Convert 0-30V 3A PSU to 5A or more

[audioguru]

ugly planes don't fly.

Sorry to be off-topic for a minute: :-[
I am making a very ugly looking plane that I expect will fly very well. Instead of using a radio control system, I'm going to use adjustable timers and PWM for its electric engine's speed and elevator control:
1) Full power for about 15 seconds for its takeoff roll and climb.
2) Up elevator for about 1 second when the plane is halfway down the runway.
3) About 1/2 power for cruising for about 1 minute. It won't go straight, it will circle like a hawk.
4) About 1/10th power for about 15 seconds for a gentle landing.
5) Over-current sensing of its motor to cut power if my plane hits a tree or me! ;D

I won the ugly thingy as a door prize at the very 1st model airplane club meeting I attended. ;D ;D

[maksar]

Hi!

Can I use zener diode 1N4735A instead of BZX79C5V6? If the answer is yes, which resistances I need?

Thanks for the answer.

Maksar

[audioguru]

Hi Maksar,
Why?
They are completely different animals:
1) The BZX79C5V6 is rated at 5.6V, while the 1N4735A is rated at 6.2V.
2) The BZX79C5V6 is rated and tested at 5mA, while the 1N4735A is rated and tested at 41mA.
3) The BZX79C5V6 has a low differential resistance at low currents for good voltage regulation, while the 1N4735A has a high differential resistance for lousy voltage regulation.

The 1N4735A appears to be a pretty good voltage regulator at 41mA, so why can't we change the circuit so it operates at 41mA?
1) It is driven by an opamp. Opamps can't output anywhere near a current so high.
2) At 41mA, it dissipates about 1/4W. Therefore it will heatup a lot causing its voltage to change. D8 is the project's main voltage reference. You don't want a voltage reference in the project that changes its voltage.

In my comparison chart, I included the BZX79V6V2 so you can make a direct comparison of 6.2V zener diodes. Since the BZX79V5V6 has a lower zener voltage, its differential resistance is higher. ;D

[maksar]

Hi!

Ok, thanks audioguru. The problem is that I don't have these diodes and my local electronics store don't have them too. So I tried  ;) ;D ;D ;D  But it seems that I'll need to search BZX79C5V6 in the other stores  :-\

By the way, I understand nothing in planes, however, your model doesn't look ugly at all. Pretty, small plane... But I believe to expert's word  :P ;D ;D

Maksar

[audioguru]

Hi Maksar,
Don't buy special stuff in local stores, order nearly anything online.

I'm painting my plane the colour "Red Hot" right now, to make it extra ugly. It's styrofoam with bamboo spars, so I hope the paint (deluxe acrylic) will make it stronger. ;D
I'll have an empty phoney "radio control transmitter" so people will think the timers' actions are me controlling it.

[maksar]

Hi!

Do you think the heatsink 234-2520 on www.rsisrael.com (use search) for each 2n3055 is suitable or it is too big?  ???

Maksar

[audioguru]

Hi Maksar,
With this project driving 5A into a low voltage or short, its 3 output transistors will dissipate a total of 190W, or 63W each.
The thermal resistance of a T03 2N3055 is 1.5 degrees C/W, an insulator is 0.7 and that heatsink is 1.4. The total thermal resistance would be 3.6 degrees C/W.
If the ambient temp is 30 degrees C, then with 63W dissipation the transistor's junction will be 257 degrees C. Its absolute max is 200 degrees C so more cooling is needed.
Even if you remove the insulator more cooling is needed.

You could use a high velocity fan to allow that little heatsink to be used for each transistor.
You could use a heatsink with a thermal resistance of 1.2 degrees C/W without an insulator (insulate the heatsink from the chassis) and the transistor would be exactly at its absolute max temp.
Or you could use a heatsink with a thermal resistance of 1.0 degrees C/W and the transistor's junction will be still very hot at 187.5 degrees C.

