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Thread: FETS instead of relays for track connections

  1. #16
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    Denmark
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    Hi Don,

    As a mock-up that will prove the concept your prototype is absolutely fine. In the real world the IRF630's will not survive, even if you have 2 in parallel, given that you will use a decent power supply for the track. Also the internal resistance of the 630 is 0,3-0,4 ohm, and overall this set-up will be beaten by a good relay. 9A is for sure not enough. The IRF has an internal diode that is able to handle 9A.

    I think that this is your way forward:

    1) Select a FET (or a pair) that will be able to handle all the current from the power supply that is used. Is the PSU a 2A Scalextric 9A is enough, is the PSU 40A/lane IRF1404 (maybe in pairs) is enough, and if the track is run by batteries that can deliver several hundeds of amps while they are being drained you will have to have something that can handle that, may several IRFB7530. Be aware that somebody at some time will short the track (and maybe burn a controller) pulling alle the current avaiable from the supply. It's impossible to underestimate the talents that are trying running slot-cars. Remeber that FET's die when really being abused, while relays just degrade.
    2) Make sure that you switch the FET's fully ON. If your track-voltage is 13,7 and you are swithing the gate on with a 24V-supply you will have a little bit more than 10V between gate and source. You might be better of with 15V or more. The FET's can handle 20V.
    3) The current needed to switch on the FET's is insignificant. A resistance to each gate of 1kohm will be absolutely fine, and the FET will still switch on 1000 times faster than a relay.
    4) Connecting the FET to the wiring harness of the track might be the biggest challenge. Clamping the drain-tap to a bar with a screw is fine, but even if you can get small connectors that fit perfectly a TO220 the will probably not (correction: for sure) handle the current needed. I would suggest that you make a small module on a printed circuit-board, and then this is what need to be changed if (or when) something burns.

    Good luck

    Steen

  2. #17
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    Jan 2008
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    mechanicsville Virginia
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    Thank you for that detailed response.

    Who can tell me the relay of choice that most tracks are using? We need to consider that option as well.
    Anything worth doing is worth overdoing

  3. #18
    Join Date
    Feb 2005
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    Steen,
    Your last post, while very detailed, doesn't make a very good argument to ditch relays for FET's, to turn on the track.
    "Get Yourself Retrofied"
    Chicagoland Raceway
    17B West Ogden Ave.
    Westmont,Il. 60559
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  4. #19
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    Indeed Mike, relays are looking better. What do you have on your tracks?
    Anything worth doing is worth overdoing

  5. #20
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    I have 2 sets of relays on my King.

    60 amp MDI mercury relays, for the serious race power, bought when I first built the track, and was "curing world peace", setting "World Records", with Wing cars.

    IIRC, they were $60-$70 each, 13 years ago.

    For the separate power per lane, for my casual rentals and parties, I use common 30 amp relays.

    These particular ones, I got from O'Reilly auto parts, for $7 or $8 each.
    https://www.oreillyauto.com/detail/d.../84601/4176379

    I have the same 30 amp, "cheap" type on my flat track, for 11 years, and they have worked fine.

    I'll post a few pics, if I can figure out the maze, you have to go through.

    20180310_115937-1.jpg

    20180310_115720-1.jpg

    20180310_115836-1.jpg
    Last edited by swiss; 03-10-2018 at 11:44 AM.
    "Get Yourself Retrofied"
    Chicagoland Raceway
    17B West Ogden Ave.
    Westmont,Il. 60559
    Phone-708-203-8003
    mikeswiss86@hotmail.com

    Check out Chicagoland Raceway on facebook.

  6. #21
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    Mike,
    FET's can certainly be better than relays, but you have to be carefull about which FET's you choose. The right fet's wiil be cheaper than the best relays and will give much better performance throughout the years.
    A brand new auto-relay of 30A has a resistance of 0,005 ohms, but that is increasing with use. I have seen that by measuring a lot of controllers.
    But at the same time its clear that the difference in performance can easily be exaggerated. A differen in resistance of 10mohm is not very much, and it might not be worth the trouble eliminating it.
    We use relays on our track, but if we were to build a new we might consider FET's.

    Steen

  7. #22
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    mechanicsville Virginia
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    If I lived nearby this track I would do it with fets. This discussion, and Steen's outline at post 16, has pointed the way if any track owner seeing this want to be the first to say his track has solid state switching. (Mike...?....mike...I know you want to do it...it would be up there with curing world peace.)

