Power supply voltage drop

Discussion in 'General Electronics Chat' started by Mike Bedford, Apr 4, 2015.

  1. Mike Bedford

    Thread Starter New Member

    Feb 10, 2015
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    Hello all. I have a power supply that I threw together using a linear replacement switching regulator made by CUI, part number is V7805-1000. I followed the reference circuit in the datasheet and added the caps that it recommends. I feed it 12VDC and I get a really nice and consistent 5VDC on the output. So, I would say the regulator works well.

    My issue is when I attach my load to the power supply. My load is an intelligent LCD touch screen by 4D systems, part number uLCD-70. According to the data sheet, I supply 5VDC to it and they recommend a minimum of 500mA but preferred up to 1A for current. I am right within those specs and am even at the preferred max current of 1A. For the most part, everything works and the screen runs fine. I measured actual current draw of 610mA and it holds pretty stable at that.

    The issue or question is, I happened to notice that I read a lower voltage of 4.3VDC when the screen is hooked up. As soon as I unplug the screen, it jumps right back to 5VDC and is pretty stable there. I am not driving any other loads with this supply/regulator, it is just for the screen. Is this normal to see this much of a voltage drop on my supply when my load is hooked up? Is there anything I can/should do to prevent this? Like I said, the screen still runs fine but I am .7VDC under their 5V requirement so I am concerned that the screen could act up.

    I have attached datasheets for the regulator and the screen. Any ideas would be appreciated!

    Thanks!

    Mike
     
  2. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Please observe the output EMF while powering a 5w to 10w resistive load (e.g. 1 Ohm power resistor or 4.5v torch [aka 'flashlight'] bulb) --- this will 'rule out' faulty transient response...

    Best regards and good luck!:)
    HP

    PS: It may be advisable to refrain from connection of the uLCD-70dt module pending attainment of proper PS operation -- Test loads are 'cheaper'! ;)
     
    Last edited: Apr 4, 2015
  3. AnalogKid

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    Aug 1, 2013
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    Is the 12VDC source capable of supplying all of the needed current? It might be sagging under load so much that the regulator has no choice but to follow it downward.

    ak
     
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  4. Papabravo

    Expert

    Feb 24, 2006
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    The linear replacement switching regulator has to work pretty hard under those conditions.
    5V * .61 A ≈ 3 Watts
    Assuming 80% Efficiency
    3 Wattts / .8 = 3.75 Watts
    3.75 Watts / 12 V = .312 Amps

    What are the specifications on the 12 Volt supply?
    Is it a wall-wart by any chance?
     
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  5. Mike Bedford

    Thread Starter New Member

    Feb 10, 2015
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    All, thank you so much for the responses and help. HP, I will take your suggestion and check EMF. I have to be honest, I am not sure how to test that but I am sure Google can help.

    Ak and Papabravo, Understood on the comment that the switching regulator may be working too hard and also agree that if the 12VDC supply cannot source the current needed, it will cause issues down the line. Let me answer that question to help shed some light and maybe bring answers. The 12VDC supply is NOT a wall wart. I am using a 40VA transformer to step down my 120VAC to 12VAC. I am then running through a bridge rectifier (full wave) model number GBU4B-BP. The rectifier is rated for 100V/4A. Finally, I do run through a 470uF/25V cap before running into the switching regulator.

    Given this, the only things that could cause current supply issues on the 12VDC supply in my opinion is the transformer or the rectifier. The rectifier is 4A and the 40VA transformer is A = 40 / 16Vrms which is about 2.5Amps. So, I am not sure the 12VDC supply (which is actually around 16rms) should be having a current supply issue to my 1A regulator.

    Any comments or thoughts after learning more about this?

    Thanks!

    Mike
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    How about a link to the datasheet for the V7805-1000.
    Can you measure the input at +12V± x, where x is a small number, when the output is at 4.3V
     
  7. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    I apologize that you found my post lacking in perspicuity:(

    EMF=Electromotive Force The unit for said quantity being the Volt (and denominations thereof) hence, informally, 'EMF' may be substituted with 'Voltage' :)

    Again, sorry for the unintentionally obfuscatious formality
    Very best regards
    HP
     
    Last edited: Apr 4, 2015
  8. Mike Bedford

    Thread Starter New Member

    Feb 10, 2015
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    Papabravo, I attached the datasheet to this post when I started it, you should have it. Agree, I can check those voltages. One minute for the results.

    HP, You didn't do anything wrong to apologize for. I am embarrassed that I didn't catch that and said I would have to Google it. In case it already isn't painfully obvious, I am fairly new to this stuff. These projects and things I do are just part of my hobby. Having said that, I can check the test you asked for as well.

