Outer Limits 4x4 Board

Hey all,

Im after a 5v supply coming from 12v that will give approx 3amps or more.

possible? also cheap as possible..

ive tried the LM7805 reg using jaycar kit and works ok but not enough amperage.

thanks

Definitely possible.

I can't say if this really lives up to its specifications but it is the sort of thing you need I think? http://cgi.ebay.com/Car-12V-18V-24V-48V ... 0433293519" onclick="window.open(this.href);return false;

Hi nicbeer, do a google for LM317T semiconductors.

These are 3 amp variable voltage regulators and there are plenty of schematics on the web for make power supplies using any number of these in conjunction with your LM7805 which is used to set the reference voltage, and you can then build up the number of LM317T regs till you have the current you require.

DC-DC Converter : http://www.amtex.com.au/power_pdf/TT.pdf" onclick="window.open(this.href);return false;

Doing this with a linear regulator is quite inefficient, but still possible.

Your biggest problem is heat dissipation. With a 12V input, you're dissipating 21W at 3A. If it's an automotive application, you've got 27W to 30W to dissipate, depending on alternator voltage, and possibly a warmer ambient to dissipate into - it gets trickier.

Most devices with a T03 case will handle 3A (or more), as long as you have suitable heatsinking. That's the key.

You can either find a T03 cased regulator, or use a bypass transistor to boost the output of your regulator (regulator sets voltage, transistor provides current and dissipates most of the power).

The first T03 regulator I found has a thermal resistance of 2.5 Deg C per Watt. Worst case (30W) your junction temperature (the silicon inside) will be 75 Deg higher than the case temperature. With a maximum junction temperature of 125 Deg C, you would need to keep case temperature to 40 Deg C. In an automotive application, that would be tricky. A simpler solution would be two of these puppies in parallel, on a decent heatsink. Or a single regulator and T03 transistor.

-Scott- wrote:Doing this with a linear regulator is quite inefficient, but still possible.

Your biggest problem is heat dissipation. With a 12V input, you're dissipating 21W at 3A. If it's an automotive application, you've got 27W to 30W to dissipate, depending on alternator voltage, and possibly a warmer ambient to dissipate into - it gets trickier.

Most devices with a T03 case will handle 3A (or more), as long as you have suitable heatsinking. That's the key.

You can either find a T03 cased regulator, or use a bypass transistor to boost the output of your regulator (regulator sets voltage, transistor provides current and dissipates most of the power).

The first T03 regulator I found has a thermal resistance of 2.5 Deg C per Watt. Worst case (30W) your junction temperature (the silicon inside) will be 75 Deg higher than the case temperature. With a maximum junction temperature of 125 Deg C, you would need to keep case temperature to 40 Deg C. In an automotive application, that would be tricky. A simpler solution would be two of these puppies in parallel, on a decent heatsink. Or a single regulator and T03 transistor.

In that case probably looking at fan cooling maybe ?

One of the biggest problems with any air cooling is the ambient temps in a car.

Seems a DC-DC converter module may be easier and more efficient ?

What's it powering ?

nicbeer, If you really want to go that route I have one of the TO3 high current 317s, not sure exactly which one, would have to check the part no. but it may be the one scott linked. I never got round to using and probably never will, you might as well have it. That is if I can find it in my parts stash.

Let me know because I have to clear the stuff out in a few days so can have a look then.

edit: It's a 338, adjustable regulator 5A with 7A peak.

Hey Dools,

Yeh for sure if u arent using it..

Its for a USB power only sockets i have installed into a custom console, running 3 sockets and trying to allow about 1-1.5 amps each socket as almost killed the 1amp kit on the weekend charging my phone and mp3 player.

PM me on perth4x4 if u like, im on there more often..

Scott, i knew you would come through with the elekky jargon. i'll have a read through and go from there. Going to be mounted inside a centre console with decent space for a heatsink/board if needed and i could fan it also but would not be the best airflow in there.

cheers

What sort of sockets did you get? I have been looking for some that look okay on a console. It seems like it's time to ditch cig lighter sockets and put usb sockets in instead.

Not sure if' you've got this covered but some new devices require the data pins to be bridged in order to recognise the port as a charging port. I was puzzled for a while when a cheap in-car socket would charge one phone but not another. I gather bridging the data pins has been added to the standard now that the Europeans have forced phone manufacturers to use usb as a standard for chargers.

havent had the bridging pin issue yet.

im using some ones i ripped out of old PC front panels, they usually screw in and have terminanton cables,

USB standard is a MAX of 500 ma per outlet. 3 x .5 = 2 amp tops with safety margin

My guess is it was a way below spec power supply you had originally.

Switchmode would be best for efficiency and heat generation, not to mention size.

http://www.powerstream.com/dc-dcz0303.htm" onclick="window.open(this.href);return false;

DAMKIA wrote:USB standard is a MAX of 500 ma per outlet. 3 x .5 = 2 amp tops with safety margin

My guess is it was a way below spec power supply you had originally.

Switchmode would be best for efficiency and heat generation, not to mention size.

http://www.powerstream.com/dc-dcz0303.htm" onclick="window.open(this.href);return false;

Not sure how authoritative this is, but devices on a "dumb" usb power supply can apparently attempt to draw up to 1.8 amps.

USB power usage:

Bus-powered hubs: Draw Max 100 mA at power up and 500 mA normally.
Self-powered hubs: Draw Max 100 mA, must supply 500 mA to each port.
Low power, bus-powered functions: Draw Max 100 mA.
High power, bus-powered functions: Self-powered hubs: Draw Max 100 mA, must supply 500 mA to each port.
Self-powered functions: Draw Max 100 mA.
Suspended device: Max 0.5 mA

Dedicated charger mode:

A simple USB charger should short the 2 data lines together. The device will then not attempt to transmit or receive data, but can draw up to 1.8A, if the supply can provide it.

DAMKIA wrote:USB standard is a MAX of 500 ma per outlet. 3 x .5 = 2 amp tops with safety margin

My guess is it was a way below spec power supply you had originally.

Switchmode would be best for efficiency and heat generation, not to mention size.

http://www.powerstream.com/dc-dcz0303.htm" onclick="window.open(this.href);return false;

Yeah but they're way too expensive...

National Semi and Texas Instruments make devkits and demo boards that would be perfect but as an example one by National they've set the output to 1.8v by default and to change it you have to desolder and solder in new SMD resistors. They're only $15-25US.

Like the LMZ12003 would be perfect.

I'd say it wouldn't be below spec. the 7805 IIRC is only good for 1A maybe 1.5 and that is if it's heatsinked properly. I mean they're pretty bulletproof but it may have gone into protection.

Dooley wrote:

DAMKIA wrote:USB standard is a MAX of 500 ma per outlet. 3 x .5 = 2 amp tops with safety margin

My guess is it was a way below spec power supply you had originally.

Switchmode would be best for efficiency and heat generation, not to mention size.

http://www.powerstream.com/dc-dcz0303.htm" onclick="window.open(this.href);return false;

Yeah but they're way too expensive...

National Semi and Texas Instruments make devkits and demo boards that would be perfect but as an example one by National they've set the output to 1.8v by default and to change it you have to desolder and solder in new SMD resistors. They're only $15-25US.

Like the LMZ12003 would be perfect.

I'd say it wouldn't be below spec. the 7805 IIRC is only good for 1A maybe 1.5 and that is if it's heatsinked properly. I mean they're pretty bulletproof but it may have gone into protection.

You would have to severely thermally derate the capacity - in reality about 750ma at 40-50 degrees with no airflow (underdash area). Know them well (ubiquitous) and have used them in my Ch9 days in similar automotive situations.