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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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10th Sep 2020, 9:54 am | #41 |
Dekatron
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Re: Toroidal inrush
I have no direct information on the transformer VA, but since I need to take the lid off to replace the toroid fixing screw I'll measure its dimensions. I removed the fixing when I was using it as a subwoofer amp, because the transformer buzzes annoyingly in a hifi application. That is not an issue in its current use incarnation.
Craig |
10th Sep 2020, 10:18 am | #42 | |
Dekatron
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Re: Toroidal inrush
Quote:
McMurdo's post #21 says that the running current of the transformer is 90mA, and further on, that with no audio and the amplifier powered-up the whole thing takes 150mA. On 240V supply, that's equivalent to 1.6kΩ. So a surge limiter of 160Ω, which can't possibly pass more than 1.5A, should be fine. After a few seconds of virtually no load the reservoir capacitors will be fully charged, and if this is shorted the transformer will see a rise of, crudely, 218V to 240V. Minuscule toroidal surge will result. In practice, it's likely to be even less of an increase, because when settled down with the surge limiter in place, the transformer voltage and the surge limiter voltage will be nearly in quadrature so the transformer could even be seeing 238V. And after the surge limiter is shorted, the amplifier can have signal applied, and do its job. |
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10th Sep 2020, 10:41 am | #43 |
Dekatron
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Re: Toroidal inrush
Summarising:
The amp blows fuses even when started on a variac at working voltage. The amp comes up OK when the variac is wound up slowly. The quiescent current does not seem excessive. Have you tried monitoring the rail voltages with a scope? Seems likely the transient from the relay engaging is causing a power amp fault. |
10th Sep 2020, 11:25 am | #44 |
Dekatron
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Re: Toroidal inrush
I haven't, but with amps this size, very few faults in the audio circuits that involve tripping the mains are recoverable; they normally go off like fireworks.
PTC inrush thermistors are often used because if there's an overcurrent at power up, the soft start does not complete and the PTCs go high resistance, minimising colateral damage. Obviously a bypass is always used with PTCs, though not always with NTCs, providing they are mounted so the heat isn't a problem either normally or under fault conditions. I do have another of these in the shed that was scrapped for an output blow-up; I'm tempted to salvage that transformer and see what happens.
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10th Sep 2020, 11:25 am | #45 | |
Octode
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Re: Toroidal inrush
Quote:
Mike |
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10th Sep 2020, 12:08 pm | #46 |
Octode
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Re: Toroidal inrush
If another toroid is available, try that, it will at the very least prove a point one way or another.
If it does it again, it's clearly destined for the scrap yard.
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10th Sep 2020, 5:00 pm | #47 |
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Re: Toroidal inrush
If the amp is working normally without blowing it's plug fuse, when powered through the variac and only trips the 32A house mcb when plugged straight into the mains supply, I would suggest changing the 32A house mcb to a type 'c'. Also check the issue without anything else on the came 32A circuit, as it might be a combination of loads upsetting the 32A house mcb.
We used to have issues where I worked with 3Kw 240v to 110v isolation transformers that would trip the 16A type b mcb's and the solution was to replace them with type the c kind. There was also a 3 phase mcb that would trip occasionally and once we found that it only happened when a certain piece of equipment was switched on, we realised that the motor in the equipment was the cause. Not faulty just a rather large inrush current, that was cured by changing the mcb to a 'd' curve type. Here is a link to a helpful web page that explains about the different mcb curve types. https://www.electricalclassroom.com/...b-trip-curves/ Dave |
10th Sep 2020, 5:07 pm | #48 |
Hexode
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Re: Toroidal inrush
Fitting a type C 32A MCB might be a solution, however before doing this I would check the circuit and measure the prospective fault current to ensure that the MCB trips within the time specified in BS7671. I would only do this as a measure of last resort. C type MCBs are not very common in domestic wiring systems.
Here in Germany all sockets are wired in a radial system nowadays with 2,5mm² conductors and are on type B 16A MCBs. My isolation transformers often tripped the workshop MCB at switch on. I bought a couple of DIN rail mounted soft start modules from a well known auction site and have had no problems since.
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10th Sep 2020, 7:22 pm | #49 |
Dekatron
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Re: Toroidal inrush
OK - the transformer in the C-Audio is 150 dia x 75 high, which I reckon is 750VA.
Craig |
10th Sep 2020, 7:41 pm | #50 |
Hexode
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Re: Toroidal inrush
This is a most interesting thread but may I expand on the reference alluding to the type of MCB installed.
Here it has to be borne in mind that over current protection is not only configured for the circuit protection but also a need to ensure discrimination throughout the system upstream. It is for this reason that most if not all manufacturers provide a range of over current protection curves to cater for the differing types of application to allow grading to be undertaken. Merlin Gerin for example as most others provide three types of curves namely b,c & d as given below. Correspondingly the trip times are easily calculated as per their web based examples. These would suggest that one may need to seek an alternative MCB device for the application at hand. |
10th Sep 2020, 7:55 pm | #51 |
Hexode
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Re: Toroidal inrush
I forgot to add in my last message that circuit breakers unlike contactors or relays or switches or disconnectors are designed to interrupt fault currents. Maloperation in the form of contacts welding together and mechanical failure attributable to magnetic forces can cause explosions.
