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  1. #331
    If I add a precharge circuit to this, could I use a type B mcb I guess the capacitors have a large inrush which might trigger the type b mcb but when charged you would have better protection with a type b than a type c? I have some adjustable time delay relays (0-30mins) 10amp I could use so the precharge circuit always comes on say 1 minute before the PSU powers on.

    If so how the hell do I make one I read something about precharge resistors but have no idea where to start.
    http://www.mycncuk.com/threads/10880...60cm-work-area My first CNC build WIP 120cm*80cm

    If you didn't buy it from China the company you bought it from did ;)

  2. #332
    Came today but missing the top metal plate! I think the one on my old transformer will work fine but still poor maplins I do have both rubber protectors.

    I was hoping it would straight swap but not so sure it will I might just get away with it and a hammer ;)

    Click image for larger version. 

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    Not going to wire it up until I have an MCB, replacing the rectifier as well, going to leave the caps in place.

    Think I traced the problem to a poor crimp causing the initial problems and we think my friend reconnected the bridge rectifier backwards on the output but no one can be sure.
    http://www.mycncuk.com/threads/10880...60cm-work-area My first CNC build WIP 120cm*80cm

    If you didn't buy it from China the company you bought it from did ;)

  3. #333
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 2 Hours Ago Has been a member for 9-10 years. Has a total post count of 2,037. Received thanks 224 times, giving thanks to others 5 times.
    There's a reason transformer manufacturer's usually recommend slow blow/time delay fuses, and this thread has just proven why.

    Fit a 10A MCB to a 1KVA transformer, and there is a high probability the transformer winding will fail before the MCB trips.
    1KVA roughly equals 4 amps, so taking a type B MCB, it takes a minimum of 6 seconds at double the rated current for it to trip. So taking that 10A MCB, that's 20A, 5 times the transformer rated current, or over 4KW flowing for 6 seconds. Do you still think that MCB will do much good?

    I personally just use a time delay fuse rated around 120% of the transformer rating. If you blow it, you're doing something majorly wrong, and it's far cheaper than a MCB.

    As for cheap multimeters, they rarely have a fuse for the amp setting, which is likely what will of caused the problems. A sudden short on the power supply, as well as causing the transformer to fail, will of likely burnt something out in the power side of the AM882. If you were to strip it down, you could at least check the main power supplies are still functioning.

    Also, if you strip the outer covering of the transformer, you might find the fuses near the outside of the windings, so it could potentially be a relatively easy repair.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

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  5. #334
    I am going to have a look at fixing the transformer, there's a good vid on you tube it doesn't look that hard if the fuse is near outside but at 41 for a 1kva I couldn't resist. Maplins still have a new 625va transformer (50v) for 30 which I'm seriously tempted (Have enough bits to make a 2nd PSU) and also try and fix the original one or they have an 800va (45v) for 35. I have 7 caps now and 2 new rectifiers so it's cheap PSU for betty bigger sister Already planning ;)

    Would these be suitable slow blow fuses?
    http://www.ebay.co.uk/itm/5-CERAMIC-...A3euImkU3PFBZg

    These look plug compatible in which case I'll put the PSU on a plug for ease.

    When I first got the Am882 it had a dodgy connection I changed the plug and it was fine but I do wonder if the connection is dodgy on the board open it up couldn't see anything from the top need to take it out properly and look underneath

    I am going to complain to Chinese I bought it off as they "offered" 1 year warranty but also got it on my CC so will see if I can get a refund through them.

    At the moment this is going to a normal plug socket which is connected to an MCB in the board (16amp) on a side note my shitty 24v psu tripped it so I chucked the PSU in the bin was 7 ebay special. But it didn't trip whenthe psu blew in fact it didn't even blow the 13amp fuse in the plug and my pc stayed on connected to same power in.
    Last edited by Desertboy; 11-11-2017 at 07:25 AM.
    http://www.mycncuk.com/threads/10880...60cm-work-area My first CNC build WIP 120cm*80cm

    If you didn't buy it from China the company you bought it from did ;)

  6. I would not bother with pre-charge, this supply is not big enough to need it, as long as the rectifier is big enough to take the inrush as well as a slow-blow fuse in the input it should work fine.

    Pre-charge is easy with a resistor in the output before the rectifier, maybe 10r 25W and a timer relay which shorts out the resistor after maybe 2s would do it but i wouldn't bother myself. Pre-charge is more used with supplies that have very large capacitors/higher voltages.

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  8. #336
    Quote Originally Posted by Davek0974 View Post
    I would not bother with pre-charge, this supply is not big enough to need it, as long as the rectifier is big enough to take the inrush as well as a slow-blow fuse in the input it should work fine.

    Pre-charge is easy with a resistor in the output before the rectifier, maybe 10r 25W and a timer relay which shorts out the resistor after maybe 2s would do it but i wouldn't bother myself. Pre-charge is more used with supplies that have very large capacitors/higher voltages.
    Last edited by Desertboy; 11-11-2017 at 02:12 PM.
    http://www.mycncuk.com/threads/10880...60cm-work-area My first CNC build WIP 120cm*80cm

    If you didn't buy it from China the company you bought it from did ;)

  9. #337
    Right wired up properly and tested 78.2v out so a little high what's the best way to reduce this voltage my AM882's 80v is max input.
    Click image for larger version. 

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    I daren't plug it in to the AM882's.

    Got it wired to a 13 amp switch finally so I can turn it on/off without killing power to pc.
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    Got a free enclosure some damage to the front but better than what I have, will transfer the AM88's and BOB's across PSU will stay in PC case.

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    Last edited by Desertboy; 11-11-2017 at 06:49 PM.
    http://www.mycncuk.com/threads/10880...60cm-work-area My first CNC build WIP 120cm*80cm

    If you didn't buy it from China the company you bought it from did ;)

  10. Quote Originally Posted by Desertboy View Post
    Right wired up properly and tested 78.2v out so a little high what's the best way to reduce this voltage my AM882's 80v is max input.
    If it were my supply i might unwind a couple of turns off the transformer secondary, but thats just me ;)

    Not too many ways to fine tune the voltage really.

    Maybe also look at an autotransformer in the primary circuit - a multi-tapped primary only transformer with the input going to the normal 240v tap and then take an output from the neutral and a tapping a little lower like 220v.

    https://en.wikipedia.org/wiki/Autotransformer

    Just an idea though. ;)

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  12. #339
    Or just run it at 78v's and hope ;) I turned it on off a few times and tested the voltage each time, and I noticed my input was 250v at the time it seems to vary at work from 238 to 250v this is the highest reading I've had so far and I think the max allowable is like 252v.

    I need to check what the ac voltage going into the rectifier is didn't think that but was gutted as I was expecting to get something like Joe Harris 68v giving he has 25v output coils in series and I have 50v in parallel.

    I wonder why I'm getting such a high reading is it because I have a high input voltage? This is mains fed, to an industrial unit, is it possible other things could affect voltage on the line? I'm going to have a dedicated line put in for the machine I seem to remember getting 240v when I was working on the 3 phase sockets (I tapped 1 phase for my plasma cutter)

    Could I be getting a high reading because it's Saturday and the demand is lower on the estate?
    Last edited by Desertboy; 11-11-2017 at 07:35 PM.
    http://www.mycncuk.com/threads/10880...60cm-work-area My first CNC build WIP 120cm*80cm

    If you didn't buy it from China the company you bought it from did ;)

  13. Quote Originally Posted by Desertboy View Post
    Or just run it at 78v's and hope ;)

    I wonder why I'm getting such a high reading is it because I have a high input voltage?
    Yes, output is directly related to input, that is a very high input.

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