    Thanks!

    Mike
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    I see the datasheet now. Did we mention anything about input and output capacitors? The datasheet recommendations are not exactly clear. Tell me what you have.

    I don't know what is going on. My best current hypothesis is high frequency oscillations that look like 4.3VDC because your meter is averaging them. Can you borrow an oscilloscope?

    Looking at the schematic for the LCD module it says the +5v input can go all the way down to 3.3V
     
    Last edited: Apr 4, 2015
  10. Mike Bedford

    Thread Starter New Member

    Feb 10, 2015
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    Papabravo, Just ran the test. Keep in mind as I mentioned before, even though my transformer is a 10:1 and theoretically should give me 12V out with 120VAC input, I actually get around 16VDC. As I understand it, this is because of the rms voltage after rectification. So, I misspoke earlier when I said I feed the regulator with 12VDC. It is actually upwards of 16VDC. However, the regulator can handle up to 32VDC so this is not an issue.

    What I will say though is after the last test I did, when I hooked the load up and dropped from 5VDC to 4.3VDC on the output, the input also dropped from 16VDC down to 14.3VDC (1.7VDC!!!). I assume this is pointing to the issue that it is not the regulator that is sagging but actually the input? Two things I still don't understand though, even though the input sags 1.7VDC, it is still well above the the minimum input voltage for the regulator and I would expect is should still easily provide my 5VDC out consistently? Am I missing something or is this pointing to something more going on that I don't understand?

    HP, I looked around for a flashlight bulb and unfortunately, all my flashlights these days are LED. I am going to have to use a resistor. You did say only 1 ohm? I assume I need a resistor that can handle that wattage? I am going to have to dig for that too...I only have typical 1/8 and 1/4 watt resistors. I will keep trying to do this test though and will post the results as soon as I have them.

    Thanks!

    Mike
     
  11. Mike Bedford

    Thread Starter New Member

    Feb 10, 2015
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    Papabravo, I have a 100Mhz digital scope and a Fluke 87V true RMS DMM. As for the caps, I followed the reference table from the datasheet on page 4 of 7 (Typical application circuit) and used 10μF/50V for C1 and 22μF/16V for C2.

    Thanks!
     
  12. Hypatia's Protege

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    Mar 1, 2015
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    --Edited at 4/5/2015 for accuracy and applicability--

    Two points:

    1) Although a 5 ohm resistor (connected to the output of the power supply) will dissipate Ca. 5W, significantly less power handling capability will be required for the brief time required to take the measurement.

    2) The power-handling capability of networks of like connected equal value resistors is equal to the sum of the power ratings of all constituent resistors...

    For example; A 4.7 Ohm network comprised of ten paralleled 47 Ohm 1/4 watt devices will exhibit a (continuous) power handling capability of 2.5 watts...

    The described input drop is about what I'd expect -- Thus it seems the trouble is in the regulator circuit itself -- in my experience construction or procurement of a non-reactive test load (as described above) is a good 'first step' in troubleshooting 'wonky' SMPS units...

    Best regards
    HP:)
     
    Last edited: Apr 5, 2015
  13. Mike Bedford

    Thread Starter New Member

    Feb 10, 2015
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    Okay, I was able to do two things. First, I ran some tests using the scope and I have attached the scope images on this post. I named the file names to be descriptive as to what was measured on the given screenshot. Second, HP, I finally found a flashlight bulb to use as a load to test and it is one of the scope screens attached. Note, similar result with the voltage drop on the bulb as well.

    Papabravo, agreed that the screen appears (at least in the datasheet) to be able to run lower than 5VDC.....down to 3.3VDC possibly. However, I have noted two issues with this. First, they state that while it can run with a lower voltage that the LED back light brightness will be affected (which I don't want). Second, I have started to notice a very intermittent issue where the screen flashes on and off. Seems to be mostly during first power on and once it is on, it seems to be fine. The flashing is almost as if it is trying to start and doesn't have enough to start.