I would use larger heatsinks without an insulator, and use a fan. ;D

[maksar]

Hi AUDIOGURU!

Thanks for this explanation. I didn't think that it is so simple to calculate and determine the needed heatsink  :o  (After your post I got some reading on this topic).

It seems like I'll have to use fan. But does an airflow has big influence on the effective heatsink resistance? What will be the temperature of 2n3055 if I will put the 80mm computer fan on the 1.4 C/W heatsink?
Maybe I'll do termal control of this fan (or fans..) by using Maxim's temperature sensor.

By the way, should I make aluminium case for psu? It has low termal resistance and is
easy to work with, but it won't too soft?

Maksar

[maksar]

Hi!

And one more question.  :)
According to the datasheet of 2n3055, after rising of the temperature above 100C the power dissipation falls under ~65W. So in this project 100C for 2n3055s is the max?

Maksar

[audioguru]

Hi Maksar,
In the datasheet they show a derating graph for the amount of power a 2N3055 transistor can dissipate when its case is at a certain temp and its junction is at its absolute max temp.
My graph shows 115W if its case is 25 degrees (impossible), about 65W if its case is 100 degrees, and 0W if its case is 200 degrees. I never operate things at their absolute max.

Datasheets from heatsink manufacturers show the improvement in thermal resistance of their heatsinks with a fan at a certain airflow that is measured in cubic feet/min.
I went to www.wakefield.com and they have a heatsink similar to the one you posted.

An aluminum case would be a good heatsink for the driver transistor Q2, the rectifier bridge and R7. It wouldn't be good enough to be a heatsink for the output transistors. ;D

[Thomas]

Hi anyone

I've got a suggestion to make the project cheaper and perhaps less temperature-dependent. TI's TL431 makes a temperature-compensated Z-Diode, which output voltage can be adjusted by two (temperature-stable) resitors. That can replace the expensive U1.

There is a problem, though, because the thing can only stand 37VDC. That could be solved by putting a resistor in series with it, and a Z-Diode of, let's say 15V, parallel.

This way, we would get our 11.2VDC reference cheaper, with less space needed and with better stability over the temperature range.

What do you think?

The datasheet: http://www-s.ti.com/sc/ds/tl431.pdf

[audioguru]

Hi Thomas,
A BZX79C5V6 zener diode is fairly temperature stable but you can use a TL431 reference if you want.

You have a better way[/b[ to make the modified project cost less. ;D
The output voltage of U1 is only 11.2V so will work fine with a cheap 18V/0.5W zener diode across it and a 1.8k/1W resistor feeding them from the 40V unregulated supply. A 100uF capacitor should be used to decouple and filter the 15V. Then a cheap TL081 or just about any opamp can be used for U1. ;D ;D

[narog]

Hello.

I am electronic engineering student and for 3 weeks I have been studding this power supply. Now I understand 98% of the PSU, and I decided to design a symmetric PSU. Here is my schematic. This is not a finished job, but maybe can help you. If you have Electronics Workbench you can simulated and maybe understand how it works.

Right now this schematic has a big problem in the current limiter. The problem is very simple the current limiter only works if the load is connected to ground. If you have a load connected to +32 -32 V the PSU don’t work very well.  Maybe some one can help and figure the solution out.


Sorry I am Portuguese and my English is not the best.

PSU.zip

[audioguru]

Hi Narog,
Welcome to our forum. ;D
I don't have Electronic Workbench so I didn't see your zipped file.

I like the way you added an opamp then sensed the current in the active supply, instead of sensing the ground current in our project. ;D
But the opamps sensing both supply's currents have their inputs reversed. Or the opamps they are driving have their inputs reversed. :(
I would use red LEDs instead of green to warn that current regulation is reducing the output voltage.

