Is your unoptainium power switch worth $5 and an hour or so of your time?

Discussion in 'Solid State' started by K7sparky, Feb 26, 2013.

  1. EchoWars

    EchoWars Hiding in Honduras

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    Well, the manufacturer calls them Thyristors, so who am I to argue?

    In addition, they are referred to as:
    Alternistors
    High Commutation Triacs
    Bi-directional Solid-State Switch
    The little Fender switch can be used with a snap-switch if you can figure out how to mechanically actuate the switch. Get your brain in gear and see if you can find a way to make something like one of these work:
    http://www.digikey.com/product-detail/en/TMCGD6SP0040C/CKN1434-ND/484221
    http://www.digikey.com/product-detail/en/TFCJF5VT2540Y/CKN9915-ND/2055193
    http://www.digikey.com/product-detail/en/LCGHF5A10RC/CKN1426-ND/484150
    http://www.digikey.com/product-detail/en/TMCGD6SA1040C/CKN10057-ND/484219
    http://www.digikey.com/product-detail/en/ZMSL03130T11SSC/CKN10168-ND/2044454
    http://www.digikey.com/product-detail/en/TF2CFF5ST1440C/CKN10556-ND/5051903
     
  2. PViking

    PViking Member

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    @roger2, thanks for the reply. My problem has been getting the original switch re-assembled with all the internal parts in the right place. Twice it seemed I had it just right and then when operating the switch lever something comes loose internally and the lever is stuck. I think one of the metal "teeter totters" works loose from its pivot and gets jammed, which might be what the original problem was. The tiny red nipples and their springs seem in good shape. And now that I've opened up the switch three times I'm afraid the metal tabs will be breaking off from fatigue. But I might try again.
     
  3. EchoWars

    EchoWars Hiding in Honduras

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    Might note that I'd be very leery of trying to pass 120V through that switch. It was never meant to carry that kind of voltage.

    Safety first, pls.
     
  4. PViking

    PViking Member

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    @EchoWars, thanks for the idea. I was considering using one of those, called "micro switches" often for some reason, by pushing it with the original switch. I will get my brain in gear (1st) and see if there's a way to physically use the Fender to push the actuator. Might work.
     
  5. roger2

    roger2 . Subscriber

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    Agreed, that is a tricky operation. I have done it using a pair of vise-grips holding the back part of the switch steady (and masking tape holding the vise grips to my work surface). A vise mounted to a bench would be much better. That way both hands could be used to position the other half of the switch. I don't recall, I may have also used tape to temporarily hold the switch position the way I wanted it...

    Yes, everything has to be positioned perfectly and not jostled during re-assembly.
     
  6. elnaldo

    elnaldo Addicted Member

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    Back to this thread, since I have a Crown DC300A with a LOUD thump at power off, (it even burned a 1A fuse) caused by the power switch...

    And I remember the Peavey CS 800 uses this circuit:

    [​IMG]
     
  7. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    Technically speaking, for the non-engineer observers following this thread, a thyristor is any semiconductor switch exhibiting bistable action dependent upon p-n-p-n regenerative feedback. Thyristors include a variety of devices: SCRS, GTOs, TRIACs, LASCRs, etc. Hence, a triac is a thyristor, but a thyristor is not a triac.

    Now, while a triac is a very good choice, it is not the only thyristor suitable for this application. Paired, back-to-back, SCRs would work quite well, but require different gating circuitry. There are other thyristors that could be designed in as well. just know that whatever device you design in, you will require enough gating current to accommodate the the necessary di/dt (rate of change of current flow through the device over time) that occurs upon initial power-up of the amp (charging the big power supply caps), or you will eventually (possibly a significant period of time) punch through the thyristor through accumulated burned paths. If the gating current is too low, you will not get conduction paths, as the thyristor is turning on, of sufficient area to handle the inrush currents. Higher gate current results in more rapid expansion of conduction paths through the device as it is turning on each cycle.

    Another competing parameter would be the on-state current necessary to ensure that the thyristor latches early enough in each 1/2 cycle for full cycle conduction, or you will get buzzing and noise. If you have insufficient current through the device, in its on state, for the device to latch, or stay latched, it will drop out, and the gate will drive it back on, and on and on, until good latching takes place, resulting in noisy hash at the beginning of each cycle of conduction (and possibly at the end of each cycle).

    Devices that can tolerate higher di/dt, such as that experienced during amp power supply cap charging, generally require higher gating current and latching current for reliable full-cycle turn on and conduction. If you are taking percentages of the voltage you are switching, you must also consider what kind of gating current will be available at peak voltage, as well as at the desired start of conduction per half-cycle. Higher gate current at the start, may result in excessive available current at peaks, and early demise of the device.

