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Building a linear power supply

Started by Andrew Wagstaff November 13, 2011
Hi

I'm looking at building a linear power supply. I'm thinking of using a 
12V 300VA torodial and linear regulators. I really want a power supply 
that can be used to power both AT and ATX computer motherboards, has got 
variable current limiting (3.3V, 5V and 12V) and displays current. I 
haven't seen anything in the marketplace, so I was looking at building 
my own.

The voltage and current ratings I'm looking at currently are:
3.3V 10A, thinking a 5A variable reg and using a pass transistor to get 
up to 10A.
5V 5A-8A, thinking a 5A variable reg and a pass transistor.
12V 5A, thinking a 5A variable reg.

The voltage drop from 12VAC (~17VDC using a bridge rectifier) down to 
the lower regulators (3.3V and 5V) was concerning me, as I was looking 
at producing around 60W-100W of heat just for the 5V reg.

Was also considering having 4.5V, 6V and 9V. Was going to use another 
variable reg - only need around 1A and using a switch to change between 
them.

Another problem is how do I get the -ve voltages for the 5V and 12V 
(only need ~500mA), do I just use some sort of polarity inverter or do I 
need a centre tapped transformer?

I already have most of the components (except the torodial), that's why 
I was thinking of going with linear. I do have some components to make a 
5V switchmode, but I wanted to keep the power supply as quiet as I could.

Thanks

Andrew
Andrew Wagstaff wrote:

