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Author Topic: Variable Power Supply - Questions  (Read 154 times)

Online Rusty_S85

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Variable Power Supply - Questions
« on: November 27, 2017, 12:47:06 pm »
I have been a long time follower of TrekWorks and I know he uses a Variable Power Supply for testing his LED circuits.  For me having an automotive background I could use one for building custom circuits so it is more justifiable for me and not just a tool id use once or twice and that's it.

With that said how ever I had a few questions about them.

First off is there anything a first time user should know before purchasing or using one?

Second is it possible to use this as a tool to find out what voltage/current you need to obtain a brightness you want on a LED?

Third can it be used on a completed light circuit to find out what your minimal current requirements are to properly size a power supply for your model?

Fourth is about the actual unit I found.  It is $67 on amazon and it appears to be the same style if not the same one that has been shown briefly on TrekWorks?

https://www.amazon.com/dp/B00SWK6M0M/_encoding=UTF8?coliid=I2JR9ZNKS4NJV8&colid=2D0FT1SY2DG1Q

I probably will have other questions that I will probably think of later on but just trying to get an idea of what to expect for this as I got a few models I want to light up and this would make it easier on me I think to run the circuit for 48 hours to test for failiures as well as to help me build circuits without having to pick a power supply first then build the light circuit to the power supply.

Offline LynnInDenver

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Re: Variable Power Supply - Questions
« Reply #1 on: November 27, 2017, 07:06:55 pm »
First, test with something that you don't care about losing in terms of if you can power it up with the circuit attached, or if you need to turn it on and THEN attach the circuit; I destroyed an Atmel chip by doing the former with my (admittedly vintage analog) bench power supply, as it spikes hard on power up.

Second, in constant voltage mode, it should tell you what current is being drawn. Voltage is what can cause an LED to vary in brightness.

Third, yes, it can indeed give you a reasonable idea of how much current is required to drive your completed circuit. Note: Always buy at least 20% ABOVE that for the permanent power for the project.

Online Rusty_S85

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Re: Variable Power Supply - Questions
« Reply #2 on: November 27, 2017, 09:07:40 pm »
First, test with something that you don't care about losing in terms of if you can power it up with the circuit attached, or if you need to turn it on and THEN attach the circuit; I destroyed an Atmel chip by doing the former with my (admittedly vintage analog) bench power supply, as it spikes hard on power up.

Second, in constant voltage mode, it should tell you what current is being drawn. Voltage is what can cause an LED to vary in brightness.

Third, yes, it can indeed give you a reasonable idea of how much current is required to drive your completed circuit. Note: Always buy at least 20% ABOVE that for the permanent power for the project.

Thanks for the reply.  I wonder if the power supply will spike hard on power up even if you have everything switched to low.  Guess its not a big deal as could always switch it on then connect.  Not like its going to be bad to make the connection after powerup.

So without seeing actual instructions I take it if I switch the voltage to 9 volts for example then that is in constant voltage mode and would show what current is being drawn?  I was thinking the current knob would have to be fine tuned along with the voltage to get a balance.

Offline Decoman

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Re: Variable Power Supply - Questions
« Reply #3 on: December 01, 2017, 06:20:45 am »
As interesting as it might be to buy a decent power supply, the thought of spending that much money makes me wonder if i should buy something else instead. Because the cheap stuff seem scary to me.

There are some worrisome aspects to buying a cheaper variable power supply. The good stuff is probably tested, and tested to be safe I am sure. The cheap stuff, probably isn't tested by independent parties.

In a news article recently, it was pointed out that something like 90% of the electronics could in worst case lead to electrocution in some cases where things to wrong, because the things weren't built to be safe as I understood it, also because these are cheap items, things that connect into the power socket at home. I forgot what products was mentioned. I think it was a mix of usb chargers and whatnot.

