L-Adapter

[JMMF2019]

Understood. Thanks.

 

[Dymaska]

Good morning,
I would like to create a psu with the L-adapter schematic for a Raspberry PI4b, could you please help me in choosing alternatives to the NJW0302G and MJE15030G transistors, as I can't find them?
If I use single LEDs instead of the LED bar could I run into problems?
Thanks so much for your help.
Dimitri

 

[Salas]

You would likely need a small number of LEDs to reach 5V with a little help from the trimmer. You could try TIP2955 and BD243C. Same size but not directly equivalent spec parts, it will possibly work on those too, but no guarantees. By the way, the MJE15028G is also applicable as it is just an 120V rated MJE15030G.

 

[Dymaska]

Thank you very much Salas,
in truth I would only need the replacement of the NJW0302G, since of the NPN, I can perhaps find the version without G (MJE15030), which perhaps could be quite equivalent.

Thanks again

 

[Salas]

Only difference between 15030 and 15030G is G being Lead Free (RoHS)

 

[Dymaska]

Then, as soon as possible I will try the TIP2955 with MJE15030 and a pair of LEDs + trimmer and let's see how it behaves, I will keep you updated.
Thanks again

 

[Salas]

What Leds color are you planning to use? Better three red than three green not to overshoot 5V. Vout (when the trimmer sits at zero) is VfLedsTotal-Q1Vbe.

 

[Dymaska]

Thanks Salas for the information.
I was thinking of using 2 red LEDs believing that they would be enough. I have some of it with VF of 1.89-1.92V, but maybe if I insert 3 I won't exceed the final voltage of 5V?

 

[Dymaska]

Hi, in this case (l-adapter for PI4) would it be possible to use these rectification diodes: BYW 80 200?
Thanks in advance.
Have a good day

 

[Salas]

1.9x3=5.7V minus Vbe 0.6V is 5.1V. Two seem short for the 5V goal. Look for Vf at 5mA on their datasheet curve not just their nominal Vf spec.
Those BYW are 8A rated so they are fine for the Pi4 job.

 

[Dymaska]

thank you so much Salas!!

 

[jdkinase]

I built two L-Adapters using the parts from Tea-Bag, nice easy build (!), to power, separately, an RPi 4 and an external Topping DAC.
-120-120/9-9 VAC 50 VA toroidal transformer (ANTEK AS-0509)
1) 5VDC no-load output using 25VA gives 50 mA output with a 100 Ohm/5W resistor
2) #1 with 50 VA = same
3) using heavy load (soldering iron), initial 5V/14 mA drops to 0V/0 A
4) a 25VA transformer (ANTEK AN-0209) in #1 configuration gives 50 mA output

So, shouldn't I be getting 1-2 amps @ 5 VDC? (Yes, semi-newbie in electronics).

 

[Salas]

Is it a USB micro-soldering iron? What is its nominal power? Those are usually 8W so 8W/5V=1.6A. The transformers you use should have the capacity for such an iron. Maybe it has many more W initial quick heat up peak demand? Do you use some USB tester in-between PSU & load for measuring current that maybe goes in protect mode? Is there a prerequisite for floating or grounded DC output for that iron? LEDs on L-Adapter also extinguish? Measure PSU's AC input (i.e. transformer's output) when the PSU's DC output drops to zero.

 

[jdkinase]

This was a old-fashioned 50W/120 VAC iron. I just tried it without the multimeter in series, to measure current, and LEDs do not extinguish, transformer output is 9.5 VAC, L-A output stable at 5.0 VDC. I’ll let it run for a while. Multimeter is not good?
Nonetheless, is it normal to be getting 50 mA on a 5W 100R resistor load with 50 or 25 VA transformer?

 

[Salas]

Seems normal, 5V/100R is 0.05A or 50mA.

Probes maybe weren't connected in the proper sockets for 10 ampere mode on the meter?

By the way, better don't risk the meter's internal fuses with such heavy experiments because they can be high rapture capacity special ones in the better brands. Expensive fuses. It also takes to disassemble the cover in many meters to replace fuses.

 

[jdkinase]

Seems normal, 5V/100R is 0.05A or 50mA.

Probes maybe weren't connected in the proper sockets for 10 ampere mode on the meter?

By the way, better don't risk the meter's internal fuses with such heavy experiments because they can be high rapture capacity special ones in the better brands. Expensive fuses. It also takes to disassemble the cover in many meters to replace fuses.

Issues, continued:

• No-load: 5.0VDC output on 9.8VAC input from 50 VA transformer, 3LEDs lit. 50 mA with 100 Ohm load.
• Connected to RPi 4 via USB C: 1.4 VDC at output and 7.3 VAC input, LEDs extinguished. Current off-scale (new Fluke meter). Could not adjust output voltage at pot.

I'll accept I may have wired both units incorrectly or components were defective or failed. Any ideas on where to start troubleshooting would be greatly appreciated. Unfortunately I do not have a scope available.

 

[Salas]

Test Q1 & Q2.

 

[Ghoostknight]

hello guys, where i can find the schematic and bom?

 

[Salas]

In 1st post

 

[jdkinase]

Q1 and Q2 test results