Rectifier Problem

Discussion in 'General Electronics Chat' started by LostTime77, Jan 23, 2011.

  1. LostTime77

    Thread Starter Member

    Dec 9, 2010
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    Hi everyone:

    I apologize if this has been asked before. I did do a bunch of research... but there is a difference (as far as I can tell) between my problem and what my research came up with.

    I am trying to build a straight 120VAC RMS to 160 / 170V DC bridge rectifier that can provide 5 - 10 amps. The schematic is as shown on wikipedia under the basic operation section:
    http://en.wikipedia.org/wiki/Diode_bridge

    I know it is dangerous to work directly with AC mains. Many people have posted this in this forum. However, in this application, I cannot use a transformer for isolation and whatnot because of a weight constraint on the circuit. A transformer would add unneeded weight (around 3 pounds from my research). This circuit is 'mobile' and is to be put in a high mileage vehicle. It is also only to be plugged in at certain times, not all the time. ANYWAYS... onto the problem

    For the two source terminals, I connect my AC wall power. One side of the rectifier goes to the hot wire (black), and the other side of the rectifier goes to the neutral wire (white). Now when I turn the power on (plug it in)... BANG.. top right diode always explodes. Ive tried the circuit 3 times replacing the diodes each time. Every time its the same exact diode that blows. The first time the circuit blew is because I connected a scope ground terminal of a probe to the 'ground' of the bridge. That was stupid... after research I see why. The second time.. I didn't connect anything and just had the neutral and hot connected with open circuit DC terminals. The third time I just put a couple of 2 megaohm resistors inside the bridge to make sure that no current could possibly go from one side of the hot to the other side directly.
    This baffles me as literally, the DC outputs of the bridge are open (open circuit). For the rectifier diodes I am using 4 1n4004 general purpose diodes. In the datasheet it says that these are rated for a repetitive peak reverse voltage of 400 volts. I don't exactly know what the problem is.. because I am literally just hooking up a neutral and the hot wire to the bridge. The DC output is not grounded or connected to the third wire in a wall house plug (earth ground) or the green wire.
    Another question. I need to add just a small amount of safety to the inherently unsafe circuit. Would a 5 - 10 amp circuit breaker at the input of the hot or neutral be alright? I don't exactly want to use a fuse as it is not resettable, but I will if suggested to. But seeing how things turned out today, I would have blown 3 fuses, which we do not have many of.

    Any help is appreciated. I reiterate, I can't use a transformer for this application. Ultimately this is to be the base of a 0 - 120V selectable SMPS power supply.

    Thanks
     
    Last edited: Jan 23, 2011
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Can you post a photo of how your bridge is wired?

    If this is a mobile application, the 120V would be from an inverter and already isolated.
     
  3. LostTime77

    Thread Starter Member

    Dec 9, 2010
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    I will post a photo as soon as I can, when I get back into the lab tomorrow.
    The supply is not isolated as far as I know. My 120AC RMS is coming from a standard household wall outlet and not from the vehicle itself. I think you may be thinking that I have a 12V battery which an inverter is connected to. This is not the case.
    The application calls for being able to plug the vehicle into a wall household outlet for testing / battery charging / other requirements. The application is mobile in that the car will not be plugged into the wall when driving.

    Thanks
     
  4. SgtWookie

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    Jul 17, 2007
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    For virtually all applications, a transformer is required to galvanic isolation from the mains.

    In the case of an SMPS, the incoming AC mains power is inductively coupled to the secondary via what is effectively a broadband transformer, and when designed properly, provides the galvanic isolation that is always required for safety.

    LostTime77 wrote:
    You neglected to mention which image that you are going by.

    Is it the "hand made diode bridge" photo of a very poorly constructed circuit with no documentation on what goes where?
    If you are connecting the hot and neutral across a single diode, during one half-cycle the current flow will be blocked; during the other half-cycle the diode will conduct for a brief time until it approaches the temperature of the sun, eventually rupturing into a miniature fireball of expanding gas.

