chris_ivan wrote:
> Steve at fivetrees wrote:
> > To be blunt, I think you're going about this the wrong way. For that kind of
> > application, I would strongly recommend you use a switching regulator.
> > However, with respect, if you're struggling with a linear regulator, you'll
> > be in all kinds of trouble with a switcher. Why not just buy a switching
> > module?
> >
> > Steve
> > http://www.fivetrees.com
>
> Hi Steve,
>
> thanks for the recommendation. To be honest, I usually build power
> supply module for microcontroller system whose current need around 1A.
> So far I don't find any difficulties with linear regulator. This is the
> first time I build 3A adaptor. I'm gonna start learning the alternative
> using switching regulator. Ah, also in the city where I live in,
> electronic shops typically sells adaptor around 1A, if I need more
> current, I ought to build it by my own. I also looked for switching
> transformer to reduce the size of the stuff, but I hardly found it.

You can easily build one for a few bucks.   A 30V 7A MOSFET
costs around $1.   Unfortunately, most controllers are very small and
difficult to build without a PCB.  Everything else can be hand wired,
includind the MOSFET.  We are thinking about build an adaptor just
for the controller (lm3485), expanding from MSOP-8 to DIP-8.

>
> This may seems a stupid question, but I would be if I don't ask. I just
> wonder whether it is possible to build 3A adaptor using 1A linear
> regulators connected in parallel. In theory it's possible, isn't it?

No, it's not.  They all see overloading conditions and shut down
itself.
You can use by-pass MOSFET or IGBT.  But why not using switchers
with 80% to 90% efficiency?

> 
> Thank you for the suggestion, it really helps.
> 
> Ivan

"chris_ivan" <chris.ivan@gmail.com> wrote in message 
news:1139906299.586337.132050@g44g2000cwa.googlegroups.com...
>
> This may seems a stupid question, but I would be if I don't ask. I just
> wonder whether it is possible to build 3A adaptor using 1A linear
> regulators connected in parallel. In theory it's possible, isn't it?

It is possible in theory, but in practice you'll have problems with current 
sharing - i.e. it will almost certainly prove tricky to get the parallel 
regulators to share nicely. It can be done rather better with a single 
regulator reference/feedback/comparison circuit and a number of series-pass 
MOSFETs in parallel and bolted onto the same heatsink... but a switcher is a 
far better approach. A linear regulator providing 3A is, no matter which way 
you do it, going to waste an awful lot of energy as excess heat which will 
need dumping somehow.

Steve
http://www.fivetrees.com 

chris_ivan wrote:
> thanks for the recommendation. To be honest, I usually build power
> supply module for microcontroller system whose current need around 1A.
> So far I don't find any difficulties with linear regulator. This is the
> first time I build 3A adaptor. I'm gonna start learning the alternative
> using switching regulator. Ah, also in the city where I live in,
> electronic shops typically sells adaptor around 1A, if I need more
> current, I ought to build it by my own. I also looked for switching
> transformer to reduce the size of the stuff, but I hardly found it.

Learning is great.

http://webench.national.com/appinfo/power/webench/index.cgi

Kelly

Steve at fivetrees wrote:
> To be blunt, I think you're going about this the wrong way. For that kind of
> application, I would strongly recommend you use a switching regulator.
> However, with respect, if you're struggling with a linear regulator, you'll
> be in all kinds of trouble with a switcher. Why not just buy a switching
> module?
>
> Steve
> http://www.fivetrees.com

Hi Steve,

thanks for the recommendation. To be honest, I usually build power
supply module for microcontroller system whose current need around 1A.
So far I don't find any difficulties with linear regulator. This is the
first time I build 3A adaptor. I'm gonna start learning the alternative
using switching regulator. Ah, also in the city where I live in,
electronic shops typically sells adaptor around 1A, if I need more
current, I ought to build it by my own. I also looked for switching
transformer to reduce the size of the stuff, but I hardly found it.

This may seems a stupid question, but I would be if I don't ask. I just
wonder whether it is possible to build 3A adaptor using 1A linear
regulators connected in parallel. In theory it's possible, isn't it?

Thank you for the suggestion, it really helps.

Ivan

"chris_ivan" <chris.ivan@gmail.com> wrote in
news:1139845595.181759.142310@g44g2000cwa.googlegroups.com: 

> Hi Ian,
> 
> Sorry for my late reply.I've got problems retrieving my files from my
> external HDD.
> 
> Ian Shef wrote:
>> Please provide your schematic.  Does it look anything like the figures
>> in the datasheet, especially either Figure 1 or "1.2V-25V Adjustable
>> Regulator" ?
>>
>> What are the resistor values that you used?
> 
> I follow the schematic given at the LM350T datasheet. for the variable
> resistor I use 200ohm. Is it right to do that? As far as I know, the
> datasheet doesn't specify the specific value for the varistor. 
The data sheet assumes that you will calculate the value of the resistors 
based on your needs and the various datasheet parameters that appply.  
However, if you look at the other appplication circuits, you will see that 
R1 is typically 75 - 240 ohms.  On page 5 under "Load Regulation" the data 
sheet says that R1 is usually 240 ohms.