    When powering an audio amp via thyristor switches, you MUST consider the fact that there is a very wide range of currents drawn by the amp, from inrush currents of 10s to 100s of amps at initial power-up, to milli-amps at very low output levels. It is much easier to design thyristor switch circuits for constant/predictable loads like lighting systems or industrial controls. Depending on the amp model, it might require quite a delicate balancing act to design a suitable switch, and impossible to design one that will work for most or all amp models.

    For reliable control of power thyristors, where low "control" currents or voltages are available, you might consider an opto-triac (an opto-coupler with a small triac as the output device).

    Also know that if you want to power an audio amp via thyristor switch, you are feeding a transformer, which is significantly inductive. Because of that, you will have a shifting of phase between the current through the switch and the voltage across the switch. If there is enough phase shift, you will have trouble with commutating the switch (getting it to turn off), also resulting in hash noise. A properly designed snubber may be required.

    In my opinion, a better choice would be an adequate switch, or use a relay, controlled by a less than adequate switch. The NRE for a solid-state switch exceeds the value of the resulting switch, unless the value is in the designing exercise, itself.

    Carry on gentlemen. Good luck.

    Enjoy,
    Rich P
     
    Last edited: May 4, 2015
  8. PViking

    PViking Member

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    Thanks roger2.
    And yet after seeming to get it all lined up properly more than once something wiggled loose and the switch got jammed up. That's what's so frustrating -- it seemed good until I toggled the switch twice (each time it was once or twice) and it stopped moving freely.

    It is certainly possible I put it together wrong. More than once I backed out and started over because I could see a part had moved or I suspected that. But it did seem correct those times I closed up the switch thinking I had done it right. I'm pretty mechanical and it seemed good to me.

    I'm going to try once more to rebuild the switch using the remaining good contacts. We shall see if it works.
     
  9. jessbob

    jessbob Active Member

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    A dumb question to all of you I am sure, I think I am reading the drawing properly and the way I am seeing it I need to have two new short pieces of wire to run from the switch to the triac. Is this correct? I am pushing 30yrs as a automotive mechanic but I am a toddler in this area this site has always been a great help to me and none of the people have ever talked down to me or made me feel,well lets just say not so smart and I am thankful for that and your time. Bob. I love my Lafayette LR-9090. Next mission 120 db.
     
  10. EchoWars

    EchoWars Hiding in Honduras

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    Well, from the switch to the triac, and one lead from the switch needs to go to a resistor.
     
  11. jessbob

    jessbob Active Member

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    Thanks for the clarification. I have been reading as many of your posts I can, well until my eyes start to burn. You are a very intelligent man and I thank you very much for responding I know the question was a no brainer to you but I wanted to be sure I was interpreting the hand drawn pic correctly. Bob.
     
  12. K7sparky

    K7sparky AK Subscriber Subscriber

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    Jessbob - my LR 120 still has the interposing relay. It was the last relay mod I did before starting the TRIAC mod and have never looked back. Actually it was the LR 120 that pushed me. The first ice cube relay (4PDT) 3 contacts in parallel welded once (only one set) and I next tried a 2PDT ice cube contacts parallel contacts have higher current rating. The 120 transformer has more inrush than the 9090. I haven't had the hood off the 120 again, but it will get a TRIAC when I do. A good thing about the 120 is you have 100% backup switch contacts.

    Relay contacts in parallel do not make and break at exactly the same instant. You do not really get the multiple make / break values you'd guess. There are specialty relays (and switches) with make before / after break and matched make / break times for the contacts. The ones I've had experience with were rated at ma level currents and cost BIG$:yikes:

    BTW Bob long ago we found new hire motorheads and farmers made the best trouble shooters. Your background makes you a natural.

    PViking The peak gate current listed on the spec sheet is the MAX current you can inject into the gate and not exceed the device failure threshold. Not what you get in this application

    The gate circuit is just a resistor in series with the switch to the gate. If your ohmmeter measures the contact as close to zero Ohms then it should be fine with this mod. The resistor value was taken from Little Fuse as minimum current to assure reliable gating as close to zero crossing as possible. The 100 Ohm resistor gets you about 120 ma depending on your AC line level.

