How to measure the output of a DC/DC converter? - Page 1
Forum section: electronics / beginners

author: DivineChaos
author_rank: Regular Contributor
timestamp: February 01, 2016, 02:39:26 am
content: Hello everyone.So I have successfully wired up a TPS60205 by Texas instruments to power an LED. How can I measure if it is working properly? IE putting out a constant 3.3v and the current that it is supposed to be outputting?I have tested the device without load IE no LED and I see 3.3v, but when I connect an LED the output reads exactly the voltage drop of the LED and not 3.3v. Is this working as intended?Also to measure the current coming out I put a multi-meter in series with the LED and I read basically hardly any output at all, nowhere near 100mA.Is there a way to measure under load if the device is putting out 3.3v and 100mA? It seems to only put out what the device I am powering, needs.

author: Brumby
author_rank: Unknown
timestamp: February 01, 2016, 03:17:22 am
content: 33 ohm 1/2 watt resistor.Measure the volts. If it says 3.3v, then it's doing the job.If you are connecting the LED directly across the 3.3v output, then you are likely running into an overload situation. LEDs will want to keep their forward voltage drop and it will drag the supply down (or die trying). This is why dropper resistors are used with LEDs.When you insert a meter for current measurement, you add a voltage drop - the 'burden' voltage. This changes current flows significantly - especially for low currents at low voltages.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 01, 2016, 03:25:26 am
content: You need an oscilloscope to do this properly, since you'd probably like to know the magnitude of ripple sitting on top of the DC output of the boost converter.BUT... you also need to realize that the supply voltage is the property of the power supply, but the _current_ is the property of the load resistance. In other words... say your power supply is providing a solid, regulated 3.3 volts and isn't current-limited internally. The current through the circuit will then be determined by Ohm's Law using the resistance of the load. I = V/R.Now... with LEDs.... these are very non-linear devices. The LED specification will have a "forward voltage" Vfwhich is the voltage drop across the LED at some small current. If you supply more voltage than this, the current through the LED will go up. And different kinds/colors of LEDs will have different Vfvalues. So your LED is drawing whatever current it draws at 3.3 volts.The "100 mA" rating of the boost converter is the rated maximum current output when the supply voltage is 2 volts. This does not mean that it will always push 100 mA through whatever load ! The _load_ determines the current draw at a given supply voltage. Also, when you use a DMM as ammeter, you are putting its shunt resistance in series with your circuit, so this will actually affect the current as well. For measuring very small currents, I think it is generally better to look at the voltage drop across a small Current Viewing Resistor in series with the circuit, like a 0.1 ohm precision resistor. The voltage drop reading across this resistor will be converted to a current reading by Ohm's Law, I = V/R again.What is the Vfof the LED you are using?

author: Brumby
author_rank: Unknown
timestamp: February 01, 2016, 03:31:04 am
content: https://youtu.be/u2yRR4G3yTA?t=201This is cued to the starting point of Dave's talk about 'burden voltage' and his uCurrent project.

author: DivineChaos
author_rank: Regular Contributor
timestamp: February 01, 2016, 05:07:50 am
content: The forward voltage is 3.5v and I'm reading 3.3. If I choose an led with 1.6v I read 1.6. The 3.3v LED is rated for 20mA and with the DMM I'm reading about 15mA. The 1.6v LED is rated for 100mA and I'm reading 50mA

author: Brumby
author_rank: Unknown
timestamp: February 01, 2016, 07:08:24 am
content: Burden voltage!If you have a second meter - measure the voltage across the terminals of the meter you are using to measure the current.Then do the math!