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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 14th Apr 2018, 8:40 am   #41
GrimJosef
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Default Re: Source impedance of mains, a workaround?

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Originally Posted by astral highway View Post
... And then your point about what it means if a component says ‘50R’ ‘...doesn’t mean I am 50R, it means connect me to 50R’. Thank you!

That’s probably a ‘source’ of confusion to many! ...
You can see how the confusion arises though. My favourite audio signal source is an HP8903A which contains a low-distortion sine oscillator whose output amplitude can be dialled up to anything between 1mV RMS and 6V RMS. The signal leaves via the connectors on the extreme left of the front panel, shown here https://www.mikrocontroller.net/atta...01/HP8903A.jpg. If you squint you can probably see that it says 600Ω alongside those connectors.

In this case it really does mean that the source impedance is 600Ω. The unit will only work 'properly', in the sense that it will deliver the voltage that you've dialled up, if you connect it to a load which is much higher in impedance than the value written on the front panel. If you connect it to 600Ω you will instead get only half the volts shown on the big bright display.

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GJ
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Old 14th Apr 2018, 10:43 am   #42
astral highway
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Default Re: Source impedance of mains, a workaround?

That’s a good illustration of a strict condition.

Years ago now, when I first mended (I won’t say ‘restored’) a brown box 60s valve radio, I didn’t know anything about transferred impedances. I figured it would be smart to disconnect the loudspeaker of my set for initial power -up after fixing a motorboating problem.

There was a bright blue glow in the output valve and then I thought, ah, I’ll measure that ovevoltage! So I disconnected the mains, lifted the primary hot side and then had a look. I was stunne that I cold draw an arc from the lifted lead to anode connection. I had a small but effective line output transformer!

Suddenly I understood while the bypass cap was rated 1200V.

I had the sense to stop the experiment pronto, but it was some years before I began to actually understand transformers beyond this naive level and learned about the Thévenin equivalents of a transformer circuit.
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Last edited by astral highway; 14th Apr 2018 at 10:46 am. Reason: Clarity
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Old 14th Apr 2018, 8:23 pm   #43
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Default Re: Source impedance of mains, a workaround?

In the London area, earth loop impedances are quite often lower than the rest of the UK due to the larger numbers of substations, most MCB's in a domestic scenario are rated at 6 thousand amps withstand, however, there are some 10,000 amp ones available. Although a breaker may be rated at say, 32A, in the time it takes to trip, you can get massive currents flowing, potentially in the order of a few thousand amps, enough to do major damage! one important thing to remember, never open a switch or breaker on a fault, if you are ever unfortunate enough to close one and then realise there's a problem, DO NOT try and open it, let the fuse/breaker clear the fault,there are more injuries caused by attempting to open a circuit under fault than there are closing one. We were always taught the "left hand rule", if the handle on the unit was on the right, then operate it with the left hand, that way your body is to the right, and not in front of the unit, very handy if anything happens to go wrong, especially if the circuit is a few hundred amps.
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Old 15th Apr 2018, 12:22 am   #44
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Default Re: Source impedance of mains, a workaround?

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I am about 5 metres above the main distribution board for the properties here - I've seen the giant busses when the door has been left open!!! I am 1KM from the main substation.
Are you able to identify the make, model and ratings of the CBs in the main board?

Is your aim to definitely use an in-rush limiting scheme - in which case your local upstream (or in equipment) CB can be sized to just manage operating rms level, and the trip characteristic to just meet the softened in-rush level expected.

It is also in your best interest to try and use a local CB from the same manufacturer as used in the main board CB, and to confirm co-ordination between your local CB and the main board CB via that manufacturer's technical data.
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Old 15th Apr 2018, 12:35 pm   #45
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Default Re: Source impedance of mains, a workaround?

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Are you able to identify the make, model and ratings of the CBs in the main board?
Hello, I have absolutely no qualms about the quality and robustness of the CBs in the main distribution board and MCB my consumer unit. I didn't get a look at the make but the quality of the work in the main distribution board to the building looks outstanding.

Both the building CB and my MCB are just five years old and without question, comply with to the letter or exceed the latest regs. The overall build was by a major and well-known contractor - and I moved in when some of the second fit and inspections were going on in the adjacent part of the development that was still under construction.

There is nothing to worry about in terms of the integrity of this protection.

To be clear:

1) Yes, I will be putting adequate and appropriate inrush limiting in the DC bus circuit that I'm building.

2) In a fault condition (overcurrent), the circuit will be protected by overcurrent detection circuitry that will trip out in just over two RF periods, say 5uS. There's another thread I started on the design of the current transformer I am making to be the feedback path for this current.

This is approximately 20,000 faster than what I understand is a typical trip time of 0.1S =100mS, for a consumer MCB. So no worries in a fault condition.

Not related to safety, so much, I am considering measures in line with Hugo's reflections on conduction peaks and a zero crossing solution
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Old 15th Apr 2018, 10:39 pm   #46
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Default Re: Source impedance of mains, a workaround?

Al, does that indicate you will just use the 13A domestic feed CB as the over-current protection to your equipment, or are you planning to use an additional fuse/cb within your equipment?

The thread infers that subsequent to pre-charge of main filter caps, then there will be no current limiting parts between the mains and the rectifier diodes/filter caps apart from interconnection cables and CB's upstream of your equipment.