    Is the lever switch you pictured in POST 38 an ALPS?:saywhat: A picture or 2 would help if you are still having problems. If it is not an ALPS dimensions and hole spacing would be a good thing to know :yes:

    The first LR 9090 I developed :dunno: this mod on has almost no silver contact nub left and after more than a year it is still OK. The good thing about the Alps rotary switch on the LR 9090 is that the NC contact for shut down never carried much current and is always in new shape. Worst case is a totally fried power contact. Just add an ice cube relay N/C for the shut down trip circuit and use the old ALPS N/C contact in the N/O position for the TRIAC gate circuit. Contacts are interchangeable. I used the almost burned up one as a test.
     
    Last edited: May 25, 2015
  13. K7sparky

    K7sparky AK Subscriber Subscriber

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    PViking
    If you are a scrounger like me, another source of nifty little snap action switches is old washers, dryers and the like. Top / door closed usually have levers attached that can be easily modified.
     
  14. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    OK please bear with me here because I'm pretty good with low voltage DC stuff like cars and FA systems but anything else I'm pretty useless at but am willing to learn.

    Would this mod be appropriate for a unit that may be operated on either 120 or 240VAC? Specific application is a Kenwood KR-9940 receiver; the power switch apparently directly switches the AC and it's fused at least twice (can't shut it off now.) Am looking for a solution to the issue, if I have to put a label on the back of the unit that it is now "120V only" I will but if it could be a transparent mod that would be even better.

    Also, I haven't read the whole thread yet (but I definitely am saving it for later) but could someone explain to me the purpose of the 100 ohm resistor? And would its value change if used on other than 120VAC?

    thanks!
     
  15. K7sparky

    K7sparky AK Subscriber Subscriber

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    The 100 ohm resistor sets the gate current. I think I got the value from the data sheet which you can download from Mouser or any of the data sheet sites. " Data Sheet Archive" is my favorite of the sites. Make sure you get the Littlefuse Alternistor data sheet. Way too much time has past for me to remember for sure.

    Littlefuse has lots of information on the Alternistor Triacs on their site. E4 is 6 to 40A units. AN1007 is a good general read. Note: Alternistor Triacs are made to work with high inductive loads and the R/C across them is not needed. I think this is where the 100 ohm came from.

    I did spend a fair amount of time with a scope across the Triac and a Variac on the incoming line to make sure the gate current was adequate for 120 VAC line. I seem to remember Spec sheet is for a pulse train trigger.

    If you can switch the resistor with the input line change you would have the better solution. I am not a great believer in gate leads dependent on switch contacts. So if it were mine I would just keep it on 120VAC.

    If you feel like a bit of lab work you might try a 150 ohm and a scope across the Triac if it is OK on 120 line, them rum it for several months on 240 and see if it fries. Probably go with a higher current unit as they require more gate current.
    The 100 ohm resistor sets the gate current. I think I got the value from the data sheet which you can download from Mouser or any of the data sheet sites. " Data Sheet Archive" is my favorite of the sites. Make sure you get the Littlefuse Alternistor data sheet. Way too much time has past for me to remember the source for sure.

    Littlefuse has lots of information on the Alternistor Triacs on their site. E4 is 6 to 40A units. AN1007 is a good general read. Note: Alternistor Triacs were designed to work with high inductive loads so the R/C filter across them is not needed. AN1007 may be where the 100 ohm value came from.

    I did spend a fair amount of time with a scope across the Triac and a Variac on the incoming line to make sure the gate current was adequate for 120 VAC line. I seem to remember Spec sheet is for a pulse train trigger.

    If you can switch the resistor with the input line change you would have the better solution. I am not a great believer in gate lead current path dependent on switch contacts. So if it were mine I would just keep it on 120VAC.

    If you really desire dual voltage and feel like a bit of lab work you might try a start value of 150 ohm and a scope across the Triac to see if it is still OK on 120 line, them rum it for several months on 240 and see if it fries. Probably go with a higher current unit as they require more gate current and in general can dissipate more energy. General setup would be a decade resistance box for gate resistor and a scope across the Triac. You need to look at the zero crossings of the sin wave and make sure the voltage at triggering is staying about 1.5VAC - the minimum for 2 junctions under load. A variac to lower the 120 VAC to make sure you are not too close to minimum triggering current. If you are going to use you amp in a cold location during the winter be aware the gate current will need to be a bit higher. The spec sheet rating is for the worst case of the worst of production still functioning - so you would have a good chance of success.:thmbsp:

    Looks like you have some mechanical repair time ahead with the switch. The TRIAC gate current is the only thing the switch would need to carry after this mod so if you can patch it up for low current you win:D
     
    Last edited: Jun 22, 2015
  16. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    OK, I *think* I get it. So you need the resistor to lower the current to the trigger so if I can find a value that results in an acceptable current value for both 120 or 240 I use that... right?

