[SOLVED] Advice on power supply schematic

[jmx66]

Hi all,

I need advice on the following schematic:

https://electronics-diy.com/high-current-power-supply-lm338.php


It's my third power supply, previous projects were '' small '' projects, with no more than 5A.

I only want to know:

- schematic with no mistakes?
- if adding 4 more LM338K with SK88 heatsinks from Fisher, mandatory to place them in a box with 2 cooling fans?

4 x SK 88 Length: 250 mm Rth : 0,7

**broken link removed**


Pcb is already drawn with Kicad, high amperage being withstood by cooper plates and 10 AWG - 6 mm 2 wires -


Advices about this project would be great.


Thanks a lot.


jm

 

[D.A.(Tony)Stewart]

Make output caps low ESR type.

Not very efficient but simple.

 

[jmx66]

Hi all,

Thanks SunnySkyguy for your reply.

Is there a more efficient way, without using Switch Mode Power Supply schematics?


Thanks a lot.

jm

 

[D.A.(Tony)Stewart]

here's a few ideas

- drop the 0.3R to 0.1R if you mount all active devices on a CPU cooler with low speed fan.
- use a series pass transistor as PWM controlled charger to maintain a lower Voltage drop on the regulators or use a stock SMPS to maintain a 2 V drop on the linear regulator.
- use a lower LDO regulator.

Lambda used another method on 100V 100A linear PSU's and smaller... was use an SCR bridge on the primary to pre-regulate a voltage sufficient to regulated secondary at minimal voltage drop. Ramp Phase control sampled using a reference and regulating voltage drop.

 

[jmx66]

Hi all,

Thanks SunnySkyguy for your fast reply and complete advice.

About LDO, like LT1084V from STMicroelectronic, by example?

Now after crude maths for cooling, finding right heatsink is really difficult!!!!


Power: 20 A x 24 V = 480W



Rth Case to Junction: 3° C/W

T junction limited to 110 ° ( My choice )

T ambiant: 40 ° ( Inside a box with cooling fans )

8 x LD1084D 480 / 8 = 60 Watt per item


First trial with Latex - must be great opensource :grin:...... - :

\[ Rth _ {jc} = {3^0}C/W \]


\[ T _ {case-maxi} = {110^0}C\]


\[ T _ {ambiant-maxi} = {40^0}C\]


\[ P _ {dissipated} = ({{T _ {case-maxi}-T _ {ambiant}}) / {Rth} \]


Some basic maths:

With 8 x LD1084D so 60 Watt by regulator .....

\[ Rth={(110-40)}/{60} \] \[ \Rightarrow \] \[ Rth _{total} = {1,2^0}C/W \]


With these results, must ask Pacific Ocean to perform the job ???????????????? :sad::sad::sad:

Any idea ?:idea:


Regards.

jm

 

[Orson Cart]

It is not uncommon to have very large heatsinks for linear type high current power supplies, often one or more fan(s) is used to reduce h/sink size.

It is easy to do power conditions for worst case dissipation from (Vin - Vout) x current

e.g. Vin typ 30V say, Vout min (controlled) = 12V say, I = 50A, dissipation in series pass devices = 18 x 50 = 900W or most of the power being supplied by your 1.5kVA transformer.

If you have 26V ave in and 24VDC out at 50A then the diss will be 100W. Quite a difference.

Transistors need to be seriously derated to be inside their SOA curves for this type of application, emitter res a must! 30 -40W per TO3 can on a decent heatsink.

 

[jmx66]

Hi Orson,

Thanks a lot for your reply, now I'm going to google to find TO3 can Low Dropout Regulator.

Regards.

jm

 

[D.A.(Tony)Stewart]

Brute force linear power supplies are inefficient. Why use them? You can use LC filters to reduce the noise on SMPS so why ignore SMPS?

 

[jmx66]

Hi SunnySkyguy,

To be honest after googling and googling alot, about pro and cons of smps versus linear ones, i'm going to build a smps.

Up to now my main concern is about winding transformer, but found many links about this.

There's a great site about smps,mainly directed to hi-fi guys:

**broken link removed**


Others:

https://www.smps.us/


With a homemade built transfo, how to be sure 3KV isolation is really effective by exemple????


Thanks a lot for your reply, as usual it makes sense.


Jm

 

[D.A.(Tony)Stewart]

mylar tape has good hipot performance

http://www.pulseelectronics.com/download/3836/switch_mode_transformer_platform_overview_2013

 

[jmx66]

Hi SunnySkyguy,

As usual thanks for your reply.

Regards.

jm

 

[ROCKET SCIENTIST]

I heard some guyz to say that, they've destroyed their MCUs supplied from smps. I guess linear is more jitter free than smps. Well, a personal problem. I need to power up two peltier coolers (each rated 12v, 5A, 60W) from a transformer rated at output (12v-0v-12v, 6A). what shold I do? paralleling the coolers on (12v/10A) or connecting them in series with (24v,5A) ?? any idea?

 

[jmx66]

Hi rocket scientist,

Read this link:

https://www.eleccircuit.com/regulator-12v-10a-by-ic-7232n3055/

To short 12 x 1,44 = 17,28 V minus 2 V from bridge rectifier so 15 V before 7812 voltage regulator.

Should be sufficient enough for 7812 ,for low current I think. View attachment 7812-datasheet.pdf

But as you see in above link, there is other stuff for more than 1,5 A....



Others links to know why there is a dot on transformer: - read carefully -

- https://en.wikipedia.org/wiki/Transformer
- https://www.electronics-tutorials.ws/transformer/transformer-construction.html
- https://www.smps.us/magnetics.html



For short:

- paralleling 2 transfos but 12 V AC from trany for 12 V DC after rectifying stage, safety margin ?????
- paralleling 2 coolers but see line above?????
- apply above schematic


I am a beginner too, so gurus in this field, will help you; and if i made mistakes they help me too :wink:




Hope this helps.

jm

 

[ROCKET SCIENTIST]

Hi Jim, thanks a lot for the circuits. nice stuff. Well I just wanted to make a very very simple DC power supply, so I think maybe I should gor for the series connection. Also the secondary has current limitation according to the wire gauge.
well It's nice to have a 10 Amp power supply.

RS..

 

[D.A.(Tony)Stewart]

Dont forget old PC ATX power supplies have plenty of 12V capacity and are easy to make work. Some old ones require a 10% preload on the 5V, to make the 12V work, but not the newer ones in last 5 yrs.

**broken link removed**

 

[jmx66]

Hi SunnySkyguy,

Thanks a lot for your numerous replies and advices, i'm now on modifying one schematic found on web site.

jm

 

[jmx66]

Hi all,

As planed, i 've just posted a new one '' Advices needed for modifying smps power supply '' , Power Electronics forum.

I don't know, if the best practice is to continue this post, or to begin a new one?

Thanks.

jm