Hi
I've done some calculations and think they're right, I want to build a 5v 5A power supply. So my parts are a 230v toroidal with 2 12v 6.6A secondary outs. I'll only be using one at the moment. so the 12v 6.6a goes into a rectifier then 2 4700uf caps and a 1uf ceramic cap all parallel, then a 5v 5a regulator then the same caps again then a 1k resistor for bleed.
are my calculations way out or should this give me a decent linear supply?
another question is what awg wire can handle 7-5 amps? is 22 too low? what about 18? i keep reading things and can never get a definitive answer.
I've done some calculations and think they're right, I want to build a 5v 5A power supply. So my parts are a 230v toroidal with 2 12v 6.6A secondary outs. I'll only be using one at the moment. so the 12v 6.6a goes into a rectifier then 2 4700uf caps and a 1uf ceramic cap all parallel, then a 5v 5a regulator then the same caps again then a 1k resistor for bleed.
are my calculations way out or should this give me a decent linear supply?
another question is what awg wire can handle 7-5 amps? is 22 too low? what about 18? i keep reading things and can never get a definitive answer.
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If you take a 6 Volt 6A AC transformer, you will find about 9 Volt DC after rectifier and capacitor.
This is fine for a regulated 5 Volt supply.
If you use 5 A from the 5 V supply, you will burn the difference to the 9 Volt DC.
Now you see it is no good idea to use as 12V AC, which will show 17V DC.
Anyway, a switch mode supply is only a fraction the problems of a linear DIYS.
This is fine for a regulated 5 Volt supply.
If you use 5 A from the 5 V supply, you will burn the difference to the 9 Volt DC.
Now you see it is no good idea to use as 12V AC, which will show 17V DC.
Anyway, a switch mode supply is only a fraction the problems of a linear DIYS.
At full power, the transformer RMS voltage for 5V DC would be approximately 5VRMS. However, at lower currents, this voltage would rise due to the (non-ideal) voltage regulation of the transformer and you'd need an electronic voltage regulator to get exactly 5V at all conditions.
The recommended current density is usually around 3-4A/sq.mm, giving approximately 1-2 sq.mm (16-18SWG) wire for a 5A current. For centre-tapped transformer only half of the average load current flows through one of the windings, allowing you to use slightly thinner wire.
The recommended current density is usually around 3-4A/sq.mm, giving approximately 1-2 sq.mm (16-18SWG) wire for a 5A current. For centre-tapped transformer only half of the average load current flows through one of the windings, allowing you to use slightly thinner wire.
the regulator is this https://uk.farnell.com/microchip/mi.../2851565?CMP=e-email-sys-shipping-GLB-Product
I'm a bit worried though as my 4700uf caps are only 16v rated. the output from 12v on the transformer will be 17v so probably pop them. i think if my maths is right a 9v secondary out of a transformer will give me 12.7v
I was also looking at a twin 15v power so would my 12v otputting 17v volts be ok for that?
I was also looking at a twin 15v power so would my 12v otputting 17v volts be ok for that?
I understand, you want to use an existing 12V transformer. There is just one problem, the tiny little regulator you have choosen is not able to deliver 5 Volt and turn the rest of the 12 V into heat. Maybe with an expensive, large heatsink. To buy the correct 6V 5A transformer would be much cheaper than that heat sink.
Please read the data sheet of the regulator. It is a "low drop" regulator, the unregulated DC should be about 1 Volt higher than the output, not 12 Volt more.
You may not understand how the linear regulator works. If you use 25 Watt from it with your transformer, it will use 85 Watt and turn the difference into heat.
Please read the data sheet of the regulator. It is a "low drop" regulator, the unregulated DC should be about 1 Volt higher than the output, not 12 Volt more.
You may not understand how the linear regulator works. If you use 25 Watt from it with your transformer, it will use 85 Watt and turn the difference into heat.
Electrolytic capacitors need to be rated at twice the average DC voltage to be reliable in the long term.
The design of a linear power supply must also consider the full range of line voltage variation in your area. Not only the specified limits from the power company but the actual variation present in your neighborhood throughout the seasons, which can be worse than the specs. This will assist in selecting the right transformer secondary voltage. If the secondary is too low (because of low line voltage and/or the load is high) the regulator can drop out or quit. Most transformer secondary voltages are specified at the full rated current load, and with the primary at a specified voltage. With these two voltages you can estimate the transformers' voltage ratio, Vout/Vin and then estimate the secondary voltage range produced by line voltage variation. The raw DC produced will be about 1.4 times the secondary voltage for full wave rectification. This must be higher than the regulator droput voltage when the line voltage is at it's lowest point. When the line voltage is on the high side the regulator will be less efficent, wasting power and the filter caps must be able to handle that voltage, as you already noticed.
