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'[EE]how does a power supply provide current limiti'
2008\04\18@123251 by gardenyu

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Recently I'm purchasing some power supplies. What surprises me is that for some very similar power supplies, MFJ-4225MV
www.mfjenterprises.com/Product.php?productid=MFJ-4225MV
and VS35M
http://www.astroncorp.com/linear.shtml
 The latter one has an adjustable current limiting function while the previous one has not. The rest are quite similar. The latter one is a 29 lb and the previous one is only 3.7 lb! The dimension is also doubled.
What makes this big difference, the MFJ one does not have a transformer inside?
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2008\04\18@124532 by David VanHorn

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> What makes this big difference, the MFJ one does not have a transformer inside?

Switching supply Vs Linear.

You will also notice that the MFJ is MUCH cooler than the astron.

IMHO, neither of these is well made.  Astrons have circuitry that's
pretty much lifted from National Semi app notes, with a crowbar
circuit that is RF sensitive, and can be more trouble than it's worth.
 MFJ products are frequently rather sloppily assembled, and the
company has the moniker "Mighty Fine Junk" among the ham radio
community.

2008\04\18@124928 by Marcel Birthelmer

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>
> What makes this big difference, the MFJ one does not have a transformer
> inside?
> _________________________________________________________________
>

Hi,
one of them (the 2nd) is a linear supply, while the other one (the first) is
a switching supply.
In a linear supply, excess power is dissipated (meaning it needs to be big
for heatsinking, plus a big line transformer), whereas in a switchmode
supply, the voltage conversion is very efficient, ideally 100%, so there is
no need for that much heatsinking, and there is also no need for a line
transformer.
Current limiting can be easily implemented in both cases (generally it's
done by using a sense resistor with a very small value and measuring the
voltage drop across it), but in one case I guess they chose not to.
- Marcel

2008\04\18@130254 by Bob Axtell

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gardenyu wrote:
>  
> Recently I'm purchasing some power supplies. What surprises me is that for some very similar power supplies, MFJ-4225MV
>  
> www.mfjenterprises.com/Product.php?productid=MFJ-4225MV
>  
> and VS35M
>  
> http://www.astroncorp.com/linear.shtml
>  
>  
> The latter one has an adjustable current limiting function while the previous one has not. The rest are quite similar. The latter one is a 29 lb and the previous one is only 3.7 lb! The dimension is also doubled.
>  

> What makes this big difference, the MFJ one does not have a
transformer inside?

That handles the weight issue, yes.

as for the current limit, it is a cheaper product. It could be
current-limited as well.

--Bob A


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2008\04\18@133126 by gardenyu

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I went to Wikipedia and it tells that for a switching mode power supply, it is a AC-DC-AC-DC process, and since the latter conversions are at high frequency, thus they need a much smaller transformer. This makes sense to me, although I still don't know which one might have a higher efficiency. They must have their own reason of living in the world.
Would you recommend any stable and trustable power supply brand?  I need about 12-15V DC adjustable  and 15-20A at output.



> Date: Fri, 18 Apr 2008 09:49:27 -0700> From: spam_OUTmarcelTakeThisOuTspamcarrietech.com> To: .....piclistKILLspamspam@spam@mit.edu> Subject: Re: [EE]how does a power supply provide current limiting function> > >> > What makes this big difference, the MFJ one does not have a transformer> > inside?> > _________________________________________________________________> >> > Hi,> one of them (the 2nd) is a linear supply, while the other one (the first) is> a switching supply.> In a linear supply, excess power is dissipated (meaning it needs to be big> for heatsinking, plus a big line transformer), whereas in a switchmode> supply, the voltage conversion is very efficient, ideally 100%, so there is> no need for that much heatsinking, and there is also no need for a line> transformer.> Current limiting can be easily implemented in both cases (generally it's> done by using a sense resistor with a very small value and measuring the> voltage drop across it), but in one case I guess they chose not to.> - Marcel> -- > www
.piclist.com PIC/SX FAQ & list archive> View/change your membership options at> mailman.mit.edu/mailman/listinfo/piclist
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2008\04\18@134426 by David VanHorn

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> Would you recommend any stable and trustable power supply brand?  I need about 12-15V DC adjustable  and 15-20A at output.


What are you running with it?

For my ham gear, I use a 12V gel battery, and a 3A power supply which
is adjusted to the batterys float charge voltage.  The assumption is
that while I need >10A during transmit, I don't actually need to
transmit all that often.  I can do 50% duty cycle for several hours,
draining the battery, and the power supply refilling what it can while
I listen, and when I turn it off, the power supply completes the
battery charge.

Different solutions for different problems! :)

2008\04\18@135758 by John Ferrell

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I have the MFJ-4245MV sitting on the back of my desk. It provides power for my 100 watt Ham Transceiver and whatever else I might be inclined to add to it. It is light  weight and quiet in spite of the fact that the fan has been stuck on since the BIG lightning strike the equipment took several years ago.

You can buy a power supply that is too big but I don't think you can buy too much capacity.

