andyr Posted March 2, 2014 Posted March 2, 2014 Following on from guru's thread about building a mains DC blocker, I am interested to know whether the following idea would work for an isolation transformer (which will also prevent DC from passing): Yes, I could buy an isolation transformer from Tortech or Harbuch but the minimum size is 300VA and they are $150+. I did buy a smaller one from RS Components ... but it produced 273v on the output (from 244v input) - which was useless, so I returned it. I happen to have a pair of 'normal' power toroids spare ... 240v, 100VA/25-0-25 secondaries. Can anyone see anything wrong with connecting up these 2 transformers in series, arranged so that the secondaries of each toroid are joined? Surely this will deliver 240v at the output (or whatever the input voltage happened to be at the time)? I will only have about 20ma of current flowing (for a TT PSU) so, as a 100VA/25-0-25 transformer allows for 2a, I am not stressing the transformers. Thanks, Andy
VanArn Posted March 2, 2014 Posted March 2, 2014 If you measure the secondary voltage of an isolation transformer that is unloaded,it will measure a higher voltage than when it is working as intended. Are you actually having a buzzing problem with your TT PSU? If so , a little more investigation would not go astray to ascertain the cause.Sometimes it is simply too high a supply voltage that is the cause and series resistance,rather than additional transformers or a blocking network ,can be used as a cure.Precautions apply as you are dealing with the mains supply.You can use your existing toroidal transformers back to back as you propose to create an isolation transformer.If there is indeed sufficient DC on your mains,you might hear one of them buzz.
andyr Posted March 3, 2014 Author Posted March 3, 2014 (edited) If you measure the secondary voltage of an isolation transformer that is unloaded,it will measure a higher voltage than when it is working as intended. Are you actually having a buzzing problem with your TT PSU? If so a little more investigation would not go astray to ascertain the cause. Sometimes it is simply too high a supply voltage that is the cause and series resistance, rather than additional transformers or a blocking network, can be used as a cure. Precautions apply as you are dealing with the mains supply. You can use your existing toroidal transformers back to back as you propose to create an isolation transformer. If there is indeed sufficient DC on your mains, you might hear one of them buzz. Thanks, VA. I have a 1KVA, EI isolation transformer made by Soros (now Dyne Industries) which measures the same on the output as the input, when unloaded. I tried that out last night with the TT PSU which is giving me trouble - and it solved the problem. My "problem" is not actually DC on the incoming mains - it appears to be that my TT PSU is injecting hash back into the mains - which is getting into my preamp or phono stage and causing a hum to appear when the TT is powered up (which didn't occur when the TT PSU was plugged into the isolation transformer). Hence, I need to isolate the TT PSU from the mains. I am using my big isolation transformer elsewhere in my system, so I don't want to keep the TT PSU plugged into it - hence the need to get a second isolating transformer. I'll set up the 2 toroids later this week and see if it works as well as my 1KVA unit. Thanks, again, Andy Edited March 3, 2014 by andyr
Zaphod Beeblebrox Posted March 3, 2014 Posted March 3, 2014 Toroidal transformers are a poor choice for your application. Use EI or 'R' core types, as these will provide far better isolation of HF crap. Toroidals have a very wide bandwidth and are thus the worst choice for HF isolation. Hence my constant recommendation NOT to use toroidal for any low level application. EI and (particularly) 'R' core types are the best choices.
andyr Posted March 3, 2014 Author Posted March 3, 2014 Toroidal transformers are a poor choice for your application. Use EI or 'R' core types, as these will provide far better isolation of HF crap. Toroidals have a very wide bandwidth and are thus the worst choice for HF isolation. Hence my constant recommendation NOT to use toroidal for any low level application. EI and (particularly) 'R' core types are the best choices. Yes, ZB, I know that toroids are not ideal, due to their bandwidth. But I don't know where I can buy EI or R transformers - can you suggest any Oz mfrs of, say, 100VA or 160VA, 25-0-25v EI or R transformers? Thanks, Andy
Guest rondine Posted March 3, 2014 Posted March 3, 2014 1. Fix your TT power supply 2. If using back to back transformers you need to take into account magnetising current and losses. EI type will get hotter and 240 to 25 to 25 to 240 arrangement will not produce 240 out. ron
Number 9 Posted March 3, 2014 Posted March 3, 2014 (edited) Andy, Is it possible that you have connected the earthy side of the power supply to your system ground? Edited March 3, 2014 by Number 9
andyr Posted March 3, 2014 Author Posted March 3, 2014 Andy, Is it possible that you have connected the earthy side of the power supply to your system ground? Hi Steve - fear not, this is not your TT PSU! It's one I bought about 6 months ago, which works with my 110v Linn Premotec. And no, my signal grounds are not connected to mains earth, anywhere. Regards, Andy
andyr Posted March 3, 2014 Author Posted March 3, 2014 1. Fix your TT power supply 2. If using back to back transformers you need to take into account magnetising current and losses. EI type will get hotter and 240 to 25 to 25 to 240 arrangement will not produce 240 out. ron Alas, I cannot fix the TT power supply, Ron. (It's a relic from the 90s and I don't have a circuit diagram.) OK, EI will get hotter - so maybe that's one good 'pro' for a toroid - but I suspect when the TT PSU only draws 20ma, max, I suspect it is not going to get very hot. Can you explain why 240 to 25 to 25 to 240 won't produce 240v out (if the input is at 240v)? Surely there's a mathematical relationship between the number of coils on the primary and the number of coils on the secondary that produces 25v on each secondary, for 240v on the primary? Where "losses" come in is that if I'm drawing 20ma consistently out of this "isolating transformer" then I will be drawing, say, 30ma in from the wall? Regards, Andy
VanArn Posted March 3, 2014 Posted March 3, 2014 The " losses" in the voltage output of a transformer are due to the effective series resistance of each winding and have to be considered in the design of a transformer and for the application it is to be put to. Higher current models (High VA ratings) will have lower resistance windings as a result of heavier gauge wire and of course more transformer core material.
