9v, 3 Amps psu, limit @ 2.7 Amps

Discussion in 'The Projects Forum' started by Tera-Scale, Jan 7, 2012.

1. Tera-Scale Thread Starter Active Member

Jan 1, 2011
164
5
What is the best technique to design a 9V, 3Amps power supply that limits current at 2.7 Amp. I would like to stick to linear technology but design a circuit that is also efficient with best accuracy I can get. Any hints?

2. kbyrne Member

Dec 10, 2011
84
2
From a newbi I write a sudjestion. Go to http://knol.google.com & search till you find a
knol by a Author called Max Iskram. I have a 17 chapter set of down loads where he guides me thru a power supply simular. The difference is 12VDC @ 5A. As I see it change
voltage and amperage per his set of chapters and let me know because I can also guide you to searching for another site.

3. Tera-Scale Thread Starter Active Member

Jan 1, 2011
164
5
Excellent source! thanks a lot. I will recommend it to others as well.

4. wayneh Expert

Sep 9, 2010
12,403
3,250
One thing you'll want to do is better define your specifications. Source power? Allowed ripple size (as a function of current load)? Is normal operation at constant voltage (your 9V) or constant current (2.7A)? Do you want to vary that specification? What do you want the supply to do when the load is excessive, such as a short (blow a fuse, sound an alarm, light an LED, shut down)? The experts can't help you much unless they know what you want.

5. Tera-Scale Thread Starter Active Member

Jan 1, 2011
164
5
First of all I want it regulated. I'll be using 240V to 12Vrms transformer. The output would be constant voltage unless the 2.7A limit has been reached with a load impedance of less than 3.33Ω (9V/2.7Amps). Then obviously the output voltage will drop below 9V to leave 2.7Amps through the load - Ohm's Law. For the indication, I already designed a comparator with a zener voltage reference which will indicate any voltage drop on the output (by an LED).

Last edited: Jan 7, 2012
6. R!f@@ AAC Fanatic!

Apr 2, 2009
8,793
771
If the output of the transformer is going down to 9V @ 2.7A then you are overloading it.

It will fail eventually if you want to build a PSU at around 2.7A

7. R!f@@ AAC Fanatic!

Apr 2, 2009
8,793
771
Load the transformer secondary while measuring current and voltage simultaneously.

If unloaded ACV is 12VAC, then load it till it drops to 11.4VAC.

@ 11.4VAC the measured current will be your continues load for that transformer

8. Tera-Scale Thread Starter Active Member

Jan 1, 2011
164
5
.. i am not connecting any load directly to the transformer. I will use a voltage regulator.

9. wayneh Expert

Sep 9, 2010
12,403
3,250
Your power supply IS the "load", from the transformer's point of view.

10. Tera-Scale Thread Starter Active Member

Jan 1, 2011
164
5
Oki.. so if I take a transformer behind a FW Bridge rectifier that will supply a continuous 2.7 Amp and the transformer has a sufficient VA rating. What is wrong with that? Now let us consider a scenario where a voltage reg such as 7809 is used with a bypass transistor and short circuit protection. This will mean that the load current will never exceed 2.7Amps athough at some point the voltage will have to drop below 9v. Will this load the transformer if the transformer has a VA rating of: (12V sec. X 3 Amps) = 36VA ?

Last edited: Jan 8, 2012
11. R!f@@ AAC Fanatic!

Apr 2, 2009
8,793
771
U are confusing ur self now.

If the transformer can supply 2.7A ( which you presume is that it is rated to that current ) then the voltage will not drop to 9V. It will maintain it's rated output voltage at it's rated current.