What determines whether a switching supply will properly drive a given load?

[William Sommerwerck]

The following is unduly long -- especially as I want a short answer -- and
is partly an excuse for bragging about an expensive luxury I own. But the
question is serious, and the answer likely useful to many in this group.

The most-common external power supply (wall wart, if you like) ever made is
likely the Sony AC-E455D. This little black cube pumped out 4.5V at 500mA,
to power hundreds of thousands of Discmans and other portable devices.
They're so common that you can easily find them on eBay for a few bucks
each.

Jump to STAX. A few years back STAX introduced the SR-001 electrostatic
earphone (not headphone) system. These little gems are driven by a tiny
battery-operated amplifier/energizer that puts out 580V DC polarizing
voltage (!!!) and about 300V of AC drive signal. They aren't cheap, but the
sound justifies the price -- if you want really good sound when listening to
portable equipment.

http://www.audioreview.com/mfr/stax/headphones/sr-001-mk2-system/PRD_143823_2750crx.aspx

Now, the amplifier/energizer is powered by two AA cells. But there's a jack
on the back for 4.5V DC -- whose diameter and polarity just happen to match
the AC-E455D's. Though STAX sells its own terribly overpriced power supply,
one can't help but think that STAX was deliberately making the SR-001
compatible with a very common AC adapter.

Hang on, we're getting there...

Sony also produced a switching version of the AC-E455D, which operates from
100V to 240V, and puts out 1000mA. Two years ago I bought a dozen of these
on eBay for a buck apiece. As they were thinner and lighter than the
"linear" version, they became the power supplies I carried with my Discman
and STAX SR-001 system.

The interesting thing (finally!) is that most of the switching supplies
won't power the STAX energizer. The energizer's red "low voltage" light
comes on, the energizer tries to turn on (briefly flashing the green "good
voltage" light), but then drops back, and the cycle repeats, producing a
"thump-thump-thump" in the earphones.

Only one of the switching supplies I have will power the earphones -- and it
often has to struggle a bit to get going.

It's not an issue of voltage or current capacity, per se. What, exactly, is
going on?

 

[William Sommerwerck]

Just a wild guess .. a load resistor in parallel with the device

will fix the problem. It seems that some SMPS need a minimum
load to function as expected.

That's what I suspected. However, the energizer is not a "light" load -- it
runs slightly warm. Whereas the CD player draws next to nothing, and the
supply has no problem with it.

Nevertheless, I might just stick a 200 ohm resistor across the power jack,
just to see what happens.

 

[Charles]

Just a wild guess ... a load resistor in parallel with the device will fix
the problem. It seems that some SMPS need a minimum load to function as
expected.

 

[Phil Allison]

"William Sommerwerck"

The interesting thing (finally!) is that most of the switching supplies
won't power the STAX energizer. The energizer's red "low voltage" light
comes on, the energizer tries to turn on (briefly flashing the green "good
voltage" light), but then drops back, and the cycle repeats, producing a
"thump-thump-thump" in the earphones.

Only one of the switching supplies I have will power the earphones -- and
it
often has to struggle a bit to get going.

It's not an issue of voltage or current capacity, per se. What, exactly,
is
going on?

** The STAX energiser obviously contains a high frequency ( supersonic at
least ) inverter to generate the high DC voltage. Betya anything it draws a
heap of current when it first gets going.

Problem is, SMPSs usually have over-current protection which typically shuts
the supply down pronto - then it may try to re-start itself. Hence the
hiccupping noise.

OTOH - a battery supply or iron transformer supply can provide brief bursts
of high DC current that few small SMPSs can manage.



..... Phil

 

[William Sommerwerck]

The STAX energiser obviously contains a high frequency

(supersonic at least ) inverter to generate the high DC voltage.
Betya anything it draws a heap of current when it first gets going.

Problem is, SMPSs usually have over-current protection which
typically shuts the supply down pronto -- then it may try to re-start
itself. Hence the hiccupping noise.

OTOH -- a battery supply or iron transformer supply can provide
brief bursts of high DC current that few small SMPSs can manage.

That makes the most sense. Even with the supply that works, the red
(low-voltage) light comes on for quite some time before the energizer
switches to green, noticeably longer than batteries or the conventional
power supply.

Thanks for the info. I have to decide whether I want to accept things as
they are (I use the headphones fairly often, and am not sure want to carry
around a pile of rechargeable batteries), or modify the energizer with (say)
a 10-ohm series resistor.

 

[William Sommerwerck]

Otherwise, get some over-2000 mAh NiMh AAs and a good

charger, and carry freshly-charged spares. This has been the
only reasonable solution for keeping my digital cameras portable.

Actually, I have a bunch of 2500 and 2750mAh AA cells, from PowerEx (MAHA).
I use the MAHA C9000 charger, which I highly recommend. I have two, both of
which were purchased when Thomas Distributing had them on sale. There's an
(optional?) car-battery cable that permits "off line" recharging, a great
help for those "in the field".

