Voltage drop from PSU to device - Page 1
Forum section: electronics / beginners

author: Silenus
author_rank: Contributor
timestamp: January 05, 2015, 03:20:34 am
content: I am trying to power the equivalent of 5 parallel resistances at 5V with my Tenma 72-10495 digital PSU and am finding a considerable voltage drop at the terminals of my resistor network at 4.6V. At the terminals of my PSU, its 4.98V but at the end of my alligator clip leads it drops down to 4.6V. Should I just dial up the voltage higher to make it 5V at the input of my device? Or is there something I should change about how I'm powering the device?

author: devanno
author_rank: Regular Contributor
timestamp: January 05, 2015, 04:46:13 am
content: You can try heavier leads from the PSU to your DUT. Shorter is also better. I'm assuming you have highly trusted metering, too. Are you metering the device end after the 'gator clips, or on the 'gator clips? Might have a less than desirable contact surface area with the clips. I don't know what kind of current you're drawing either, but if the leads are getting warm, they're too small.

author: Silenus
author_rank: Contributor
timestamp: January 05, 2015, 05:13:20 am
content: Quote from: devanno on January 05, 2015, 04:46:13 amI narrowed it down a little. I am going from the PSU > alligator clips > breadboard wires > breadboard > wires > sensor > wires > GND. The sensor ishttp://www.pololu.com/file/0J309/MQ2.pdfand if I put 5V across it via this configuration, it says 4.7V across the sensor and every other part after the PSU terminals, so I'm guessing its the alligator wires? The current is on the order of 200mA.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 05, 2015, 05:27:46 am
content: You did not disclose either the resistance of the load(s) nor the current you are using.And you did not reveal the gauge of the wiring you are using. So we have one equation and 3 unknowns.Alligator clips are typically NOT suitable for high-current circuits.

author: Silenus
author_rank: Contributor
timestamp: January 05, 2015, 05:53:57 am
content: Quote from: Richard Crowley on January 05, 2015, 05:27:46 amJust did a quick smaller test.100 Ohm resistor in circuit, 4.98V at PSU, 4.89V over resistor at 47mA so it seems there is quite a bit of drop through the ally clips. Would banana leads be more appropriate?

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 05, 2015, 06:36:37 am
content: Banana plugs are much more suitable for high current than any kind of clips.HOWEVER...5V into 100 ohms is only 50mA, quite a low current.You should NOT be seeing that kind of voltage drop even with aligator clips.Something else is wrong with your connections or wiring, etc.

author: jeroen79
author_rank: Frequent Contributor
timestamp: January 05, 2015, 11:56:49 am
content: Are you using aligator clips on the PSU's terminals as well?

author: Alex30
author_rank: Regular Contributor
timestamp: January 05, 2015, 12:09:21 pm
content: Use your DMM and measure the output plugs from your PSU, the digital display on the PSU may be out.

author: rr100
author_rank: Frequent Contributor
timestamp: January 05, 2015, 12:13:16 pm
content: Probably there's some oxidation on the aligator clips or on the parts they clip on. Move them around, try to add some scratches to the surface.Also with such a setup you can measure the voltage drop directly over the affected part so you don't need to do the math and make assumptions, just use your regular (surely high-impedance) multimeter and see where those 0.09V missing are.

author: Seekonk
author_rank: Super Contributor
timestamp: January 05, 2015, 02:00:15 pm
content: I picked up a packet of clip leads somewhere, they were cheap. I decided to cut one length in half and put banana leads on. There were only three strands of wire inside. That may be where all the resistance is.

author: steve_w
author_rank: Regular Contributor
timestamp: January 06, 2015, 05:19:55 am
content: Post a picture of your setup, sounds fishy to me.regardsSteve W

author: Silenus
author_rank: Contributor
timestamp: January 06, 2015, 07:44:25 am
content: Pretty sure I figured it out. The wires that I am using look thick enough, but most of it is just plastic, there are like 3-4 tiny strands of copper in each, and quite a bit of length to them, trying to pass hundreds of milliamps. Makes sense that they couldn't handle that high of a current. Thanks for the input, I'm sure it's a combination of all the things you mentioned.

author: electr_peter
author_rank: Unknown
timestamp: January 06, 2015, 09:18:54 am
content: I have a set of cheap 0.5-1 m alligator clips. Each of these have a consistent resistance of 0.8-1 Ohm! This could negatively impact some measurements or power delivery.Always check your equipment and cables just to be sure. Outer looks may be misleading.