I find all of this somewhat depressing, and it would be categorically dangerous opening a unit, even with the power off, because of how power is stored in some of the components, which aren't normally discharged after use, or after plugging out the unit from the power grid. I know too little about this, but one would be wise to not award yourself the proverbial Darwin award, nor to anyone in your family, if they end up using it without knowing the risks.

Having watched youtube videos of some of the cheaper variable power supplies, there seem to be a variety of issues: parts misplaced and found loose inside the unit possibly causing a short circuit at some point, unit outputting power even when not indicating this, unit outputting a lot more power than what is shown on the display, voltage spike on power up (and maybe when using stored presets), badly assembled units with stuff left loose inside the unit, one youtube apparently had a unit where the power on/off stopped working and the unit remained outputting power, and probably other stuff as well i forgot about.

I think if I go buy a discount power supply, I will have to take a look inside an unpowered unit without touching anything, just to see how good or bad the unit was assembled.

I don't know enough about this topic, so i cannot recommend any variable power supply model at all. Presumably quality brand names would be a lot more trustworthy, and the cheap stuff won't be trustworthy at all.


I will probably buy a variable power supply, but I haven't made up my mind on any model.

Btw, here's a fun youtube channel for electronics stuff, like electronics equipment: EEVblog
https://www.youtube.com/user/EEVblog/videos
« Last Edit: December 01, 2017, 07:53:34 am by Decoman »

Offline Decoman

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Re: Variable Power Supply - Questions
« Reply #4 on: December 01, 2017, 11:34:18 am »
The only new variable power supply that I have come across that doesn't cost a fortune, that apparently is a well known brand (I think), is the

Rigol DP711
http://int.rigol.com/Product/Model/85/487 (product home page afaik) (30V/5A)

Note; There is also a DP712  model that looks the same (50V/3A).

Heh, I don't like the looks of the Rigol power supply though personally.

This one seem to cost about three times as much as the cheaper stuff.

Hm, this Rigol product is labeled EAC on the back side. EAC seem to refer to:
https://en.wikipedia.org/wiki/Eurasian_Conformity_mark
« Last Edit: December 01, 2017, 11:56:51 am by Decoman »

Offline Tankton

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Re: Variable Power Supply - Questions
« Reply #5 on: December 01, 2017, 02:21:58 pm »
Live Long and Prosper

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Offline Markeno

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Re: Variable Power Supply - Questions
« Reply #6 on: December 01, 2017, 11:15:09 pm »
First let me say, yes there is something to "cheap" electronic gear very possibly and maybe likely to "not be safe".

----

My suggestion is the best thing to have would be a powersupply that lets you set the Voltage and Current.  Many variable powersupplies will only have a Voltage adjustment and no Current adjustment.  Although they will have a Current "readout" on the display telling you how much current is "currently" flowing though the circuit.

In the past I used a Variable Powersupply that I built as a school project 20 odd years ago.  I still works just fine.  It does have a Voltage and Current readout, but it lacks a constant current adjustment.  Even at that it was very useful, it was trickier to use.  To check/verify the voltage that an LED is pulling the listed 20mA, I just slowly turn the voltage up.  Once the Current readout hit 20mA, I knew that what the Voltage listed was the typical voltage of that LED.   Btw, I suggest driving LEDs in use at like 10mA, maybe some at 5mA or less in some cases, you don't "have to" drive them at 20mA, it just means you won't damage them if you drive them that hard...


I now have a Variable Powersupply with both Voltage and Current limit/set adjustments.  This is a bit easier to use, provided you keep the Current limit set at a safe level.  Like Lynn mentioned some types of power supplies can have a "power on Surge" that can damage or destroy your circuits, "vintage" but maybe something a bit newer in some cases, I wouldn't expect a new modern unit to though.  Typically I will turn on the power supply without the load attached.  After setting my wanted Voltage level (if I know what that is already), then I attach my load.  If my perticular powe supply finds the load is drawing more Current than the Current setting is set to allow, it automatically dials the voltage down to where the current won't go over the setting.   So if you used such a power supply, and you set it for 20mA and attach an LED across it, the voltage would set itself and read out at it's Typical Voltage drop at 20mA.  Just a note that LEDs typical voltage at 20mA will vary a bt .1.2 volts or something one way or the other for a batch.  You can use one of those methods to check the typical voltage of a number of LEDs in a batch if you are trying to match them really closely for some reason.