The only thing wrong with this charger is that it won't take C or D cells
(MAHA refuses to produce adapters), so I use an Eveready nicad/NiMH charger
that takes all sizes from AAA to D, plus 9V. It works well, but it doesn't
have any of the C9000's features (choice of charging current, controlled
discharge and conditioning, capacity measurement, etc).

My first digital camera was the Olympus D-620L (still have it -- I'll sell
it for $100 to anyone who wants a cheap zoom DSLR for eBay shots, etc),
which used loose AA cells. My newer DSLRs take custom Li-ion battery packs.
I hope Olympus and Canon have the common decency to provide replacement
packs for at least the next decade. If not, may they (and other camera
companies) be struck with class-action lawsuits!

In case it hasn't been mentioned here... Do not purchase third-party Li-ion
battery packs, even from well-known companies (eg, DigiPower). They simply
do not last. Their capacities quickly decline and the charger starts
blinking or reporting "battery won't charge". The ones I bought for my Yaesu
VX-5R and Olympus E-500 "went bad" very quickly -- a complete waste of
money. (The OEM batteries for both still work fine.)

 

[William Sommerwerck]

Some switching supplies do no do well in the presence

of a RF field. Does the STAX emit this type of interference
through the power lead?

The STAX energizer is an audio product, and it's unlikely any switching
circuits in it operate above 50kHz or so.

 

[bob urz]

The following is unduly long -- especially as I want a short answer -- and
is partly an excuse for bragging about an expensive luxury I own. But the
question is serious, and the answer likely useful to many in this group.

The most-common external power supply (wall wart, if you like) ever made is
likely the Sony AC-E455D. This little black cube pumped out 4.5V at 500mA,
to power hundreds of thousands of Discmans and other portable devices.
They're so common that you can easily find them on eBay for a few bucks
each.

Jump to STAX. A few years back STAX introduced the SR-001 electrostatic
earphone (not headphone) system. These little gems are driven by a tiny
battery-operated amplifier/energizer that puts out 580V DC polarizing
voltage (!!!) and about 300V of AC drive signal. They aren't cheap, but the
sound justifies the price -- if you want really good sound when listening to
portable equipment.

http://www.audioreview.com/mfr/stax/headphones/sr-001-mk2-system/PRD_143823_2750crx.aspx

Now, the amplifier/energizer is powered by two AA cells. But there's a jack
on the back for 4.5V DC -- whose diameter and polarity just happen to match
the AC-E455D's. Though STAX sells its own terribly overpriced power supply,
one can't help but think that STAX was deliberately making the SR-001
compatible with a very common AC adapter.

Hang on, we're getting there...

Sony also produced a switching version of the AC-E455D, which operates from
100V to 240V, and puts out 1000mA. Two years ago I bought a dozen of these
on eBay for a buck apiece. As they were thinner and lighter than the
"linear" version, they became the power supplies I carried with my Discman
and STAX SR-001 system.

The interesting thing (finally!) is that most of the switching supplies
won't power the STAX energizer. The energizer's red "low voltage" light
comes on, the energizer tries to turn on (briefly flashing the green "good
voltage" light), but then drops back, and the cycle repeats, producing a
"thump-thump-thump" in the earphones.

Only one of the switching supplies I have will power the earphones -- and it
often has to struggle a bit to get going.

It's not an issue of voltage or current capacity, per se. What, exactly, is
going on?

I would find it interesting that a supply of 1000ma would go into
current shutdown. If you had a peak hold DMM or some other way to
plot inrush current draw, it would be helpful to know what the device is
actually drawing peak. If it is an over current situation, maybe you can
get a NTC resistor to cut down the peak surge current. It might be
a challenge to find one with that low of rating.

on the flip side, if its shutting down from not enough load, you need
to put a small light bulb or such in parallel with the device for some
additional start up load.

bob

 

[Jamie]

That makes the most sense. Even with the supply that works, the red
(low-voltage) light comes on for quite some time before the energizer
switches to green, noticeably longer than batteries or the conventional
power supply.

Thanks for the info. I have to decide whether I want to accept things as
they are (I use the headphones fairly often, and am not sure want to carry
around a pile of rechargeable batteries), or modify the energizer with (say)
a 10-ohm series resistor.

If you would like, use a power PNP transistor on the (+) output and bias
the base via the Hfe of the transistor to limit the max current of what
the supply can handle. The Collector would then be the output and the
Emitter to (+) of the power supply output. Base gets biased with
appropriate R to maybe just below the max current of the supply.

The beta will shift upward a bit when the transistor gets warm but
I don't think i'll matter after that..One could always place a Si diode
on top of the transistor to help clamp the bias voltage at the base to
stabilize it.

I've used simple PNP current limited circuits like that in many cases
to fix inrush currents on small circuits.