    Last, you guys aren't going to believe this one. During the testing with the scope, I noticed two things one of which may be my issue. First thing I noticed is that my meter is in fact off from what my scope says by about .2 - .3 volts. The scope still confirmed (as you can see in the screens) that I have an issue with the power supply sagging and I am sure the scope is more accurate than my DMM but just wanted to point out that I did note a difference. Second (and this may be the issue), I did not mention throughout this thread (because I did not think/realize it could be a problem) that just before the screen hookup to the power supply I had installed a self resetting PTC fuse. Made by Bourns, model number MF-R050-2. I did this to try to protect the power supply/regulator circuit from issues. During testing tonight, I found that my issues appear to only happen AFTER the fuse. If I take measurements BEFORE the fuse, I don't see a sag at all, seems to hold steady at 5VDC as I was expecting. So, it would appear that when placed under load, the PTC fuse drops the .7VDC. Does that make sense? I know that the PTC fuse does have an internal resistance which might explain this but I thought the resistance was too low to cause this kind of drop. So, if we all agree this is an issue, I now have a new set of questions:
    1. Did I just pick the wrong PTC fuse and maybe there is one with better specs that won't cause the issue or at least the issue will be minimal.
    2. Do I even need a PTC fuse? The regulator is short circuit protected but not over current protected. This is why I thought I needed a fuse. I could have gone with a fuse that blows but I thought self resetting was a nice feature.
    3. Is there something different I can be doing to protect the regulator from over current that would complement the protection already built into the regulator and give me all i need without using this type of fuse OR if this fuse is good, is there a better way to utilize it? Similar to number 1, maybe I just need to spec a different one?

    That is all for now. Again, thanks everybody for the help!

    Mike
     
  14. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    @Hypatia's Protege
    A 1 ohm resistor across a 5V source will dissipate 25W.
    Presumably a 5 ohm resistor was your intended recommended load.
     
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  15. Papabravo

    Expert

    Feb 24, 2006
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    Interesting results, and I commend you on your industry.

    One thing that happens to PTC fuses is that they age. In the sense that they do not return to their same low resistance after a fault. After a number of faults they return to a value which is much higher than the one they came from the factory with. I'd be comfortable with getting rid of it, or moving it to the other (input) side of the regulator. You have some haedroom on the input side for some voltage drop, and if the output goes overcurrent the input will have to do the same.
     
    Last edited: Apr 5, 2015
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  16. Hypatia's Protege

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    Mar 1, 2015
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    Sorry about that!:oops: --- I really must refrain from 'drowsy typing'!!!:rolleyes:

    To elaborate:
    5v/1 Ohm=5A
    5A*5V=25W
    Or, succinctly: (5v^2)/1 Ohm = 25W (Generally: P=E^2/R)

    Hence, via rearrangement:

    The resistance dissipative of 5W at a 'drop' of 5V:
    5v^2/5w=5 Ohms (Generally R=E^2/P)

    Hey, Mike -- Again, sincere apologies should my lackadaisical posting and negligent proofreading have contributed to what must already be an annoying situation!:( --- And thank you, "t n k", for catching my error before any harm was done!:)

    Personally I'd would 'go with' a 'standard' fuse -- As regards the described application, over-current events will be anomalies -- indicative of serious malfunction -- to which you should be alerted, moreover, as you have discovered, 'polyfuses' tend to be 'quirky'...

    Best regards
    HP:)
     
    Last edited: Apr 5, 2015
  17. dl324

    Distinguished Member

    Mar 30, 2015
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    Hi Mike,

    I'm of the opinion that you should let the power supply do what it's designed to do. From what I read of the specs for the switching regulator, it has sufficient built in protection.

    If you're trying to protect your circuits from over current, consider investing in a bench power supply that will let you set a current limit.

    BTW, that's some fancy oscilloscope you have. I would have spent money on the basics before splurging on a scope. I rarely need a high bandwidth scope and get by with a 200MHz analog variety (40 years old, IMO Tektronix really new how to build them).

    Regarding the suspected inaccuracy in your voltmeter, it may be that the supply voltage after the PTC was changing at a rate that your voltmeter couldn't handle. After all, it's a digital meter and doing successive approximations on a changing voltage can produce strange results. Analog meters are more appropriate for measuring "varying" DC voltages, but even they have a relatively slow response time.

    What current is your PTC rated for? They will increase resistance before they get to their rated current.
     
  18. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Indeed they did! Just for fun I purchased a 545B equipped with a Pentrix L30 (10Ghz SA [i.e. FDO] 'plug-in-unit') at Dayton -- To my amazement it not only functions but, incredibly, is in good calibration -- despite tell-tale layers of dust! :D:D:D

    FWIW HP/Agilent are 'second to none' as well:)

    Best regards
    HP
     
  19. dl324

    Distinguished Member

    Mar 30, 2015
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    Hi HP,
    Maybe now. When I worked at HP Labs in the late 70's, we wanted Tek scopes but were only allowed to use HP. At the time, Tek had a 1GHz sampling scope and HP didn't. This was in ancient times when 300MHz logic was considered fast and computers had 5V supplies you could weld with.
     
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  20. Papabravo

    Expert

    Feb 24, 2006
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    And "Fast Eddie" the technician got his name by welding a Craftsman screwdriver to the terminals of a big blue capacitor the size of a Budweiser can.
     
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