With either of my power supplies, I can read on them what Current I am using to know what my power draw is of a completed circuit or the individual portions I have powered at the time.   The Current Limited power supply can be nice as well, by setting it to 20mA or slowly ramping it up when powering a full circuit to see where the current goes up to.  If I hook a completed circuit to my non current limited power supply it can try to draw 5+Amps instantly and that much current will destroy the circuit if something is wrong.  Say you have a short across a positive and negative rail or something, it is just going to blow something up, but if you do that and have it limited to 20mA, you wouldn't likely hurt a thing and just have to look it over to find why it isn't working or where your short is or where you have something backwards.

The current limited supply I have is very cheap, I wouldn't suggest getting it.  It was a diy kit, and has poor fine control on the adjustment, and a latency between "releasing" the "current limit" and "reengaging" the "current limit".  Say I have a loose main power lead to my circuit, and I bump it, it quickly looses connection then reestablishes it.  In that case when it looses connection, if I happened to have the voltage set at 12volts, but I have a 2volt LED attached and it set to 20mA.. the loss of the load on it let the voltage go back to 12volts from 2volts... the LED gets quickly reattached and is blown instantly before the current is drawn down to 2volts again.  I had that happen a number of times (maybe 3 or so) until I realized what was going wrong, it is a bit like the "power on" surge Lynn mentioned.  It is when the regulator is not yet "regulating" the power.

Personally I like a power supply with at "Load switch".  This lets me disconnect the power from the circuit without pulling the leads either from the power supply or from the circuit itself. It is a switch that disconnects the power output jacks when it is thrown.

Also when I shut off my power supply, I will either turn the voltage and current to minimum or a very low setting.  I also typically disconnect the power leads from the power supply.  I don't leave my equipment plugged in to electric when not in use either (or have them turned of by turning off the power strip at least).  For one unplugging or otherwise cutting power to the unit saves electric ("Turned Off" is rarely a true "off" condition in today's push button electronics, they still usually have power going through part of them, but also a bit more importantly, in the case of a fault or failure in the unit they won't become a safety or fire hazard.  Also in the case of a power surge or lightning strike they won't be damaged or become a fire hazard if they are unplugged (a surge protector or power strip that is plugged in can still be damaged by a lightning strike etc).
« Last Edit: December 01, 2017, 11:18:33 pm by Markeno »

Offline Decoman

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Re: Variable Power Supply - Questions
« Reply #7 on: December 02, 2017, 09:32:40 am »
I ordered a eh variable power supply. I am conflicted about saying anything about it, as I am not qualified to evaluate it.

I can say that, once I get it, which might takes a couple of weeks, I will have to:

* gently shake the unit to listen to any rattle (or just carefully flip it over, and anything loose will probably make some noise inside)
* never power up the unit, until I get to check it out first
* not touching ANY of the components, even though the unit wouldn't be plugged into the power grid
* test the unit with a multimeter for very basic functionality, to make sure it doesn't output more voltage that the unit should, and that it doesn't output voltage when it is not supposed to

Heh, hopefully I won't end up killing myself doing this. I should be ok if I am very careful I think.
I will take some photos of the inside of the unit and post those on this electronics forum I joined recently (EEVblog forums), for others to see, in case there are upgrades or something obviously faulty or weird with the build quality.

Offline Markeno

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Re: Variable Power Supply - Questions
« Reply #8 on: December 02, 2017, 05:19:58 pm »
Here is a video "review" of essentially the powers supply that Rusty was asking about.

https://www.youtube.com/watch?v=bpRN-MyxBEw

Watching it, I would say it looks pretty decent for what usage I would expect for hobby usage.  The only real fault I see in it is the resolution of the Current readout as shown at the end of the video.   For LED work, it would be best for a .001 Amp resolution, and this unit is a .1 Amp resolution.  So the lowest it can display is .1Amp or 100mA, which is 5 times at minimum too much to be of any great use. 