John Ferrell    W8CCW

"All that is necessary for the triumph of evil is for good men to do nothing." -- Edmund Burke
http://DixieNC.US

{Original Message removed}

2008\04\18@143124 by gardenyu

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Well, It's an inductive charging system that requires 50W at output, due to possibly low coupling issue,  a continuous 15-20A will be required

> Date: Fri, 18 Apr 2008 13:44:04 -0400> From: microbrixspamKILLspamgmail.com> To: .....piclistKILLspamspam.....mit.edu> Subject: Re: [EE]how does a power supply provide current limiting function> > > Would you recommend any stable and trustable power supply brand? I need about 12-15V DC adjustable and 15-20A at output.> > > What are you running with it?> > For my ham gear, I use a 12V gel battery, and a 3A power supply which> is adjusted to the batterys float charge voltage. The assumption is> that while I need >10A during transmit, I don't actually need to> transmit all that often. I can do 50% duty cycle for several hours,> draining the battery, and the power supply refilling what it can while> I listen, and when I turn it off, the power supply completes the> battery charge.> > Different solutions for different problems! :)>

2008\04\18@145808 by Spehro Pefhany

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Quoting gardenyu <EraseMEgardenyu2004spam_OUTspamTakeThisOuThotmail.com>:

>
> I went to Wikipedia and it tells that for a switching mode power  
> supply, it is a AC-DC-AC-DC process, and since the latter  
> conversions are at high frequency, thus they need a much smaller  
> transformer. This makes sense to me, although I still don't know  
> which one might have a higher efficiency. They must have their own  
> reason of living in the world.
>
> Would you recommend any stable and trustable power supply brand?  I  
> need about 12-15V DC adjustable  and 15-20A at output.

Where are you? GW isn't too bad (Taiwan company). A lot of the supplies,
including US makes, are re-branded mainland Chinese supplies and are a bit
rough around the edges.

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
s...spamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2008\04\18@184808 by Jeff Stevens

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On Fri, 2008-04-18 at 16:32 +0000, gardenyu wrote:
> Recently I'm purchasing some power supplies. What surprises me is that for some very similar power supplies, MFJ-4225MV
>  
> www.mfjenterprises.com/Product.php?productid=MFJ-4225MV
>  
> and VS35M
>  
> http://www.astroncorp.com/linear.shtml

As others have said, the MFJ is a switching power supply while the
Astron is a linear supply.  The closest Astron equivalent to the MFJ
above is probably the SS-25M.

Not being particularly versed in high current power supplies, I can't
really comment on the quality of either the MFJ nor the Astron products
from an engineering perspective.

I own three Astron supplies though and they have served me well.  An
RS-35M serves as the main supply for my radio equipment.  Previously I
used a 10-15 year old RS-20.  I use an SS-25M for traveling because the
switching power supply easily fits in my luggage.

The RS-20 is my current lab power supply.

There are a lot of mixed thoughts about both the Astron and MFJ
products.  Plenty of EE folks, who I suspect know what they are talking
about, say they are all poor designs.  At the same time, Astron supplies
have been the workhorse of amateur radio for quite some time.

Likewise, the "Mighty Fine Junk" products also come with mixed reviews.
I own a number of their products and I treat them like they are as
fragile as solar cells.  While they may not be the best built products
out there, in some cases, MFJ is the sole manufacturers of particular
products at a price level affordable to a hobbyist.  For anything
commercial, I'd stay away from their products all together.  They do
fill a hobbyist niche though.

-Jeff


2008\04\19@003000 by Sean Breheny

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My experience with Astrons has been positive. Yes, in some situations
RF can get into them and do strange things (like make it think it's in
current limit when it isn't). However, they seem to soldier on for
years and years and are not that expensive (although very bulky and
heavy).

One problem with using Astrons for lab work, though, is that they
usually have foldback current limiting. This is great for unattended
operation because it means that when a short circuit is detected, it
will stop supplying max current and only supply enough current to tell
when the short has been opened. This prevents tons of heating and
possible fire risk.

However, lab work often requires the ability to supply a known current
or to limit gracefully (i.e., without folding back to low current
suddenly).

Sean


On Fri, Apr 18, 2008 at 6:47 PM, Jeff Stevens <@spam@jeffKILLspamspammossycup.com> wrote:

{Quote hidden}

2008\04\19@004346 by Sean Breheny

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As others have said, switching power supplies are almost always more
efficient. That is their main advantage. Their light weight is a
secondary advantage.

Here is a quick comparison:

Weight - switcher is lighter
Size - switcher is smaller
Noise - linear power supply is usually quieter in terms of electrical
output noise, also usually radiates much less RF noise
Efficiency - switcher is more efficient
Cost - Below a certain power level, linear is cheaper. Above that,
switchers are cheaper.
Simplicity - Linear is simpler

Switching power supplies use PWM along with inductance to convert from
one DC voltage to another (up or down) and power is not intentionally
wasted. That is to say that the product of the input voltage and
current will be close to the product of the output voltage and
current. Regulation is accomplished by adjusting PWM duty cycle to
produce a varying buck (step down) or boost (step up) voltage across
the main inductor or transformer primary. As you said, this can often
be much smaller than a 60Hz transformer because it usually runs at
around 100kHz PWM.

Linear power supplies convert from one DC voltage to a lower DC
voltage. Their input current is close to their output current, so that
they accomplish regulation by wasting power on purpose (i.e., if the
input voltage is 10V and the output is 5V at 2 amps, then the input
power is 20W, the output power is 10W, and the extra 10W is dissipated
as heat by the regulator transistors.

Both of these techniques describe DC to DC conversion along with
regulation. The front end of the power supply is what plugs into the
wall. For a linear supply this almost always involves a 60Hz (or 50Hz)
power transformer to step down. The AC is then rectified and filtered
before going into the linear regulator.

For a switchmode power supply, the input CAN still have a transformer,
but it usually instead is directly rectified and filtered to produce
rough 100 to 200V DC, which is then stepped down by the switching
converter/regulator.

This seems to have a good explanation:

http://www.hills2.u-net.com/electron/smps.htm

Sean

2008/4/18 gardenyu <KILLspamgardenyu2004KILLspamspamhotmail.com>:
{Quote hidden}

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