Volts, amps and electric power supplies/components

[TomSawer]

Oh dear!

Look at this - one of many sites that give basic electrical calculations

[pamela]

Thanks for explaining. I hadn't realised that change had happened. I seem to recall the UK IEE were a bit, I dunno what the word is, perhaps "low-key".

[tabbypurr]

Er, the average value is zero.

NT

[tabbypurr]

no he's not :)

NT

[TomSawer]

The average value refered to is the half-cycle average.

[The Natural Philosopher]

actually it isn't. Its the root mean square average

Defined as 'the DC voltage that would heat a resistor up the same as what this AC stuff does.'

[TomSawer]

Actually it is!

It is the average of the Instantaneous values of the half-cycle.

[The Natural Philosopher]

Oh, are you bending the argument to reference the .636 value?

[TomSawer]

What argument are you going on about?

As far as I can see the posts are about Sine waveforms. The average value for the half cycle of such a wave is 0.637 of the peak. A simple graph of a sine values between 0 and 180 degs will show you this.

Also see

for a detailed explanation.

This is not "bending" any argument!

[John Rumm]

Na, it made it far more entertaining ;-)

[John Rumm]

Its ok to equate the voltage with being the "push" - the higher the voltage the more push if you like.

What you are pushing is electrical charge.

Current is a measure of the rate of flow of charge.

Too much voltage may well damage the driven circuit - either by exceeding the maximum voltage specification of some of the components, or by pushing too much current through them (typically resulting in their overheating)

A reasonable power supply would be designed with some built in protection such that if too much power is drawn (i.e. the product of the voltage and current) it will shut down, or blow a fuse etc.

It depends a bit on what it does when overloaded, and how long you run the circuit that way. It may be that its voltage output sags but it soldiers on, it might try to maintain voltage, but then shutdown thermally or blow a fuse, or it might just be that the ripple on the output goes way out of spec. The latter could in time damage the input circuity of the driven circuit.

In general terms, not too far of the mark.

[Tim Streater]

Which stands for what?

[Bob Eager]

The Institution of Engineering and Technology. Basically, the IEE merged with the IIE (Institution of Incorporated Engineers) back in 2006. It's a bit of a behemoth now.

[NY]

Ah, that solves a mystery. I saw the IET credited as supplying archive material for Prof Jim Al-Khalili's programme "Shock and Awe" about the history of electricity and its uses, and I thought I'd never heard of the IET. But it seems it's the IEE (and IIE which I'd never heard of) merged.

[Bob Eager]

I probably only knew because I'm a member.

[Johnny B Good]

Well, that saved me the bother of putting you straight on that 'schoolboy howler'. :-)

Assuming the speaker is rated to handle 7KW transients, you might have to endure 50Hz at high SPL for several seconds before the plugtop fuse blows or the voice coil releases its magic smoke with a side order of pyrotechnics.

More realistically, with 50 to 100 watt rated units, the magic smoke with pyrotechnic display event is likely to occur in less than quarter of a cycle (5ms) so more a 'crack' and a flash of light with a side order of magic smoke than a deafening thump.

It really depends on the size and rating of the speaker drive unit involved. With smaller 2 inch half watt units, the flexible voice coil tails will probably act as fusewire, rapidly curtailing any speaker cone excursions and pyrotechnics.

It's only interesting the first time around with most domestic sized speaker drive units. Beyond that, it all becomes somewhat predictable and therefore a bore, unless you're paying Russ Andrews prices - then the transactions on your bank account will be where the real action lies. :-)

Even assuming the use of a 250Vac X class capacitor (typically also rated for a maximum stress rating of 630VDC and intended to be connected directly across the mains input terminals) in a mains filter designed to act as an LPF with a cut off transition frequency of 90 or 130 Hz, you're likely to suffer a spectacular failure from voltage magnification generating kilovolts within the filter circuit.

I could've added that rather obtuse acronym, DAMHIK, made more obtuse by the lack of the qualifying phrase, "I just know :-)"[1] but I won't :-).

