Getting 10V/1.5A output from a 19V/4.7A PSU.

Discussion in 'General Electronics Chat' started by LETITROLL, Oct 17, 2015.

  1. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    Hello everybody .

    I have a laptop psu delivering 19 v and 4.7 amp .

    Now i need to replace a battery charger PSU but the original one was delivering a 10 v / 1.5-2A at the output . also the 10 volts was regulated with the tl431 , and the psu was used to charge a 9.6 v/ 3800 mah battery pack .

    Is there a way to get the same output from a laptop PSU ( 19v/4.7A) using the tl431 or any other regulator but getting at least 1.5 A ??
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Battery chargers are different than power supplies. The design depends on whether you are charging Lithium ion, nickel metal hydride or NiCd or lead acid batteries. In summary, yes you can use the laptop supply but you need more than a voltage regulator to use it as a charger.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The electronics between the laptop supply and the battery should contain both a current-limiter (to protect the battery and the power supply), and a voltage regulator (to protect the battery).
     
  4. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    Hey thanks a lot for the interest .

    The battery type is lithium ion like in the picture . And the goal is to convert this laptop PSU into a charger but almost similar to the original one.

    the original charger uses the TL431 so i guess the current limit is incorporated on it .

    Is there a way to take a normal laptop psu and add an extention to make it a charger ?
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    No picture in either of your posts...
    Without a schematic, no one can say whether substituting a higher voltage power supply is okay.
    Download the datasheet so you don't have to guess. The TL431 doesn't mention any safe area protection.
    Yes, but you need to implement the correct charging profile. If you just charge with constant voltage, battery lifetime will be impacted.
     
  6. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    Hi dl324.

    I will try to draw the output schematic of the original charger .

    Also for the battery . Its actually a ni mh type .

    [​IMG]
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    NiMh charging requires constant-current and time; not voltage limits. This was recently discussed in this thread. (follow my suggested links)
     
    Last edited: Oct 17, 2015
  8. dl324

    Distinguished Member

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    If the charger was for NiMH, you just need to determine whether a higher input voltage will be tolerated. If not, a quick and dirty approach would be to insert a power zener diode in series to drop 19V to around 10V. Or you could try to hack the adapter to reduce it's voltage. Or you could use a voltage regulator to adjust the voltage into the battery charger.

    Trying to design your own battery charger can be interesting, but it's faster and more economical to buy. But, you can't always count on commercially available products to perform as advertised...
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    @dl324 , you could learn something by following the previous thread. NiCad and NiMh are not charged with a voltage-controlled charger.
     
  10. dl324

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    Non sequitur. OP said "i need to replace a battery charger PSU but the original one was delivering a 10 v / 1.5-2A at the output". The charger just needs a new power supply...
     
  11. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    The third option seems good , but what type of regulators should i use to avoid current losse ? also how do i control the charge time , i mean how do i know when the battery is fully current-charged ?

    I should recall that the battery is 9.6 v 3800 mAh pack .
     
  12. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    Yes but that 10 v is given with the TL431 so its both a charger and a psu in case the battery is not in the device using it.

    NB : the current i mentioned is not accurate i just decided to use the minimum current to avoid any damage for the battery , since its a 3800 mAh i guess 1.5 A is safe for charging it.

    It would be a nice idea to know the charging rate of this type of batteries .
     
  13. GS3

    Senior Member

    Sep 21, 2007
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    NiMH is fairly easy to charge. Fast chargers are more complicated and stress the batteries more but you can slow charge them easily and the battery benefits. NiMH will take C/20 overcharge indefinitely without damage which means you can charge at C/20 with no special precaution. You can charge at C/5 for a while, say two or three hours, and then switch to C/20. Al you need is a resistor to limit the current. This is how I charge all my batteries and it works fine as long as you are not in a hurry. People are impatient and want fast chargers but this is much more stressful for the battery. Better have two sets of batteries and charge one set while the other is charging.

