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Thread: What's your favorite 1 ohm current sense resistor?

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    What's your favorite 1 ohm current sense resistor?

    What's your favorite 1 ohm current sense resistor and why? Do you prefer something small that will act like a fuse or a big sucker that is unbreakable?

    How precise?

    I've used a few different ones but I'm contemplating the purchase of a hundred or so of something nicer like the Dale precision resistors.

    jamie

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    I'm guessing that no matter how consistent the resistor, it's fusing current is less well controlled or predictable than a fuse's opening current.

    Lord knows that's bad enough to work with.

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    Old Timer J M Fahey's Avatar
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    It will not be as reliable , precise, and definitively as fast as a fuse.
    It might take quite a few seconds to open and in the meantime, turn your amp on fire.
    Fuse wires are more exact and encased in glass/ceramic, much safer.
    In the old times, BW tube TV sets used "fusistors", large flat power resistors , encased in ceramic and dusted in sand (looked like a post-nuclear wienerschnitzel, sort of) which were designed to crack open as soon as overloaded.

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    Thanks for the replies. It sounds like no one is in favor of the fuse theory! I'd seen it mentioned somewhere but I couldn't recall what the story was.

    I didn't really intend any fuse action, I really just was curious what other people prefer. I'm leaning toward using a pair of 1 ohm resistors connected to test lead sockets like those pictures of the inside of a Komet amp that are out there in internet land. For my own uses I'd provide ground, plate supply and cathode test points as well as external access to the bias set pot. I was just curious about opinions of what kind of resistors you would use.

    jamie

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    If it's even theoretically going to fuse, it's more prone to drift from heating. 1% wire wound resistors are much more stable.

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    Senior Member redelephant's Avatar
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    I'm guessing this is for cathode current sensing? In that case I'd use carbon comps. They are incredibly pulse tolerant, non-inductive.

    Get a hundred little demons before they are unavailable, rumor has it they are no longer in production.

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    Senior Member redelephant's Avatar
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    Actually to be precise, a wirewound in parallel with a carbcomp is my fav. But if the choice is only one, I'd use CC.

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    CC does have a big pulse tolerance. But CC drifts, which ruins the accuracy of the current measurement you're trying to make. MOX is similar to CC on pulse tolerance. Wire-wound in low values has a low inductance, and you can get non-inductive WW if it's an issue. The leakage inductance of the OT on the plate of the output tube you're measuring current for is probably much bigger than the self-inductance of a 1% wirewound.

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    Thanks for your input. What part do you use, if you use anything?

    Sorry to be asking for specifics. Surely I'm not the only one that uses 1 ohm current sense resistors. I've used metal and carbon film and I kinda want something better but not a giant overkill 3 or 5 watt part.

    jamie

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    Mouser Electronics has in stock multiple 1 ohm, 1% wirewound resistors, some of which are the size of a 1/4W resistor but are rated at 0.4W to 1W. There are others in a selection of sizes even at the 1W level. I was going to put in a part number, but there are a bunch.

    Mouser has 1 ohm CC resistors, but the best tolerance is 5%, at least that they normally stock. They also stock 1 ohm 1% metal film in 1/2W sizes.

    Note that the peak operating power in a 1 ohm cathode resistor would only reach 1W with 1A of current running through it. Even a modest fuse in the B+ would keep a shorted tube from letting 1A through for an appreciable time. And normal operating power is trivial.

  11. #11
    Gaz
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    Specifically, these are what I've been using

    RS01A1R000FE70 Vishay/Dale Wirewound Resistors - Through Hole

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    Noodle of Reality Steve Conner's Avatar
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    I like the green enamel-coated Welwyn ones.

    http://www.welwyn-tt.co.uk/pdf/datasheet/V700.PDF

    They're British made and may not be easily available elsewhere.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Supporting Member tubeswell's Avatar
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    I like those little 1% 0.6W MF ones ('Made in Taiwan' I believe) cost about 2c each
    Building a better world (one tube amp at a time)

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    Senior Member Wombaticus's Avatar
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    If you look at modern commercial schematics for these, you will notice that they usually specify flame proof (fp) units, and often parallel them with a beefy failsafe diode.

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    Senior Member redelephant's Avatar
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    I'm really nerdy when it comes to my passives, not audiophoolic, but still I have fetishes...

    In the cathode I use a 3W 10ohm WW in parallel with a .05W 20ohm CC, sometimes I vary the total resistance, but always have a CC in there. From experience...
    I've had Vishay 3W WWs blow up on me a couple of times but never CCs (during torture testings). Even when using the parallel combo, I've had WWs blow up twice, and the little .5watt CC kept soldiering on. The last time this happened the CC increased in size by about 2x, and it's resistance changed to about 50ohms, but it didn't open up.
    Now this wont happen during normal ops, but it proved to me that the CCs are good to have. Besides I like the fact they are good to look at.
    MF and CF I will never use in such a position.
    I'm not going to give a lecture on resistors, since if u're interested u find all u need on google, if not interested ure already sleeping. Besides I suck at technical writings. But the CCs are really good components in some places, and the cathode is one of them.
    The CCs do have some nasty characteristics, but not so bad that they should get the shaft. Even 1000ppm tempco is only 0.1% per degree, and how much does the resistor go up in temp due to 50mA going thru? 50mA times 50mV aint a whole lotta love is it?
    Tubes, and the total circuit as a whole, drifts more than the little CC in that position. Your readings will be plenty accurate enough to determine bias.
    Granted, if only used for measuring steady state current, a MF or better a WW is best for getting the most accurate readings. But I want my CCs if passing signals thru!

    Not that this is anything to discuss into the everlasting BS ending in hurt feelings and sour smells, but that's my view.

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    I didn't realize there was this much to discuss about it! That's pretty cool!

    Maybe these:
    RS01A1R000FE70 Vishay/Dale Wirewound Resistors - Through Hole

    or these:
    MRS25 1 1%TR Vishay/BC Components Metal Film Resistors - Through Hole

    JT

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