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5v linear voltage regulator design problem.

Discussion in 'General Electronics Discussion' started by Damien, Apr 19, 2015.

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  1. Damien

    Damien

    55
    1
    Jan 28, 2015
    Hey guys

    I was wondering if someone has a spare moment if they would would mind sharing their ideas as to why a makeshift 5 volt regulator transistor had a lot of heat being thrown off it with such a small load.

    Scenario:

    We're about to head to a camping spot to go camping, and I decide that the 4 port Cigarette lighter female socket hub should be wired into a live circuit instead of the circuit that is only powered when the notch on a an automotive ignition barrel is clicked onto the spot just before accessories so that things such as mobile phones, vape batteries etc can be charging off a single medium power 5v regulator (CV05 or somethingarather). Long story short - accidentally reversed polarity going to the hub as I just figured y'know, the black wire should really be negative and the black and white wire should be positive - this is standard practice right? Well, turns out it was the opposite for some reason which blew firstly the 5 volt regulator in G.P.S male plug. secondly the U.S.B cigarette lighter socket to u.s.b adaptor regulator and thirdly, an internal voltage regulator (Part of the hub) that has an external 5v u.s.b adaptor female socket. Lots of smoke, many sad. So 3 regulators blew and I had about 20 minutes to put together something that could power the g.p.s for a 5 hour round trip as well as at least one female u.s.b port for charging nicotine vaporizer batteries while camping.

    So on a breadboard (Yep it was very unpretty!) with u.s.b female plugs duct taped to the breadboard lol. (Any port in a storm right? ;-))
    I used a cv something 05 transistor & wired it with 12vin comming from anode from car battery/alt circuit into the gate/base terminal pin and tied the common ground to the drain/collector and emitter/source to positive 5v out.

    This worked but the heat sink got mega hot, way too much than I would expect just to run a g.p.s and charge a vape battery. (Not sure how much they draw, maybe not much? I really should measure it sometime) Anyhow I only wired it this way as per a circuit diagram advised to wire voltage regulators but this is very different as to how an lm317 operates yet it still works? o_O *Scratches head*

    Had a look at an N.P.N schematic of bipolars or mosfets and the way it was wired I can't see how either of these could work properly unless I am overlooking something. Are these preset voltage regulator transistors in their own separate group and the wiring is completely different?

    Any help would be appreciated - I don't think it should be throwing off so much heat that the heat sink is very hot to touch. (Pretty big heat sink for a medium power tranny too)

    Regards -

    Damien
     
    Last edited: Apr 19, 2015
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Give us a sketch of the circuit.
    Did you use a regulator or transistor or a combination of the two.
    Was the supply connected the wrong way round or were the regulators killed by voltage transients?
     
  3. Damien

    Damien

    55
    1
    Jan 28, 2015
    Soz Duke just got back from trip, exhausted might draw up a schematic and scan it tomorrow with scanner. Is there any circuit drawing programs that are good so I don't have to draw them by hand? Not that I mind it would just be quicker with a mouse perhaps.

    The 3 regulators were all destroyed by reverse polarity due to wiring being incorrectly colored by the manufacturer of the cigarette lighter female socket hub.

    I just used a regulator.. It was getting bloody hot so I decided to add a current limiting resistor (150 ohm) between 12V+ and Gate/Base and although it kept the transistor cool, heat started building too much in the resistor so I opted to just run it without a current limiting resistor and just put a big ass heat sink on. Upon further inspection of the datasheet for this particular regulator, I fear that I may have just wired it up incorrectly as per the pinout diagram but will have to double check this tomorrow morning.

    Datasheet is here - http://pdf1.alldatasheet.com/datasheet-pdf/view/22629/STMICROELECTRONICS/L7905CV.html


    I used the TO-220 and pinout is as follows

    Pin 1. - GND
    Pin 2. - Input
    Pin 3. - Output

    I think I accidently used pin 2 as ground..perhaps...
    How could this still work? I was getting a steady 5v from the output pin on pin 3. I think I may have mixed up pin 1 and 2 but I will clarify this tomorrow morning, If I have the wiring correct then I'm still scratching my head but i'm kinda hoping I had it incorrect as this would mean that this circuit is actually useful instead of getting so hot that it burns breadboards and drains the battery in the car when charging a battery or whatever from the regulator overnight.
     
  4. Damien

    Damien

    55
    1
    Jan 28, 2015
    Nope wiring was all correct just checked it then.

    Sooo...

    Pin one = 12v input source
    Pin two = ground
    Pin three = 5v output

    Would the huge gap in voltage cause all that heat to dissipate? I am thinking perhaps I should go down the switching mode regulator path but surely these little l7805cv transistors must have some use?? I just checked it then and only drawing like 0.1 amp at 5v so....half a watt? What the hell is going on here. Surely there must be a way to use these little regulators so they don't heat up so much. Should I try a current limiting resistor in series at the input? I would imagine this would heat up also..Seems like alot of energy is being wasted and turned to heat what are your thoughts?
     