    Many cameras and other devices take two batteries. I build simple battery chargers using a AA or AAA battery holder, a limiting resistor and a USB connector. Plug it into any USB port or charger with USB connector and you're charging. A 22 ohm resistor gives you about 100 mA charge.
     
  14. dl324

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    Mar 30, 2015
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    NiMH batteries are not charged with constant voltage or constant current. Read this article from batteryuniversity.com on charging NiMH batteries. Clearly not worth rolling your own...
     
  15. dl324

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    In your original post, you said you wanted to replace the power supply in your charger. Are you now saying you want to build your own charger??
     
  16. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    Just like dl324 said , i better use an adjustable regulator to output 19v / 1.5 A just to turn on the device , after that i will deal with the charging time limit.
     
  17. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    The problem is that the charger belongs to a special device ( A mobile Triphase harmonics analyzer). its not easy or its near impossible to find one doing the same job ( in the charging part)

    What i want to do is add a charging circuit after regulating a laptop psu voltage .
     
    Last edited: Oct 17, 2015
  18. dl324

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    Mar 30, 2015
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    I need clarification. You initially stated that the power supply for your charger needed to be replaced and that you wanted to use a higher voltage/power laptop supply to do that.

    Are you now saying that the charging circuit itself needs to be replaced and you want to build your own? If that's the case, that isn't a simple task. If the charging circuitry in the existing charger can be salvaged, it's probably a DIY project. From scratch, you'd be better off looking for an off-the-shelf charger. 9.6V NiMH battery packs aren't that rare, neither is the connector on the battery picture you posted.
     
  19. LETITROLL

    Thread Starter Member

    Oct 9, 2013
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    Ok sorry for not explaining that well , i try to explain from the beginning.

    The charger am trying to recover is incorporated into a device similar to a mobile oscilloscope .

    The problem first was simply that the charging indicator on the LCD screen shows that no battery is recognised and the charging indicator is displaying this symbol (?) instead of showing blinking bars . the same battery was used in another device and it worked .

    So i decided to take a look at the power supply section , and what i noticed is that there is two PCBs on the device , the main motherboard for the LCD display and other functions , and small one incorporating a smps power supply , and at the output there is a tl431 based circuit.

    So my first impression was that the charging problem i got was caused by something on the TL431 ( soic-8) section since that ic is used for that purpose .

    Before i replace the tl431 the device was turning on even if the battery was not charging , and the psu card was outputting a clean 10 V dc but i had no idea about its output current.

    I replaced the so-8 version of the tl with a to-90 version and that's when trouble began.

    After turning the device on with the new soldered component there is no display like before , just black screen .

    So then i checked the PSU again to see the changes , and found that there is no output like before and a tvs diode in parallel with the transformer's primary was shorted out , also a buzzing sound from the transformer , i replaced that tvs diode and the problem is still the same but the buzzing sound has gone .

    I wished i could upload some pictures of the actual PSU and also the battery pack, but the device it not here at the moment , ill do that later for sure.

    The questions i got :

    - Is the TL431 circuit the main charging controller ?
    - Also is there any regulator that would regulate a laptop's PSU voltage to 10 volts accurately and also delivering enough current for charging the NI-MH battery
    withing at least 2 hours ?
    - Also how to know when the battery is full in order to control the charging time ?

    I appreciate your time and hope i explained the way it should be .
     
  20. dl324

    Distinguished Member

    Mar 30, 2015
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    What you're asking for now is completely different than what you had in your first post:
    Probably not; can't tell without a schematic for the charger in question. TL431 is a shunt regulator. It is used to regulate voltage which *could* be used to control charge characteristics.
    There are a number of ways to reduce 19V to 10V, but that has nothing to do with charging a NiMH battery in a couple hours.
    Did you read the article I linked to at batteryuniversity.com? Charging an NiMH battery can't be done with constant voltage or current. They require one of a small number of charging profiles and, based on the questions you're asking, it is not a DIY project.
     
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