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  1. #41
    Quote Originally Posted by the great waldo View Post
    Hi Doddy
    I'm using a 500va 2x18volt toroidial transformer with 3 or 4 4700uf 100 v caps. 35 amp rectifier in my power supply, would the elektor circuit be ok with that power supply ? or should I hunt around for another soft start circuit ? Thanks in advance.

    If I were you I'd buy the one from Conrad, or something similar ready made. In my opinion it is not worth the effort and is safer and guaranteed to work. The reason I linked to that circuit is because you asked for an example. Anyway, if you decide to build it then in your case I think it needs some modification, so I'd modify the following:

    1. The four resistors, as Doddy mentioned already. It is better to use more powerful type, even if the 5W may work for such application because of the short time current if flowing trough. In fact, I think a single resistor would be better, something like this:

    That one is 47 Ohm, so the current will be limited a little more, to 4.9A Instead of 5.75A with the 40 Ohm, but that doesn't matter.

    2. The relay in the example is 8A, I'd take a more powerful one, for example this one:

    3. The fuse should be 10A slow type.

    4. The art work in my opinion should have more powerful tracks, especially on the K1, K1 and the relay part. No need to waste copper by removing more than what's necessary to provide isolation.

    5. Resistors R1, R2 and R3 should be metal film, 0.6W or 1W type. Even if the current trough is low, it is good practice to use better resistors than coal film type.

  2. #42
    Quote Originally Posted by Neale View Post
    I watched those videos - the first one was quite entertaining. But did no-one spot the value of the capacitor he was using? 2600F! Farad, not microfarad. That's about 3,000,000 times bigger than the usual smoothing capacitors we are talking about! Yup, quite a lot of stored energy. Not entirely relevant, though. My machine is wired according to good practice with separate feeds to each stepper driver from the PSU. In effect, I have four separate permanently-connected bleed devices connected. A bleed resistor in this situation is a waste of energy, quite literally. Those capacitors will not hold charge after switch-off except in an extraordinarily unlikely combination of faults. The only time, in practice, that a bleed resistor might be useful is when testing off-load, and in this case you just need to be aware of the possible problems. The mains input connections are a much bigger danger if you poke around with a finger or have loose trailing wires.

    And as several people have said, certainly up to about 625VA toroidal transformers, a 16A B-curve or 10A C-curve MCB is perfectly happy without any inrush limiter.

    The engineer says, if you don't need it, don't put it in!

    You are more likely to get problems with things like the RFI filter on the VFD. These can cause out-of-balance live-neutral currents that trip RCDs.
    I mentioned how irrelevant the first video was because of the type of experiments he makes (basically, playing like a kid, as I played with capacitors when I was 7-8 years old), and also because the very large capacitor, which is actually not only large, but also only 2.5V, totally irrelevant and nothing but entertainment.

    The second video was a little more facts, but too much monkeying. The guy is too busy playing an idiot instead of explaining facts. At the end of the video, the guy drops the PSU, pretending he received an electric shock from the capacitors... but what he forgets to mention is that it is because his PSU is 170V, so of course, one should be more careful there, but still, a bleed resistor is not necessary because the capacitors will normally be discharged after a few seconds, or a few minutes later if something wrong with the circuit after the rectifier. Playing an idiot is popular on YouTube, it generates MANY views. Boring facts are not as popular.

    I agree, what's not needed should not be put in, but I disagree regarding the slow starter. I hate the bang which the toroidal transformer cause without the current limiter, so in my opinion, that is necessary in these type of PSU.

    The engineer in me doesn't understand why bother with a dedicated bleed resistors at all in a PSU, because normally there are other electronics connected to it, which will automatically ALWAYS discharge the capacitors after a few seconds maximum.

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