PSU

[zachtheterrible]

Can anybody provide me with a schematic for a PSU that will output 0 - 20V or more? And at least 3A?

 

[Someone Electro]

This meaby:
https://www.electronics-lab.com/projects/power/003/index.html

0-30V at 0,002-3A (Its curent limtiting)

Btw: we have google

 

[zachtheterrible]

Electro, thanks a bunch, that was exactly what I was looking for.

I did try google and couldn't really come up with anything.

 

[audioguru]

Hi Zach,
That circuit doesn't work. Its tiny rectifier diodes and driver transistor melt at full output and it doesn't provide anywhere near 30V.
Look in that site's forum for a discussion of mods for it to make it work properly. A link to the discussion is at the bottom of the project.

Here are the mods I recommend in its parts list:

 

[zachtheterrible]

Hi audio, thanks for that mod list, you just saved me from blowing my money and time :lol:

I cannot find a transformer that you recommend on mouser. The closest I can find is 28VC.T , 3.6A. Those ratings are given in RMS. It is 100VA. By the way, what does VA mean? Would that suffice?

I have an old microwave oven transformer. Perhaps I could wind my own? I would imagine it to be pretty easy because I would be using large gauge wire with not very many turns.

 

[Someone Electro]

VA is just an nother way of saying W.

Yea an mircowave cod work.(I wod like to see it work whith the original winding.LOL)

 

[zachtheterrible]

Yeah, I'll probably have to use the microwave.

I cannot find op-amps labeled opa445ap. Is that the right one??

Wow, this is the biggest order I have ever made from anyone, I'll probably spend around $100. I'm ordering stuff for three different projects: the power supply, PIC programmer and PICs, and my LED light. The power supply being by far the most expensive.

Can anybody recommend to me any certain kind of heatsink?

 

[Someone Electro]

The one whith the lowest C/W ratio.And you cod get an fan too to draw even more juce out of your PSU.

Oh and you can get PICs for free over microchips free samples

 

[zachtheterrible]

What is C/W ratio? I was thinking about getting a fan for this, I'm sure its going to generate a good amount of heat.

I thought that they quit giving out free samples due to abuse. I'll definetaly check, thanks :lol:

 

[audioguru]

I cannot find a transformer that you recommend on mouser. The closest I can find is 28VC.T , 3.6A. Those ratings are given in RMS. It is 100VA. By the way, what does VA mean? Would that suffice?

With only 28VAC, if your transistors are weak, you might have a max DC output voltage of 28V at full load. Trying for a higher output voltage would result in high ripple (hum).
VA is volt-amps and is the fully loaded rating of the transformer. 28VAC has a peak voltage of 39.6V then the rectifiers lose about 2V and heat-up due to their current. So the transformer can produce 100VA divided by the peak of 39.6V = 2.53ADC from the supply.

I have an old microwave oven transformer. Perhaps I could wind my own? I would imagine it to be pretty easy because I would be using large gauge wire with not very many turns.

Other people have tried microwave oven transformers. Because they don't have enough turns, their inductance is low so they use a lot of current without much load. It doesn't matter for a 900W oven but it could heat itself with 200W in addition to the power of the supply project.

The project's 2N3055 transistors heat with about 60W each with the supply providing 3A at a very low voltage or shorted. The 2N3055 has a max chip temp of 200 degrees C so the max thermal resistance allowed is calculated like this:
1) Assume a balmy ambient temp of 30 degrees C.
2) 200 - 30 = 170 degrees C for the thermal resistance of the transistor's chip to its case, an insulator and the heatsink.
3) The thermal resistance of a 2N3055 transistor is 1.5 degrees C/W. The thermal resistance of an insulator is 0.7 degrees C. so the total thermal resistance without the heatsink is 2.2 degrees/W.
4) 60W x 2.2 degrees = 132 degrees + 30 degrees ambient = 162 degrees. 200 degrees (transistor's max) - 162 = 38 degrees rise for the heatsink to dissipate 60W. Two transistors dissipate 120W.
5) 38 degrees/120W = a thermal resistance rating for the heatsink of 0.32 degrees C/W. Impossible without a gale-force fan.
6) Try the above calculation without an insulator and insulate the heatsink from the chassis instead. Then the thermal resistance rating of the heatsink will be 1.02 degrees C/W which is available, but it would still be a pretty big heatsink. Check the ratings of smaller heatsinks with a fan.