These and the opamps they drive are operating with a 39V supply, but their absolute max rating is only 36V. >:(

Where did you find little 2N2369 VHF transistors? I have some that are about 40 years old! They are rated for only 15V Vce, and will quickly melt if your circuits supply 5A at a low voltage. :'(  ;D

[audioguru]

Hi Again,
I think your output transistors will also quickly melt if your circuits supply 5A at a low voltage. :'(

[narog]

Hi, the transistor/Ampops in the schematic are not the final ones. These components are there only for simulation.

The Electronics Workbench has a big library but is a little old.

The power transistor has to be symmetric so I have been search the web and I found:

NPN 2n5631 PNP 2n6031      16A 140V 200W
NPN 2n3772  PNP 2n6609      16A 140 V 150W
NPN 2n3055  PNP MJ2955    15A 90V 114W


For current sense and voltage reference generator ampops you can use tl081 if you build a regulated power supply of 15V 0V -15V, “LM7815 LM7915” this is because the opa445AP cost 8.40euros “Portugal Price”.

For the 2n2369 sorry, my mistake…..

But the opamps sensing both supply's currents have their inputs reversed. Or the opamps they are driving have their inputs reversed. :(

The positive current sense is well connected, but in the schematic you cant see that. The wires are very close, sorry. For the negative one I need a negative output. If you invert the inputs the ampop will not give you – X.XX V because  -31.74V – (-34V) = 2.26V and not -2.26V.

Thanks audioguru.

[audioguru]

Hi Narog,
Your positive current sensor opamp has positive feedback and therefore won't work. :(
Your negative current sensor opamp circuit is correct. ;D

The 1k resistors in the current sensor circuits will get mighty hot. Use 10k.

The 0.47 ohm current sensor resistors will also get mighty hot and waste valuable voltage. Use 0.27 ohms or less, then re-do the current setting voltage divider.

The output transistor will dissipate 170W if its circuit has a low (shorted?) output voltage at 5A. With a huge heatsink a power transistor might be able to dissipate only 60W. Therefore at least 3 paralleled output transistors are needed like our proposed circuit.
When the output transistors are paralleled, their individual current is lower, therefore their gain is higher and the driver transistor won't need to supply them with as much current and will be cooler. ;D

[BigPoppa]

Hello.
I am building this project and have some questions.
-i have 26VAC/150VA transformer?Is this ok?I think i will never use the psu at it`s limits...
-can i change TIP31A with BD241C?
-my R7 is 0,47E/9W.Is this ok?
-C1 will be 10,000uF/50V
-op-s will be OPA445
-bridge rectifier is 35A,type that fixes on the heatsink
-should i use 2 or 3 2N3055`s

Is there anything else that i should be aware of?

Thanks in advance,BigPoppa

[audioguru]

Hello.
I am building this project and have some questions.

Hi BigPoppa,
Welcome to our forum. ;D
This thread is about modifying the original 0-30V/3A project to 5A. Your little transformer won't power this supply to 30V nor to 5A.

-i have 26VAC/150VA transformer?Is this ok?

The project's voltage control's range should be changed to match your transformer since it will allow only about 27V but the control will try to force it higher and its output will become full of ripple and poor regulation.

-can i change TIP31A with BD241C?

Yes, any BD241 is the same or better than a TIP31A.

-my R7 is 0,47E/9W.Is this ok?

It will allow your project to supply 3A like the original modified project will do. Its max dissipation is 4.2W so it won't get too hot.

-C1 will be 10,000uF/50V
-op-s will be OPA445
-bridge rectifier is 35A,type that fixes on the heatsink

Excellent for all.

-should i use 2 or 3 2N3055`s

Use two with emitter resistors. A fairly large finned heatsink should be used.

Is there anything else that i should be aware of?

Use BZX79C5V6 low-current zener diodes and change R4 to 1k.
We suggested increasing the power ratings for many resistors in the other thread.

[BigPoppa]

The project's voltage control's range should be changed to match your transformer since it will allow only about 27V but the control will try to force it higher and its output will become full of ripple and poor regulation.

