# How to read this transformer label?

Discussion in 'Datasheets, Manuals and Component Identification' started by Farukh Khan, Sep 6, 2019.

1. ### Farukh Khan

157
2
Jun 12, 2015
Hello Guys,

I have a transformer which I bought quite long ago, and now suddenly want to use it for a project. So, I really want to know the different specs for this transformer, but I do not really know how to read these labels on the sticker properly. The main thing I want to know is, how much continuous current I can draw from this transformer (@Constant 5V output) using a proper bridge rectifier and filtering circuit where the bridge rectifier I am going to use is: https://my.cytron.io/p-bridge-rectifier-w08m-1.5a800v and a switching regulator (consider efficiency 100 percent for calculation).

2. ### Harald KappModeratorModerator

10,585
2,360
Nov 17, 2011
left to right, top to bottom
Just joking

10 V 6 VA -> I = 6 VA / 10 V = 600 mA on the AC side.
10 V Ac is the RMS value. Peak value is 14V (10 V + sqrt(2)). The rectifier will drop approx. 1.4 V (2 diode voltage drops) which give a peak DC voltage of 12.6 V. Assuming filter capacitors large enough you may assume an input voltage to the DC/DC converter of ~ 11V on average (with ripple due to discharge of the filter capacitors).
The max. input power available to the DC/DC converter is thus 6.6 W (11 V * 0.6 A). Use 6 W to stay within the limits of the transformer.
Assuming 90 % efficiency (100 % is totally not realistic!) the output power of the DC/DC converter is 5.4 W. This results in a max. current of 1.08 A @ 5 V.

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3. ### Farukh Khan

157
2
Jun 12, 2015
@Harald Kapp

Thanks a lot for the useful information. I did some testing with my DMM and come across these values and these seems to match whatever you calculated.

After the bridge rectifier I have four 470uF 25V SMD electrolytic caps. and one 35V 1000uF normal electrolytic cap. When I measure the voltage on the parallel caps I get: Bridge Source = 11.17 V - SCC = 2.13 A (SCC = Short Circuit Current through DMM)

Then I hooked up a 12V regulated switching power supply and I get these results: Output = 10.13 V - SCC = 1.73 A (and a humming sound for some reason in this switching regulator's inductor, not sure why.) This regulator is rated for [email protected] (SCC = Short Circuit Current through DMM)

Then I hooked up a 5V regulated switching power supply and I get these results: Output = 5.0 V - SCC = 3.0 A (It works normally) (SCC = Short Circuit Current through DMM) This regulator is rated for [email protected]

I want to power up a Raspberry Pi 3 Model B with this 5V output, but it seems like the pi is indicating bad power supply even though the buck converter and the transformer seems adequate for the pi voltage and amp requirements. It seems like voltage is dropping rapidly when the pi is using more current during CPU stress. So, what exactly is the culprit? transformer? Bridge Rectifier? regulators? or the wires? Can't seem to figure it out.

4. ### Harald KappModeratorModerator

10,585
2,360
Nov 17, 2011
This measurement does not give you an indication how much current can be delivered reliably while still maintaining 5 V. You need a variable load (e.g. a hand full of power resistors) and increase load current until the output voltage of the regulator falls below the nominal output (e.g. 5 V - 10 %).
According to the documentation the Raspi 3B required between 700 mA and 1 A, a 2.5 A power supply is recommended. According to my calculation your setup should be able to deliver 1 A. However, peak power requirements of the Raspi may exceed 1 A. Try adding an electrolytic capacitor (>= 1000 µF / 16 V) at the output of the step-down regulator. This capacitor can deliver peak current way beyond the nominal power of the transformer (note: 1000 µF is by rule of thumb 1000 µF/A, not an exact calculation).
That is generous. You can use the 1000 µF / 35 V capacitor from this part of the circuit on the output of the step-down regulator if don't have another one lying around. 4*470 µF are sufficient on the input side.
Rectifier and regulator are one possibility, but unexpected. What kind of wiring do you use? Measure the voltage drop across the wires during your short ciruit test to see whether you may need thicker wires.

5. ### Farukh Khan

157
2
Jun 12, 2015
Well, I was going for a more complicated setup than what I mentioned earlier. Sorry for the mistake. Actually on the 5V buck output I do have a 470uF 25V SMD cap inline where I did the wire and microusb joint. See the picture for a better view.

Then I am planning to connect this microusb to the simple Li-ion Charge Controller for charging the 1 cell parallel battery and then take the output from the battery charge controller and put it inside a [email protected] rated boost converter as you can see in the picture. And the boosted output is directly connected to the rpi GPIO power pins. I am currently running a stress test on the pi with an underclock of 1.1GHz on the processor, and the pi seems to be working fine, provided the batteries are almost 70 percent charged. The pi is running totally headless.no USB devices or hdmi. Just the processor,ram,wifi and sd card in use.

http://175.144.229.24:8888 stats for the pi running on a docker container.

@Harald Kapp

6. ### Farukh Khan

157
2
Jun 12, 2015
I did a small testing with dc cpu fans on the boost converter 5V output. After connecting the load, I measure the voltage drop across one wire and then measured the current draw of the load, and then came up with 0.07 Ohm resistance on the wire. Then I calculated the total voltage drop for 1.3A max current draw of the pi using the resistance value of the wire and I came up with ~0.7V drop, which seems quite a lot. So, should I consider that my wiring is the main culprit here? @Harald Kapp

7. ### Harald KappModeratorModerator

10,585
2,360
Nov 17, 2011
Obviously. 0.7 V is a lot. Use thicker wire.

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8. ### Farukh Khan

157
2
Jun 12, 2015
Got it. Thanks for info, Harald. Problem Solved.