Voltage regulator output current

Discussion in 'The Projects Forum' started by Michael.Riley, Mar 3, 2015.

1. Michael.Riley Thread Starter New Member

Jan 3, 2015
15
2
I have built a voltage regulator based on the following circuit:
http://www.circuitstoday.com/3a-switching-regulator
with a few alterations: Q1 is a TIP42G; L1 is an unknown but large value; D1 is a Schottky dual 15 A common cathode; everything else is essentially the same. The problem I'm encountering is that I am not getting the amperage expected through the regulator. I could only get 450 mA through a charging circuit to a Kindle from a solar panel putting out 2.5 amps. I suspect that the power transistor is not fully turned on, but I'm not sure. Any advice?

2. wayneh Expert

Sep 9, 2010
12,379
3,235
Your panel was not putting out 2.5A unless you have a short somewhere not shown in the diagram. What was Vin while you measured 450mA?

3. Denesius Member

Feb 5, 2014
89
14
While you're checking the Vin per Wayneh, I'd also check the DC resistance of your 'large value' inductor....

4. Michael.Riley Thread Starter New Member

Jan 3, 2015
15
2
Vin was 14.5V, and the 2.5A was my short circuit measurement at that time. The max short circuit I've gotten has been 3.6A. The dc resistance of the coil is 0.2 to 0.3 ohms. (My meter oscillators between .2 and .3 ohms.) The coil is toroidal, approx. 74 turns, ~16mm od core, ~4mm x 4mm, possibly ferrite.

5. wayneh Expert

Sep 9, 2010
12,379
3,235
Well the Vin is good. But the 2.5A - how did you measure that? I'm guessing you placed the leads across the panel to short it? You need to place the leads in series with the panel, so that the current is measured while everything is operating. What you'll likely find is that the current into 14.5V is much lower than the short-circuit current. It should be very close to your output current.

6. Michael.Riley Thread Starter New Member

Jan 3, 2015
15
2
I did get the 2.5 A directly across the panel, just to see what current would be available. I didn't test the current on the input side while the regulator was operating, but I assume it would need to be somewhere close to the output current. What I don't understand is why I only get 450mA going to a Kindle, which is capable of taking a 1.8A charge. If the panel is capable of putting out 2.5A at that level of sunlight, shouldn't the regulator be able to pass as much as the device is able to take?

7. wayneh Expert

Sep 9, 2010
12,379
3,235
The panel cannot drive 2.5A into 14.5V, only into zero volts - a short.

That said, I think I would expect more than 450mA. You should verify what the panel can do to drive current into a car battery at ~13V. It's worth the experiment unless you already have the I-V curve in the panel's tech literature.

I'm not sure what's going on with that switcher circuit. If you assume it is not switching, it should still pass quite a lot of current into the load.

8. Michael.Riley Thread Starter New Member

Jan 3, 2015
15
2
The solar panel is one I built myself, using 3 x 6 cells rated for 3.6 A at 0.5V. To test the actual output, I wired five 10 ohm resistors in parallel across the panel. In full sun, I got 7.2V across the 2 ohm resistance, giving 3.6A of current, just as the cells were rated for. The ammeter readings confirmed that by the test across the panel, but I don't know what voltage was developed across the meter probes during that test.
What I can't see is the emitter-base current through the transistor, which I presume controls the degree to which the transistor is turned on. It can/t have reached saturation, because the only other obstruction to output current that I can see is the resistance of the coil, which is very low. (I posted earlier that the coil read 2 - 3 ohms, but I tried another meter and got only 1 ohm.)

9. ronv AAC Fanatic!

Nov 12, 2008
3,393
1,455
Maybe the Kindle charge regulator doesn't want more than 450 ma.?

10. wayneh Expert

Sep 9, 2010
12,379
3,235
I agree. Smart devices goes through some sort of negotiation with the source to establish a charging current. If the source doesn't give the right response, current is limited.

11. Michael.Riley Thread Starter New Member

Jan 3, 2015
15
2
Wow. That concept would never have occurred to me. OK, next step is to test the regulator against a low impedance and see what it can deliver for amperage.
Thanks for the input, wayneh and ronv. I'll let you know what transpires.

12. Michael.Riley Thread Starter New Member

Jan 3, 2015
15
2
Update: Finally got some Sun back, set up my smaller panel (1.4 A peak output) feeding my voltage regulator, with two 10R10W resistors in parallel across the regulator output. It maintained a steady 5.36 V, giving 1.07 A that heated up the resistors nicely. Next step, find out why the Kindle won't accept more current.

13. Michael.Riley Thread Starter New Member

Jan 3, 2015
15
2
Update: Finally got some Sun back, set up my smaller panel (1.4 A peak output) feeding my voltage regulator, with two 10R10W resistors in parallel across the regulator output. It maintained a steady 5.36 V, giving 1.07 A that heated up the resistors nicely. Next step, find out why the Kindle won't accept more current.

14. wayneh Expert

Sep 9, 2010
12,379
3,235
The USB specifications themselves might give some indications, although there is no guarantee that Kindle follows them to the letter. But I bet with some searching you can find what others have learned on the details of Kindle charging.