    By the way, did I mention that the "hand made diode bridge" is very poorly constructed AND unsafe for use with high voltage?

    If you connect a rectifier bridge incorrectly, you'll burn up at least one or more diodes every time.

    I suggest that you stop trying to connect your bridge directly to mains power at the moment, because you're going to wind up being injured or killed - or at least run out of fuses and diodes pretty quickly. Your local Radio Shack carries some low-voltage transformers that will help minimize the damage when you connect something up incorrectly. You really need to concentrate on safer, low-voltage circuits for a while before you attempt to tackle moderately complex projects like switch-mode power supplies. It's not so much the complexity - it's all of the theory behind the operation of such things.

    You mentioned 170V @ 5A to 10A, but you're using 1A diodes in the bridge. Where were these extra 4A to 9A supposed to come from?

    Why don't we shelve the conversation about the bridge for a moment, and you explain what it is that you're actually attempting to accomplish as the end goal? About the only thing I can think of that you would need for a vehicle would be to recharge the battery, but a typical automotive battery is best charged at under 10A. Commercially available battery chargers in the 5A to 10A range are pretty well constructed and not very expensive.

    But I have a feeling you're looking to do more than charge just a single car battery.

    What are you wanting to do?
     
  5. LostTime77

    Thread Starter Member

    Dec 9, 2010
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    Sorry for me not explaining properly. What I meant by connecting to one side and the other is: the bridge is in a diamond configuration. The diodes are connected as shown on the wiki page. Each corner represents a connection point. One would usually connect the AC mains to two oppositely connected corners of the diamond. This is what I meant. I don't connect AC mains across a single diode, that would be absurd.

    As I have stated in my initial post, a transformer is a no go if the application stays the way it is now. This is due to weight constraints on the circuit. I realize that until the circuit is down pat, connecting / testing is unsafe. I have been shocked by wall power before by touching the hot wire to one finger and the neutral to another finger on my hand. I didn't mean to do this as I was measuring the AC voltage with a voltmeter and the leads slipped. This was awhile ago.

    Pardon my bluntness, but I actually have much more experience in DC than I do with AC, especially with switch mode power supplies. I am basically just starting with AC circuits practically. I have known a ton of AC theory for awhile, it is just I have never done many practical experiments and circuits with AC. I am also unfamiliar with household wiring to an extent in terms of isolation with transformers and the like. I would like to point out that just because I have blown a few things playing with this AC circuit, because I am just starting out with AC (in the practical sense), does not mean I lack in other areas such as SMPS.

    The 1n4004's are being used for testing purposes. They will not be the final diode used in the design. Yes.. 1A rated and I want 10A. I apologize for not stating that I was building a test circuit with this first. I do not intend to put 10A through the bridge for testing purposes as the diodes are 1A rated.

    The end goal is an SMPS that can provide a selectable DC voltage from 0V to whatever. I can of course go out and by myself a switch mode power supply, but I believe you are missing the point. Most designs use transformers (more like all). Most designs are also bulky and are heavy (0.5 - 5pounds). Since my application deals directly with weight, why go out and buy something to integrate into the vehicle that weighs even 0.5 pounds for something like a 0.2Amp output? However if you look at what I am trying to do: converting 120AC to a DC selectable voltage with a diode bridge, a controller, inductor, few capacitors, a few resistors, and a few transistors. Weight < 0.5 pounds.

    The vehicle is not a standard vehicle that you may be thinking of. The vehicle will be designed with a highly customized electronics system that has the option of being powered for testing purposes from the AC wall outlet. Now in light of your suggestion for safety, as I have determined since this post, I have a better idea. I think for now the best option would be to build a customized SMPS power supply that lies outside of the car instead of trying to integrate it into the car. Since the car will not be driven while it is hooked into the AC wall outlet, the customized 'wall wart' we can call it can provide the proper voltage to the car ~16 volts. The onboard regulators then just have to worry about converting 16 volts or so. This also takes away the weight restriction, and I can use a transformer.

