Is this a dirty nasty DC power supply? (Using it on Arduino projects) - Page 1
Forum section: electronics / beginners

author: loiphin
author_rank: Contributor
timestamp: August 04, 2014, 07:11:39 pm
content: Hi all,I have a Ripmax 13.8v DC power supply that I have used in the past for model aircraft battery charging. As it was lying around I thought it would be a good idea to use it as a bench supply for my Arduino's.When I connect it to a homemade Arduino running blink, the LED blinks very erratically. I decided to put the output of the Ripmax supply, via a 7805 voltage reg and two 10uf caps, onto my scope....And this is what I see... (see attachment for scope pic)Should I bin the power supply?? When I use a 12v lead acid battery via the 7805 it runs nice and clean and the scope shows only around 40mv deviation.DP832 time ?Thanks,loiphin

author: David Hess
author_rank: Super Contributor
timestamp: August 04, 2014, 07:46:35 pm
content: The 100 Hz repetition leads me to guess is that that is switching noise getting into the chassis ground when the input rectifier of the power supply is conducting. The oscilloscope shares the same ground so if the output of the power supply is not floating (or even if it is), there is a ground loop through the probe ground.I am inclined to think that the power supply is defective by design lacking common mode RFI filtering on its input. This would not matter when powering a floating load not connected to anything else.I am also not impressed by the Rigol's index graded display but maybe I am just not used to it.

author: loiphin
author_rank: Contributor
timestamp: August 04, 2014, 07:51:18 pm
content: Thanks for the replyI am a newbie in all of this, so dont quite understand it all. However just to add to the original story, I have the power supply plugged into the same power outlet as the scope. And unfortunately the power socket it not grounded ( my crap house wiring), so neither power supply nor scope have an earth. So I don't know if that will make matters worse ?And a link to the power supply....http://www2.ripmax.net/Item.aspx?ItemID=O-IP2000&Category=O0170loiphin.

author: David Hess
author_rank: Super Contributor
timestamp: August 04, 2014, 08:52:27 pm
content: At high frequencies long ground connections become ineffective because of inductance.One thing I would try is changing the position of the probe cable and running my cupped hand along it. If the character of the signal changes on the oscilloscope, then that indicates a high frequency ground loop problem.

author: ozwolf
author_rank: Regular Contributor
timestamp: August 05, 2014, 12:03:13 am
content: What a co-incidence! I've just been doing some work with Arduino Nanos, and blew up 2 of them (over two days) by switching on the AC supply to my 12vdc laptop brick which I had connected to the Nanos using the Vin pin and Ground pins. I thought this was OK as Nanos are supposed to accept up to 12vdc for power supply. Turns out this power supply outputs a raw DC anywhere from 12 vdc with spikes up to 20 vdc with little or no loads. I scoped the output of the power supply to see the horrible waveform. By putting a 47uF electrolytic capacitor on Vin and Ground the supply cleaned up tremendously.Later that week I purchased an old copy of Silicon Chip magazine and they had a great article on regulating the output of wall adaptors. I'm at work right now, so will post some more information tonight.In the meantime, try an electrolytic capacitor of at least 47uF/25v capacitor (watch polarity) on input to 7805 and a 0.1uF ceramic capacitor on output side of 7805. So no, don't bin the power supply (all (most) things are repairable).Together we learn.

author: tautech
author_rank: Super Contributor
timestamp: August 05, 2014, 12:18:50 am
content: QuoteIf the scope shows 40 mV ripple with a pure DC 12V supply, you are doing something very wrong.Is it the way you are using the scope, the regulator oscillating or outside interference?

author: loiphin
author_rank: Contributor
timestamp: August 05, 2014, 05:20:53 am
content: Quote from: tautech on August 05, 2014, 12:18:50 amUmmm good question. I am such a newbie I don't know what I am doing wrong? I am using the scope with DC coupling.

author: loiphin
author_rank: Contributor
timestamp: August 05, 2014, 05:38:43 am
content: Quote from: ozwolf on August 05, 2014, 12:03:13 amI already have a 10uf electrolytic on the input side of the 7805 (and the pic from the scope is through the input 10uf cap, the 7805 and the output 10uf cap). Would a 47uf really make a big difference? I can give it a try this evening.

author: mij59
author_rank: Frequent Contributor
timestamp: August 05, 2014, 05:52:25 am
content: Quote from: loiphin on August 04, 2014, 07:11:39 pmNo, just don't use it to power electronics, its probably are very noisy smps.What is the voltage of the battery?Was the minus pole of the power supply still connected to the battery?

