Just Want To Get off The Ground Power Supply

Thread Starter

The Floating Brain

Joined Mar 7, 2018
16
Hello, second post, sorry its so quick after the first.

I really don't know much about what I am doing, I am new at this, and I really just want to get my project off the ground.
I have been struggling for about a week. I want to hook up some SBC's to wall power and give it a consistent 3.3VDC.
I have a wall wart which seems to output about 5VDC that but it also seems to output 10 ~ 13 VAC. I tested a battery (restarting the DMM) and
I get ~3.3 VDC as well as 5 ~ 6 VAC. I'm very confused, my control of 2 AA batteries should not output any VAC. Should I ignore this?
I don't understand why I just cant just hook up some resistors to my wall wart and plug it directly into my SBCs. I have an old
computer PSU, to get things going, would it just be easier to do that? I read somewhere on the EE stack exchange that
some old volt meters can output double VAC when working with DC which seems somewhat approximate to what I am experiencing
(my DMM is a CEN - TECH 98025 https://www.harborfreight.com/7-function-multimeter-98025.html ).

Any help to this n00b is very appreciated >_<
 

dendad

Joined Feb 20, 2016
4,426
How do you mean it outputs 5VDC and 10...ACV?
Are you just measuring it on the DC and then AC range?

Don't run your SBC until you have a good power supply. working to avoid smoke.
More details....
What is the SBC you are wanting to use?
The current needed?
What is the power supply?
Can you post a picture?
Details, details! It is really hard to help with such vague questions.
 

Thread Starter

The Floating Brain

Joined Mar 7, 2018
16
Thank you for the reply! Yes I am measuring it in the dc then AC range.

SBC: Onion Omega 2
Current: 500 ~ 2000 mA I think should be fine
Power Supply: Control: 2 AA batteries in series, One I am trying to use: Wall Wart (see image below)



Sorry about that Il try to learn what types of info people in EE ask for :)
 

spinnaker

Joined Oct 29, 2009
7,830
Thank you for the reply! Yes I am measuring it in the dc then AC range.

SBC: Onion Omega 2
Current: 500 ~ 2000 mA I think should be fine
Power Supply: Control: 2 AA batteries in series, One I am trying to use: Wall Wart (see image below)



Sorry about that Il try to learn what types of info people in EE ask for :)

So why are you trying to measure AC on a DC volt output?

And you are trying to power an AC wallwart with 2 AA batteries???
 

Thread Starter

The Floating Brain

Joined Mar 7, 2018
16
No, the AA's are just a "control" in my test, they measure ~3.3 VDC out. But they are also registering some VAC as well. I was testing both VDC and VAC on the wall wart to be sure it wasent giving off any VAC so I wouldn't damage the SBC, especially since this one in particular say just OUTPUT: 5V @ 1000 mA, doesn't necessarily specify VDC or VAC, but since I see VDC on most wall warts I assume it means VDC, but it registers both.
 

dendad

Joined Feb 20, 2016
4,426
It appears the multimeter is not a really good design. it should block DC on the AC range but yours does not.
So just ignore the AC reading. If it is giving you a reading of AC on the batteries, I think you need to find a new meter. Or at least be aware of that failing.
To get 3.3V from the 5V supply, the easy, but not the best way is to add a couple of 1n4004 diodes in line. That will drop the volts down to a bit over 3.3V.
Better is to use a regulator.
What is the board you are trying to use?
 
Last edited:

Thread Starter

The Floating Brain

Joined Mar 7, 2018
16
Thank you both for your replies! I will probably look into getting a new multimeter, but for the time being is there a way I can tell if there is AC voltage?
It appears the multimeter is not a really good design. it should block DC on the AC range but yours does not.
So just ignore the AC reading. If it is giving you a reading of AC on the batteries, I think you need to find a new meter. Or at least be aware of that failing.
To get 3.3V from the 5V supply, the easy, but not the best way is to add a couple of 1n4004 diodes in line. That will drop the volts down to a bit over 3.3V.
Better is to use a regulator.
What is the board you are trying to use?
Thank you, I may open a separate thread if my rectifier gives me too much trouble but I am working on one (however I am quickly leaning toward the second suggestion). Just for knowledge sake, if I were to have a device that I wanted to have ~5 VDC or above, and it were sensitive to VAC and would produce magic smoke if I gave it VAC, could I plug my wall wart directly into it without damaging the component? What I am trying to understand is: is this effectively equal to a battery, outputting DC voltage, except with a more unstable/unpredictable voltage?
 