> Hi > > I'm looking at building a linear power supply. I'm thinking of using a > 12V 300VA torodial and linear regulators. I really want a power supply > that can be used to power both AT and ATX computer motherboards, has got > variable current limiting (3.3V, 5V and 12V) and displays current. I > haven't seen anything in the marketplace, so I was looking at building > my own. > > The voltage and current ratings I'm looking at currently are: > 3.3V 10A, thinking a 5A variable reg and using a pass transistor to get > up to 10A. > 5V 5A-8A, thinking a 5A variable reg and a pass transistor. > 12V 5A, thinking a 5A variable reg. > > The voltage drop from 12VAC (~17VDC using a bridge rectifier) down to > the lower regulators (3.3V and 5V) was concerning me, as I was looking > at producing around 60W-100W of heat just for the 5V reg. > > Was also considering having 4.5V, 6V and 9V. Was going to use another > variable reg - only need around 1A and using a switch to change between > them. > > Another problem is how do I get the -ve voltages for the 5V and 12V > (only need ~500mA), do I just use some sort of polarity inverter or do I > need a centre tapped transformer? > > I already have most of the components (except the torodial), that's why > I was thinking of going with linear. I do have some components to make a > 5V switchmode, but I wanted to keep the power supply as quiet as I could. > > Thanks > > Andrew
why not simply use an existing computer supply and have inline current limits on the outputs of choice? The current limit circuits can be designed using power fets to pass the source voltage with current sensing to drive a comparator that drives the power mosfet. With a little hysteresis you can keep the fet out of the linear mode and force it to simply shut off if current limit is reached. Pots on a panel can set the (I) range of each output.. Jamie
On 14/11/2011 12:32 PM, Jamie wrote:
> Andrew Wagstaff wrote: > >> Hi >> >> I'm looking at building a linear power supply. I'm thinking of using a >> 12V 300VA torodial and linear regulators. I really want a power supply >> that can be used to power both AT and ATX computer motherboards, has >> got variable current limiting (3.3V, 5V and 12V) and displays current. >> I haven't seen anything in the marketplace, so I was looking at >> building my own. >> >> The voltage and current ratings I'm looking at currently are: >> 3.3V 10A, thinking a 5A variable reg and using a pass transistor to >> get up to 10A. >> 5V 5A-8A, thinking a 5A variable reg and a pass transistor. >> 12V 5A, thinking a 5A variable reg. >> >> The voltage drop from 12VAC (~17VDC using a bridge rectifier) down to >> the lower regulators (3.3V and 5V) was concerning me, as I was looking >> at producing around 60W-100W of heat just for the 5V reg. >> >> Was also considering having 4.5V, 6V and 9V. Was going to use another >> variable reg - only need around 1A and using a switch to change >> between them. >> >> Another problem is how do I get the -ve voltages for the 5V and 12V >> (only need ~500mA), do I just use some sort of polarity inverter or do >> I need a centre tapped transformer? >> >> I already have most of the components (except the torodial), that's >> why I was thinking of going with linear. I do have some components to >> make a 5V switchmode, but I wanted to keep the power supply as quiet >> as I could. >> >> Thanks >> >> Andrew > why not simply use an existing computer supply and have inline current > limits on the outputs of choice? > > The current limit circuits can be designed using power fets to pass the > source voltage with current sensing to drive a comparator that drives > the power mosfet. With a little hysteresis you can keep the fet out of > the linear mode and force it to simply shut off if current limit is > reached. > > Pots on a panel can set the (I) range of each output.. > > Jamie > > >
Hi Jamie I have tried before to use a computer power supply as a high current 12V supply before, but found that unless the 5V line was loaded at around 1.5-2A, the 12V line dropped down to around 11V. I might not be using the 5V line when I want to use the 12V or 3.3V lines. The current limiting I was thinking of using, was using a 5A linear regulator wired as a current regulator. Thanks Andrew
On 14/11/2011 1:33 PM, Andrew Wagstaff wrote:
> On 14/11/2011 12:32 PM, Jamie wrote: >> Andrew Wagstaff wrote: >> >>> Hi >>> >>> I'm looking at building a linear power supply. I'm thinking of using a >>> 12V 300VA torodial and linear regulators. I really want a power supply >>> that can be used to power both AT and ATX computer motherboards, has >>> got variable current limiting (3.3V, 5V and 12V) and displays current. >>> I haven't seen anything in the marketplace, so I was looking at >>> building my own. >>> >>> The voltage and current ratings I'm looking at currently are: >>> 3.3V 10A, thinking a 5A variable reg and using a pass transistor to >>> get up to 10A. >>> 5V 5A-8A, thinking a 5A variable reg and a pass transistor. >>> 12V 5A, thinking a 5A variable reg. >>> >>> The voltage drop from 12VAC (~17VDC using a bridge rectifier) down to >>> the lower regulators (3.3V and 5V) was concerning me, as I was looking >>> at producing around 60W-100W of heat just for the 5V reg. >>> >>> Was also considering having 4.5V, 6V and 9V. Was