You can do this with a NPN if you want to float the (-) from the
collector.. Myself, I would go with the PNP..

 

[Phil Allison]

"William Sommerwerck"

The STAX energizer is an audio product, and it's unlikely any switching
circuits in it operate above 50kHz or so.

** Hang on a mo !!!

Current technology in audio power uses PWM amplification.

Switching frequencies are typically from 250 kHz up to over 1 MHz.

The A to Ds in most current audio uses sampling rates of 2.6 MHz.

Hi-fi FM radio is also audio - operates in the VHF range.

Radio microphones are considered part of "audio" too - frequencies used
range from VHF up to several GHz.



..... Phil

 

[William Sommerwerck]

I would find it interesting that a supply of 1000mA would go into

current shutdown. If you had a peak hold DMM or some other
way to plot inrush current draw, it would be helpful to know what
the device is actually drawing peak. If it is an over current situation,
maybe you can get a NTC resistor to cut down the peak surge
current. It might be a challenge to find one with that low of rating.

On the flip side, if its shutting down from not enough load, you need
to put a small light bulb or such in parallel with the device for some
additional start up load.

I think is the issue is "too much" load. The power supply operates a
Discman -- which draws 1/10 the current -- with no problem whatever. Ergo...

I don't have a peak-hold meter. However, I did measure the battery current
with a Fluke 87 -- it's around 180mA. But at the moment you connect the
battery, the digital bar-graph display goes WHAM!, all the way to the right
in a fraction of a second. I suspect the unit is drawing several times 180mA
at turn-on.

The AC adapter input is spec'd at 300mA -- presumably the maximum capacity
required, rather than the average drawn.

If I can find a matching plug/socket pair, I might experiment with small
series resistors (1 to 5 ohms) to see what happens. At the moment, it's not
a pressing issue.

 

[William Sommerwerck]

The STAX energizer is an audio product, and it's unlikely

** Hang on a mo !!!
Current technology in audio power uses PWM amplification.

Who said this was a PWM amplifier?

The design dates back to 1996. It's highly unlikely the unit uses PWM -- why
should it? The amp drives electrostatic earphones whose cross-sectional area
is a bit larger than 1 square inch -- plus the cable capacitance. You don't
need much current drive for that. What would be the advantage of PWM
amplification?

Now, it might use a switching power supply to get the 580V DC needed for the
polarizing voltage. But that's a different issue.

 

[William Sommerwerck]

If you would like, use a power PNP transistor on the (+) output

and bias the base via the Hfe of the transistor to limit the max
current of what the supply can handle. The collector would then
be the output and the emitter to (+) of the power supply output.
Base gets biased with appropriate R to maybe just below the
max current of the supply.

The beta will shift upward a bit when the transistor gets warm
but I don't think it'll matter after that..One could always place
a Si diode on top of the transistor to help clamp the bias
voltage at the base to stabilize it.

I've used simple PNP current limited circuits like that in many
cases to fix inrush currents on small circuits.

You can do this with a NPN if you want to float the (-) from
the collector. Myself, I would go with the PNP...

Thanks for the suggestion, but the amplifier is quite small, and I'm not
going to rip into it for a modification of this sort.

 

[William Sommerwerck]

** Read the WHOLE fucking post.

I did. Who said you knew how to write?

 

[Phil Allison]

"William Sommerwanker"

Who said this was a PWM amplifier?

** Read the WHOLE fucking post.

IDIOT !!

 

[William Sommerwerck]

What makes you think I take anything you say seriously? Or that I'm bothered
that you so crudely disagree with anyone here, or am pleased when you do?

I couldn't care less. You are less than nothing, Mr. Allison.

 

[Phil Allison]

"William Sommerwanker"

I did.

** Nope.



...... Phil

 

[PeterD]

What makes you think I take anything you say seriously? Or that I'm bothered
that you so crudely disagree with anyone here, or am pleased when you do?

I couldn't care less. You are less than nothing, Mr. Allison.

If you "Couldn't care less" why'd you post this?

 

[Baron]

Rheilly Phoull Inscribed thus:

Perhaps a few hundred mF as well ??

I've seen a lot of laptop SM PSU simply cease to supply power to the
load because the output caps have developed high ESR. Adding a
suitable cap across the output often will confirm the problem.

 

[Bob Larter]

The interesting thing (finally!) is that most of the switching supplies
won't power the STAX energizer. The energizer's red "low voltage" light
comes on, the energizer tries to turn on (briefly flashing the green "good
voltage" light), but then drops back, and the cycle repeats, producing a
"thump-thump-thump" in the earphones.

Only one of the switching supplies I have will power the earphones -- and it
often has to struggle a bit to get going.

It's not an issue of voltage or current capacity, per se. What, exactly, is
going on?

My guess would be that the switcher is generating lots of hash, & it's
upsetting the oscillator in the phones. Try looking at the DC with a 'scope.