If the diodes and caps were ideal parts, and the mains effective series impedance was really low, and there was a significant load on the filter caps, then the mains charging current pulses would trend to being of very short duration and very high crest factor, as was generically described by Schade (1943).

At some peak level of mains current pulse, the first upstream cb will trip, and even with a common D-curve cb, that CB may require a higher than expected rated current. Eg. a 10A cb could trip if the pulse peak exceeds 100A. Unfortunately cb trip characteristics are not well defined for short duty-cycle pulses.
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Old 16th Apr 2018, 4:18 am   #47
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Default Re: Source impedance of mains, a workaround?

Laboratory type signal generators usually have a feedback amplifier with very low output impedance followed by a series resistor to create an accurately controlled output impedance of whatever is needed, 50/75/600 Ohms. This is then followed by switched attenuators usually in 10dB steps to allow coverage of low output levels. The drive level to the output amp is variable to allow interpolation of the attenuator steps, and the application of AM if needed.

Makers varied in how they specified the output levels. As a power level, dBm (decibels with respect to one milliWatt) the displayed figure only makes sense into a matched load. power into anything else can be calculated and included as a mismatch loss.

As a voltage, then some makers specified the EMF, the driving voltage at the input to an ideal impedance-setting series resistor. It's usually thought of as the off-load terminal voltage, but with such a mismatch, you may have to consider the effect of the mismatched transmission line length from the impedance setter to the connector if you're working at high frequencies. Some makers specified the PD, Potential Difference voltage, which would apply into an ideal impedance load.

This all created quite a bit of confusion to put it mildly. Modern gear with microprocessors usually let you switch between the different output level formats. EMF to matched PD is a simple 2:1 ratio so you can easily do it in your head. 0dBm is close to 224mV RMS into 50 Ohms, or close to 775mV into 600 Ohms but these test your mental arithmetic a bit more.

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Old 16th Apr 2018, 11:00 am   #48
astral highway
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Default Re: Source impedance of mains, a workaround?

Phil: that's all interesting stuff. Thank you. Yes, I can see why earth loop impedances are low in London - (as noted, not only do I live 1KM from a main Siemens sub-station, but 5 metres away from the building's distribution busbar. All good points about the MCB - I leave my MCB well alone and only re-set it when I know what the fault condition is!
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Old 16th Apr 2018, 11:29 am   #49
astral highway
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Default Re: Source impedance of mains, a workaround?

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... does that indicate you will just use the 13A domestic feed CB as the over-current protection to your equipment...
Erm, no, it doesn't!! That would win a Darwin award. But just to be clear, in case it wasn't before.

1)...of course, there'll be a plug dedicated to the power side, with a suitable fuse. I have no idea where the impression might have come from that this would be omitted!!!

2) If there is a fault condition (load drawing too much current instantaneously, which in effect means in less than two RF periods), then a current transformer will be the feedback mechanism to logic circuitry I've designed that will shut down the drive to the whole circuit. As I say, it will do this 20,000 times faster than the MCB. (0.1mS, for the sake of example/5uS)

In this event, until the fault clears, the total mains current will at that point be defined by the miniscule leakage current of the capacitors plus the tiny current flowing in the safety (bleeder) resistors in parallel with each. Not very much at all, in other words.

The overcurrent protection circuitry is, of course, a separate issue to the charging currents in the caps, but there has never been any confusion over this anywhere in the thread.

You repeat another point:

Quote:
Originally Posted by trobbins View Post

If the ...mains effective series impedance was really low, and there was a significant load on the filter caps, then the mains charging current pulses would trend to being of very short duration and very high crest factor, as was generically described by Schade
I get your concern, but, in the nicest possible way, we're just going over exactly the same ground as before.

What you're stressing here is that even if (and it is a given)... even if I have inrush current limiting circuitry, then the mains charging current pulses in the diodes would have a very high (your are proposing too high) crest factor.

Yes, this is relevant...but... also please note that Hugo addressed this, along with a solution (charging reactor/inductor) earlier on. (See post no 32.) We also discussed the possible role of a zero crossing switch. This is a good idea, although one suitably rated costs around £100 here.

But...I not going to wait until I've built the whole power side before I investigate the nature of the charging pulses into the caps. I'll run a simulation, e.g in LT Spice, and then build and test that subassembly (maybe a single diode and a single cap - each with the properties of the ones in the final design - to start with) before I go on to the rest of it.

Schade, writing in 1943, would likely fall off his chair if he knew that 80 years or so later, there'd be silicon diodes and bridge rectifiers with surge current ratings of 500A, and for a few £!

Hope things are all clearer now?
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Last edited by astral highway; 16th Apr 2018 at 11:52 am.
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Old 16th Apr 2018, 11:35 am   #50
astral highway
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Default Re: Source impedance of mains, a workaround?

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Originally Posted by Radio Wrangler View Post
This all created quite a bit of confusion to put it mildly. Modern gear with microprocessors usually let you switch between the different output level formats. EMF to matched PD is a simple 2:1 ratio so you can easily do it in your head. 0dBm is close to 224mV RMS into 50 Ohms, or close to 775mV into 600 Ohms but these test your mental arithmetic a bit more.
Thanks, David. This is interesting stuff indeed, relevant and thought-provoking.
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