going to use another >>> variable reg - only need around 1A and using a switch to change >>> between them. >>> >>> Another problem is how do I get the -ve voltages for the 5V and 12V >>> (only need ~500mA), do I just use some sort of polarity inverter or do >>> I need a centre tapped transformer? >>> >>> I already have most of the components (except the torodial), that's >>> why I was thinking of going with linear. I do have some components to >>> make a 5V switchmode, but I wanted to keep the power supply as quiet >>> as I could. >>> >>> Thanks >>> >>> Andrew >> why not simply use an existing computer supply and have inline current >> limits on the outputs of choice? >> >> The current limit circuits can be designed using power fets to pass the >> source voltage with current sensing to drive a comparator that drives >> the power mosfet. With a little hysteresis you can keep the fet out of >> the linear mode and force it to simply shut off if current limit is >> reached. >> >> Pots on a panel can set the (I) range of each output.. >> >> Jamie >> >> >> > Hi Jamie > > I have tried before to use a computer power supply as a high current 12V > supply before, but found that unless the 5V line was loaded at around > 1.5-2A, the 12V line dropped down to around 11V. > I might not be using the 5V line when I want to use the 12V or 3.3V lines. > The current limiting I was thinking of using, was using a 5A linear > regulator wired as a current regulator. > > Thanks > > Andrew
Just revised my design a bit and are now thinking of using the 9V 300VA torodial, instead of the 12V one. Thinking I'll use a low dropout reg with a pass transistor to get the required current at 12V. This should make the heat production from the 5V and 3.3V a bit less, but still not sure how to get the -5V and -12V. Thanks again Andrew
Andrew Wagstaff wrote:
> Hi > > I'm looking at building a linear power supply. I'm thinking of using a > 12V 300VA torodial and linear regulators. I really want a power supply > that can be used to power both AT and ATX computer motherboards, has got > variable current limiting (3.3V, 5V and 12V) and displays current. I > haven't seen anything in the marketplace, so I was looking at building > my own. > > The voltage and current ratings I'm looking at currently are: > 3.3V 10A, thinking a 5A variable reg and using a pass transistor to get > up to 10A. > 5V 5A-8A, thinking a 5A variable reg and a pass transistor. > 12V 5A, thinking a 5A variable reg. > > The voltage drop from 12VAC (~17VDC using a bridge rectifier) down to > the lower regulators (3.3V and 5V) was concerning me, as I was looking > at producing around 60W-100W of heat just for the 5V reg. > > Was also considering having 4.5V, 6V and 9V. Was going to use another > variable reg - only need around 1A and using a switch to change between > them. > > Another problem is how do I get the -ve voltages for the 5V and 12V > (only need ~500mA), do I just use some sort of polarity inverter or do I > need a centre tapped transformer? > > I already have most of the components (except the torodial), that's why > I was thinking of going with linear. I do have some components to make a > 5V switchmode, but I wanted to keep the power supply as quiet as I could. > > Thanks > > Andrew
It's going to take up a lot of bench space and produce a lot of waste heat. It'll be heavy. And it will cost you a lot. Your proposed 12V transformer won't work. You'll need an 18V transformer for the 12V 5 amp supply**. If you use the 18V transformer for all three supplies your dissipation will be about 400 watts under full load. Better would be 3 transformers, or a single transformer with 3 secondaries. You'll still produce a lot of heat, no matter what you do, if you go with a linear design. A linear supply to power a PC is a really *bad* choice. ** Regarding the voltage: 12VAC input to a bridge rectifier and capacitive filter will produce 16.97 VDC, ignoring the voltage drop in the diodes. Under a heavy load, you'll get a drop in the diodes totaling about 2.2 volts, reducing the voltage to 14.77 max. Ripple will reduce that further. The LM338 (5 Amp Adjustable Regulator) needs a minimum of 2.7 volts overhead at 5 amps, so it will drop out. In fact, it will only regulate for the briefest of moments when the AC is at peak. Say we figure about 3 volts ripple and about 2.2 volts drop in the diodes. That means we need 3 + 2.2 + 14.7 or 19.7 volts DC peak across the filter caps. That implies a 14 volt AC transformer which will produce ~19.79 DC peak across the caps. However, there will be some voltage drop in the transformer under load, and the input line voltage can vary, too. Say the line voltage is 110 instead of 120. With an 18 volt transformer fed by 110 instead of 120, we get 18*110/120 or 16.5 volts AC. We can drop to 14 volts AC and still work. If the transformer drops 10% under load, it still yields 14.85 volts AC and will charge the caps to 21 volts. Even if the ripple was 4 volts, we'd still have 15 volts input to the LM338 which is plenty of headroom. The above is in no way intended to encourage you to build the supply you described. OTOH, I don't want to discourage experimentation/learning, but the proposed project has too many downsides to be worthwhile without some compelling reason(s). Ed
Andrew Wagstaff wrote:

> On 14/11/2011 12:32 PM, Jamie wrote: > >> Andrew Wagstaff wrote: >> >>> Hi >>> >>> I'm looking at building a linear power supply. I'm thinking of using a >>> 12V 300VA torodial and linear regulators. I really want a power supply >>> that can be used to power both AT and ATX computer motherboards, has >>> got variable current limiting (3.3V, 5V and 12V) and displays current. >>> I haven't seen anything in the marketplace, so I was looking at >>> building my own. >>> >>> The voltage and current ratings I'm looking at currently are: >>> 3.3V 10A, thinking a 5A variable reg and using a pass transistor to >>> get up to 10A. >>> 5V 5A-8A, thinking a 5A variable reg and a pass transistor. >>> 12V 5A, thinking a 5A variable reg. >>> >>> The voltage drop from 12VAC (~17VDC using a bridge rectifier) down to >>> the lower regulators (3.3V and 5V) was concerning me, as I was looking >>> at producing around 60W-100W of heat just for the 5V reg. >>> >>> Was also considering having 4.5V, 6V and 9V. Was going to use another >>> variable reg - only need around 1A and using a switch to change >>> between them. >>> >>> Another problem is how do I get the -ve voltages for the 5V and 12V >>> (only need ~500mA), do I just use some sort of polarity inverter or do >>> I need a centre tapped transformer? >>> >>> I already have most of the components (except the torodial), that's >>> why I was thinking of going with linear. I do have some components to >>> make a 5V switchmode, but I wanted to keep the power supply as quiet >>> as I could. >>> >>> Thanks >>> >>> Andrew >> >> why not simply use an existing computer supply and have inline current >> limits on the outputs of choice? >> >> The current limit circuits can be designed using power fets to pass the >> source voltage with current sensing to drive a comparator that drives >> the power mosfet. With a little hysteresis you can keep the fet out of >> the linear mode and force it to simply shut off if current limit is >> reached. >> >> Pots on a panel can set the (I) range of each output.. >> >> Jamie >> >> >> > Hi Jamie > > I have tried before to use a computer power supply as a high current 12V > supply before, but found that unless the 5V line was loaded at around > 1.5-2A, the 12V line dropped down to around 11V. > I might not be using the 5V line when I want to use the 12V or 3.3V lines. > The current limiting I was thinking of using, was using a 5A linear > regulator wired as a current regulator. > > Thanks > > Andrew
That's not a problem, have yourself a front panel indicator lamp operating from the 5 Volt supply with a little extra load on it, that will satisfy most computer supplies and give you an idiot light, also add a 5 volt cooling fan.. Jamie
On Nov 13, 10:38=A0pm, ehsjr <eh...@nospamverizon.net> wrote:
> Andrew Wagstaff wrote: > > Hi > > > I'm looking at building a linear power supply. I'm thinking of using a > > 12V 300VA torodial and linear regulators. I really want a power supply > > that can be used to power both AT and ATX computer motherboards, has go=
t
> > variable current limiting (3.3V, 5V and 12V) and displays current. I > > haven't seen anything in the marketplace, so I was looking at building > > my own. > > > The voltage and current ratings I'm looking at currently are: > > 3.3V 10A, thinking a 5A variable reg and using a pass transistor to get > > up to 10A. > > 5V 5A-8A, thinking a 5A variable reg and a pass transistor. > > 12V 5A, thinking a 5A variable reg. > > > The voltage drop from 12VAC (~17VDC using a bridge rectifier) down to > > the lower regulators (3.3V and 5V) was concerning me, as I was looking > > at producing around 60W-100W of heat just for the 5V reg. > > > Was also considering having 4.5V, 6V and 9V. Was going to use another > > variable reg - only need around 1A and using a switch to change between > > them. > > > Another problem is how do I get the -ve voltages for the 5V and 12V > > (only need ~500mA), do I just use some sort of polarity inverter or do =
I
> > need a centre tapped transformer? > > > I already have most of the components (except the torodial), that's why > > I was thinking of going with linear. I do have some components to make =
a
> > 5V switchmode, but I wanted to keep the power supply as quiet as I coul=
d.
> > > Thanks > > > Andrew > > It's going to take up a lot of bench space and produce a lot of > waste heat. It'll be heavy. And it will cost you a lot. =A0Your > proposed 12V transformer won't work. =A0You'll need an 18V transformer > for the 12V 5 amp supply**. =A0If you use the 18V transformer for all > three supplies your dissipation will be about 400 watts under full > load. Better would be 3 transformers, or a single transformer with > 3 secondaries. =A0You'll still produce a lot of heat, no matter what > you do, if you go with a linear design. =A0A linear supply to power > a PC is a really *bad* choice. > > ** Regarding the voltage: > 12VAC input to a bridge rectifier and capacitive filter will > produce 16.97 VDC, ignoring the voltage drop in the diodes. > Under a heavy load, you'll get a drop in the diodes totaling > about 2.2 volts, reducing the voltage to 14.77 max. =A0Ripple > will reduce that further. =A0The LM338 (5 Amp Adjustable Regulator) > needs a minimum of 2.7 volts overhead at 5 amps, so it will > drop out. In fact, it will only regulate for the briefest of > moments when the AC is at peak. =A0Say we figure about 3 volts > ripple and about 2.2 volts drop in the diodes. =A0That means we > need 3 + 2.2 + 14.7 or 19.7 volts DC peak across the filter caps. > That implies a 14 volt AC transformer which will produce ~19.79 DC > peak across the caps. =A0However, there will be some voltage drop > in the transformer under load, and the input line voltage can vary, > too. Say the line voltage is 110 instead of 120. With an 18 volt > transformer fed by 110 instead of 120, we get 18*110/120 or > 16.5 volts AC. =A0We can drop to 14 volts AC and still work. If the > transformer drops 10% under load, it still yields 14.85 > volts AC and will charge the caps to 21 volts. =A0Even if the > ripple was 4 volts, we'd still have 15 volts input to the > LM338 which is plenty of headroom. > > The above is in no way intended to encourage you to build the > supply you described. OTOH, I don't want to discourage > experimentation/learning, but the proposed project has too many > downsides to be worthwhile without some compelling reason(s). > > Ed
A few months back there was a similar discussion on DIY audio and I raised the points you just made. The idea was that the audio from the PC would be quieter because of the linear power supply. Of course someone pointed out that all motherboards have switching supplies on board to get the CPU core voltages so who cares about a linear supply TO the motherboard? Just as an FYI we had a power problem last year that caused the line voltage to drop to 67 VAC. One computer and monitor still worked so I could go online to get the power company. Problem was a missing phase about 1/2 mile away. A linear supply would _never_ work with that low an input. G=B2
"Andrew Wagstaff"  wrote in message=20
news:w5%vq.5942$NR2.3139@viwinnwfe01.internal.bigpond.com...

