Doubling 7805 regulator current is effective?

[wuemura]

Hello again guys

My nefiew has a PSP, here we go, so he wants to play games in his father car without running out of battery. I know that there are alot of cheap adapters to do this for about less than $10 bucks, but here at Brazil is not so easy to get this stuff with the same price, and game stores that do sell it, is too expensive, including tax over tax the $10 bucks adapter can go easly to about $150, $180, $200... not good. So i thinking in creating a car adapter using the 7805 in parallel.

The PSP adapter is 5VDC 2A, when the adapter is used the PSP drain about 1.5A~1.8A, my idea is to use 3 7805 in parallel to provide about 5V/3A, in theory, it should work fine but i dont know how effective this could be for real.

I know that there is better way to do this but my goal is to use cheap parts that i have here.

Thanks!

 

[audioguru2]

Hi Wellington,
If you parallal three 7805 regulators, they each will have a slightly different output voltage, so the one with the highest voltage will take the entire load then current-limit and its voltage will drop down to where the next lowest voltage regulator will begin to work, etc. Lousy voltage regulation.

The datasheet for the 7805 shows a PNP booster power transistor added. A second transistor is shown to limit the current if the output gets shorted.

The booster transistor is going to get pretty hot so it will need a pretty big heatsink.

View attachment 40090

 

[wuemura]

I was afraid of that, so i will have to say a BIG sorry also to this project because with the heatsink, board and transistor the circuit can be bigger than PSP.

Thank you Audioguru.

 

[audioguru2]

Switching regulators are nice and cool. But they are very complicated.

 

[mvs sarma]

Hi
Perhaps an LM323 can do your job but very costly compared to LM7805

In fact earlier MC78T05 (TO220) were available - I saw few of them -- these devices used to give 3 amps laod provided properly heatsunk.

if you get hold you may try

 

[wuemura]

Man, to see my nefiew cry is heart breaking. :-\
He cryed alot when i said that i won't do it.

So here we go again....

I know that switching regulators can do this job well, the problem is not how to build because there is alot of books about it, the realy problem to me is to get my hands on one. Also the LM323, could not find any near by. So i have to do it the old way.

Here is my schematic...

F1 has no value yet because i have to build first and mesure later, i dont think R1 could get that hot but 1W resistor wil be a safe value to work with, the two TIP42 should get enought current to PSP without getting also too hot.

So what you think?

View attachment 40103

 

[audioguru2]

Hi Wellington,
Two transistors with the same part number have a different current gain and a different bese to emitter voltage. So if they are in parallel then the one with the highest gain will have the lowest base-emitter voltage and will hog all the current then overheat and fail. Then the next one in parallel will fail. Low value emitter resistors (0.22 ohms) are used to make the transistors operate nearly the same. But you don't need two transistors.

Your circuit will have an input from the charging car battery of up to 14V. The output is 5V so the transistor will have 9V across it. The load is 1.8A and the regulator will take about 80mA of it. So the transistor will heat with about 15.5W and will need a medium size heatsink. The heatsink must be away from things for air to move across it.
Don't use the inductor.
Use a slow blow fuse so it doesn't blow when the 1000uF capacitor first charges.

 

[wuemura]

Man, we dont have a resistor of such low value here, the lowest value that i could find is 1 ohm. The main idea of using two transistors is to let then work as cooler as possible, because this circuit will be in the hand of a 7 years old kid in a closed metal box, even if i install this inside the dashboard it could not get too much hot because, inside the car dashboard is hot already and there is no ventilation.

 

[wuemura]

Suggestions?

 

[audioguru2]

Hi Wellington,
Here we can buy anything with any value. I can order anything online before 8:00PM and it is delivered the next morning.

You have 15.5W of heat to get rid of. One transistor will make a heatsink the same temperature as two transistors. The only advantage for using two heatsinks is that you can use two pretty big heatsinks to share the heat, then each one will be cooler.

The heatsink fins must be mounted on the outside of the box.
Here is one that is a reasonable size. Its fins will be at 66 degrees C if the ambient is 30.

View attachment 40104

 

[mvs sarma]

Man, to see my nefiew cry is heart breaking. :-\
He cryed alot when i said that i won't do it.



Here is my schematic...

F1 has no value yet because i have to build first and mesure later, i dont think R1 could get that hot but 1W resistor wil be a safe value to work with, the two TIP42 should get enought current to PSP without getting also too hot.

So what you think?

Hi Wuemura ,

Replace the D1 with 3 amp diode-- Go ahead-- try mount the transistors on the body of the poer supply cabinet ( if Metal) and insulate them with TO220 type packing kit. If plastic,  try a  heat sink of aluminum sheet of adequate thickness if you dont get the type suugested by Audioguru--you may try for 200 mA thro' 7805 by reducing the R1 value to 3.3ohms and 1/2 or 1 watt.
fuse can be at 3amps slow blow as already recommended by GURU

 

[wuemura]

Hello Guys!

Thank you for the sugestions, but i think is too dangerous to let a kid handle this.
So, i installed a small DC/AC converter (12VDC > 120VAC) inside the dashboard and he can use his original PSP AC Adaptor, now he can charge the PSP battery and play, also is much more safe for him and the car because the converter does not get hot and is small.

Thank you!

 

[ante1]

Hi wuemura,

Can you get hold of a MAX 724 or an LT 1074 where you live?

 

[wuemura]

Hello Ante!

Don't know, let me check.

 

[wuemura]

Sorry to keep you waiting.
I could not find the MAX724, only the LT1074CT and LT1074HVCT.

 

[AN920]

I know there are many arguments about this issue of paralleling regulators. Looking at the internal structure of the LM7805 it looks possible to do this.  Some time ago I experimented with true national 7805's and found that they do indeed share current when run near or at their maximum current ratings. I could not see any significant loss in voltage regulation running in this mode. The regulators that I tested had good thermal coupling close on the same heat sink. At lower currents the current sharing was poor. I don't know how well regulators from other manufacturers will perform.

View attachment 40127

 

[audioguru2]

Hi AN920,
The paralleled regulators will be slightly different. The one with the highest voltage will feed the higher voltage to the lower voltage one. The negative feedback of the lower voltage one will force it to turn off, because the output voltage is too high for it.

Maybe your regulators were nearly the same. They could be 4.8V to 5.2V.

 

[AN920]

They could have been from the same batch but I can't remember. I have also seen a frequency standard product that one company sold many of, that paralleled 3 or 4 of these regulators because of space problems. I own one and it's working for the last 5 years without failure. Can't say if they selected them especially for that purpose.

 

[ante1]

Sorry to keep you waiting.
I could not find the MAX724, only the LT1074CT and LT1074HVCT.

Any of these will work fine and does not generate much heat!