Tweakers' Asylum

Be careful as the cathode may be connected to the tab...

71.131.220.233

[ Follow Ups ] [ Post Followup ] [ Thread: Display All Email ] [ Tweakers' Asylum ]

Al Sekela

(A)

In Reply to: Hex Fred diodes help..... posted by ABliss on December 31, 2006 at 11:12:47:

If your diodes look like the one in the link below, beware that the mounting tab is electrically connected to the cathode (-) side of the diode. Any heat sink attached to it will also be electrically connected unless you mount the diodes with insulating mica wafers. You can get TO-220 mounting kits that contain the mica washers and insulating sleeves for the mounting bolts from electronics supply houses.
Heat-sink mounting is a good idea, as HexFRED diodes do not have overload capacity like conventional diodes. The inrush current upon power-up may burn them out.
You can solder them with the skinny legs (with care), but the diode can then vibrate easily unless it is held down in some fashion.

I-R datasheet (pdf) for HexFRED (Open in New Window)

Follow Ups:

Re: Be careful as the cathode may be connected to the tab... - ABliss 16:24:21 12/31/06 (4)
AL, thats exactly how it looks. But mine has DESEI12 writen on them.
So you are saying, I should make sure however I mount them, that heatsink tap must not be grounded to the chasis?
Ok, I got that, now how do I tell which leg is in or out?
I hope these are really worth the effort, I have been thinking of tring these for years, 'till now the price doubled.
Thanks again.
- A couple of ways to identify polarity... - Al Sekela 11:09:00 01/01/07 (3)
  The easiest is with a multimeter that has a diode function. These read out a voltage that comes from applying about three volts through a resistor to the device attached to the test leads. The diode will read the open-circuit voltage in the reverse direction, and some value between 0.5 and 1.2 volts in the forward direction. HexFREDs will read a little higher than conventional silicon rectifier diodes in the forward direction. Compare the diodes to a known-polarity rectifier to be sure of the meter polarity.
  If you lack such a multimeter, but have a simple one, you can gin up your own test circuit with a couple of dry cells and a resistor of about 1000 ohms or higher. Test for the voltage across the diode as above.
  If you know the manufacturer of your diodes, you can likely find a web page similar to the IR page I referenced. It is likely your diodes have the same pinout as the IR diodes, since they are sold into the same market.
  HexFREDs and similar rectifiers have a good reputation because they reduce a common problem in linear power supplies. Conventional (p-n junction silicon) recifier diodes store some electrical charge as part of the process of forward conduction. When the applied voltage reverses, this charge is swept out of the diode as a reverse-current pulse with a very steep trailing edge. The trailing edge steepness contains a lot of RF energy that stimulates the transformer and remainder of the AC supply circuit to ring. The ringing is a resonant phenomenon that induces noise in nearby audio circuits.
  HexFREDs were designed to minimize the stored charge because this is a significant power loss mechanism in switching power supplies. These types of diodes are good for audio, but the DIY audio market is far too small to have supported their design and production. More and more switching power supplies are being used, so I'm sure the demand for these diodes is growing.
  There are now some high-voltage Schottky rectifiers made of silicon carbide. The Schottky diode does not store charge in the same way as silicon p-n junction diodes. The down-side of these is that silicon carbide is a refractory material and very difficult to make in single-crystal form. CREE makes silicon carbide Schottky rectifiers, but I've heard of reliability problems with the early versions.
  - Re: A couple of ways to identify polarity... - ABliss 14:27:56 01/01/07 (2)
    Al, thanks for confirming what I discovered last night as I used my trusty Tenma 72-4025 meter and checked out the Hex Fred.
    I found that the heatsink is connected to the left leg as you said, so I figure thats the negative leg.
    Going across the two legs with the black common lead on the neg leg, and the red lead on the right, the meter reads .6 volts.
    When I reverse the leads it shows open.
    So thanks again for your detailed help. Now I just have to figure out the mounting in the tight space I have to work with.
    From the other info you gave, it seems like they are really worth the effort.
    What do you think of what the guy above said about only needing to change one in the bridge to get the full benifits?
    That dosen't seem logical to me.
    AB.
    - He's wrong. - Al Sekela 14:45:51 01/02/07 (1)
      All the diodes in the bridge store charge if they are junction diodes. The stored charge is the source of the noise. Changing out one diode would reduce the noise by no more than 25%.
      - But aren't we all? ...at times - pmkap 13:05:00 01/05/07 (0)
        
        In Reply to: He's wrong. posted by Al Sekela on January 2, 2007 at 14:45:51:
        
        It would have been nice if Bare had actually explained wtf he was talking about. I guess its one of the double super secret probation things, and he'd have to kill us if he told us. Ignoring the implicit threat, I'll opine that he was (non)suggesting that one could put a single extra bespoke diode on the b+ output of the rectifier bridge, and the result would be an extra diode drop as well as imparting the turnoff characteristic of that added rectifier to the whole bridge. Dunno if it would actually work, but this has been discussed over in the TubeDiy Asylum.
        As to the need for heatsinking the hexfreds, in this specific application, I don't see the need for it. According to AES, the PH1 consumes 20watts. Conservatively assuming 4 watts for the heaters, one is left with about 16watts consumption. At a conservative B+ voltage, lets guess 100ma through the bridge. If there is a point of failure, its going to be the turn on, when the discharged caps represent a load of close to their ESR. The propensity to blow at turn on is going to depend on exactly where in the AC cycle you turn the PH1. One could model this in PSUD and check the first cycle parameters against the rectifier's 'nonrepettitive peak surge current'from the spec sheet. Regardless, added heatsinking isn't going to help in dealing with the non intermittant turn on current pulse, it will either fail at that point, or not.
        FWIW,
        Paul

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

You can not post to an archived thread.