> I have tried before to use a computer power supply as a high > current 12V supply before, but found that unless the 5V line > was loaded at around 1.5-2A, the 12V line dropped down to > around 11V. > I might not be using the 5V line when I want to use the 12V > or 3.3V lines. > The current limiting I was thinking of using, was using a > 5A linear regulator wired as a current regulator.
A single 16 VCT transformer can be used for all of the required = voltages. A=20 FWCT rectifier with two diodes and a big capacitor can provide the raw = 10=20 VDC for the 5V linear regulator (and 3.3V). And another, with diodes and capacitor reversed, can be used for the -5 = VDC=20 supply, which is usually much lower current. You can also get an adjustable buck supply (1.25-30V, 2-3 amps) on eBay = for=20 about $4-$6 including shipping: http://www.ebay.com/itm/LM2596-DC-DC-Step-down-Adjustable-Power-Supply-Mo= dule-/260901219660 For the + and - 12 VDC supplies, you can use a voltage doubler made with = two=20 capacitors and two diodes for each polarity, which generates about 25 = VDC=20 open circuit, and is also inherently current limiting because the=20 transformer is capacitively coupled to the output. And perhaps the easiest solution is to add a load to the +5 VDC output = of=20 the ATX supply. It's about 20W but for a test setup efficiency and heat = are=20 not much of an issue. Use a 5 volt fan as part of the load. Paul=20
On Nov 14, 12:32=A0am, Andrew Wagstaff <andrew_spam_me_...@hotmail.com>
wrote:
> On 14/11/2011 1:33 PM, Andrew Wagstaff wrote: > > > > > On 14/11/2011 12:32 PM, Jamie wrote: > >> Andrew Wagstaff wrote: > > >>> Hi > > >>> I'm looking at building a linear power supply. I'm thinking of using =
a
> >>> 12V 300VA torodial and linear regulators. I really want a power suppl=
y
> >>> that can be used to power both AT and ATX computer motherboards, has > >>> got variable current limiting (3.3V, 5V and 12V) and displays current=
.
> >>> I haven't seen anything in the marketplace, so I was looking at > >>> building my own. > > >>> The voltage and current ratings I'm looking at currently are: > >>> 3.3V 10A, thinking a 5A variable reg and using a pass transistor to > >>> get up to 10A. > >>> 5V 5A-8A, thinking a 5A variable reg and a pass transistor. > >>> 12V 5A, thinking a 5A variable reg. > > >>> The voltage drop from 12VAC (~17VDC using a bridge rectifier) down to > >>> the lower regulators (3.3V and 5V) was concerning me, as I was lookin=
g
> >>> at producing around 60W-100W of heat just for the 5V reg. > > >>> Was also considering having 4.5V, 6V and 9V. Was going to use another > >>> variable reg - only need around 1A and using a switch to change > >>> between them. > > >>> Another problem is how do I get the -ve voltages for the 5V and 12V > >>> (only need ~500mA), do I just use some sort of polarity inverter or d=
o
> >>> I need a centre tapped transformer? > > >>> I already have most of the components (except the torodial), that's > >>> why I was thinking of going with linear. I do have some components to > >>> make a 5V switchmode, but I wanted to keep the power supply as quiet > >>> as I could. > > >>> Thanks > > >>> Andrew > >> why not simply use an existing computer supply and have inline current > >> limits on the outputs of choice? > > >> The current limit circuits can be designed using power fets to pass th=
e
> >> source voltage with current sensing to drive a comparator that drives > >> the power mosfet. With a little hysteresis you can keep the fet out of > >> the linear mode and force it to simply shut off if current limit is > >> reached. > > >> Pots on a panel can set the (I) range of each output.. > > >> Jamie > > > Hi Jamie > > > I have tried before to use a computer power supply as a high current 12=
V
> > supply before, but found that unless the 5V line was loaded at around > > 1.5-2A, the 12V line dropped down to around 11V. > > I might not be using the 5V line when I want to use the 12V or 3.3V lin=
es.
> > The current limiting I was thinking of using, was using a 5A linear > > regulator wired as a current regulator. >
Hmmm. Just so happens we use a similar set up on one of our devices. The PS is a basic computer switching PS with the 5V loaded with a big honking resistor. They put out approx. 21 amps with no issues at 12 VDC. We had a question as to how much they would do consistently and had one loaded to that figure. It hung in there all day long with no problems.
> > Thanks > > > Andrew > > Just revised my design a bit and are now thinking of using the 9V 300VA > torodial, instead of the 12V one. Thinking I'll use a low dropout reg > with a pass transistor to get the required current at 12V. > This should make the heat production from the 5V and 3.3V a bit less, > but still not sure how to get the -5V and -12V. > > Thanks again > > Andrew