If it were me, I would set up the simultaneous equations so that:
Vout = Vref x (1 + R2/R1)
I1   = 10 mA = Vref / R1   (see figure 1 for the definition of I1)

(I use 10 mA because the requirement is at least 5 mA and I like lots of 
margin.  It is still small compared to the load of the monitor.)

Then choose the closest standard value resistors, substitute back in and 
check the results.  If close enough, you are done.  If not, then iterate.

By "done", I mean that you are now in a position to calculate the power 
dissipated by the resistors.  If you don't like the answer, then you need 
to go back and iterate some more.

I'm
> gonna send you my schematic (JPG format) if you allow me, coz I don't
> know if I could post attachment to this group.
Send to izshefz 
at
earthzlink
dot
znet

but first remove every z from the address.  Sorry!
> 
There are two resistors, and you gave me the value for one.
As I understand it, you made R2 = 200 ohms.  Then is your R1 something 
close to 19.6 ohms?
The good news:  If you did this, then you have a minimum load of about 64 
mA, way beyond the requirement of 5 mA.
The bad news:  R2 is going to dissipate about 0.8 watts.  I hope you are 
using a 1 watt (or more) resistor.

A better set of choices might be R2 = 2000 ohms, R1 = 196 ohms (or as close 
as you can get).  Another choice would be R2 = 1224 ohms (or as close as 
you can get), R1 = 120 ohms.

<SNIP>

-- 
Ian Shef     805/F6      *    These are my personal opinions    
Raytheon Company         *    and not those of my employer.
PO Box 11337             * 
Tucson, AZ 85734-1337    * 

"chris_ivan" <chris.ivan@gmail.com> wrote in message 
news:1139847739.044518.291310@g44g2000cwa.googlegroups.com...
>
> Paul Carpenter wrote:
>> What no bypass or output filtering caps?
> It does, using 10uF/16V tantalum cap, as stated in the datasheet.
>
>> Another indication of marginal design is if he could get 14V out at 3A 
>> from
>> 25.5V input that regulator is going to COOK without a very good heatsink.
>>
>>         (25.5 - 14) * 3 = 34.5W
>>
>> Nice little heater he is designing, if the cap is still working.
>
> hmm.. as hot as 30W solder gun! I also put 2 fans to cool it down using
> different transformer. is it helpful?

To be blunt, I think you're going about this the wrong way. For that kind of 
application, I would strongly recommend you use a switching regulator. 
However, with respect, if you're struggling with a linear regulator, you'll 
be in all kinds of trouble with a switcher. Why not just buy a switching 
module?

Steve
http://www.fivetrees.com

Paul Carpenter wrote:
> What no bypass or output filtering caps?
It does, using 10uF/16V tantalum cap, as stated in the datasheet.

> Another indication of marginal design is if he could get 14V out at 3A from
> 25.5V input that regulator is going to COOK without a very good heatsink.
>
>         (25.5 - 14) * 3 = 34.5W
>
> Nice little heater he is designing, if the cap is still working.

hmm.. as hot as 30W solder gun! I also put 2 fans to cool it down using
different transformer. is it helpful? 


ivan

Hi Ian,

Sorry for my late reply.I've got problems retrieving my files from my
external HDD.

Ian Shef wrote:
> Please provide your schematic.  Does it look anything like the figures in
> the datasheet, especially either Figure 1 or "1.2V-25V Adjustable
> Regulator" ?
>
> What are the resistor values that you used?

I follow the schematic given at the LM350T datasheet. for the variable
resistor I use 200ohm. Is it right to do that? As far as I know, the
datasheet doesn't specify the specific value for the varistor. I'm
gonna send you my schematic (JPG format) if you allow me, coz I don't
know if I could post attachment to this group.

>
> Caution:  It is risky to use your expensive LCD monitor for testing.  It
> would be better to use a 5 ohm 50 watt (!) resistor.

will do. thanks for the advice.

> Wrong.  The datasheet guarantees 3.0 Amps.  The 4.5 Amp value is what the
> "typical" part is capable of.  Your part may or may not be typical.  In any
> case, your design is marginal, you are on the edge of the guaranteed limit
> of the part.  Oh, and this limit only applies if the difference between
> input voltage and output voltage is less than or equal to 10 volts.
> Otherwise, the limit is worse (lower current).  More on this below.

