Miscellaneous questions (mainly bridge rectifiers)

Discussion in 'General Electronics Chat' started by Gump, Jul 21, 2010.

  1. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hello,

    I have a couple of miscellaneous questions - if somebody would mind taking some time to answer them (or whatever help they can provide), it would be appreciated. :)


    1) Note: I got 3x RMS value from the page at http://www.kpsec.freeuk.com/powersup.htm#rectifier

    When building a regulated AC to DC power supply, why does bridge rectifiers Vrrm have to be 3x the RMS value? I'm in the UK, so the AC RMS here is 230V (see below for further question on this), giving a peak of (1.414*230) 325.22V, so why should I need a Vrrm of 690V if the peak is only 325.22V?

    Actually, come to mention that, take a transformer with the following stats:

    If the output voltage is only 6V, then why should a bridge rectifier Vrrm need to be more than (1.414*6) 8V? Similarly, the transformer has a current rating of 167mA, yet it seems the vast majority of bridge rectifiers start from 1A, why?


    2) It seems that the Max Vf for bridge rectifiers don't seem to really go beyond 3.3V - the most common values seem around the 1V mark, so how can this be used on a microchip that may require say 5V?


    3) Why would a bridge rectifier ever have more than 4 pins, although they aren't common at all looking at Farnells stock, I'm not seeing how there even can be anything other than 4 pins?


    4) The UK mains is 230V ± 10% (207V - 253V), does this mean any equipment that I design (or rather the DC power supplies) should really work at the top end of 253V, so when buying a transformer I should get one rated at least 253V rather than 230V?


    That should keep me going a while. :)


    Thanks in advance,
    Gump.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Some quickies

    1. You always want to have headroom in the rectifiers. Double the expected peak voltage is about right. Line transients cause voltage spikes.

    2. Vf is the voltage dropped across the PN junction of the diode. It is simply a loss to the system. It means the voltage on the filter capacitors will be a volt or so less than the prediction going from RMS to peak. A voltage regulator is the device to regulate the output voltage to the load.

    3. Our Ebook talks about polyphase bridge rectification.

    4. The ratings are for RMS voltages - the various manufacturers know to factor in peak voltages.
     
  3. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hi BeenThere,

    Thanks very much for the response.

    What I'm not quite understanding is why the rectifier has so large values (see the original question sentence starting with "Actually, come to mention that")?


    I was under the impression that diodes often have a voltage drop of 0.7V, so if I use a bridge rectifier, wouldn't this give a constant drop of 1.4V? How come there are so many different bridge rectifiers with different voltage drops?


    Very sorry, I don't think I've explained myself correctly. Our RMS voltage isn't necessarily a constant voltage of 230V, it can be anything between 207V - 253V, these aren't peak values. So if the RMS voltage can be up to 253V does this mean that any transformers I buy must be rated up to that amount?

    What I find strange is for example my mobile phone charger simply states:
    INPUT: AC 100-240V/50-60 Hz

    How come they seem to be capping things to 240V, yet the standard is 230 ± 10% (meaning it can go 13V past their 240V input?)
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Can you give some examples? As long as they rectify, what is the significance (showing my practical side here)?

    As far as the input upper limit, I would imagine that the various manufacturers are aware that the 240 VAC is nominal, and allow for the variance. They like to have some overhead on their stuff, too, so they aren't always in court explaining why the transformer failed at a line voltage of 254 VAC.

    I do not know what the insulating enamel is rated for, but most transformers in the US have a guaranteed rating of 5 KV between the input and the output windings. The things that might fail in a transformer would be the wire to wire insulation in the primary coil. That difference is not going to be very much at all.

    In the same way as in England, our distribution has been characterized as 117 or 120 VAC (it used to be 110). I measure 124 VAC in my line with a Fluke 23. There is always going to be some variance from nominal depending on you distance from a distribution transformer and the time of day - your line will almost certainly measure a bit higher late at night when most loads are off.
     
    Last edited: Jul 21, 2010
  5. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    A quick look on the Farnell site bridge rectifier page (http://uk.farnell.com/bridge-rectifiers) gives a list although I suppose as you say as long as they do their job it doesn't overly matter, I was just curious as to why there were so many different variations away from what I would expect from the standard 1.4V.

    Hm, fair enough, although what I'm a little confused over is the whole idea of the bridge rectifier being such high values (see the original post, first question, second part).

