Dear, Gurus
I need help with... actually help and have some question, which might be a little or too stupid for this forum, but please bear with me because I really need to know and make it and almost have no idea, even after reading many threads of this forum for over a year or so. Please do help me...
I would very much like to make a linear power supply that could output ±26.79V and 6Amps after rectification by four diodes. I have read the thread optimising tda7294 audio and would really like to make an amplifier using that chip. Danielwritesbac has helped me a lot with the amplifier design. Actually its his design, sorry. Ok, the problem is there is no transformer I could find that rated at 19-0-19 volts, atleast not in my area. They have got very pricey 20-0-20v toroidal ones but I just could not afford them. I would like to know if:
#1 I could make a cheap 24-0-24v 5Amp laminated core transformer (they are really cheap here) to act like a 19-0-19 volts transformer without turning my psu into an smps?
#2 also could there be a way, to increase the amperage by one unit to 6Amps?
Thank you, so much!
PS my mains are 220-230V AC.
I need help with... actually help and have some question, which might be a little or too stupid for this forum, but please bear with me because I really need to know and make it and almost have no idea, even after reading many threads of this forum for over a year or so. Please do help me...
I would very much like to make a linear power supply that could output ±26.79V and 6Amps after rectification by four diodes. I have read the thread optimising tda7294 audio and would really like to make an amplifier using that chip. Danielwritesbac has helped me a lot with the amplifier design. Actually its his design, sorry. Ok, the problem is there is no transformer I could find that rated at 19-0-19 volts, atleast not in my area. They have got very pricey 20-0-20v toroidal ones but I just could not afford them. I would like to know if:
#1 I could make a cheap 24-0-24v 5Amp laminated core transformer (they are really cheap here) to act like a 19-0-19 volts transformer without turning my psu into an smps?
#2 also could there be a way, to increase the amperage by one unit to 6Amps?
Thank you, so much!
PS my mains are 220-230V AC.
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The transformer's AC output is different from the power supply's DC voltage.
If you want 19VDC output from the power supply, the transformer should have 14VAC windings.
Then 14VAC x sqrt 2 =19.6VDC [actually a volt or so less] when rectified and filtered.
You need greater than a 250VA transformer, perhaps 300-350VA.
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I am sorry rayma, I edited my thread. I made a mistake. Please do look at it again. Thank you.
If you want +/- 27VDC, each secondary should be 27/1.414 = 19VAC.
The transformer should have two 20V secondary windings, connected in series,
with the center tap grounded, into a 35A diode bridge rectifier and a pair of filter capacitors.
http://www.interfacebus.com/diode-bridge-split-supply.png
Power rating for the transformer is 350VA minimum, more VA preferred.
If you want to use the cheap local transformer, an extra transformer with about
45VAC secondary voltage could buck the voltage difference by lowering the input VAC.
The 45VAC secondary in connected in series with the main transformer's 230VAC primary,
with a polarity that reduces the output of the main transformer's secondaries.
Power rating for the extra transformer would be 100VA.
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Is there a way to do it with 24-0-24v transformer instead. I have got 15-0-15 5amp, 18-0-18 5amp and 24-0-24 5amp laminated core transformer. Also, found one 20-0-20v toroidal but atrociously priced. Impossible for me to buy.But 24-0-24 transformer really in my budget though.
Thank you.
If you want to use the cheap local transformer, an extra transformer with about
38VAC secondary voltage can buck the voltage difference by lowering the input VAC.
The 38VAC secondary in connected in series with the main transformer's 230VAC primary,
with a polarity that reduces the output of the main transformer's secondaries.
Power rating for the extra transformer would be 100VA.
Connection diagram: Bucking transformer
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Thanks for the reply. I am now trying to grasp concept of bucking the transformer now. Will reply as soon as I get it.
Is there any other way to convert the 24-0-24 to 19-0-19?
> ±26.79V and 6Amps.... tda7294 ....
TDA7294 will survive +/-50V at idle (but you prefer under +/-40V). +/-35V at 8 ohms, +/-27V at 4 Ohms.
> 24-0-24v 5Amp laminated core transformer (they are really cheap here)
Makes about +/-34V DC, except at no- or low-load it may be 5%-7% high, maybe +/-36V. This *appears* to be safe for the chip.
A 240VA transformer clearly will not support two channels delivering 100 Watts. There's loss in the AC-DC conversion, and loss in the amplifier. We sure would "like" a 300+VA transformer. And I would like a 4-foot concrete foundation under my garden shed. However a couple small concrete blocks are working fine. In speech/music hi-fi amplifiers, the average power output is 1/10th of the maximum output. I would expect a 240VA transformer to do OK. It will sag a bit on loud passages. It will sag significantly on full power test tones, and may not make its expected power. And it will get awful hot in an hour. But this is not how we use hi-fi systems.
If this iron is really cheap, get one. Use a 400V 10A bridge rectifier and two 2,000uFd 50V caps. Measure the actual output DC voltage. If well under +/-50V, the chip will be OK at idle. Hang two 100 Ohm 10W resistors on the output. They will get HOT (13 Watts), so only do it for a minute. Measure the actual output DC voltage. If it settles near +/-35V, it will probably be OK for two channels. If it settles far under +/-30V, sag will be bad even for speech/music work.
TDA7294 will survive +/-50V at idle (but you prefer under +/-40V). +/-35V at 8 ohms, +/-27V at 4 Ohms.
> 24-0-24v 5Amp laminated core transformer (they are really cheap here)
Makes about +/-34V DC, except at no- or low-load it may be 5%-7% high, maybe +/-36V. This *appears* to be safe for the chip.
A 240VA transformer clearly will not support two channels delivering 100 Watts. There's loss in the AC-DC conversion, and loss in the amplifier. We sure would "like" a 300+VA transformer. And I would like a 4-foot concrete foundation under my garden shed. However a couple small concrete blocks are working fine. In speech/music hi-fi amplifiers, the average power output is 1/10th of the maximum output. I would expect a 240VA transformer to do OK. It will sag a bit on loud passages. It will sag significantly on full power test tones, and may not make its expected power. And it will get awful hot in an hour. But this is not how we use hi-fi systems.
If this iron is really cheap, get one. Use a 400V 10A bridge rectifier and two 2,000uFd 50V caps. Measure the actual output DC voltage. If well under +/-50V, the chip will be OK at idle. Hang two 100 Ohm 10W resistors on the output. They will get HOT (13 Watts), so only do it for a minute. Measure the actual output DC voltage. If it settles near +/-35V, it will probably be OK for two channels. If it settles far under +/-30V, sag will be bad even for speech/music work.
Dear PRR
The truth is while reading "optimising tda7294 output" I came to know the tda7294 has gone a design change and its more like a tda7296 now, just could not take more voltage. Also, my room is really small 110sqft and so was thinking of undervolting the chip but increase the current for a sturdy and higher resolution amp. My dear friend suggested 18-0-18v 5A transformers but I wanted to go a little further: so 18-0-18v 5amp transformers didnt cut it but I just couldnt do it with the 24-0-24 either and so I ask. So, PRR is there a way to make the 24-0-24v 5amps transformer output 19-0-19v 6amps before rectification. I would be really grateful to you. Also did you read the previous post by rayma about bucking transformers? Is there another way?
Thank you
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You can reduce the voltage by using bucking, as already explained by others.Jishu said:
To increase the current rating you would need to cool the transformer by blowing cold air around it. This might be noisy.
Alternatively, get the secondaries rewound by a local transformer or motor rewinder.
This seems like a good idea, bucking would be costly but rewinding seems doable, if I find a rewinder. I will report as soon as possible, if I am successful. Thank you.
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