I agree.. I'm gonna check the input and output voltage of the
regulator.

> Another concern:  Your transformer puts out 18 VAC Root Mean Square.  When
> rectified, this is about 25.5 volts peak.  You are stressing your 4700 uF
> filter capacitor - verify by using a DC voltmeter to measure the voltage
> across it.
>
> If your input voltage is 25.5 volts and your output is 14 volts, you are
> beyond the 10 difference that limits the output current.  The guarantee of
> 3.0 amps no longer applies.

you are absolutely right. I'm gonna check it right away. thanks.

> Ian Shef     805/F6      *    These are my personal opinions
> Raytheon Company         *    and not those of my employer.
> PO Box 11337             * 
> Tucson, AZ 85734-1337    *

vze24h5m@verizon.net wrote:
> You've said that the transformer secondary is 18 volts, but at what
> current?  According to the National Semiconductor web site
> (www.national.com), the LM350T requires a _minmum_ of 10mA of load
> current.  A LED with a series resistor, connected to the output and to
> ground, should satisfy that requirement.

I'm quite sure that the LCD monitor draws current more than 1A. I've
tried to use 1A adaptor to light up the LCD, and the LED indicator of
the monitor flashes, then it goes off.

> Try 35 volt caps, instead.

Will do. Thanks anyway for the response. Appreciated much.


ivan

On Thursday, in article
     <Xns976586D37CF54vaj4088ianshef@138.125.254.210>
     invalid@avoiding.spam "Ian Shef" wrote:

>"chris_ivan" <chris.ivan@gmail.com> wrote in news:1139505907.756795.187500
>@g43g2000cwa.googlegroups.com:
>
>> Hi everybody,
>> 
>> I have a project to build 14V/3A adaptor to drive an LCD monitor.
>> 
>> the block diagram can be described as follow:
>> main plug (220VAC) --> step down transformer (18VAC) --> diode bridge
>> using 1N5402 --> cap 4700uF/25V --> voltage regulator LM350T -->
>> Voltage Divider --> DC Jack

What no bypass or output filtering caps?

>Please provide your schematic.  Does it look anything like the figures in 
>the datasheet, especially either Figure 1 or "1.2V?25V Adjustable 
>Regulator" ?
>
>What are the resistor values that you used?
>> 
>> Using my DMM, I measure the unloaded output voltage is 20VDC. After I
>> load the adaptor with rhe LCD monitor, the output voltage of the
>> adaptor drops to 2.5VDC. 
>Sounds like you are trying to use a voltage divider to directly drive your 
>load.  A voltage divider only provides the rated voltage if the load on it 
>is small compared to the load of the resistors in the divider.  When used 
>in conjunction with a three terminal regulator (LM350T), the divider should 
>be hooked up like Figure 1 in the datasheet.

Also as stated in the paragraph below Figure 1, if insufficent load the output
will rise. As someone else has said put an LED across the output with a
resistor calculated on at least 21V at output for 5 to 10mA LED current.

I think he should scope his inputs and outputs as I doubt his levels being
measured are truely DC (within normal noise parameters).

>Caution:  It is risky to use your expensive LCD monitor for testing.  It 
>would be better to use a 5 ohm 50 watt (!) resistor.
>
>> At first I think it's because the regulator
>> can't drive upto 3A, but the LM350T datasheet states that the device
>> can drive output current max 4.5A.
>Wrong.  The datasheet guarantees 3.0 Amps.  The 4.5 Amp value is what the 
>"typical" part is capable of.  Your part may or may not be typical.  In any 
>case, your design is marginal, you are on the edge of the guaranteed limit 
>of the part.  Oh, and this limit only applies if the difference between 
>input voltage and output voltage is less than or equal to 10 volts.  
>Otherwise, the limit is worse (lower current).  More on this below.
>> 
>> does anybody have suggestion how to solve this? Any comment is
>> appreciated. Thank you.
>
>Another concern:  Your transformer puts out 18 VAC Root Mean Square.  When 
>rectified, this is about 25.5 volts peak.  You are stressing your 4700 uF 
>filter capacitor - verify by using a DC voltmeter to measure the voltage 
>across it.
>
>If your input voltage is 25.5 volts and your output is 14 volts, you are 
>beyond the 10 difference that limits the output current.  The guarantee of 
>3.0 amps no longer applies.

Another indication of marginal design is if he could get 14V out at 3A from
25.5V input that regulator is going to COOK without a very good heatsink.

        (25.5 - 14) * 3 = 34.5W

Nice little heater he is designing, if the cap is still working.
 
-- 
Paul Carpenter          | paul@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/>    PC Services
<http://www.gnuh8.org.uk/>              GNU H8 & mailing list info
<http://www.badweb.org.uk/>             For those web sites you hate