    The reason that I wanted the AC circuitry to be onboard in the first place was for simplicity. When hooking up AC power to the vehicle, we would simply have to worry about a single AC power cord (C cable) that plugs into the car. These are available as a dime a dozen. On the other hand having a specialized power source outside of the vehicle requires us to have a more customized system. In this sense anyone who has wants to run the car from a wall socket has to get the specialized supply first However, this option is the safest.

    Now in this method I have a few questions. I can essentially buy any old transformer that steps down the AC voltage. Would putting a say 5 or 10 amp rated circuit breaker on the hot wire into the top half of the transformer protect against say a short circuit? Am I correct in saying that the top half of the primary winding would be connected to the hot wire and the lower half would be connected to the neutral? What if the transformer was center tapped? Do I then have to connect earth ground to the center? As I have said, I am not super familiar with household wiring.

    Thanks for your responses.
     
    Last edited: Jan 24, 2011
  6. SgtWookie

    Expert

    Jul 17, 2007
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    OK, let's go back to the "hand made diode bridge" photo.

    There are four connections made to that bridge:
    Upper left, upper right, lower right, lower left.

    To what two connections did you wire neutral and hot?

    [eta] I had to look at the Wikipedia image again; the 1N5333B diodes used are 5W 3.3V Zeners. I'm sure the creator of the bridge got some interesting fireworks when they connected power to it.
     
    Last edited: Jan 24, 2011
  7. LostTime77

    Thread Starter Member

    Dec 9, 2010
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    Lets see.. looking at that poto.


    Upper right was hot, lower left would be neutral. This causes the bridge to pop with nothing connected to the open circuit DC terminals (upper left, lower right portions).
     
  8. SgtWookie

    Expert

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    OK, and you had NOTHING connected to the positive (upper left) and negative (lower right) junctions?
     
  9. LostTime77

    Thread Starter Member

    Dec 9, 2010
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    That is correct. I will try and take a photo of the blown circuit tomorrow.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Good.
    Please use Windows Paint or other tool to label your connections.

    In the meantime, a brief intro to mains wiring in homes as it is in most of the USA.

    From the power company, you get 3 wires coming into your breaker panel; L1, L2, and Neutral.

    L1 and L2 are the opposite "ends" of a center-tapped secondary on a transformer, and Neutral is the center tap. If you measured between L1 and L2, you would get nominally 240VAC RMS. If you measured from Neutral to either L1 or L2, you would get nominally 120V RMS.

    The Neutral line is connected to earth ground inside the breaker panel. The earth grounding wire is a heavy-gauge (usually AWG-4) solid wire that connects to one or more 8-1/2' copper clad rods pounded into the earth. This is for safety, as otherwise you could wind up with very high voltage on the wiring if there was a fault in the power company's transformer.

    The breaker panel is the ONLY place that earth ground should ever be connected to Neutral. Earth ground is there for the sole purpose of protecting human life, and ground wires are never supposed to conduct current; if they ARE conducting current, there is a fault someplace that needs to be corrected.

    I've attached a very simple schematic just to show you what occurs at the output of the bridge. I've used a signal generator to simulate L1 and a grounded Neutral (N) input to the circuit.

    C1 charged fully during the first half-cycle of L1. Normally, you would want some kind of current limiting scheme, as if C1 was much larger, it would "look" electrically like a near-dead short, and the high current flow would burn up the bridge.

    Note that the distance between plot lines (C) and (D) remains constant after the first half-cycle (170V), but the low side of C1 varies between -170v and 0v, and the high side between 0v and +170v. This is why you "zapped" the bridge when your O-scope ground touched either the + or - outputs.

    This is just one of the reasons why we seem to be so insistent on using transformers, as you can safely ground the secondary side.

