Combo Power Supply with overcurrent protection

Thread Starter

TCOP

Joined Apr 27, 2011
94
Hi all,
I need to power up a project of mine with +12v to drive some relays and +5v for the rest of the board.
I am thinking of using a 7812 and a 7805 in series and to add a 500ma fuse just before the 7812.
The actual thing would be something like this:

+15v DC in ---[300ma FUSE]----[ > 1N4004]------[7812]-----[7805]----> +5V


The whole thing seems a little bulky. Do you have any other suggestions?
 

Thread Starter

TCOP

Joined Apr 27, 2011
94
There is no schematic since I've not designed it yet. I am just trying to collect some info. The diode is supposed to protect from reversed polarity.
The Relays should need no more than 150-200ma and all the rest must be less than 80ma (meaning PIC + leds), so I think that 300ma fuse will be fine.
 
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spinnaker

Joined Oct 29, 2009
7,835
There is no schematic since I've not designed it yet. I am just trying to collect some info. The diode is supposed to protect from reversed polarity.
The Relays should need no more than 150-200ma and all the rest must be less than 80ma (meaning PIC + leds), so I think that 300ma fuse will be fine.

Every design starts with a schematic. No one is going to help you unless you express your ideas clearly.

Can a mod please move this thread? It does not belong here.
 

Thread Starter

TCOP

Joined Apr 27, 2011
94
Every design starts with a schematic. No one is going to help you unless you express your ideas clearly.

Can a mod please move this thread? It does not belong here.
I am not sure what you don't understand. I clearly stated that i need to design a power supply for a project of mine that needs +5 and +12v stating clearly the current I will draw.
I asked help for designing this, thus helping me on designing the schematic.
I you feel that this has no interest for you and you don't want to share info then don't. Don't just repeat over and over the same things.
 

Thread Starter

TCOP

Joined Apr 27, 2011
94
Then engineers do design work they have to act in accordance with the information given in the datasheet(s). If you have not downloaded them yet please do so. Then draw your schematic. A photo of a paper and pencil drawing works fine.
Here is a link to the datasheet for both 7812 and 7805
http://www.fairchildsemi.com/ds/LM/LM7805.pdf
Thanks for the info. I already downloaded the datasheets.
My question though was , if i should use two linear voltage regulators for doing this? It looks bulky to have two of them.
I thought of using two 5V relays and get rid of the 12V power source but then I will need much more current to drive them.
Are there any alternatives? Can I use a 7812 and then use a voltage divider to drop to +5V?
I don't know what is more appropriate to do.
 

t06afre

Joined May 11, 2009
5,934
Thanks for the info. I already downloaded the datasheets.
My question though was , if i should use two linear voltage regulators for doing this? It looks bulky to have two of them.
I thought of using two 5V relays and get rid of the 12V power source but then I will need much more current to drive them.
Are there any alternatives? Can I use a 7812 and then use a voltage divider to drop to +5V?
I don't know what is more appropriate to do.
Yes you can go ahead with your plans. It could be that the 5 volt regulator can benefit of a TO-220 type heatsink. Like this one
http://www.futurlec.com/Heatsinks/TO220LGBA.shtml
At least put it in your designs plans. Then you can add it if the 5-volt regulator gets to hot. The 7812 need at least an inputvoltage equal 2+12 to maintain a 12 volt output (typical value at 500mA) So with a series diode it can be somewhat tight. But I will guess a 12 volt relay will operate properly at voltage 12 volt +/- 10%. Remember to include a protection diode http://www.azatrax.com/relay-contacts.html
Also before you go ahead post your final schematic for audit here
 

ErnieM

Joined Apr 24, 2011
8,108
Using two supplies the way you have them is fine. The only simplification I can see is to use 15V relays directly off the unregulated supply, then just one linear reg to make the 5V.

I've always seen it as a toss up if the 2nd reg runs off the first or direct to the source. It's basically a power dissipation issue, though having one reg run off the other does add a bit of protection to the 2nd reg.
 

Thread Starter

TCOP

Joined Apr 27, 2011
94
Yes you can go ahead with your plans. It could be that the 5 volt regulator can benefit of a TO-220 type heatsink. Like this one
http://www.futurlec.com/Heatsinks/TO220LGBA.shtml
At least put it in your designs plans. Then you can add it if the 5-volt regulator gets to hot. The 7812 need at least an inputvoltage equal 2+12 to maintain a 12 volt output (typical value at 500mA) So with a series diode it can be somewhat tight. But I will guess a 12 volt relay will operate properly at voltage 12 volt +/- 10%. Remember to include a protection diode http://www.azatrax.com/relay-contacts.html
Also before you go ahead post your final schematic for audit here

What do you think about this?
 

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Thread Starter

TCOP

Joined Apr 27, 2011
94
Using two supplies the way you have them is fine. The only simplification I can see is to use 15V relays directly off the unregulated supply, then just one linear reg to make the 5V.

I've always seen it as a toss up if the 2nd reg runs off the first or direct to the source. It's basically a power dissipation issue, though having one reg run off the other does add a bit of protection to the 2nd reg.

DAMN!
that seems quite a better choice!
So forget the 7812 and use only 7805 with no heatsink. Then drive the relays directly from the powerjack.
 

t06afre

Joined May 11, 2009
5,934
It may very well be that 15 volt is inside the specification for acceptable control voltage for your relays. The relay data sheet will give you this information. Heatsink will be needed for the LM7805. For every Watt the LM7805 have to dissipate not using any heatsink. The component will heat up 65 degree Celsius. And even at a modest 100mA at 15 volt input. You will have to dissipate 1 Watt.
As a tip, most modern cell phone chargers can supply around 500mA and 5 volt. Perhaps you can use an old one and 5 volt relays for your project. The output voltage and output current should be written some place on every charger by law.
 

Thread Starter

TCOP

Joined Apr 27, 2011
94
It may very well be that 15 volt is inside the specification for acceptable control voltage for your relays. The relay data sheet will give you this information. Heatsink will be needed for the LM7805. For every Watt the LM7805 have to dissipate not using any heatsink. The component will heat up 65 degree Celsius. And even at a modest 100mA at 15 volt input. You will have to dissipate 1 Watt.
As a tip, most modern cell phone chargers can supply around 500mA and 5 volt. Perhaps you can use an old one and 5 volt relays for your project. The output voltage and output current should be written some place on every charger by law.
How about if I lay the 7805 on the pcb and screw it on the ground plane. Could I get away with no extra heat sink then?
 

ErnieM

Joined Apr 24, 2011
8,108
How about if I lay the 7805 on the pcb and screw it on the ground plane. Could I get away with no extra heat sink then?
How much current at a 10V drop will it supply? That's how much current you need plan for.

The 7805 is one rugged device and it will shut itself down before you overheat it, so it's safe to test without a heatsink.
 

spinnaker

Joined Oct 29, 2009
7,835
If that 12V supply you have is a battery then I think you are going down the wrong track with a linear regulator.

Someone just asked about a 5V supply from 12V here:

http://forum.allaboutcircuits.com/showthread.php?t=82736

See my mention of switching regulators. TI also has a fixed 12V version of the LM2675, plus they have an adjustable version too.

Their regulators are so easy to use, I don't even know if it is worth it to consider a switching regulator any more except maybe in the case where you have very low current and want to save space.
 
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spinnaker

Joined Oct 29, 2009
7,835
After thinking about this I realized 12V out is one of the voltages that you want. I would still consider the switching regulators. It would solve any heat issues you might have.


There is whole series on power supply design videos here:

http://www.eevblog.com/episodes/

That you might want to review.
 
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