author: loiphin
author_rank: Contributor
timestamp: August 05, 2014, 05:54:07 am
content: Quote from: mij59 on August 05, 2014, 05:52:25 am

author: tautech
author_rank: Super Contributor
timestamp: August 05, 2014, 05:58:21 am
content: Quote from: loiphin on August 05, 2014, 05:20:53 amIf you want to measure ripple with a scope, set the Ch input to AC coupling to remove the DC component of any waveform that may be present.For the PSU, you may have to add 1000 uF or more before the regulator to get ripple low.If ripple is from the mains aLinetrigger setting will produce a stable waveform.

author: ozwolf
author_rank: Regular Contributor
timestamp: August 05, 2014, 10:57:21 am
content: Quote from: loiphin on August 05, 2014, 05:38:43 amTry this linkhttp://tronixstuff.com/2010/06/16/lets-make-a-cheap-5v-power-supply/just drop the bridge rectifier as your supply is DC.You haven't told us which Arduino board you are powering and are you connecting to the 2.1mm power plug? If so, 5 vdc is probably not enough for proper output on the PWM outputs. I use 9 vdc for reliable operation.

author: loiphin
author_rank: Contributor
timestamp: August 05, 2014, 11:05:41 am
content: Quote from: ozwolf on August 05, 2014, 10:57:21 amI have built a barebones arduino on a breadboard. Just the ATMEGA328P, some leds, a crystal, some 22pf caps for the crystal..... And the 7805 and two 10uf caps are acting as the 5v powersupply... no different from the tronixstuff.com.My DC 13.8V supply appears to be the sh!tty component in all of this, which is to supply 13.8V to the 7805 regulator.loiphin.

author: mij59
author_rank: Frequent Contributor
timestamp: August 05, 2014, 11:18:43 am
content: Try this linkhttp://tronixstuff.com/2010/06/16/lets-make-a-cheap-5v-power-supply/just drop the bridge rectifier as your supply is DC.Low budget without heat sink, very cheap.

author: loiphin
author_rank: Contributor
timestamp: August 05, 2014, 11:19:56 am
content: Quote from: mij59 on August 05, 2014, 11:18:43 amRead above!

author: ozwolf
author_rank: Regular Contributor
timestamp: August 05, 2014, 11:45:43 am
content: Quote from: loiphin on August 05, 2014, 11:05:41 amI'm almost sure the problem is with your 10uf caps, try the 470uf/25v on input side and 0.1uf/75v on output side. It will make a difference. A heat sink is a good idea.

author: loiphin
author_rank: Contributor
timestamp: August 05, 2014, 11:50:55 am
content: Quote470uf or 47uf? You mentioned 47uf earlier...

author: ozwolf
author_rank: Regular Contributor
timestamp: August 05, 2014, 12:39:54 pm
content: Yes, I did mention 47uf, as my Freetronics "Eleven" board (see attached circuit diagram - power supply) uses those. However, the link to tronixstuff uses 470uf, and tautech mentions up to 1000uf to eliminate input ripple. When you look at some other circuit diagrams such as Silicon Chip February 2014 issue page 46. they use a 1000uf/25vdc capacitor. I believe the variance is due to the supply conditions, and yours isn't great, but fixable.

author: loiphin
author_rank: Contributor
timestamp: August 05, 2014, 06:43:58 pm
content: So I added 440uf worth of caps on the input and it made no difference to the interference.But I did find something else out. If I switch the 13.8V DC supply on and connect either the +ve or the -ve lead (just the one lead connected) to the input side of the 7805, I see the same nasty interference on the output of the 7805.loiphin.

author: ozwolf
author_rank: Regular Contributor
timestamp: August 05, 2014, 08:31:57 pm
content: Really sounds like a break in the ground circuit then. Whilst you may have already done this, I'd recheck the layout on the breadboard. Perhaps one of the connections is out by one pin (I've done that myself before).

author: ozwolf
author_rank: Regular Contributor
timestamp: August 05, 2014, 11:25:03 pm
content: Quote from: loiphin on August 05, 2014, 06:43:58 pmHere's a really easy test to confirm your circuit is working. Get a 9vdc battery and connect to the input of the 7805. If the circuit works as expected, then we'll continue working on filters to clean up the 13.8V DC supply. If the circuit doesn't work on 9 vdc battery, then fix that first, then re-try the supply.(edit) - Woops, I just noticed that you mentioned this in the first note. Oh well.