Definition of IHF Burst Peak Output (Music Power)

Can someone please define the term "IHF Burst Peak Output (Music Power) as quoted in the TwinTen (February 2005) project. I was under the impression that peak music power specifications were worse than useless and as such have little place in a good technical journal.

This impression was further reinforced by the Publishers Letter in the January 2005 edition which to quote, stated " ....in fact, if a piece of audio or home theatre equipment makes any reference to "PMPO" (peak music power) it is a fair bet that it is junk..."

Now I know that there is no reference specifically to PMPO in the TwinTen article, nor is the project junk but please, what is the relevance of IHF Burst Peak Output (Music Power) and how does it differ from PMPO?

Malcolm Fowler,
Beaumaris, Vic.

Comment: rather than being specified as IHF Burst Peak Output, it should have been simply called Music Power. This is measured using the 20 millisecond tone burst method specified for measuring Dynamic Headroom in the American Institute of High Fidelity IHF-A-202: 1978 specification.

It effectively measures the audio power that an amplifier can deliver with typical music signals. It is a legitimate measurement and one which gives a good guide to the amount of power that an amplifier can deliver under typical conditions.

PMPO (peak music power output) ratings seldom have any connection to reality and typically far exceed the maximum power consumption of the device, sometimes by as much as 100 times. For example, a device with a maximum power consumption of say, 5W, might have a PMPO rating of 200W!

USB-Up power switch a beauty

I recently built the USB-Up power-up kit and fitted it to a 4-outlet powerboard, a $2.50 special. This unit is slightly narrower than your PC board so I shaved a millimetre off each side without any damage to the printed tracks. For safety reasons, the 240VAC cable had to be re-routed to enter from the opposite end. The cable clamp was omitted but drilling a hole just smaller than the 240VAC cable sheath in a spot where the top plastic cover when screwed on acted as a clamp worked fine. It may not meet the modern cable clamp standard but as this device when installed is rarely touched, very secure cable clamping is not an issue.

Once I had assembled the PC board and then marked where to cut the base unit of the powerboard for the USB connectors I realised that fixing this PC board via screws was inappropriate as it was held in place very tightly due to the snug fit in the plastic base and the top cover pressing on the tops of the USB connectors.

Anyway it worked perfectly when powered up, so thanks again for a beaut kit.

J. M. Taylor,
Donvale, Vic.

Why no 240V door switches?

Could someone please explain why simple 240VAC door switches don’t seem to be readily available. Having had our kitchen renovated just recently, I was less than impressed with the ‘state-of-the-art’ solution for an automated pantry light. This consists of an infrared detector switch connected to a 12V halogen light. Yes, the light comes on when you open the door. Having opened the door, however, you are standing there stroking your chin and wondering what you will extract to satisfy your appetite then click - the light promptly goes out again, leaving you in the dark with a rumbling stomach..

Yes, you can extend the period for which the light stays on but this is not a good solution. All told, I regard the infrared switch idea as just plain dumb and vast overkill into the bargain.

This eventually prompted me to replace the IR detector with a door switch. It was just a matter of buying one from the local hardware store, or so I thought. No such luck! Door switches seem to be unheard of (except on fridges, cars and house alarms). Eventually I settled on a 240V micro-switch from Jaycar, mounted inside an insulating black plastic box, which I then had to paint white. It works well but it was a lot of fiddling around. Kitchen renovation companies sometimes use micro-switches but they do so with reluctance. I now understand why.

Why is it that 240V door switches seem to be unavailable, even from kitchen renovation companies? Is there some law or regulation that prevents manufacturers from coming up with such a device? After all, door switches have many applications - not just kitchen pantries.

Rod Ryburn,
Fadden, ACT.

Large low-speed fan better for PC cooling

I recently came across the July 2004 issue and the article about silencing a noisy PC. I have found that a huge fan running slowly is actually far better and cheaper than the cooling options available today. I use a 15-inch 3-speed floor-standing low-profile fan, set to its slowest speed, blowing into the open side of the computer case. Both side panels of the PC case are removed, allowing a nice airflow around all components including the hard-drives, the AGP card and capture card.

My machine uses an AMD 3000+ XP CPU, two 120GB 7200RPM hard-drives, 8x AGP with 128MB video RAM and Pinnacle AV/DV video capture card. I have HDD TEMPERATURE installed and without the floor-standing fan, the normal operating temperature of the HDDs is approximately 42°C with an ambient temperature of 25°C. With the fan at its slowest speed, the reported temperature drops significantly to around 30°C.

Graeme Rixon,
Otago, NZ.

Comment: the lead picture in that story showed a PC with a huge fan – we included it as a sight gag. But apparently it really is true! So there you go.

Raw prawns and persiflage

I have two comments: (1) page 66 of the January 2005 issue states that "..twenty or thirty bucks for a kilo of prawns..". By my reckoning that is extremely CHEAP, unless they are very small in which case they could be mini shrimps. After all, a kilo of prawns (or other things like apples etc) means you should have 1000 of them! So 1000 (typical sized) prawns for the $20 or $30 would be a real bargain!

