Input current measurement of LM2596 - Page 1
Forum section: electronics / beginners

author: Dave99
author_rank: Newbie
timestamp: February 06, 2017, 12:36:14 pm
content: Hello,I'm trying to measure the input current of an LM2596 regulator but having some issues. Without a load on the LM2596 I'm measuring just over 4mA which seems reasonable.The issue is when I connect a load the current readings are fluctuating quite significantly between 20mA and over 150mA, it never settles on a value. I'm thinking this could be caused by the way a DC-DC convert or works?Additionally I have tried to measure the current on the regulated output before it goes into the device but unfortunately the device thinks no power is connected when the multimeter is in series, it must be changing something the device doesn't like.The device is a wifi dongle that I need to power for a long period of time so I have removed the battery and connected the regulator directly. I had to connect a 10k resistor between the T terminal and ground to trick it into thinking the thermistor in the stock battery was connected. Voltage is set to 4V and battery source is a 3 cell lipo.Any advice appreciated.Dave

author: danadak
author_rank: Super Contributor
timestamp: February 06, 2017, 01:27:56 pm
content: You are not experiencing a lot of coupling to on/off pin ? Ie. its terminatedto ground.Your ammeter leads plugged into correct jacks, you are just measuringIload. Using a shunt R in series with Iload to sense current, measuringshunt V and converting to current via I = Vmeas / Rshunt. This vs havingIload flow thru meter and its long leads and the internal shunt of meter ?Load resistive in nature or complex, re. L or C ?Layout has solid grounds all grounds merging at board edge. Short leads,traces, to power source. What is power source ?You have observed the ESR requirements for Cin and Cout, especially Cout...Regards, Dana.

author: Benta
author_rank: Super Contributor
timestamp: February 06, 2017, 03:18:38 pm
content: Which meter are you using for measurement?

author: Dave99
author_rank: Newbie
timestamp: February 06, 2017, 04:56:48 pm
content: Quote from: Benta on February 06, 2017, 03:18:38 pmA Uni-T 61E

author: Benta
author_rank: Super Contributor
timestamp: February 06, 2017, 05:04:34 pm
content: Quote from: Dave99 on February 06, 2017, 04:56:48 pmShould be OK. Can you post a schematic of the LM2596 circuit?

author: 2N3055
author_rank: Super Contributor
timestamp: February 06, 2017, 06:17:04 pm
content: Buck switching regulators are basically energy pumps that pump little bursts of current into output capacitor, building the voltage in it.Once voltage reaches wanted level, regulator stops pumping energy.. In normal mode where you draw some current from it, regulator can regulate output voltage within it's normal regime using PWM. That is called continuous mode. If you don't draw any current, output capacitor will charge to setpoint voltage, and then regulator have no choice than to stop pulsing completely, until something discharges output capacitor. If you draw very small current from regulator, it can pulse every few seconds... That is called discontinuous mode..You can easily see it with an oscilloscope, or if you don't have one, multimeter with frequency range put on the output will show either switching frequency (150kHz in this case)..If frequency is lower than that, you are running in discontinuous mode..Also, you might want to measure input current before input capacitor, so your multimeter has easier life...

author: Benta
author_rank: Super Contributor
timestamp: February 06, 2017, 07:27:32 pm
content: QuotePrecisely. That's why I asked for the schematic.

author: Dave99
author_rank: Newbie
timestamp: February 06, 2017, 11:15:32 pm
content: Thanks for all the replies, I don't fully understand them all but have a good avenue to do more reading. The shunt resistor principle seems worth pursuing. I don't have a schematic unfortunately, it was just bought from eBay. I think next purchase will be an Oscilloscope.

author: danadak
author_rank: Super Contributor
timestamp: February 07, 2017, 12:07:30 pm
content: If you put the Rshunt in the high side of the load you want to makea differential measurement across the resistor. If in ground leg themeasurement single ended from load/shunt connection to ground.If you want to do a low drop (low R shunt) you can use a diff amp withsome gain, keep in mind it must have CM range that accommodates thevoltages involved. Or you can use a scope, 2 channels, and config itas add the two channels, invert one. That's not a high accuracy measurement,but satisfactory for many examinations. Note this method does not havereal high CM rejection, depending on frequency 20 - 50 db typical. Ifyou need more than use OpAmp based diff amp.http://www.analog.com/en/education/education-library/dh-designers-guide-to-instrumentation-amps.htmlRegards, Dana.