If it will be stable to about 24 volts,than i am happy. ;D I almost never need voltages higher than this,so it is not important to me.

I am using this partlist:

Parts List for 3A Variable Power Supply

*T1 = 30VAC, 4.3A, 130VA stepdown power transformer
*R1 = 2,2 KOhm 2W 
R2 = 82 Ohm 1/4W
R3 = 220 Ohm 1/4W
R4 = 4,7 KOhm 1/4W
R5, R6, R13, R20, R21 = 10 KOhm 1/4W
*R7 = 0,47 Ohm 10W
R8, R11 = 27 KOhm 1/4W
R9, R19 = 2,2 KOhm 1/4W
R10 = 270 KOhm 1/4W
R12, R18 = 56 KOhm 1/4W
*R14 = 1,2KOhm 1/4W
*R15 = 100 Ohm 1/4W
R16 = 1 KOhm 1/4W
R17 = 33 Ohm 1/4W
*R22 = 3,9 KOhm 1W
RV1 = 100K trimmer
P1, P2 = 10KOhm  linear potentiometer
*C1 = 10,000uF/50V electrolytic
C2, C3 = 47uF/50V electrolytic
C4 = 100nF polyester
*C5 = 220nF polyester
C6 = 100pF ceramic
C7 = 10uF/50V electrolytic
C8 = 330pF ceramic
C9 = 100pF ceramic
*D1, D2, D3, D4 = 6-10A/50V diode bridge module
D5, D6 = 1N4148
D7, D8 = 5,6V Zener
D9, D10 = 1N4148
D11 = 1N4001 diode 1A
*Q1 = TIP31A, NPN transistor, no heatsink
*Q2 = TIP31A NPN transistor
Q3 = BC557, PNP transistor or BC327
*Q4 = Two paralleled 2N3055 NPN power transistors with 0.1 ohm, 1W emitter resistors each
U1, U2, U3 = OPA445AP, operational amplifier
D12 = LED

"*" is a change from the original design.

Do i need to change something,since my transformer output voltage is to low?Or is this partlist ok for stable 0-24V and about 2-3 amperes?

Thanks for the zener suggestion,i will change it if i can find some around here.  :D

Thank you very much for your answers. 8)

[audioguru]

Hi BigPoppa,
I mentioned the zeners and R4 that feeds one, but someone said that R2 gets very hot. Here is the latest parts list:

[BigPoppa]

Just one more question...What`s the function of R2 and C2?

[audioguru]

What`s the function of R2 and C2?

They are part of the half-wave rectified negative supply.
C2 charges through D5 when the tranformer connection #2 swings positive. R2 limits its charging current so a small capacitor and diode can be used.
C2 discharges through R2, D6, R3 and D7 to make the regulated -5.6V negative supply for some of the opamps, when the transformer connection #2 swings close to ground.
The negative supply is one reason why the opamps must have a very high supply voltage rating.

[Kain]

Ok, finally some late update on this project from me. As you already know I assembled the PSU and had some problems with the maximum output voltage. I finally found the problem - there was an unetched copper on the board between 2 traces that looked so good that one can never tell it's not supposed to be there (looked like part of the layout). I could have seen it earlier, but on dual sided boards with the parts already soldered is not exactly easy to look at. Anyways, I fixed this and now the PSU goes all the way up to 34.5V when unloaded. I will be doing ripple testing this week so the results will be here soon. What I tried so far was testing for max output current. With the most recent setup which Audioguru posted the maximum output current is arround 6.85A, and this is at any voltage BUT lower than 2.5. At lower than 2.5V output the current can't reach high values. Audioguru, do you care to explain why?  ;D

[Kain]

I just did something simple - I connected the PSU across a huge rheostat set to 6 Ohm and set the PSU to max V and A. First thing to notice - I figured I can cook on the rheostat...