(2)Your articles use the (incorrect) term ‘voltage’ for the (correct but wordy) term ‘electrical potential difference’. For consistency, why not use terms like nanofaradage for capacitance, ohmage for resistance etc? Unfortunately, this is also found in too many publications, classes, lectures and often leads to (at least) students not properly understanding the important (electric) concepts. Any comments? However, the magazine is great.

D. Sidors,

Port Vila,
Vanuatu.

Comments:
(1) A kilo of raspberries.
(2) How can you take issue
with a term like voltage?
We might have taken umbrage
at the suggestion,
from an insular personage,
that we were involved
in incorrect verbiage,
then we realised
you wished to engage
in mere persiflage.
To persist with this badinage
would give you the opportunity
to further disparage.

Canal better than desalination

I am disappointed with Leo Simpson’s March editorial concerning desalination. He quotes just a handful of figures and supports the "no-brain" technology solution. I wonder how many people have downloaded the Tenix canal brochure?

It’s not difficult to make some calculations from the rather minimal data in the proposal. Manning’s formula suggests that a grade of one metre per 100km would provide the flow required. Thus the total head required is just 40m. With fairly efficient axial pumps the power (not "energy") required is around 5MW, compared to the 25MW the desalination plant will require.

This calculation though still fails to come to grips with the real problem. Perth will use all the water that a government is prepared to lavish on it. At the same time Water Corporation pollutes the sea with many gigalitres per year of fresh secondary-treated ex-sewage water instead of recycling it. The horticultural industry still closely surrounds the city and uses a very significant amount of water that could be redirected to hungry lawns.

Leo suggests raising water pricing. While that would increase revenue to buy a bigger desalination plant it is unlikely to reduce suburban water consumption, though it would force some horticulture out of town.

The real problem is that we live in a society where the easiest solutions to implement are the buy-more-technology ones, like trucks over trains, that unfortunately cost us foreign exchange and use the most fossil fuels. These solutions are always easier to promote when we avoid looking at the whole set of relevant facts.

Kevin Shackleton,
via email.

Comment: We think you will find that even the most slowly flowing rivers have substantially more fall than 1m in 100km and even then, most slow rivers are very wide to give a large flow - just how wide is this proposed canal going to be? We have more regard for the figures on the Water Corporation website. In any case, it does not now appear that the canal will ever be built.

All Australian cities can do much more to promote water conservation and re-use.

True & reactive power explained

With reference to the letter entitled "Confusion about instantaneous power" (March 2005), I believe it is timely to clarify the difference between "True" and "Reactive" power. All resistive loads draw current that is in phase with the applied voltage, resulting in consumption of true power, measured in watts. The power factor is high, ie, unity.

All purely reactive loads (ie, capacitive or inductive) cause the current to be 90 degrees out of phase with the applied voltage so that the resultant power is reactive power, measured in vars (volt-amperes reactive). As a TAFE instructor some years back, I would demonstrate this by connecting a large capacitor across 240VAC with a series-connected ammeter in circuit, which read 10 amps. I would then pose the question, "How much power is being consumed by the capacitor?"

The answer is that an average of zero watts of true power is being consumed. However, 2400 vars of reactive power exists and the circuit power factor is at minimum, ie, zero. In reality, power is being consumed by the device for a total of half a cycle and produced by the device for the other half, as a result of the stored energy capabilities of inductors and capacitors. Thus the net average true power is zero watts. In your reply, you state that high power factor loads cause serious problems to the distribution system. I presume you mean low power factor loads, as the power companies apply penalties to consumers with low power factors.

Terry Ives,
Penguin, Tasmania.

Audio/video data is brilliant!

I always enjoy every issue of SILICON CHIP. It’s full of interesting material.

Regarding the Professional Sports Scoreboard in the same issue, I have no use for a scoreboard but the method of data transmission using video/audio modules is brilliant!

It is simple but effective and doesn’t require dedicated (and therefore pricey) data modules. I have dabbled a bit with PICs transferring wireless data with limited success.

Is there any reason why these video/audio modules couldn’t be set up to work as a full duplex data link if the transmitters and the receivers were set to appropriate channels? And what could the maximum data rate be?

Finally, the article about the start of Colour TV (March 2005) was particularly interesting.

I cut my teeth on cassette tape recorders and progressed to restoring written-off VCRs (when they were worth $1000+) and then onto fixing computer monitors.

It was just a hobby but I don’t do that any more as it isn’t worth spending the time!

David Vieritz,
Mango Hill, Qld.

Comment: We don’t see any reason why you couldn’t use two pairs of 2.4GHz AV transmit & receive modules (set on different channels) to provide a duplex data link. However the simple data modulation system Jim Rowe used is fairly limited in terms of its maximum data rate, because of the audio channel bandwidth. It’s only capable of reliable operation up to about 150 bits per second.