  5. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    All vehicles since about 1950 have used negative ground and this should be the reference point for all equipment. It follows that to reduce voltage you will need a positive voltage regulator such as a 7805 not the negative 7905 regulator.. This was probably running on reverse polarity so it is no wonder that it was getting hot, it could well have given up the ghost.

    It you put 7V 1A across the regulator, that is 7W so the regulator will need a heat sink.
    The regulator should take very little current with no load.
     
    davenn likes this.
  6. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    The 7805 is NOT a transistor, it is a regulator. It may be small but it is perfectly formed.
    It is not dropping 5V. From 12V it is dropping 7V and from 14V it is dropping 9V. The TO220 cased device can dissipate about 2W in free air or about 25W on an infinite heat sink.

    With no load it should take only a few mA and not be appreciably warm. If it does get warm with no load, it is dead or perhaps you have not fitted the recommended bypass capacitors.
    An input resistor of 5Ω would drop 5V at 1A and dissipate 5W, reducing the heat in the 7805 by this amount. Make sure that you have a capacitor close to the input of the 7805.
    A diode is often recommended across the 7805 to prevent reverse voltage should the output voltage stay high when the input voltage is removed. It must be connected correctly.

    In a vehicle with plenty of power available I would not worry about the power loss.
     
    davenn likes this.
  7. Damien

    Damien

    55
    1
    Jan 28, 2015
    I apologize as I have provided the wrong datasheet. The regulator I used is in fact the 7805 not the 7905. So 7805 no workies with 1950 studebaker? =P

    Ok so regulator's are different than transistors. What about the lm783 regulator? Is this not a mosfet transistor?

    The regulator heat sink was cold until current was drawn.

    Testing the circuit out on my desk now and simulating the conditions (v13.8 in)

    No load - Not heat whatsoever

    500ma - Starting to heat up but nothing like it was on the trip.

    Perhaps the g.p.s is drawing over an amp? o_O
    The regulator that came with it (built into the cigarette lighter male plug socket) Was tiny though! ..Would this be a switching regulator?

    I'm asking all of these questions because I am wondering if it would be feasible to put this onto a p.c.b & mount it permanently in the car feeding a multiport u.s.b hub to run devices or is this regulator too small (even with a heat sink)

    I will look into these bypass caps you speak of. I added a small filtering cap after the transistor as I figured perhaps there would be some noise after the regulator (I have no idea if there would be or not - I don't own an oscilloscope but can't wait to get one to look for these kinds of things hehe). Would an automotive alternator cause d.c ripple or would the car battery smooth this out and give a smooth d.c waveform?
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    if you wired the 7805 using the pin connections for a 7905, chances are you killed the 7805 as the pinout is different

    dunno .... what is the power connection for a studi ? have you looked at the battery and noted if it is the + or the - that is connected to the
    car chassis ??

    There is no LM783 did you mis type ? and really mean a LM7833 ?
    you are not gaining anything using it instead of a 7805

    if you need more current then use a LM338K
    http://pdf.datasheetcatalog.com/datasheets/208/49610_DS.pdf

    cheers
    Dave
     
    duke37 likes this.
  9. Damien

    Damien

    55
    1
    Jan 28, 2015
    I presume it would be positive as someone mentioned that they didn't use negative ground until after 1950 so it was just a tounge in cheek question. ;-)

    Yep, miss typed I meant lm317 variable voltage regulator.

    Did my back in so very high on pain killers and anti-inflamatory medication. I will double check the right names for things before posting.

    Thanks Dave. How much current do these 7805cv regulators handle with a heat sink?
    They say 1 amp without and 5 with If I recall? Does this current rating relate to the 5v output or the 12v input as 5v 5a is alot less than 12v 5a as we all know.

    I might wait till I'm not doped on codeine & valium until I reply to this again as it's making things very confusing for all those involved hehe.

    cheers

    - Damo
     
    Last edited by a moderator: Apr 20, 2015
  10. davenn

    davenn Moderator

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    did you mean to type something ?
     
  11. Damien

    Damien

    55
    1
    Jan 28, 2015
    Yup, not comming up for you? I can see my replies throughout the quoted message.
     
  12. davenn

    davenn Moderator

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    Sep 5, 2009

    OK I didn't see that cuz it shows as just one big quote from me
    Its always better to use the quote and /quote fort different sections that you want to reply to

    makes it a lot easier to read .... I will edit your post so you can see the difference :)

    Dave
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009
    I wouldn't dare try and pull 5A from a 7805 .... it would probably have a very short life span, even when well heatsunk
    would keep to ~ 1.5A max

    OK a LM317 will easily give ~ 1.5 A well heatsunk and I wouldn't try for anything 2A and over, hence why I suggested the LM338K which will easily do 5A when heatsunk

    no probs :) get well soon .... Endone is much much better than codeine :)
    for several of my nasty pain sources I will take an Endone and a couple of panadine forte usually has the desired effect :)

    Dave
     
  14. Damien

    Damien

    55
    1
    Jan 28, 2015
    Cool thanks for that, I'll check out what the draw of all my gadgets are before making a decision on regulator sizing.

