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Old 10th January 2012, 05:28 PM   #21
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Jointdead View Post
Are the voltage results strange and I'm doing something wrong? cause they seem wrong to me, so many different results.
have you carefully checked that you truly have only two windings?

Separate windings can give spurious voltages, due to capacitive coupling between the windings even though they are adequately insulated from each other.
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Old 10th January 2012, 05:30 PM   #22
AndrewT is offline AndrewT  Scotland
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Originally Posted by KatieandDad View Post
... but he cant see the wire diameter.....
If this is true then I refer to my earlier post.
One cannot measure the current ratings of the windings.
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Old 10th January 2012, 05:30 PM   #23
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Try loading every winding - or widing that you think you have - with a suitable resistor. That should help against capacitive coupling.
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Old 10th January 2012, 06:48 PM   #24
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How much amperes would a lm1785 stereo amp need? from the datasheet I found out that it has a high current capability of 4A, How much would it really need? and would you suggest the 32v or the 18v?
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Old 11th January 2012, 01:54 PM   #25
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Originally Posted by AndrewT View Post
If this is true then I refer to my earlier post.
One cannot measure the current ratings of the windings.
No you can't.

But you can use an estimate of the transformers ability to regulate its output as an estimate.

With an unknown transformer it is a bit of a gamble, but that is the risk that you run by using scrap components.

Most transformers will maintain their steady state output rated output voltage to around 80% of their rated output power. THIS IS A VERY GENERAL STATEMENT. Therefore if you keep loading up the secondary until you see a 5% drop in output voltage, you will have a fair idea of what that winding is capable of providing.

With multiple secondaries you must repeat the experiment with all secondaries appropriately loaded.
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Old 11th January 2012, 02:34 PM   #26
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In this case. It looks like a complex transformer. Either he needs to confirm the secondary winding configuation or spend a few quid on a transformer.

There are loads of transformers on E-Bay at the moment.

The schematic of the donor machine of the transformer might help - anyone.

I've bought a few of them off Amazon for less than 10 each.

http://www.rs-online.com also do a budget range of toroids at a VERY GOOD price.
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Old 11th January 2012, 07:09 PM   #27
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So what you're telling me is that I need to load all the secondaries or just the ones with the correct voltage for my project? and then start adding load, by small steps. Until i see the voltage drop?
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Old 11th January 2012, 07:44 PM   #28
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Default It'll work, go for it

I think people are worrying too much about the transformers output current capability. It's probably undersized, but all that means is the peak power available will be higher with dynamic material and bog down some with something more constant (boom boom boom disco etc). The main thing to be concerned about is not frying the chip with too much voltage. It will draw very little current with no audio, so just figure on the supply not putting out more than plus and minus 30 Volts with no load. The filter caps can charge up to near the peak value of the sine wave, which is the square root of 2 (1.414) times the RMS value. That 18 volt AC output is a good start. Put one end of that winding at ground, and just use a single diode to a cap each for the plus and minus outputs. (negative like positive, just reversing diode direction and cap polarity) That's half-wave, but will still load the transformer output on both half-cycles. Since amps are not a constant load, you'll be fine getting a bit more than what the winding might be intended for on audio peaks with the caps charging (and anything that warms cooling) in between. The voltage falling is no big deal, the amp can work over a very wide range, it'll just affect how much power you can get before clipping occurs. Just don't exceed the +/- 30V, don't get the polarity backwards, and make sure the chip has a heatsink. Whatever the chip in the VCR power supply was on might work Really low current windings on the transformer can be spotted by checking for a high DC resistance (measured with power off of course). So pick a winding (or series combination) adding up to no more than 20 VAC, and if you have several choices favor those with lower DC resistance. It should work no matter what, you'll just see how much power you get later. Many speakers have higher impedance in the bass range near the resonant frequency, so the actual current drain may be less than expected. 8 Ohm speakers are best since 4 Ohm would pull twice as much current at the same driving voltage. There is no need to load any unused winds/taps, just don't let them get shorted. In case the transformer does fail, it's a good idea to have a fuse on the primary. Something like 1/2 Amp slow-blow should be fine. Good luck!

Last edited by riccoryder; 11th January 2012 at 07:55 PM. Reason: for better clarity, stray text at end
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Old 11th January 2012, 08:39 PM   #29
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If stranded on a deserted island you could hack it to make sound. That's a bit different than spending the time and money on an amp and having it crippled by the power supply.

You can get surplus transformers on the cheap that are a lot more appropriate. You might find something at a trash dump, or a pawn shop, or an auction site. It need not be just a bare transformer, you can remove one from something more likely to have a suitable transformer which not so ironically might be a low wattage old amplifier.
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Old 11th January 2012, 08:53 PM   #30
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So I got a offer on a transformer scavanged from a UPS it has two outputs of 13,5V and 25V and a VA rating of 400 but it was said by the seller that with constant use it would be about 250VA for about 5 euros And another 400VA transformer with 14V output again for 5 euros. now the first one should give me 35,35V after rectifier and the second one 19.76V. Id like to get the 25V one, and since it has quite a nice VA rating maybe make 5 channel amp, Could I bring the voltage down to about 30V? And I plan on using 4 ohm speakers does that mean that 30V +- rails would be too much or just need a bigger heatsink?
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