My older power supply that I built has a .01A or 10mA resolution on it's Current meter reading.  My new unit has a .001A or 1mA resolution and is much more useful to me in hobby lighting work.  if you look at some other power supplies you may find one with a .01 or .001 resolution for Current, and that would likely prove to be more useful.

I also took note of he referenced these cheaper power supplys having a power on surge.  Which may be the case on some I guess, maybe worse on some than others depending on how well they are designed.


Online Rusty_S85

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Re: Variable Power Supply - Questions
« Reply #9 on: December 02, 2017, 08:10:39 pm »
For testing low power led circuits and possibly arduino projects that power supply is overkill.

This is  the one I have.  https://www.amazon.com/VARIABLE-average-ratings-recommend-quality/dp/B01E5RJ63U/ref=sr_1_10?ie=UTF8&qid=1512159548&sr=8-10&keywords=2+amp+regulated+variable+power+supply

If you want control over current this is the other one I have.   https://www.amazon.com/Eventek-KPS305D-Adjustable-Switching-Regulated/dp/B071RNT1CD/ref=sr_1_1?ie=UTF8&qid=1512159548&sr=8-1&keywords=2+amp+regulated+variable+power+supply

Both are simple and safe.

Well I looked at it for being able to build 12V DC circuits for automotive use as well.  If I bought a power supply for just building 9V LED light circuits for models I couldn't justify spending more than $20 for a variable power supply unit.

Offline Markeno

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Re: Variable Power Supply - Questions
« Reply #10 on: December 02, 2017, 10:48:05 pm »
The one u were looking at would be good, short of if you could find one with a more accurate current reading being more useful.

There seems to be a newer version that appears to have a .001 based on the picture of the display.  Might be good for a review video on it and see if it does if you are interested in it. 

https://www.amazon.com/Tekpower-TP3005T-Variable-Linear-Supply/dp/B00ZBCLJSY/ref=pd_sbs_60_1?_encoding=UTF8&pd_rd_i=B00ZBCLJSY&pd_rd_r=3RFP3BQQPXA529AYPQE2&pd_rd_w=Tpwby&pd_rd_wg=KWrdo&psc=1&refRID=3RFP3BQQPXA529AYPQE2

I looked at a video on the above unit and it looked far better inside than I expected for the money.  If I didn't already have what I needed with mine, I would certainly consider it.  I don't know if you want more like the 10amp capable units, with those keep an eye out on the Amps display as you loose a level of precision since they are 10.xx amp displays and not 5.xxx displays for a 4 digit display.

---
Alternately something like these potentially.

UniT UTP305D (not the "UTP305" without the "D", it only has "0.00" 10mA precision on the Amps display)
https://www.banggood.com/UNI-T-UTP305D-Precision-Variable-Adjustable-DC-Power-Supply-Digital-Regulated-Switching-Power-Supply-p-1046115.html?rmmds=search&cur_warehouse=CN

These Gophert units have 0.000 precision on the Amps readout, as well as Constant Current and Constant Voltage and appear to have a "Lock" mode so you don't accidentally adjust them when you don't want to.  Although they seem to mostly be out of stock.  There are a fair number of them with different voltages and Amp capabilities.  All of that if you are up to doing business with Banggood as I didn't find these ones anywhere else. They look pretty sharp, but maybe they aren't.  I have purchased quite a bit from Banggood and generally been happy with the equipment and supplies knowing roughly what to expect and what I expect to get for the low priced equipment there.  I have never purchased an item from them that cost as much as these supplies.
https://www.banggood.com/CPS-3205C-0-32V-0-5A-Portable-Adjustable-DC-Power-Supply-180-264V-p-974495.html?rmmds=search&cur_warehouse=CN

So there are some potentially good options out there.  Still I would suggest looking for youtube reviews by people that have had hands on the specific units and to see how they seemed to work and possibly if they look to be constructed well or not.
« Last Edit: December 03, 2017, 08:47:35 pm by Markeno »

Online Rusty_S85

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Re: Variable Power Supply - Questions
« Reply #11 on: December 06, 2017, 09:22:01 pm »
The one u were looking at would be good, short of if you could find one with a more accurate current reading being more useful.