This is where SWPS really shine with their wide range input of 85-250Vac.
This is where SWPS really shine with their wide range input of 85-250Vac.
Concerning capacitors, with 12V you should aproximate the maximum voltage after rectifier to be rated voltage x 1.4 +10%.
1.4 is short for square root af 2 what, is the exact value.
So a 12V AC transformer will never (anything else is very, very rare) show more than 19 Volt.
Which leads you to a 25 Volt electrolytic capacitor.
If you buy your components at a Chinese or Indian market, maybe buy even higher voltage and capaciy than you need. In your region you should not run into fake or recycled parts too often.
One more, general hint. Always, on any self build gear, use a fuse of correct size in the primary cirquit of the transformer. Even on a "prototype" or what ever you may call your build. Otherwise, in case of a fire, your insurance will have a perfect cause to refuse paying, even if your DIYS was not the primary source of the incident. It shows you are not qualified too handle electricty and they will argue you manipulated other parts of the installation, too.
1.4 is short for square root af 2 what, is the exact value.
So a 12V AC transformer will never (anything else is very, very rare) show more than 19 Volt.
Which leads you to a 25 Volt electrolytic capacitor.
If you buy your components at a Chinese or Indian market, maybe buy even higher voltage and capaciy than you need. In your region you should not run into fake or recycled parts too often.
One more, general hint. Always, on any self build gear, use a fuse of correct size in the primary cirquit of the transformer. Even on a "prototype" or what ever you may call your build. Otherwise, in case of a fire, your insurance will have a perfect cause to refuse paying, even if your DIYS was not the primary source of the incident. It shows you are not qualified too handle electricty and they will argue you manipulated other parts of the installation, too.
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"Concerning capacitors, with 12V you should aproximate the maximum voltage after rectifier to be rated voltage x 1.4 +10%.
1.4 is short for square root af 2 what, is the exact value." thanks for that, i remembered from uni after the recent posts it was x 1.414. i think that is the correct calculation but will use the the one you said.
I bought my components from farnell in the uk but i'm returning them and starting from scratch with the advice given here. I had also planned to put a 7a fuse on the 12v secondary output before it hit the rectifier as the stated current was 6.667a, is that the best place to put it?
All help is very much appreciated
"Concerning capacitors, with 12V you should aproximate the maximum voltage after rectifier to be rated voltage x 1.4 +10%.
1.4 is short for square root af 2 what, is the exact value." thanks for that, i remembered from uni after the recent posts it was x 1.414. i think that is the correct calculation but will use the the one you said.
I bought my components from farnell in the uk but i'm returning them and starting from scratch with the advice given here. I had also planned to put a 7a fuse on the 12v secondary output before it hit the rectifier as the stated current was 6.667a, is that the best place to put it?
All help is very much appreciated
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No, the fuse has to be in the primary 230V AC cirquit. What you do behind the secondary is your thing. 5A or even lower is fine, overdimensioning anything and all is not elegan and sometimes wrong. Like puting too much sugar in your tea...
Even a 5A slow blow will be hard to activate. Please read about fuses and their rating for current and time.
Even a 5A slow blow will be hard to activate. Please read about fuses and their rating for current and time.
An overvoltage of 250V on the primary will only lead to 6.5V instead of 6V on the secondary. For the 12V transformer this would lead to about 18V DC at the capacitor. Still 7 V to go with a 25V capacitor. To reach the 25V on the cap the mains AC has to rise to more than 330V.
Should be enough safety margain.
Should be enough safety margain.
Fuse wise do you mean on the mains plug? UK AC mains is always a 13A fuse should I put a 5amp in the plug?
if I were to use a 6v 3a 2 secondary output transformer in parallel would that work? I think that would give me 8.5v 6a single feed into the rectifier. My 16v caps would seem to be ok not sure about the heat that the regulator would create though.
if I were to use a 6v 3a 2 secondary output transformer in parallel would that work? I think that would give me 8.5v 6a single feed into the rectifier. My 16v caps would seem to be ok not sure about the heat that the regulator would create though.
it's me again. I've done some calculations following the info given. I hope i've got these right. a 6v 10a transformer would output about 8.5v ac into the rectifier. around 1.2 will drop after so it would then be 7.3dc
that is 7.3 going into 16v caps so that's below the x2 voltage of the caps.
7.3 going into the regulator is my next problem the regulator outputs 5v 5a i'm trying to work out the amps under load is it 10 or 5? is it the amps going in or amps being passed out. do i need a different regulator?
I will need a heat sink obviously
that is 7.3 going into 16v caps so that's below the x2 voltage of the caps.
7.3 going into the regulator is my next problem the regulator outputs 5v 5a i'm trying to work out the amps under load is it 10 or 5? is it the amps going in or amps being passed out. do i need a different regulator?
I will need a heat sink obviously