    Why would the bridge rectifier need to be 2x the peak voltage yet the transformer is okay at the nominal voltage? Why doesn't the transformer need to be 2x the peak voltage also to protect against voltage spikes?


    Thanks,
    Gump.
     
  6. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    Look at Schottky Diodes.

    You are almost correct that most Diodes are .7 Vfe. That is true for standard silicon diodes.

    The problem is that they are not as standard as they used to be.

    But for a 10 Amp bridge, building with lower Vfe diodes can cut the wasted power from 1.4Volts x 10 Amps = 14 Watts to .9Volts x 10 Amps = 9 Watts.

    That is a power dissipation improvement of over 50%. That increases reliabilty and decreases thermal management issues that might mean a large heatsinked package can be avoided - which actually makes the better diodes turn out to be much cheaper.
     
  7. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hi Potato Pudding,

    Thank you. :) I don't suppose you could tackle these two for me:

    Thanks very much. :)
    Gump.
     
  8. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    To understand how a bridge rectifier works, you must take into account that if you consider one of the AC wires to be the reference (let's call it "AC1"), the other wire (let's call it "AC2") swings positive and negative around that reference. If you take a look at the disposition of the diodes in a bridge, and see what happends when AC2 is positive with respect to AC1, and what happends when it's negative with respect to AC1. Then it should be obvious why there are two diode voltage drops (Vf) in the bridge.

    The diode's Vf is dependant of the diode type and current flowing thru it. It's not a 0.7v fixed value.


    Transformers are quite resistant devices. The voltage it's written is the voltage that is expected to be applied to it, and what secondary voltage is going to be present with that primary voltage. If you have a 230v/6v transformer and the input voltage is 250v, the output is going to be 6.5v.
    It's your task to design the rest of the circuit to be able to function in that conditions. The primary isn't likely to malfunction at all with such a slight increase in voltage, but as the secondary voltage is higher, it can have consequences in your circuit.
    The transformer could fail if the increase in secondary voltage causes your circuit to use more power than the transformer can deliver.

    Because 6V is the RMS voltage. The bridge needs to withstand the peak voltage. Take a look at www.allaboutcircuits.com about RMS, Peak, Average, etc. Anyways, it's bad practice to use a device that just meets the requirements. Some margin is always desirable.

    About the current rating, as bridge rectifiers are likely to be used with capacitive filtering, and capacitors are very low impedance when uncharged, when the diodes conduct, they will carry a lot of current. That's also why you need at least a 1.8A transformer to supply 1A current after bridge rectification and capacitive filtering. Transformer ratings are calculated for resistive loads, not the very brief high current spikes they need to supply when connected to a bridge rectifier with capacitive filter.

    Simulating this with any spice-based software like LTSpice would be very informative.
     
  9. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    A bridge rectifier will drop more than its expected 1.4 volts because of the composition of the diodes. A typical silicon diode will drop 0.7 volts - 0.8 volts at 1 amp, but at 10 amps that increases due to the structure of the diode, and due to the internal resistance. This is where Schottky diodes come into their own. A Schottky bridge drops less voltage; less power is wasted, so they are more efficient. However, they are more costly, so it is up to you to decide whether it is worth the improved efficiency. This is why you see the range in voltage drops.

    In the UK, and in many other places, the standard is in fact 230V -6% +10% (not ±10%), which is a range of 216.3 volts to 253 volts. Which is why you now see devices rated from 100V-265V (I've seen up to 275V before), because they utilise active power factor correction, which boosts the mains voltage to around 400V (d.c.) before switching it down into the 5V or so required to charge the phone. Although this sounds like a somewhat backwards and inefficient way of doing it, it ensures the output of the charger is consistent no matter what the input, and it is in fact much more efficient and cheaper than the old wall wart transformers (it also makes the charger lighter). Your mobile phone charger manufacturer is probably just writing on 100V-240V for the sake of brevity, or because they don't know what they're talking about; this is why you will sometimes see Pri:Sec ratings on switch mode wall warts as well.
     
  10. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hi all,

    Very sorry for not replying earlier, been reading all you've put and have been trying to absorb it. I don't think I'm quite grasping things fully, but I've taken your advice and got hold of Multisim and am going through trying to create a simple circuit for a microchip. I'll get another question up when I've gotten the hang of Multisim a little better and can make some simple circuits. :)

    Thank you,
    Gump.
     
  11. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    It has been mentioned, but you will find silicon diode Vf drops goes up dramatically as the current goes up. Multisim will help with that.
     
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