Ironically, here we are taking about high transformer inrush currents whereas one of the most arduous parameter are low transformer magnetising currents. |
10th Sep 2020, 10:09 pm | #52 |
Nonode
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Re: Toroidal inrush
You need to be very careful about swapping trip curves (type B to type C for example), the Earth loop impedance also needs to be low enough too! as Eddie CE mentioned earlier.
the magnetic trip part of the curve type letter isn't the trip speed as Mark H implied earlier per se (although it is related loosely to speeds), but rather the trip current required to trip the device instantaneously, for example a B type requires a fault current of between 3 to 5 times In (nominal trip current), so a 16A B type MCB would need between 48A and 80A to ensure an instantaneous trip (fault current rather than overload), a C type needs between 5 and 10 times the In or between 80A and 160A to achieve the same result. As always with BS7671 (regs) you need to take the worst case figures, it is inadvisable to change breakers for a higher trip curve unless you have the means to measure and calculate if the circuit is suitable to be altered/modified and that the Earth fault loop impedance is low enough to guarantee the trip current is high enough. In the case of a 32A MCB then a B type needs a fault current of between 96A and 160A, while a C type needs between 160A and 320A to achieve the same result, compliance with the regs means again the Earth Loop impedance has to be low enough to achieve the higher figure of fault current. Edit: Yes I know RCDs can lower the requirements for the Earth loop impedance part, however you still need a high enough prospective fault current, ie. Line to Neutral as well if an RCD is fitted to ensure the device can still trip in the required time.
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10th Sep 2020, 10:47 pm | #53 | ||
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Re: Toroidal inrush
Quote:
Quote:
You're asking a power supply based on a 750VA transformer to deliver around 1700VA continuously to the power amplifiers. I could just about believe that this amp - working in non-harsh conditions with a replacement (perhaps upgraded?) fan and no internal debris or blocked inlets/outlets - might have enough thermal headroom in the output stage for full power operation. But surely you can't happy about overloading the mains transformer to that extent? The transformer will of course be absolutely fine for music, but for a continuous full power sine wave? |
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10th Sep 2020, 10:53 pm | #54 | |
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Re: Toroidal inrush
Quote:
This is a useful overview: http://wiki.diyfaq.org.uk/index.php/MCB |
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10th Sep 2020, 10:59 pm | #55 |
Nonode
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Re: Toroidal inrush
Yes, I know Mark
Likewise I have also grossly simplified things too in my post, as I didn't want to write a "war and peace" post either What I was mainly trying to get across was that that the type curves shouldn't be replaced willynilly unless you can verify that it is safe to do so.
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10th Sep 2020, 11:39 pm | #56 | |||
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Re: Toroidal inrush
Quote:
Which is a lot less than I remembered without reading my own project report! Craig |
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11th Sep 2020, 10:25 am | #57 |
Octode
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Re: Toroidal inrush
I did not mean to say just change the mcb type and forget about it in this case. There are of course quite a few parameters to check before taking this action. If it was me I would consider supplying the workshop with it's own dedicated mains supply, with a 20A mcb. The type could be decided upon once the type of loading is established. This would at least eliminate any other household influences having an effect.
Something else that crossed my mind re the mcb tripping, is the variac mains earthed because if not the problem might be due to an earth leakage somewhere in the amp mains filter or insulation breakdown. Also is the supply that the amp is being tested on an isolated one. Just thinking out aloud but if it's an isolated one maybe the amp transformer is creating an excessive inrush current in the isolation transformer. Really need to know the full supply connection circuit to work this out. Dave |
11th Sep 2020, 11:27 am | #58 |
Octode
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Re: Toroidal inrush
I'd agree entirely with the above post, this was mentioned earlier in this thread but seems to have fallen on deaf ears.
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11th Sep 2020, 11:44 am | #59 |
Octode
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Re: Toroidal inrush
Just had another thought, try using another kettle lead, or if it still trips, try the dangerous method as follows: re-connect the secondary to the board, lift the earth connection off the chassis (disconnect in the plug if need be), power on again and see if it trips.
The above will have a disclaimer attached, do so at a safe distance. It'll be interesting to see if the breaker stays on or not.
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11th Sep 2020, 2:34 pm | #60 | ||||
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Re: Toroidal inrush
Quote:
Over the years I've often noticed a trend to over-estimate what pro amps are designed for and capable of. I reckon that's the legacy of the Crown DC300, which gives around 200 watts per channel into 8 ohms, and about 340 watts per channel into 4. It has a 1kVA mains transformer, so is capable of continuous running at full sine wave power - though the manual cautions that you'll probably need to provide fan cooling if you plan to do that and don't want the output stage thermal trips to operate. Of course, it was designed for non-audio applications originally, so that's why the transformer is over-engineered for audio use. Personally, I always like to use a generous transformer for my own projects - the additional cost is always minimal. I don't always have that luxury for something that's being manufactured, but recommend at least parity (e.g. 100VA for 50WPC). It's plenty for music, and fine for full-power sine wave testing providing it's not prolonged. I almost always use thermal cutouts on the heat sinks, as the cost of those is easily offset by the heat sink size reductions they permit, plus it's another marketing "tick". For external heat sinks, the regulations pretty much mandate them. For mass-produced stuff, the downward pressure on heat sinks and mains transformer is driven by the cost of shipping in addition to everything else. Anyway, back to the thread... |
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