    At this track currently, the twist timers send power to each white post from across the room. You all can get back to setting world records. I'll be shopping for some medium grade relays.
    Anything worth doing is worth overdoing

  8. #23
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    Jun 2002
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    Home of the 2007 USRA Nats and a pretty fast King Track
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    Don, for he uses you describe, a standard automotive "Cube" type relay would be fine. the link provided has a 40 amp available at reasonable price. You could even make up a system where they are plugged into a connector making them easy to change out if needed. also, don't forget the diode if you go this route.


    https://www.allelectronics.com/categ...e/power/1.html
    There are "Racers" and there are "Talkers".....which one are you?

  9. #24
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    Quote Originally Posted by smichslot View Post
    Mike,
    FET's can certainly be better than relays, but you have to be carefull about which FET's you choose. The right fet's wiil be cheaper than the best relays and will give much better performance throughout the years.
    A brand new auto-relay of 30A has a resistance of 0,005 ohms, but that is increasing with use. I have seen that by measuring a lot of controllers.
    But at the same time its clear that the difference in performance can easily be exaggerated. A differen in resistance of 10mohm is not very much, and it might not be worth the trouble eliminating it.
    We use relays on our track, but if we were to build a new we might consider FET's.

    Steen
    What "unsold" me, was the chance of complete failure , when a racer shorts a controller.

    I have racers hook wrong regularly.

    Some guys that do, after they have an issue, and there is a track call to investigate, will insist they are hooked up right, when they aren't. LOL

    That doesn't work out with a commercial raceway.

    My cheap relays on my flat track have served me well.

    10+ years of non-failure.

    That said, a little power drop on the flat track would not be noticed.

    It's twisty, and as is, only has 3 - 14 gauge taps.

    Originally, I wired the test lane, much heavier, but the cars were too hard to drive.

    I have been a victim of a bad relay.

    It caught both Wayne Bramble and myself, at a Nat's warm-up race , on the Purple Mile, in Georgia.

    We both thought we blew motors, on Yellow lane, after the power went back on, after a track call.

    Luckily, his track call was early enough in the heat, that he changed motors in time to get back on the track, and confirm it was the lane.

    As slow as the cars were, when the relay was not working right, I can't believe that they hadn't figured it out earlier.

    There had already been comments that the secret to the track, was to find a motor, that would run on Yellow.

    Luckily that situation with Wayne and I, finally woke them up to the obvious.
    "Get Yourself Retrofied"
    Chicagoland Raceway
    17B West Ogden Ave.
    Westmont,Il. 60559
    Phone-708-203-8003
    mikeswiss86@hotmail.com

    Check out Chicagoland Raceway on facebook.

  10. #25
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    Jun 2002
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    St Charles, Illinois, USA
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    How much does the FET resistance increase when they exceed 125 Deg F. - or higher up to the solder melting point? Heat is the enemy of all electronics.
    l.d. kelley, M.A. Ramcatlarry@aol.com

    50 year pin 1959-2009
    Racing slot cars in America
    USRA 2009 member 0028, 2017 #404
    IRRA, ISRA/USA, Hardbodies 1/24 & 1/32
    retired raceway owner(for now)
    Omni/Cidex service center

  11. #26
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    Jan 2003
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    Denmark
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    Larry,
    The resistance of the typical power-FET will increase by 50% when the temperature of the junction is increasing from 25 degreeC to 125 degreeC. Heat is the main enemy of FET's.

    Mike,
    Since you are running power-supplies I would think that there is som over-current protection. Most modern, switching power-supplies are switching off when the current out reaches a certain limit. In that care it is relativley easy, and cheap, to employ enough FET's to be able to handle the maximum power that the supply can deliver. 3 pcs IRF1404 are able to handle 225A continuosly (and mich more in bursts, more than 500A), and where I come from 3 pcs will cost you $3,50.

    Steen

  12. #27
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    Jan 2008
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    mechanicsville Virginia
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    I will not be going this way for the track I am helping with but For $20 each what about something like this? Seems like it would work and be better than a mercury wetted relay

    https://www.mouser.com/ds/2/427/vs-f...10-1110749.pdf
    Anything worth doing is worth overdoing

  13. #28
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    That should work.
    You just have to make sure that you switch it on properly.

    Steen

  14. #29
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    Home of the 2007 USRA Nats and a pretty fast King Track
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    Don, why reinvent the wheel? a standard relay will work fine for your purpose at 1/3 or less the cost AND not require your expertise should a problem arise....just a thought.
    There are "Racers" and there are "Talkers".....which one are you?

  15. #30
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    mechanicsville Virginia
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    Indeed RK. I'm going with automotive relays. (post 27) However, having found those big chassis mount FET switches for $20 I thought it was noteworthy for anyone setting up a world class track. Who will be the first to say they have "no loss solid state switching.

    Did you check the specs? 435 Amps continuous, over a 1000 pulsed. 650Watts. 0.001 ohm on R. Switch 8 of them with a wall wart. jus sayin.
    Anything worth doing is worth overdoing

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