Cheap lab power supply : safety and how to lower the noise ? - Page 1
Forum section: electronics / rf-microwave

author: jujun
author_rank: Regular Contributor
timestamp: December 21, 2019, 07:08:57 pm
content: Hello,I bought a cheap lab power supply that can output up to 60v, to be able to build RF LDMOS Power amplifier (they need 50V).The power supply does the job, but I feel that it worth to spend a bit of time on it to improve it.This is this power supply : aliexpress.com/item/4000114318916.htmlI noticed something strange, If I connect the ground of the power supply to the ground of my transmitter that is powered by another lab power supply (A linear one), then the current of the linear power supply change constantly, +- 20mA. I don't understand why. Even if the Chinese power supply is not powered.If the power supply is powered and the gound is connected to my transmitter, I receive a lot of noise. It seems to be the hashing frequency of the power supply with a ton of harmonics. Up to 28Mhz !My guess would be to filter more the output, with a power filter choke, maybe a common mode ? I can also add good quality capacitor to the output. What do you think about it ?About the leak from the ground, I don't really know what I can do.The screen shots show the noise from the power supply. The supply is powered on, and then off.Have you got any pointers ?Thank you

author: bob91343
author_rank: Super Contributor
timestamp: December 21, 2019, 07:31:40 pm
content: Take a look at the primary wiring of the unit. I suspect a fault causing spurious paths to the power line.Often there are components in the primary circuit to assure safety but they need to be functioning parts. Further, there is a possibility of an internal short in the primary.Some gear has low power circuits that are on all the time. Check for that as well. If the power on switch is a momentary button, that's a giveaway.I didn't investigate closely so my comments are very general.

author: madires
author_rank: Super Contributor
timestamp: December 21, 2019, 07:48:29 pm
content: Your lab PSU is a SMPSU.

author: TheMG
author_rank: Frequent Contributor
timestamp: December 21, 2019, 08:51:17 pm
content: First things first, have you opened up the power supply and examined the circuitry, particularly the input and output EMI filtering sections? Quite often with these cheap SMPS not much attention is given in the design toward EMC, in fact sometimes the filtering on the primary side is sometimes completely omitted for cost saving, and the secondary side is often bare minimum to provide a usable output ripple.Silencing a SMPS that is not very well designed to begin with, can potentially be a major headache.This is why I prefer to source surplus/used telecom/IT power supplies (they are usually 48V nominal), instead of cheap overseas power supplies of unknown design which may have questionable reliability and EMC. Ebay is a great source for such things. Recently I picked up a 48V 42A (2kW) unit for a mere $150 for a LDMOS amplifier I'm putting together. Nice thing is since these power supplies are designed and used for the telecom industry, they are very robust, reliable, and well designed, and they don't skimp on EMC. They are also rated for continuous duty.

author: jujun
author_rank: Regular Contributor
timestamp: December 21, 2019, 09:29:42 pm
content: Thank you all, I also have a cheap surplus 42V 2000W power supply, but it's not handy to prototype, I need to have current limiting.The inside looks quite good, and not too cheap.The power switch cut the Line pole.What should I measure ?I also noticed that 2 caps from the power output to earth that are missing. Top left corner.

author: German_EE
author_rank: Super Contributor
timestamp: December 22, 2019, 09:55:12 am
content: 1. Ground everything to one common point including all of your PSU ground connections and all of your negative outputs. In the old days this point used to be the negative connection of a big-ass capacitor but these days with switched mode devices you will need to adapt.2. Check for filtering circuits on the mains input of each supply and if it doesn't have any filtering then fit it using a filtered power socket something like this.https://www.interpower.com/cgi-bin/ic.cgi/p35list2.p?filter_check=X&inlet_check=X3. Fit ferrite beads everywhere including on the gate/base of each power device. If you can wind each power lead through a ferrite ring. The goal here is to stop RF getting into the supply and, at the same time, stop switching hash getting out.4. Fit 100n bypass capacitors everywhere on the LV side including across each supply output and from each output connection to chassis.That should lower noise and interference to reasonable levels, if it doesn't then you will need to look at additional filtering on the LV side.

author: radiolistener
author_rank: Super Contributor
timestamp: December 23, 2019, 06:02:08 pm
content: this is switching power supply and very bad designed. It has high noise and you cannot solve it, so be prepared that your transmission signal will be noisyif you're want to get low noise power supply, then look for linear power supply instead of switching power supply.

author: Georg - PY5ZSE
author_rank: Contributor
timestamp: January 12, 2020, 08:32:24 pm
content: On your picture of the PS I see no EMI suppressing components at all. Not on the primary side - there should be 2 chokes and a couple of capacitors - nor on the secondary. What I see there is the storage inductivity (big toroid on the upper left) and another inductivity (upper right) to suppress ripple, on the PSU's switching frequency of most likely a few tens of kHz.. This, however, is not what causes the interferences, they result from high order harmonics created when in the moment of switching current is taken over rapidly from one part of the circuit to another (eg, transistor on/off, current goes from transformer secondary to Diode). Well designed SPSU's have components that limit dI/dt in critical components. I doubt that in your PS any of that exists.The two chokes mentioned above don't behave as inductiors above a few MHz. Their parallel resonant frequency is probably around 1 and 5 MHz. Above that, they resemble capacities.I second German_EE's advices. As it looks like current sense shunts are in the negative output of the PS's circuit, I would use the point where the negative (black) output lead enters the PC board as common ground. On each output lead, put a long (2 cm or the like) ferrite bead, and then connect the 2 leads with a good multilayer ceramic 100nF capacitor. from the capacitor go on to the output terminals.Is the PS installed in a metal cabinet? If not, put it there, and ground it.From the description of events when you connect the PS (20 mA, and interference begins as soon as you switch the SMPSU on, even if your receiver stays on the other PS) it seems that you have a ground loop. These are unfortunately easy to get, as (nearly) everything is "grounded" via the PE lead in it's power cord. If you then connect signal grounds , (or DC power return, in this case) of two boxes together, you build a loop, in which stray currents can circulate.To test for that is easy: Deliberately connect the SMPSU to mains with a power cord without a PE wire, after connecting its negative output line to your Transceiver's ground terminal. Switch the PS on. If the interference is gone, or much lower than before, ypou have a valuable hint. CAUTION: This experiment violates safety regulations, do it on your own risk!. If it works (the interference goes away) build yourself a high - impedance power cord: wound a couple of turns through a big ferrite toroid.Hope that helps...

author: tkamiya
author_rank: Super Contributor
timestamp: January 20, 2020, 06:03:53 am
content: I just bought something very similar myself. If I were you, just ditch it and start with something a bit more known and reliable. Yes, this is a switching power supply, and the noise on mine was 150mV (300mV p-p). I'm keeping mine but I won't be using it as a lab supply. Just too dirty. When you say ground, do you mean real ground (chassis) or black (negative) terminal? I don't see any ground on the picture you linked to. If you have a reading, there is a return path somewhere or it is picking up RF.Obviously, you aren't going to power your high-power linear with this little thing.... If testing is the only purpose, how about a fixed output open frame power supply? Most of them have some kind of current limiting and often available for cheaply.I've seen some SMP type power supplies that goes into oscillation when certain value of LC circuit is connected to output port. I was playing with adding beads and low pass filters then. It destroyed my (cheap) distribution amplifier. So be careful as well. FULL ON oscillation has enough power to kill what's attached to it.

author: jujun
author_rank: Regular Contributor
timestamp: February 05, 2020, 06:44:07 pm
content: It seems that there is a great explantation and a simple solution here :