    I'm assuming that this is the data sheet that you used... yes?

    http://www.littelfuse.com/~/media/E...ittelfuse_Thyristor_Qxx40xx_Datasheet.pdf.pdf

    This is where it really shows that I'm not involved in electronics at all, I don't know which parameter I'm looking for on that sheet? (and I'm really sleep deprived ATM, that doesn't help either.)
     
  17. K7sparky

    K7sparky AK Subscriber Subscriber

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    You ARE getting involved in electronics. Like anything else you work your way to more knowledge. Some of us have been at it all our lives and some are just starting the journey, but we ALL are still learning! :beerchug: Knowledge is based on amount of equipment destroyed :D

    Short answer. If the Triac you use works in your circuit to your satisfaction then you are done.:thmbsp:

    The longer story

    http://www.littelfuse.com/technical-resources/datasheets-and-downloads.aspx?doctype=Product%20Manuals The bottom one has all the data and AN 1001 through AN1010. AN = application note. I think the 100 ohm came from AN1007

    look at the nameplate on your amp for total power consumption. P=IE so pick a worst case low voltage say 105VAC and calculate the max current. In general I select a device and rating of 2X that current minimum. If a buck or so would get me 10X I would go for it. Actually I bought a handful of higher current to use here and there. I would probably go with a 600 or 800 volt rating as I like LOTS of extra margin. I am lazy so go for isolated tab so I do not have to bother with the isolation installation hardware.

    The higher the current rating the more gate current so it is a trade off.

    Spec sheet using the sheet you posted

    Ig = I gate is the max current necessary to turn on any of that part number. 50 to 100 ma depending on part selected. Minimum is open could be -out of the hat - 5ma

    Igtm = max gate current without blowing it up and note it is > 10 us pulse = 4A in this case don't care

    Pg(av) = is average gate power which you care about as we are using the AC line to trigger. 0.8W would like to stay below 0.4W

    Vgt = gate voltage 2V

    If I was doing this for myself.

    I would just try a 200 Ohm resistor and see how it works on 120 VAC. If OK => Done :rockon:

    The 100 Ohm seemed low to me at the time, but since there was never a problem I did not look further.

    Among the tests to run is the voltage across the Triac in the on state. A scope to look at the zero crossing voltage would be nice if possible. Not sure if a cheapie DVM will give you accurate data or not. I use a Fluke or other good quality meter for this one. Some of the OLD Fluke hand held DVM are around for a good price. Old HP bench stuff can also be found cheap.

    A cheap laser type thermometer (like the ones at from HF) to measure temperature of Triac and long term heating of your power transformer. Transformer rule of thumb - 90% heating is from hysteresis loss and the other 10% IR loss from load. Gating problems will cause higher heating. You can also use a magnetic thermometer

    Before and after your mod leave the amp on at zero volume for 6 or 8 hours. Measure the power transformer temperature. Compare to temp before and after mod on 120VAC and should be the same. Zero volume is not the worst case, but easily repeatable and good enough for me. Don't get picky about a few degrees.

    Measure the on state voltage across the Triac at 240VAC (if you have a scope look at the zero crossings). It should be / look the same at 120 VAC around 1.2 to 2 VAC. Be aware of your meter spec for accuracy at low voltage. Lower on state voltage is always better.

    Most likely there are some holes in this and LOTS of good wonks here on AK and hopefully one will catch any mistake I made in this post
     
    bseech likes this.
  18. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    Awesome, thanks for the help and explanations.

    Now do I need to also get the resistor and capacitor shown in the wiring diagram a while back to go across MT1 and MT2 of the alternistor as shown in a diagram a while back?

    And I'm assuming from skimming this thread that I definitely should remove the capacitor that is on the stock power switch?

    Now I just need to find some time to go to the electronics supply place during normal business hours...

    For the time being, I picked up one of those super old school PC power strip things that used to sit under your monitor... am using that as the "power switch" for the receiver so I can listen to it. Other than a little weirdness in the right channel on FM stereo, it's fantastic (aux input sounds brilliant.)

    thanks again!
     
  19. K7sparky

    K7sparky AK Subscriber Subscriber

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    You do not need or want any filtering from MT1 to MT2 with an Alternistor :thmbsp:

    The cap across the switch gotta git gone. Caps pass AC. Enough MF would trigger the TRIAC :thumbsdn:
     
  20. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    So it's that easy, huh? just an alternistor and a resistor...

    guess I have to find time to swing by the electronics supply place when they're actually open (they make it kind of hard for people with jobs to shop there...)
     

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