    You probably have a shorted diode in your bridge somewhere. With no power applied to the bridge, use the diode test function on a multimeter to test all of the diodes to make certain that they are not shorted.
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    It might seem that ordinary AC power distribution got missed.
    What you are describing is a power supply that has no isolation from the line. What happens if a component fails? How do you insure that no terminal or powered appliance becomes tied to the 120 VAC supply line?
    The reason for that is that there is no way to provide safety for the user without one.

    Unless, of course, you have some means of doing so. Can you make this operate safely even with miswired and/or ungrounded outlets? It's going to be difficult without that transformer.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Beenthere,
    That's the direction I'm heading with the post I made just prior to yours; inductively coupled galvanic isolation.

    I think the simulation shows reasonably effectively that the output of a bridge wired to the mains has dangerous voltage levels on it.

    As I'd previously mentioned to our original poster, mains-powered SMPS have the output side galvanically isolated from the primary side using inductive couplers aka broadband transformers. What they want to avoid is the bulky, heavy and expensive 60Hz transformer. In order to do so, they will need to go to much higher frequencies which will permit a much smaller broadband transformer.

    I don't think our OP realizes that many modern bench supplies have SMPS's inside the box.

    I also don't think that they realize SMPS supplies are not 100% efficient (even 94% is difficult to achieve) and as a result generate heat that is proportional to the load. That heat has to go somewhere; normally dissipated by a fan-cooled heat sink.

    At this point, I'd say that they would be much better off converting an ATX form factor personal computer power supply to a bench supply, rather than attempt to build an SMPS from scratch. If they want to go the latter route, they have a long and steep learning curve ahead of them.
     
  13. LostTime77

    Thread Starter Member

    Dec 9, 2010
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    Thank you guys for the info.

    Sgt Wookie: I believe I stated this in an earlier posting, but let me state it again. The projects I have been working on for the past 3 years have been nothing but DC. I just recently started on projects involving AC. I believe it is very incorrect to assume that just because I am having a little trouble with the AC portion that we (I) have no idea about other topics such as SMPS. I have built switch mode regulators to go from DC to DC multiple times. I know what I am dealing with. Of course I know that an ATX desktop supply is an SMPS. I know power dissipation calculations. I am an electrical engineer, it is my job to evaluate currents and power dissipations on every single design down to how much power a single P or N channel mosfet will dissipate. Again, it is not correct to assume that because I am starting AC that I am just starting with electronics. I have been in the DC business for the past 5 years. I have always just used a simple battery or wall wart for power, never having to worry about doing the AC conversion myself. Usually when you start something new (AC) you make mistakes. That is why I come to this forum; so that I can get some help when I am having trouble.

    For your information we have converted numerous ATX form factor supplies into bench top supplies because they are very easy to convert. A simple 250 watt old dell supply that was going to get thrown out can easily supply a massive 20 amps at 5V / 3.3V / 12V.

    Now in light of the obvious safety issues regarding this matter, I have talked to one of the guys in charge of the project. The route we are going to go is building an SMPS outside of the car that can be plugged into the main box inside the car. This makes more sense as the SMPS circuitry serves no purpose and just adds weight to the vehicle when the vehicle is driving around. In this case we are going for safety over convenience. I will be buying a few AC to AC step down transformers to go from 120VAC to around 24VAC and then continuing building a customized SMPS supply. We want to build a customized supply as we would like a selectable voltage from say 0V to 16V. Ordinary ATX supplies are not selectable in this manner.

    In regards to questions, am I correct in assuming that once the transformer isolates the power supply (AC mains) from the primary to secondary side of the transformer, that it will become much safer in terms of the hot wire? What would be the best route to go? I think that a non center tapped transformer (4 leads) would be the best option for this design. I would wire hot to the top side of the primary and neutral to the bottom side, correct? What about grounding? In your previous schematic it showed that you grounded the lower side (N in your schematic). can I do the same with the lower side of the secondary; grounding (earth grounding) it? For current fault protection, would say a 10 amp circuit breaker in series with the hot wire be alright?
     