Quite simply, when I was trying to eliminate what I thought were troublesome 2nd and 3rd harmonics from a petrol generator emergency supply [2], I designed and made up a LPF using mains voltage rated capacitors which I'd had the foresight to test on a 6 VAC supply before trying to use them in anger. After measuring ac voltages in the range of 30 volts and above, I swiftly realised this wasn't going to be the right solution [3] for me. :-)

I assume Brian's "Mains Filter" was the more prosaic "EMI" filter designed to filter out 100KHz and above noise and transients on a 240v

50/60Hz mains supply where the magnification effect only applies to a few hundred millivolts worth of unwanted HF energy that might be present on the supply. I'm guessing Brian simply overlooked the fact that the equivalent DC voltage rating for a capacitor to safely handle 250 volts ac typically being in the region of 630 to 650 vdc. [1] This is a dig at posters who, unconscionably, use the acronym, DAMHIK *without* that all important phrase, "I just know :-)". [2] The implication, often made in the literature accompanying most UPS kit, that it's the 'poor quality' of mains voltage produced by such emergency gensets which makes them unsuited for use with their UPS kit (or a reference to a sensitivity adjustment that needs to be lowered to mitigate such problems of "poor quality" emergency supplies), is rather misleading in the extreme.

The truth of the matter is that all such gensets are extremely susceptible to capacitive loading on their output over-riding the AVR causing them to over-volt considerably (4.7microfarad capacitor across the terminals of a 230v 2.8KVA genset resulting in 280v AC ouptut - the

2KVA Line Interactive UPS in question switching some 9 microfarads across the supply when in pass mode, dropping it when switched to battery - you can imagine the consequences of this set up - I didn't have to, endless cycling between battery and 'mains power').

The problem of "Poor Quality" has nothing whatsoever to do with harmonic content (2nd and 3rd harmonic and the sub-harmonic component resulting in the use of a single cylinder 3000 rpm prime mover imposing its own 25Hz modulation along with windings slot ripple effects). Nor has it anything to do with the less than perfect frequency and voltage regulation performance of such emergency gensets. It's all to do with capacitive loading sensitivity of the AVR leading to uncontrolled overvolting.

A 'heads up' for those thinking of supplementing their whole house UPS protected circuit mains sockets with an emergency genset is that the

*only* type that's free of this capacitive loading overvolting effect is the "inverter" type such as the now classic Honda E3000 (or 3000E ... whatever!) which uses a highly efficient permanent magnet alternator to generate a DC voltage feeding a 50/60Hz 230vac inverter.

Luckily, Honda no longer hold a monopoly in the inverter genset market and it's now possible to buy a brand new 3KVA inverter genset for less than £1000 these days. The last time I checked pricing a couple or three years ago, I was contemplating a spend of 600 quid or so for one such genset before common sense prevailed.

It was one thing to buy a 2.8KVA petrol genset from Aldididle 6 or 7 years back for a bargain price of 150 quid (now down to 130 quid in more recent offers) as a worthwhile supplement to my emergency backup supply system but a totally different thing when looking to spend 4 or more times that amount to get a 'solution that will *actually* work as "imagined").

For anyone contemplating such a genset upgrade to their existing battery backed UPS arrangements, it's important to realise that a basic genset that uses a conventional 50/60Hz alternator driven by a 1500/1800 or

3000/3600rpm prime mover will be a complete waste of money unless you're considering a 20 to 30 KVA monster to backup the whole house supply where the 'protected' loading is less than a quarter of the genset's rating. [3] Have you seen the price (and size and availability) of inductors and capacitors rated to handle 30 kilovolts or more? I haven't but I figured if I had to ask, I didn't need to know. :-)

[TomSawer]

The Society of Electronic and Radio Technicians (SERT) was also part of the original merger.

[Cursitor Doom]

A rather misleading analogy, anyway. Flow rate implies a speed of some sort in real life, whereas in the quantum world, electron flow is not comparable to the flow of water. IIRC, the average person can actually run faster than electric current flows.

[The Natural Philosopher]

it actually is.

water: molecules per second electricity: electrons per second

IIRC, the average person can actually

IIRC the average person can actually run faster than most river water flows.

[ed]

Having read this thread I can see there is a bit of an argument from nowhere.

1) 230V a.c. without appreciable harmonics has the same effect on a resistive load as 230V d.c. 2) A 230V a.c. without appreciable harmonics waveform will have a peak voltage to Earth/Ground/Neutral of about 325V. 3) Since 10ms later the peak will be -ve this will have a maximum negative voltage of about -325V to Ground/Earth/Neutral. 4) The +peak to -peak voltage therefore of any one phase of 230v a.c. is about 650V and one can bridge rectify between line and neutral and get that voltage.

I think I have done the subject to death.

E