More on current transformers

I respond to the editor’s comments in the March 2005 edition of Silicon Chip on page 6 regarding the letter from David Millist about "Current transformers can be dangerous".

You are right that in normal operation the voltage across the CT (current transformer) primary is very low but that is only true when the secondary winding has a low ohmic load connected.

This secondary load is reflected to appear as the primary impedance (modified by the turns ratio). However, if the secondary is open-circuit, then the primary winding impedance by simple ratio would also be infinite.

In practice, it is not infinite, as it can only be a large as the primary winding impedance times the current. This impedance is formed from the primary winding inductance and resistance combined (ie, it looks like a choke).

So yes, very high voltages do occur with open-circuit secondary circuits of current transformers.

A single wire passed through your iron-cored toroid must complete a full turn loop back to the power source (even if it is miles away), otherwise you get no current. A common misconception is that this only forms a half-turn primary but if you do measurements it becomes apparent that it acts as one full turn.

Peter Dettmann,
Moonee Ponds, Vic.

230V change in name only

In the October 2004 Mailbag, John Hunter asked "What ever happen to 230 volts?". I work in Christchurch, New Zealand, for an Australian pump company.

We build submersible bore-hole pumps from German manufactured components supplied directly to us and held in stock in our warehouse.

The motor data sheets supplied to us from Germany show "voltage 400 +10% -10%". Occasionally, we source a motor from our head office in Australia for an urgent job and the motor data sheet that is supplied to Australia from Germany for the motor shows "voltage 415 +7.5% -12.5%".

So yes, I believe the "conversion for Australia" is largely in name only. Why would a country waste its time and money to change over for 15V when most 400V appliances be affected by the difference?

I can understand countries in Europe changing when they are using power generated by neighbour countries. New Zealand chose 230V/400V in the 1920s so when Europe settled on 230/400V it was a stroke of good luck for us.

Matthew Thomas,
Christchurch, NZ.

Current transformer clarification

Some clarification required in regard to the letter in the March issue from David Millist and your response to it.

The device described in the "Current Clamp Adaptor for Multimeters" in the September 2003 issue is not a current transformer in the sense of the devices used in electric power supply systems for measurement and protection purposes. It is more a current transducer.

A typical current transformer may consist of a bar primary conductor (ie, one turn) through the centre of a toroid and a number of secondary turns around the toroid, depending on the ratio required for the CT.

For example, a 300/5 CT would have 60 turns ideally but more likely 59 turns to compensate for losses. There would be insulation between the primary and the toroid appropriate to the primary voltage level, eg, 11kV.

When the CT is operating as intended, the primary amp turns, 300 x 1, approximately equal the secondary amp-turns, 5 x 60, and act in the opposite direction on the core (Lenz’ Law) so that only enough amp-turns to magnetise the core are present. Depending on the quality of the core, this may amount to say 0.5% to 10% of the current and is the reason that the secondary may be reduced to 59 turns, to increase the secondary current for a given primary current.

If the secondary was open-circuited, the amp-turns of the primary would still act on the core without the opposing amp-turns of the secondary and the core would be forced into saturation with resulting high voltage across the secondary winding.

Because of non linear magnetic effects this voltage may be of the order of 1000 volts on a highly specified protection CT and could be nasty or even fatal on a modest CT.

If you ever work in the power supply industry you soon learn that you never open-circuit a CT secondary winding whilst the primary is energised!

Alex Brown,
Ashburton, Vic

Comment: see our comments and editorial on this topic in the April 2005 issue.

Current transformer response ill-informed

I am writing to you regarding your response to a readers letter regarding current transformers (CT’s) in the March 2005 issue. I am appalled at the ill-informed, deprecating manner with which the respondent treated a letter voicing a well-known problem.

Clearly your response was not from an electrical engineer or anyone with capabilities in the area. Any first year electrical apprentice will tell you that you don’t leave a CT secondary open circuit, as it will overheat and destroy itself while producing dangerously high voltages.

When the secondary is in a closed circuit, the potential across the secondary is very low and the current is also low. Current transformers operate at low flux densities. When the secondary is open circuit, all the primary current is available for excitation.

The core of the transformer then saturates, this makes the flux rate of change at the zero crossings extremely high. As the potential across the secondary is the turns ratio times the rate of flux change, very high potentials can appear. Voltages in the order of 3.5 to 4kV can be generated by normal industrial devices.

This is a such a well-known phenomenon that a large number of manufacturers build-in protection for exactly this situation.

With regard to the comment on turns ratio, I would suggest your writer confines himself to facts. Any textbook will explain the fact that the primary of a CT constitutes one turn.

While I don’t expect this letter to see the light of day, it would make a good topic for a well-informed editorial.

Craig Smith,
Adelaide, SA.

Comment: see our comments and editorial on this topic in the April 2005 issue.