Power Supply Questions - Page 1
Forum section: electronics / beginners

author: MC_cycle
author_rank: Newbie
timestamp: January 25, 2021, 04:27:16 am
content: Recently I bought an oscilloscope, and first thing I've done is I measured the output of my 12 volt power supply which I'm planning to use in my project. And I was completely shocked after seeing the results.Without a load:Under load (4 Peltier elements + cooling fan, 12 Amps average current draw):Power supply is rated for 50 Amps or 600 Watts output.I'm very concerned about these almost 2.5 Volts peak-to-peak spikes under load. I want to power up an Arduino board from this power supply in conjunction with the load. And if the output is so dirty, then Arduino will be going nuts (microprocessors are usually doesn't like dirty power), and the load (Peltier elements) will not be happy either. And even LEDs (for which this PSU is intended to use for) will not be happy with that sporadic 1.3+ volt spikes and will die very shortly. So, is there any ways to filter this noise out, other than buying a new power supply (because I already have four of them and have no time and money to order another ones, and all four PSUs are needed to complete my project)? Maybe a huge electrolytic capacitor between + and - power rails will do the job (but inside a power supply there is a plenty of capacitors and inductors which must filter out everything)? I saw a bunch of videos, where people measuring power supply output with oscilloscope, but in their cases these spiked noise was in millivolts range (less than 100 millivolts). But I have volts. Is my PSUs are complete bullsh*t, or I doing something wrong?

author: james_s
author_rank: Super Contributor
timestamp: January 25, 2021, 05:05:57 am
content: Where is your scope probe grounded? I see the probe tip on the negative output but the ground is not clearly visible. A whole book could be written on proper probing technique but at the very least you need the shortest most direct ground possible. Be very careful with power supplies like this, it's common for the DC ground to be tied to earth ground, and likewise with the scope the probe ground is usually tied to earth ground, this makes it easy to blow up your scope if you don't know what you're doing.

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 25, 2021, 05:38:16 am
content: for mains connection, the brown wire is line, blue is the neutral.I see a probe ground spring on the end of the probe but no connection. The ground spring should connected to one of the negative terminals, probe tip appears okay.

author: Ian.M
author_rank: Super Contributor
timestamp: January 25, 2021, 05:46:29 am
content: The spikes are very narrow, so *IF* they really exist when you ground the probe properly, and as you probably only need 100mA or so for the Arduino, would be easy to filter out. I'd suggest a 100uH choke rated for >150mA DC current of not more than 3 ohms DC resistance, feeding a LM7808 (or equivalent) regulator (as the Arduino's on-board one is seriously power limited at high input voltages), with 100uF electrolytic in parallel with 1uF 25V ceramic decoupling caps on the 7808 input and 100nF 25V ceramic on its output.

author: james_s
author_rank: Super Contributor
timestamp: January 25, 2021, 05:59:11 am
content: Also be careful with those tiny little wires you have connected to a power supply that can deliver 50 AMPS(!) That's a load of current, it will easily set fire to those puny little wires if there is an overload.

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 25, 2021, 06:06:58 am
content: I need my eyes tested.I see a probe ground spring on the end of the probe and is connected to the -V terminal at the left of the probe tip.

author: MC_cycle
author_rank: Newbie
timestamp: January 27, 2021, 03:19:15 am
content: Quote from: james_s on January 25, 2021, 05:59:11 amIt is a prototype for testing and researching purposes. These tiny wires goes to the cooling fan with rating of 0.6 Amps max. So, overload is completely impossible (only in case of a short circuit inside a fan). And in final product everything will be protected by the automatic circuit breakers and relays, controlled by Arduino. If anything will go wrong - nothing bad will happen. Not the cooling fan, neither a Peltier elements will be operating continiously.Quote from: Ian.M on January 25, 2021, 05:46:29 amThanks for your suggestions. LM7808 is rated for 8 volts. Isn't it too low for the Arduino? AFAIK, the most comfort voltage for external power for the Arduino is around 12 volts (12 to 15). And do I need to replace the Arduino on-board regulator with this schematics, or I can connect output of LN7808 to the Vin pin on the Arduino?Quote from: WattsThat on January 25, 2021, 05:38:16 amWhat is wrong here?

author: julian1
author_rank: Frequent Contributor
timestamp: January 27, 2021, 04:12:16 am
content: Quote from: MC_cycle on January 27, 2021, 03:19:15 amAs WattsThat noted, In (edit) PSU_4.jpg the wiring is crossed. The brown wire (live) is connected to neutral input of the supply terminal. And the blue wire (neutral) is connected to live input of the supply.

author: MC_cycle
author_rank: Newbie
timestamp: January 27, 2021, 05:22:38 am
content: Quote from: julian1 on January 27, 2021, 04:12:16 amIs this such a matter for AC current? Anyway, I could connect a plug into the socket in a random position, and not always live will be live, and neutral will be neutral, even if it was wired properly. I never test a "polarity" before inserting a plug into the socket. If this power supply was mounted inside an junction box and was wired without a plugs and sockets straight to the power lines, then yes. But for the power supply, lying on a workbench, which is used for testing and prototyping purposes, I think that it doesn't matter how wires are connected.

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2021, 06:08:59 am
content: Most of the world has polarized plugs, some parts don't. It shouldn't really matter but it's good practice to use the colors the right way around anyway.