There seems to be a newer version that appears to have a .001 based on the picture of the display.  Might be good for a review video on it and see if it does if you are interested in it. 

https://www.amazon.com/Tekpower-TP3005T-Variable-Linear-Supply/dp/B00ZBCLJSY/ref=pd_sbs_60_1?_encoding=UTF8&pd_rd_i=B00ZBCLJSY&pd_rd_r=3RFP3BQQPXA529AYPQE2&pd_rd_w=Tpwby&pd_rd_wg=KWrdo&psc=1&refRID=3RFP3BQQPXA529AYPQE2

I looked at a video on the above unit and it looked far better inside than I expected for the money.  If I didn't already have what I needed with mine, I would certainly consider it.  I don't know if you want more like the 10amp capable units, with those keep an eye out on the Amps display as you loose a level of precision since they are 10.xx amp displays and not 5.xxx displays for a 4 digit display.

---
Alternately something like these potentially.

UniT UTP305D (not the "UTP305" without the "D", it only has "0.00" 10mA precision on the Amps display)
https://www.banggood.com/UNI-T-UTP305D-Precision-Variable-Adjustable-DC-Power-Supply-Digital-Regulated-Switching-Power-Supply-p-1046115.html?rmmds=search&cur_warehouse=CN

These Gophert units have 0.000 precision on the Amps readout, as well as Constant Current and Constant Voltage and appear to have a "Lock" mode so you don't accidentally adjust them when you don't want to.  Although they seem to mostly be out of stock.  There are a fair number of them with different voltages and Amp capabilities.  All of that if you are up to doing business with Banggood as I didn't find these ones anywhere else. They look pretty sharp, but maybe they aren't.  I have purchased quite a bit from Banggood and generally been happy with the equipment and supplies knowing roughly what to expect and what I expect to get for the low priced equipment there.  I have never purchased an item from them that cost as much as these supplies.
https://www.banggood.com/CPS-3205C-0-32V-0-5A-Portable-Adjustable-DC-Power-Supply-180-264V-p-974495.html?rmmds=search&cur_warehouse=CN

So there are some potentially good options out there.  Still I would suggest looking for youtube reviews by people that have had hands on the specific units and to see how they seemed to work and possibly if they look to be constructed well or not.

Thanks for the reply.  I will watch some review videos on youtube and go from there.  I honestly don't think I need 10A.  I mean after all headlight circuit on old cars were 10A to 15A @ 12v.  But amperage is higher at lower voltage.  But even at 9V I don't think for model building I would need much more than 5A.

Offline Decoman

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Re: Variable Power Supply - Questions
« Reply #12 on: December 07, 2017, 05:02:45 am »
*reads the last few posts*

I am now glad I ordered this 30V/5A variable power supply with 1mA accuracy for setting and reading off the current "setup resolution & read back accuracy". :) Though, at 220V, I suspect the output is lower, like 3A, with 5A for those with 110V.


(Edit: Ah, ok, this is already mentioned in the video linked above) Btw, I would advice against having your stuff connected to your variable power supply if your variable power supply allows current to flow when turning on the unit, because of voltage spikes on every turn-on (detected afaik only with an oscilloscope). The one i have doesn't do that, thought it is known that a faulty unit of the particular unit I ordered may pass current immediately when turned on (which would be bad and potentially dangerous).
« Last Edit: December 07, 2017, 05:54:40 am by Decoman »

 




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