    I usually take opiates for pain in the form of codeine (panadine forte) or if it's more severe like today endone which is also a synthetic derivative of opium and use something from the benzodiazepine family such as diazepam (Valium) or cerapax as I think although they are great for anxiety, they are also the best anti-inflammatory medication known to man haha. I hate the feeling of defeat I feel having to resort to taking drugs and lying down doing nothing for a day as I like to keep busy and have alot of projects on the go as well as looking after my partner who is unwell also atm.

    How annoying is human body pains! I'd really prefer to just be a robot with a human brain (cyborg) Replace defective body parts etc & no sleep required just plug yourself into a power source to recharge a battery of some kind haha.

    One can only dream! =)

    Are you a chronic pain sufferer Dave? I hope this is not too off-topic from electronics.
     
    Last edited by a moderator: Apr 20, 2015
  15. davenn

    davenn Moderator

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    unfortunately, yes, ... most of it is arthritis related. 2 wrecked knees from my years with NZPO/Telecom NZ
    and also in the upper neck that gives me really nasty headaches

    take care buddy

    Dave
    Ex Dunedin
    ( been in Sydney for last 15 yrs)
     
  16. duke37

    duke37

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    Jan 9, 2011
    Is the car positive or negative earth?
    Is it 6V or 12V?
    Does it have a dynamo or alternator? When did alternators come in? I have never heard of one made for positive earth.

    I will look into these bypass caps you speak of. I added a small filtering cap after the transistor as I figured perhaps there would be some noise after the regulator (I have no idea if there would be or not - I don't own an oscilloscope but can't wait to get one to look for these kinds of things hehe). Would an automotive alternator cause d.c ripple or would the car battery smooth this out and give a smooth d.c waveform?

    A dynamo is likely to give a smoother output than an alternator but will give brush contact noise. The battery will reduce the noise but there will be voltage drops along wires and poor contacts to chassis so allowing noise and spikes getting everywhere. The regulator will filter much of the noise but should have a capacitor at the input for stability.

    A high current regulator will not be able to provide more current unless it is heatsinked properly.
    Car voltage supply is quite dirty and high voltage spikes can occur such as when starting.

    Keep off the phenylbutazone (bute), it is good enough for the royal horses but not advisable for the peasants.
     
  17. Damien

    Damien

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    Jan 28, 2015
    All this heat sinking required, surely this is inefficient. For low current needs (below a couple of amps) Would a dc to dc buck converter be better? I am by no means ready to step into p.w.m world just yet but just out of interest, are they more efficient or just as at low currents? I saw an argument on a forum somewhere where a guy said that that p.w.m is actually more inefficient than linear devices at low current levels.

    Dave: Have you tried yoga for your neck pain? Arthritis is a bitch and something I'm not looking forward to. Do good omega 3,omega6 fats help with arthritis at all? It's amazing that they can put a guy on the moon but they can't cure arthritis hey =/.

    Duke: Cheers for all that info - I will be more mindful of filtering for automotive charging systems in future. What did you mean by by-pass cap?
    Positive earth in a car? if I'm not mistaken I think someone mentioned here that they used this pre 1950? Not sure heh.

    Getting back to the regulator thing - all the ones I've seen that are commercially made (IE for g.p.s charging which has the reg built into the plug) they are tiny and they don't seem to heat up at all. I smashed apart the plug that harnessed the inbuilt reg just to see what made it tick. Two tiny little i.c's on a tiny p.c.b & a few resistors, pass filter, zener diode...I couldn't make sense of it at all! lol but uhh would this thing be a p.w.m circuit ?
     
  18. davenn

    davenn Moderator

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    DC-DC converters are a great way to go
    and they ARE much more efficient than linear reg's

    I was getting around to suggesting them haha .... like you, the head isn't overly clear ;)

    there's a zillion of them advertised on ebay, take your pick


    Dave
     
  19. BobK

    BobK

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    Jan 5, 2010
    Your friend is correct about switching regulators at low currents, they lose efficiency when run well below their rated current. But we are talking well under 1A here.

    Bob
     
  20. Damien

    Damien

    55
    1
    Jan 28, 2015
    The reg that I'm using with a big arse heat sink will run a g.p.s doesn't seem to want to charge the battery in it, not enough current?. Will also only charge a vape battery when g.p.s is unplugged. My g.p.s uses 5v a amp (5 watts) Will this reg be able to throw more than 5 watts somehow? Is there a trick? :) Using the reg to output onto the gate of large mosfets. Ahh this thing is going to be so big that it will take up my entire center consol. I think I'll just buy some cheap ones on ebay until I learn how to screw around with p.w.m.
    & timers.
     
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