dendad

Joined Feb 20, 2016
4,426
Hook a series capacitor between the multimeter and the supply.
A 100nF cap may work as you are not really after an accurate reading but just an indication.
Put your meter on ACV and see if it reads.
Try on the batteries and you should see next to nothing after the initial charging kick.
Batteries do not output AC.
Then see what the power supply reads. I would expect a small reading of a couple of hundred mV may be seen, depending on the quality of the power supply.
If your device is rated at 5V, do not feed it anything else!
You do not need to open a new thread, I think just stick with this one.
Do you know what a regulator is and how to hook one up?
And you still have not told us what the load is in any detail.
 

Thread Starter

The Floating Brain

Joined Mar 7, 2018
16


The capacitor with the lowest capacitance that I could immediately find I had was 47 Micro Farads. I hooked it up to both power sources like shown and measured the VDC and ACV across, I got the same readings as without unfortunately.
Measurement on batteries:
~3 VDC
~5.6 AVC
Measurement on Wall Wart:
~5 VDC
~10 AVC

The 5VDC device was a hypothetical, what I need to power is a few Onion Omega 2 SBC's which take 3.3VDC and 500 mA. Just found this https://docs.onion.io/omega2-docs/hardware-prep-no-dock.html In any case I just want to make sure I have no ACV going into the board before I hook it up. I know the batteries shouldent output ACV, but I want to be able to hook these up to wall power.

Edit:

I do think I understand the concept of a voltage regulator and how to hook it up though I have never done it. Then again with this stuff, the concept seems simple but in reality I cant seem to get it to work a lot :(
 
Last edited:

dendad

Joined Feb 20, 2016
4,426
No, try the cap in series with the meter lead. one end of the cap to the + battery pin, the other end of the cap to the meter + lead. And the - battery to the - meter lead.
MM.jpg


Not the cap across the battery and meter as you have shown.
 
Last edited:

dendad

Joined Feb 20, 2016
4,426

Thread Starter

The Floating Brain

Joined Mar 7, 2018
16
Thank you!!! Readings are as expected
The DC Voltage goes down at what I am guessing is a logarithmic rate for both

AA Battaries:
VDC: 1.5
ACV: 0.0

Wall Wart:
VCD: ~5
ACV: ~5 initially then quickly down to < ~.5


To be clear (as I know you said this method is approximate), in reality, is there any ACV coming from the wall wart?

Thank you for the product links!
I have already ordered some of these https://www.digikey.com/product-detail/en/LD1117AV33/497-1485-5-ND/586006?utm_medium=email&utm_source=oce&utm_campaign=1043_OCE18RT&utm_content=productdetail_US&utm_cid=432726&so=54026158&mkt_tok=eyJpIjoiWWpGaE1XVTROR1V3TVdJMSIsInQiOiJ2VHBoT3hZWFFiY0FNa01CWVczUVM4RlNIK3FpUmJRU2NNdEVqV3Z1blArNzVKUGZ2MWdyZElsc1VlRnZFaFZyKzBaSjczVFQzXC9XWGhmdjZsVW9ZSElkSFRTV01PcEdFN2dGOUc5MlhyeGU5Z01TSytIMThQa0R4U2VaakVvVFoifQ== should they surfice?
 

dendad

Joined Feb 20, 2016
4,426
To be clear (as I know you said this method is approximate), in reality, is there any ACV coming from the wall wart?
Yes, there will be ripple, that is , an AC component from the wall wart and that is normal. Better supplies have less ripple and linear regulators are best. But for a lot of uses, a 100mV or so ripple is ok. If you are using your computer board to measure analog signals, it is best to have a good supply, at least for the analog converter reference, and than either the processor inbuilt ref is used or an external very good quality linear reg is used.
Have a search for some info on switch mode and linear power supplies.
I like this site...
http://www.learnabout-electronics.org/
 

dendad

Joined Feb 20, 2016
4,426
The LD1117 3v3 reg will be ok. Just ensure the connections are correct. They have different pinouts to then78xx series regs.
And make sure the tab is insulated from the heat sink as it is in fact the output volts. Also, you will need the caps shown in the application circuits and have them close to the reg. This is to ensure stability of the reg .
Measure the voltages before you hook them up to the load. It is often a good idea to add an LED to the output as an indicator light. Try an LED and a 120ohm series resistor on the 3.3V output. Give a yell if you need more help.
 
Top