andyr Posted March 3, 2014 Author Posted March 3, 2014 (edited) The " losses" in the voltage output of a transformer are due to the effective series resistance of each winding and have to be considered in the design of a transformer and for the application it is to be put to. Higher current models (High VA ratings) will have lower resistance windings as a result of heavier gauge wire and of course more transformer core material. Yes, I can see that now - thanks VA. So I should see what 2 x 25-0-25 back to back actually do output (at the expected current draw). This will be slightly less than 240v, for the reasons you mention ... so I then adjust the second transformer to compensate. This might become 240/24-0-24 ... or, in fact, it might just produce 235v which would be perfectly acceptable for the TT PSU? Thanks, Andy Edited March 3, 2014 by andyr
Zaphod Beeblebrox Posted March 3, 2014 Posted March 3, 2014 Yes, Andy. Given the low power consumption, you could consider a multi-tapped transformer pair, like this one: http://www.jaycar.com.au/productView.asp?ID=MM2015
andyr Posted March 3, 2014 Author Posted March 3, 2014 Yes, Andy. Given the low power consumption, you could consider a multi-tapped transformer pair, like this one: http://www.jaycar.com.au/productView.asp?ID=MM2015 Great - thank you very much, ZB. A pair of those should be just fine & dandy! One Qu, though - it says "Magnetising current 80ma". Does this mean that there needs to be 70-80ma flowing through it before it works properly? Or that 80ma is the saturation current? Regards, Andy
Number 9 Posted March 3, 2014 Posted March 3, 2014 (edited) Andy, Just Call Scott Renwick at SES, 9746 4202, Tell him your problem, tell him I sent you and he'll sort you out quick smart. Edited March 3, 2014 by Number 9
davewantsmoore Posted March 3, 2014 Posted March 3, 2014 "Core losses" (losses in the transformer due to the impedance of the windings)
hochopeper Posted March 3, 2014 Posted March 3, 2014 Losses in electrical devices are grouped as iron losses (approx load invariant) ... and copper losses (increase with load I-squared * R)
andyr Posted March 3, 2014 Author Posted March 3, 2014 Andy, Just Call Scott Renwick at SES, 9746 4202, Tell him your problem, tell him I sent you and he'll sort you out quick smart. Will do - thanks, Steve. In case anyone is interested, I connected up a pair of 30VA 240v/15-0-15 Nuvotem transformers (that I must've bought from RS Components many years ago) back to back. With the TT PSU connected and the TT running at 33rpm - so drawing what I think would be 50ma of current at max - the input voltage was 245v ... but the output was 228v. Regards, Andy
andyr Posted March 3, 2014 Author Posted March 3, 2014 Andy, Just Call Scott Renwick at SES, 9746 4202, Tell him your problem, tell him I sent you and he'll sort you out quick smart. Thanks, Steve. Actually it's 970 6 4202 but I have just talked to him and ordered a 50VA 240-240 isolation transformer. He said he used something similar to stop a drinks fridge from causing a 'click' through his stereo, every time it switched on. I will add guru's mains blocker when I put it inside a case. Thanks, Andy Regards,
lovetube Posted March 4, 2014 Posted March 4, 2014 Thanks, Steve. Actually it's 970 6 4202 but I have just talked to him and ordered a 50VA 240-240 isolation transformer. He said he used something similar to stop a drinks fridge from causing a 'click' through his stereo, every time it switched on. I will add guru's mains blocker when I put it inside a case. Thanks, Andy Regards, HI Andy. I have one that suit your need. if you want to try it. regards Duc
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