  14. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Quote - "I am an electrical engineer". Our education system at work :)
     
  15. LostTime77

    Thread Starter Member

    Dec 9, 2010
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    Unfortunately... my school does not deal with AC as required courses. Its a pity because perhaps I would have not run into this problem. This is where the DC stuff comes in. Most people I work with who have jobs at major companies as electrical engineers do not deal directly with AC either, so they don't know a ton about it. They are mostly in the embedded systems area and just do DC designs all day, much like what I am doing. Either way its one thing to know the AC theory, its another to build a practical circuit with it.
     
  16. SgtWookie

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    I see. Well, I hope that the simple schematic/simulation that I posted earlier provided some insight on what is going on with your bridge.

    You simply can't treat the mains side of an AC/DC SMPS as you would a DC-DC converter.

    I have an old converted 250W Compaq ATX sitting on my "bench"; it works just fine for fixed voltages.

    There's a recent thread where someone did in fact modify an ATX DC feedback to the SMPS controller IC to provide variable secondary voltages; the range is a bit narrower than you're considering, but it would be relatively easy/economical to modify one than build an entire supply.

    You could also use the 12v output on an ATXPLUS12 supply to feed a custom buck-boost or cuk converter; that would eliminate the entire AC switching portion from your design chores at a small cost of the efficiency overhead of the ATXPLUS12 supply. However, a COTS (commercial off-the-shelf) solution is going to be very economical to acquire and lead-time is nil. If you design your add-on SMPS with the standard 24pin motherboard connector, you won't even have to open up/modify the supply; just plug it in.

    The main ATX power board-mounted header is implemented with a Molex 39-29-9202 or equivalent. The ATX 12V board-mounted header is implemented with a Molex 39-29-9042 or equivalent.

    If you'd rather go with a single fixed-output supply of suitable output, MPJA carries a number of SMPS that are reasonably priced and readily available:
    http://www.mpja.com/products.asp?dept=465&main=1

    From what you were previously attempting, absolutely.
    However, the COTS ATX form factor supplies are already UL approved.

    Transformers designed for 120v/240v to low voltage AC frequently come with dual primaries. When operating on 120v, the primary windings are wired in parallel; for 240v they are wired in series. 4-wire primaries usually have two different colors so that you can match up the polarity of the windings. If the primary winding is just one pair of leads, they'll usually be the same color.

    When wired for 120v, you just use L1 and Neutral. For 240v, you would use L1 and L2 and not connect Neutral. You will realize a small power savings from 240v, as the current requirements for the primary are cut in half; thus losses in the building wiring and cord are reduced. You would still use a 3-prong power cord, although the prong configuration is different for 240v than it is for 120v (one of the blades is rotated 90° from the other blade.)

    The ground wire should be used on the secondary side of the transformer only. In the simple schematic I posted previously, the grounding of the neutral line at the output of the signal generator was to illustrate that at the breaker/service panel, the neutral line is grounded; however the further you get from the breaker/service panel, the neutral line may deviate from earth ground considerably due to large loads, particularly imbalanced loads, on the service panel.

    You can use a circuit breaker in the hot side. The circuit breaker should be the first item from the cord, then a power switch, then the primary side of the transformer.

    Since I don't know what your secondary side current requirements will be at this point, or what your transformer specifications are, it's kind of tough to make a suggestion at the moment. However, somewhere around 1.5 to 2x of (transformer VA rating/input VAC) would be good.
     
  17. LostTime77

    Thread Starter Member

    Dec 9, 2010
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    Thank you for your information / suggestions Sgt. Wookie

    I will consider the options :). I will first try and diagnose the problem with the bridge rectifier to see if my wiring was wrong when connecting mains directly to it. Of course, I wont be plugging it in, ill just check the circuit.

    Going with the safer transformer option or a ready built option seems like the best idea.


    I will post back if I find anything. Thanks for your time.
     
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