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

[audioguru2]

Hi Kain,
Your PCB looks very nice but I am sorry I didn't check it for you.

I think you missed my discussion of R4. Its 4.7K value allows a current of only 1.2mA in the project's main voltage reference zener diode, D8.
If you use a 1W zener diode for D8 like a 1N4734A (its part number or power rating isn't specified in the original project), its regulation and voltage is spec'd by its manufacturer with a current of 45mA!
At 1.2mA it barely regulates and its voltage will be much less than 5.6V. :
A BZX79C5V6 zener diode is much better, being rated with a current of 5mA. To use it properly, the value of R4 should be 1K for a zener current of 5.6mA. The other 5.6V zener diode D7 can be the same since it operates at a fairly low current. ;D

Recently another member of our forum pointed to an error I made with the connection of the emitter of Q1. It should be connected to the negative terminal of C1 like in the original project, not to the project's output negative terminal. :-[ :

 

[Kain]

Ok. That sounds bad now since I will end up redoing the board. Before doing so please check this schematic that I am posting now where I applied the recommended changes. I want to be sure that this is correct since dealing with that small dual layer PCB isn't exactly that easy (meaning it takes more time) and I don't want to redo it too many times 8)

Schematic.pdf

 

[audioguru2]

Hi Kain,
It is looking good. Go for it.

 

[Kain]

Here we go. Hopefully this is final ;D

PSU_revised.pdf

 

[robert field]

hy! I want to ask if that 3a psu can work with el.magnet coil/selenoids because when i conect 24v dc coil it current is 3.25 amps but the coil is overhited -does someone know what i should do to decrease heating, maybe it should be controled by freqency?

 

[audioguru2]

Hi Robert,
24V at 3.25A is 78W which is a lot of heat. Maybe the solenoid is supposed to be powered by 24V only for a moment.

I could probably also overheat the momentary solenoid that opens the trunk of my car if I applied power to it continuously! It uses so much current the lights dim.

Can you put your finger in a flame? Sure, for only a moment, but don't try it continuously. Same thing as above. ;D

 

[Kain]

I was just collecting the parts for this project and the price of C1 almost killed me - it costs as much as the transformer... Do you think I can use the same value capacitor on 50V instead of 63V, since it's almost half cheaper then the 63V one? Or maybe somebody knows a place where I don't have to pay $40 for a cap of this value. I was thinking to wire up few smaller ones in parallel but guess what - it comes on even higher price...

 

[audioguru2]

Hi Kain,
Where on earth (or maybe not?) do you buy stuff?
Where do you buy a big, powerful transformer for only $10.00?
Here is Digikey's reasonable price for the IC and their high quality (105 degrees) 10000uF/63V cap is about $7.00:

View attachment 36556

 

[Kain]

Yea, I dunno what was wrong with me. I found 10000uF/63V for 5$ each at mouser, so total is going to be 25$ which is acceptable ;D. About the transformer I don't recall saying it costs 10$. I would really like if it was 10$ though....hehe. The one I am getting will be hammond 30V @7.5A for about 50$. Hopefully this will do it 8). About the ICs I mentioned earlier that you should order them as samples from Texas Instruments as I did. I already have them too. This saved me little more than 25$ per board (since my PSU is dual output) ;D

 

[audioguru2]

Hi Kain,
He, he. Yea, I dunno what was wrong with me too. :
I was thinking of the 3A corrected version of this project that needs only a single 10,000uF cap. You don't need 50,000uF for only 5A, it would reduce the raw ripple to only about 0.4Vp-p. I think 20,000uF is fine. Your nice transformer has a little more than enough power, but I don't know what its temp will be if you run your project at its limit for a long time.

It's great you got IC samples from TI. I got an expensive telephone system hybrid transformer as a sample from Hammond, but I don't think they will give away power transformers. Hey, maybe if you are reeealy nice to them, and go to Guelph to pick it up! ;D

 

[king25]

Hi to all!

I just have one question about transformer.
I have one transformet sec. 33Vac - this spec. is for notlouded transformer. I didnt try to load it jet.
I will use OPA445 chips.
Is the transformer sec. 33Vac (notlouded) to high for this project???
Should i have less voltage,maybe 30Vac -notloaded????

I see that you use 30VAC / 225VA transformer,but i dont know if is that spec. with loaded or not loaded transformer ???

I build this project and now i have to just connect the transformer.
I just wona be shore,that my 33VAC-notloaded, is ok for this project.


Tnx

 

[audioguru2]

Hi King, your majesty,
Welcome to our forum.
To get a properly regulated 30VDC at 5A out of this project the transformer must be rated to produce 30VAC at 7.5A which is 225VA.
Of course, that is its voltage measurement when it is loaded with a 7.5A AC load. The recifier bridge and filter capacitor in the project have an extremely high peak charging current when it has a 5ADC load which makes the transformer's AC average current measurement about 7.5A.

Your 33V when unloaded transformer is fine and might be 30VAC when loaded with this project operating at its rated current. If you don't know its VA or current rating then its output voltage might drop too low and you will have a nice fire!

For high power projects like this, I recommend connecting a 50W mains lightbulb in series with the transformer's primary during the first time the project is powered without a load. If there is a problem, the lightbulb will light brightly and reduce any damage.

 

[king25]

Thanke you Audioguru for this answer.
I have 33Vac not loaded and 150VA transformer.

Tnx

 

[audioguru2]

Hello, "your majesty",
Your transformer is good for this improved project. It will allow the project to deliver up to 3.5ADC at the transformer's 150VA limit. You would need only two 2N3055 transistors instead of three that are needed for a 5A output.
Your 150VA transformer would produce a peak voltage of 42.4V when the project is delivering 3.5A. The raw unregulated positive supply would be about 39V. Perfect.
Use a big heatsink for your two 2N3055 transistors. If the project is set to current-regulate 3.5A, if its output is shorted then the 2N3055's must dissipate 68W each, a lot.

 

[king25]

Thanke you Audioguru for this informations. You helped me allot 8).
I will use 3 transistors 2N3055,because someday i will use 250VA transformer or more...

Can i use instead of 0,1/1W a 0,22/5W with 2N3055,
i wont to use 0,22E or i have to use 0,1E ???

When i will run and test this project i will send you the results ;D

Tnx

 

[audioguru2]

Hi King (your majesty, he,he),
It is good that you are using three 2N3055 transistors so they share the heat.
Your 0.22E resistors will have a voltage drop of only 0.25V and dissipate 0.3W when your project is loaded to 3.5A output and will be fine. Good luck. ;D

 

[king25]

Hy Audioguru 8)

Can i use four of 2N3055 with 0,1/1W or 0,22/1w so they share the heat??
Because i have monted 4 transistors on heat-sink yet it looks better and nice

Tnx

 

[audioguru2]

Hi King (your majesty, he,he),
Sure, 4 transistors will be cool and look cool too.
But only if you fasten sunglasses to them! 8) 8) 8) 8)

 

[king25]

Hy Audioguru.

What should i use for R7: 0,47ohm, 0,39ohm or 0,27ohm ??

 

[audioguru2]

Hi King,
Use 0.27 ohms/15W for R7. With 5A output, it will dissipate 6.75W and a 10W resistor would be mighty hot.