I would use a heatsink with a thermal resistance rating of 0.8 degrees C or less if I could afford it and had space for it. Then the output transistors would operate cooler than their max rating. Or I would use a smaller heatsink and an automatic fan.

You can't find the transformer nor TI's opamps. Shop at Digikey or Newarkinone since they have everything. :lol:

 

[zachtheterrible]

audio, I tried digikey like you said and they do have them, but they're 9.50 a piece!!
https://www.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch
Do you know of anywhere cheaper?

I was talking about unwinding and re-winding my MOT. Digikey doesn't have the transformer you specified either.

 

[audioguru]

audio, I tried digikey like you said and they do have them, but they're 9.50 a piece!!

Their price went up 2 bucks but they are good for this project.
The absolute max supply voltage for ordinary cheap opamps is 36V. Some opamps have a max rating of 44V which still isn't enough for the project. The OPA445AP is the only one I found with a high enough rating.

Digikey doesn't have the transformer you specified either.

Digikey has the Hammond 182M30 torroid transformer which is 30VAC/5.3A. It is a good one.
Newarkinone has Hammond torroids 182L30 and 182N15 which are both rated at 30VAC/4A. Their current is a bit low.
I don't know if you pay by the pound for shipping this heavy thing, and I don't know if torroids come with mounting hardware.

 

[zachtheterrible]

Looks like ill just have to suck it up and shell out the extra $27 :cry:

I was mistaken saying that digikey didn't have the right transformer. I was looking as jameco, thinking it was digikey :roll:

 

[zachtheterrible]

HOLD IT!!!! I just realized that I should check ebay. I did so and found out that I can buy a PRE-BUILT UNIT WITH BETTER SPECS for CHEAPER than I can built it!!!!!!!!

pfft, im just gonna buy one, its not worth the trouble, time, . . .

I have learned a good deal about PSUs though from examining the circuit and stuff.

 

[zachtheterrible]

What do you think about this one?

**broken link removed**

The way I see it, the transformer is going to cost me $50, plus roughly $60 for other parts, not to mention the case that I have yet to buy, I was also going to put in a way to monitor voltage and current in the system. I think this is the better buy.

 

[audioguru]

It's got good spec's and is a good price if it works.
The power supply project was supposed to work and is available as a kit. It doesn't work unless it is modified.

 

[Joel Rainville]

What do you think about this one?

**broken link removed**

The way I see it, the transformer is going to cost me $50, plus roughly $60 for other parts, not to mention the case that I have yet to buy, I was also going to put in a way to monitor voltage and current in the system. I think this is the better buy.

Wow, that thing looks good. They have a lower speced one (18V/3A) that's very tempting at $75 :shock:

The fixed voltage -19V/2.4A- laptop PSU I am using for my prototypes sells for about the same price, $90CAD! Fortunately I got it free... along with my now defunct $3000 laptop :lol:

 

[zachtheterrible]

Yeah, I saw that one. If it went to 24v I'd get that one because you see 24v used quite a bit in circuits.

I just can''t pass this up though. I can't even remember the last time I spent $124 on myself! Well, its in the mail :lol:

 

[bryan1]

Hiya Zac,
Here's a pic of my PSU, it's a Manson brand rated at 30 volts@ 3amps with current limiting and short circuit protection ( which believe me is a needed feature). I picked it up for $169 Oz and it looked the best buy for the price I could find.

Cheers Bryan

 

[zachtheterrible]

I know mine has current limiting. It didn't say short circuit protection, but I'm sure that it does.

BTW, is that a set of carbide drill bits that I see on the top shelf? :lol: