relay configured as a diode

Discussion in 'Analog & Mixed-Signal Design' started by m121212, Mar 20, 2018.

  1. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    I'm curious if this has been done before, and if anyone had any design ideas about how to do it.

    Hypothetical - let's say you wanted to make a relay actuate as a diode would.

    If there is a voltage across the contacts greater than some threshold, then the coil is driven so the contacts close. But when the contacts close, that voltage drops to zero, so you either need some time hysteresis and/or a forward current sensor.

    If the current reverses, you have to break the coil current, preferably as close to zero as possible. You need some hysteresis there as well, so you don't chatter on current noise close to zero amps.

    Anyone seem something like this before? Any design thoughts? I bet there will be some questions about why you would ever want something like this - it could be quite useful if you had high current and needed low loss.
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    It has, although using MOSFETs rather than relays (more compact, more reliable, faster operating, less power consumption, etc). Check out this "ideal diode" IC from Linear Technology. They have a whole family of such devices, including ones with an internal MOSFET as well as "ideal diode controller" chips which operate together with an external MOSFET of your choice.
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Not only did the synchronous vibrator used in old car radios rely on this...but also the drive motors of Diesel Electric locomotives before the advent of decent thyristors! (I remember as a kid the unique growl of those "mechanical rectifiers" on the trains running by our house...especially during start from a dead stop!)
     
  4. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    Neat.

    The LTC4357 (external FET) is almost a good fit.

    One problem is that I would need a continuous coil current for the main switch, different than gate drive. But maybe use a FET in between to control the coil current.

    The other potential problem is the low Vin,max. Some applications are 600V for example. Perhaps this can be divided down on both the input and load sides so the chip only sees something within its allowable input range.
     
  5. dendad

    Well-Known Member

    Feb 20, 2016
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    Switching high current with a realy and having it operate at 50 or 60Hz would wear it out pretty fast I would think. As well as anoying anyone in the area with the buzzing.
    Something like that is done just as a switch, having a triac to switch the current then a relay to short it out for low loss. The switch control circuit is aranged so the relay never switches the full load but relies on the triac to turn the load on and off. So the triac turns on at the zero crossing point, then the relay operates to pass the load current. To turn off, the triac is enabled, relay releases and the triac is then turned off and so opens on the zero current crossing.
    But this is not a diode, just a low loss switch.
     
  6. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    Correct, it's not meant to operate at 60Hz.

    With the triac concept, how do you automatically make the relay follow the triac? I think you are saying it's not automatic, and that's why it's not a diode function.
     
  7. Threeneurons

    Member

    Jul 12, 2016
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    Triacs turn OFF at zero crossing. They are turned on with gate excitation, at any time. Once ON, it stays on 'til the next zero crossing.

    The op's concept is generally called "synchronous rectification". It was done mechanically, in the old days, such as original automobile "generators" (as opposed to an alternator). As mentioned, some old car radio vibrators, also performed the rectification. The mechanical contacts where locked together, hence synchronized. Though, not all vibrators performed rectification. Many, if not most, still used tube rectifiers. Synchronous rectification made a comeback with low voltage switching supplies, using power FETs, when normal rectifier drop is wastes too much energy.

    What's the nature of the signal that requires rectification, and why don't you want to use other methods ?

    Relays, with their armatures having mass, have some delay time, from when the coil is energized until contacts close. Depending on the size of a relay, it could be 10's of milliseconds. Even tiny reed relays take 100's of microseconds to actuate.

    Are you switching power, or just a small signal ? If a small signal, why not an analog switch ?
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    You might get more focused recommendations if you describe what the problem is you are trying to solve, instead of little tidbits here and there teasing at the details.
     
  9. dendad

    Well-Known Member

    Feb 20, 2016
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    And with the old radio vibrators, the same unit mechanically generated the AC that was synchronously rectified so the contacts were on the same armature as the power switching so were synchronized. It would be a lot harder to synchronize to an external AC source, and nowadays I think impracticable, to have mechanical rectification.
     
  10. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    The diode allows for backup power connection to a 400VDC source not AC. Amperage is 400-1000A continuous.

    It's a lot of power. Not gonna try it with semiconductor, at least not for continuous flow.

    I'm not rectifying an AC signal. Just trying to connect to a backup source in case of power dropout, and prevent any possible backflow in case something goes screwy.
     
  11. ebp

    Well-Known Member

    Feb 8, 2018
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    A relay that can handle DC at 400 V and 1000 A is going to cost a huge amount of money.
     
  12. BR-549

    AAC Fanatic!

    Sep 22, 2013
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    How are you generating 400 vdc or higher at 400 to 1000 dc amps?
     
  13. dendad

    Well-Known Member

    Feb 20, 2016
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  14. Threeneurons

    Member

    Jul 12, 2016
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    Yeah, I, too, want to know, what's delivering 160K to 400KW !

    There are relays that big. They're called contactors. This one here, is ~$700:

    https://www.mouser.com/new/te-kilovac/te-kilovac-k1k/

    Back, in automotive "DC generator" days, they had a "cut-off", that disconnected the generator, when the car was idling, and its voltage dropped below the battery. Otherwise, the generator would act like a motor, in addition to drawing current.

    Check the surplus sites, and eBay, and see if you can find a rectifier module. New ones start at ~$100:

    https://www.mouser.com/Semiconducto...Tax1mbZgjdhqcZ1yopdgmZ1yokr6uSGT&Ns=Pricing|0

    It will need to be mounted in its own special bracket, and there also needs to be some cooling system. These big ones are commonly water cooled.

    But to move that kind of power around, you should already be spending a lot of money just on cabling, if not bus bars ! The most power I ever had to deal with was a 4KW laser for cutting steel. 4KW was the actual light energy of the laser beam. In addition to the laser inefficiencies, additional power was also needed to move the metal, the NC cutting head, and cooling system. In my case, the 440VAC 3 phase power came thru 4 "000" gauge cables (3 hots, & 1 neutral, "Y-hookup" [aka "star"]) according to the manufacturer guidelines. The 250A 3 phase breaker cost $1000, and the copper power cable came out to ~$2000. This was part of the customer (us) required prep work, prior to laser being delivered.
     
  15. ebp

    Well-Known Member

    Feb 8, 2018
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    Well, that contactor is a few hundred dollars cheaper than I would have expected. The lifetime rating is kind of fun:

    • Hot-Switching Performance, Resistive Load:
    50 A/1000 VDC: 25,000 cycles
    130 A/1000 VDC: 10,000 cycles
    200 A/1000 VDC: 3500 cycles
    100 A/600 VDC: 10,000 cycles
    240 A/600 VDC: 2000 cycles
    1000 A/1200 VDC: 4 cycles (Break Only)

    Well, four cycles is more than one can expect from some things - dynamite for instance.
     
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