Hi current from KA7905 & 2N3055

[papunblg]

I am trying to make an (about) 1500mA 5V power supply for a D-Link DIR-300 wireless router.

I have a 12V 2amp smps which I am using as source of DC and using a KA7905 regulator and 2N3055 power transistor(I have them in my stock).

The regulator draws current through base-collector junction. The emitter is connected to the output of 7905.

The problem is after 5 to 6 min of operation the regulator shuts off the supply.

Any help? Any better circuit using KA9705?

 

[KlausST]

Hi,

Aboout 10W power loss willbe much heating.
Maybe a thermal shutdown is activated.

Klaus

 

[pinout]

Think you might be better off trying a positive regulator.
Try putting a 7805 output on the transistor base, this will then set the output at 5V-0.6V.
Wire the collector to vin of the 7805 such that 3055 becomes a big power dropper and current amplifier.

I would also place a restive load to gnd on the 7805 along with the usual decoupling caps to ensure stability.

But what about the Vbe drop lost on the output, put a diode in the 7805 gnd to raise the output about 0.6V.

 

[papunblg]

Re: Hi current from KA7905 & 2N3055

@pinout,
Before making this post, I tried 7805 with NPN 2N3055(7805 output on the base) and It worked as expected. Used a diode to overcome Vbe drop.

But, what is annoying me is the way 7905 is acting. I have tried both the following cases :-
1. The regulator draws current through base-collector junction of NPN Transistor. The emitter is connected to the output of 7905.

2. Putting a 7905 output on the base of a transistor.

These two worked for 5 to 8 min before shut off.

Where as 7805 is working fine with both PNP and NPN.


KlausST

I also suspecting thermal shutdown as well. But with same heat sink(TO-3 metal package 2N3055 attached to an empty box of computer smps) a positive regulator 7805 working fine.

In all the circuits to increase current of 3 terminal regulators available in net are using positive regulators. In all the discussions it is mentioned that in case of negative regulators current may be increased using external power transistor (compliment of as in case of positive regulator). No circuits I have found using negative regulator.

 

[KlausST]

Hi,

No circuits I have found using negative regulator.

Just use a PNP instead of an NPN and the equivalent circuit as for the positive regulator. Isn´t there a 2955 complemenary to 3055?

*****

I´d go for switching regulator. wasting 140% of the power you need not nice.

in ebay there are a lot of cheap step down DC/DC modules.

Klaus

 

[goldsmith]

Hi papunblg

May i ask you why you don't use SMPS to avoid the high dissipation ?


Best Luck
Goldsmith

 

[papunblg]

Re: Hi current from KA7905 & 2N3055

Hi papunblg

May i ask you why you don't use SMPS to avoid the high dissipation ?


Best Luck
Goldsmith

Actually I am using a 12V 2Amp smps as source of DC and regulating it using 7805/7905. Before buying a new 5V smps I thought "Why not make my own power supply". But seeing (or may I say feeling) the high dissipation, I now think it is wiser to buy a 5V smps before I call the fire brigade. its really HOT!!

thanks All

 

[goldsmith]

Actually I am using a 12V 2Amp smps as source of DC and regulating it using 7805/7905. Before buying a new 5V smps I thought "Why not make my own power supply". But seeing (or may I say feeling) the high dissipation, I now think it is wiser to buy a 5V smps before I call the fire brigade. its really HOT!!

thanks All

Just to mention a funny memory ! when i was a little boy and i was designing my first amplifiers , i designed a class A amplifier ( not AB just A ! ) it's power was about 5 watt . you know what happened ? the dissipation was at least about 10wat ! i added a complementary stage after that ( class AB ) in order to increase the power handling and my overall wasted power became around 100 w instead of 30 watt out put power . it was absolutely funny ! ( don't laugh please ! ha ha ! i was just a little curious boy ) ( the horrible efficiency ! ) a friend of mine told me : hey buddy ! why not making an omlet on the power transistor ? i told him , come on ! it's impossible that's nuts ! he did it ! brought a dish an oil and egg and ... i was shocked ! i didn't know that , it's possible ! i still have that amplifier but i never use it i keep it just for fun ! :laugh:


Good Luck
Goldsmith