until the CV loop is tested. Ill test the circuit with a 1A schottky diode here, it's likely to alter the allowed current overshoot.  R26 might have to be changed.
I use machined IC socket pins where I expect to have to later change a component value.

Ok, thank you. I'll prototype the CV loop on a perfboard.

    Juan

I had hoped that the main current paths would have been routed directly across the PCB  with input and output capacitors on board Like I illustrated in post #126. The shunt resistors are ok where they are.
The idea is to keep the main + and - paths close to each other.

[xavier60]

I don't expect the layout to cause problems.
There is one problem with my layout that I'm finding it difficult to find a solution for. There is a bit too much track length between the top side of the shunt and the star point.

[JuanGg]

I had hoped that the main current paths would have been routed directly across the PCB  with input and output capacitors on board Like I illustrated in post #126. The shunt resistors are ok where they are.
The idea is to keep the main + and - paths close to each other.
   

The main filter cap is on a separate board, perhaps it would be a good idea to add more capacitance right on the PCB. EDIT: I see this is on #126.

Output capacitor is better on the PCB or right at the front panel binding posts?

Also, voltage sense divider could be connected to the binding posts to partially compensate for drop in the wires. Is this a good idea?

Also, star grounding as I did, is it fine? Or should I use a groundplane?. I did it single sided, so adding a groundplane is simple enough.

I'll try to make a Rev B with your suggestions.

    Juan

[xavier60]

I had hoped that the main current paths would have been routed directly across the PCB  with input and output capacitors on board Like I illustrated in post #126. The shunt resistors are ok where they are.
The idea is to keep the main + and - paths close to each other.
   

The main filter cap is on a separate board, perhaps it would be a good idea to add more capacitance right on the PCB. EDIT: I see this is on #126.

Output capacitor is better on the PCB or right at the front panel binding posts?

Also, voltage sense divider could be connected to the binding posts to partially compensate for drop in the wires. Is this a good idea?

Also, star grounding as I did, is it fine? Or should I use a groundplane?. I did it single sided, so adding a groundplane is simple enough.

I'll try to make a Rev B with your suggestions.

    Juan

I always put the output capacitor on the PCB and I have never bothered with sensing at the terminals because there will be the resistance of the power leads between the power supply and load anyway.
Theoretically, having the capacitor at the output  terminals should improve the phase margin and stability but I haven't tried it, so I can't be certain.
The star point looks fine. I have no experience with ground planes either because I make my own single sided PCB's.

[JuanGg]

I always put the output capacitor on the PCB and I have never bothered with sensing at the terminals because there will be the resistance of the power leads between the power supply and load anyway.
Theoretically, having the capacitor at the output  terminals should improve the phase margin and stability but I haven't tried it, so I can't be certain.
The star point looks fine. I have no experience with ground planes either because I make my own single sided PCB's.

Alright. I was thinking of making a perfboard prototype and then have a couple PCBs made, as I have two channels. It's tempting to use the other layer, maybe I should route all grounds on the second layer and a couple more things. It'll definitely make the layout easier. No problems to replicate that on perfboard.
    Juan

[JuanGg]

Here is Rev B. There is still some tweaking and double checking to be done. I have routed all grounds on the botom layer. I have used traces as thick and as short as I could and did my best to follow guidelines in #126. I'll add the fan circuitry and some connectors to the schematic so I can see how much room I have left (size of the perfboard I have, 8 x 12 cm, but not much more left inside the case). Also I have attached some CAD screenshots.

    Juan

[xavier60]

Here is Rev B. There is still some tweaking and double checking to be done. I have routed all grounds on the botom layer. I have used traces as thick and as short as I could and did my best to follow guidelines in #126. I'll add the fan circuitry and some connectors to the schematic so I can see how much room I have left (size of the perfboard I have, 8 x 12 cm, but not much more left inside the case). Also I have attached some CAD screenshots.

    Juan

That looks good.
I have been working on a problem that I have just noticed. If the output voltage is set higher than the unregulated rail can supply, the CV op-amps goes into saturation, output pin goes to full 8 volts. This causes C4 to over charge causing the CC loop to lose its quick limiting response to the output being short circuited.
Putting a 1A schottky diode across R10 minimizes the problem.  See if you can add the diode, anode to ground.

Extra: It looks like R13, 10K on the layout.