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FET switches
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how many amps do you want to switch
5
0%
 0%  [ 0 ]
10
0%
 0%  [ 0 ]
20
11%
 11%  [ 1 ]
30
11%
 11%  [ 1 ]
50
33%
 33%  [ 3 ]
100
22%
 22%  [ 2 ]
150
0%
 0%  [ 0 ]
200+
22%
 22%  [ 2 ]
Total Votes : 9

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Valen
Experienced Roboteer


Joined: 07 Jul 2004
Posts: 4436
Location: Sydney


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FET switches

there seems to be some interest in fet switches over relays this thread is to guage interest and see what is required.
advantages of fet package over relays
higher current rating
solid state so no moving parts to randomly turn on/off
solid epoxy package (if desired) to make it almost bullet proof
easy to repair (if its not encased in epoxy)

disadvantages
potentially slightly higher cost
static sensitive device (as is the IBC)

a back of the envelope calculation seems to indicate that for the moderate current and up devices (30A+) the fet switch and relay are about the same cost.

several options availablle
50A parts cost ~ $25 or so
100A $30
150A $45

what other features do you guys want?
some kind of controller so you can plug these into servo lines and go direct from that?
possibly having multiple switches per chanell

Post Thu Aug 26, 2004 8:21 pm 
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timmeh
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Joined: 20 Jul 2004
Posts: 2523
Location: Victoria


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Personaly id like something i can just plug into the reciever to turn on a motor something 100amps or more in epoxy or whateveri d be willing to pay $40
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Post Thu Aug 26, 2004 9:01 pm 
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Spockie-Tech
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Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia


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Something like these widgets might be worth considering for your Fet switch design. I've been looking at them for a possible future motor controller with current and thermal limiting and no requirement for HIP chips.

http://www.irf.com/product-info/hexfet/ips.html

They look like they would be fairly indestructible and easy to drive and arent too expensive.

If you want to go for a little microcontroller on board so you can plug it directly into a servo, the pic-axe08's are nice'n'cheap and can do the job, plus they're easy to program with RC pulse decoding commands built in. Dont forget to include failsafe code though for weapon switches.
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Post Thu Aug 26, 2004 9:14 pm 
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Valen
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Joined: 07 Jul 2004
Posts: 4436
Location: Sydney


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not to be narky but
"Ground noise generation of less than 10mA is the industry's lowest figure."
kinda puts me off with all thats happened lol

i was just going to low side switch

cmos inverter for drive or TTL OC output for the 30V ones
swtich from gnd to 12V

Post Thu Aug 26, 2004 10:38 pm 
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JohnMuchow



Joined: 25 Aug 2004
Posts: 21
Location: New York City, USA


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quote:
Originally posted by Spockie-Tech:
Something like these widgets might be worth considering for your Fet switch design. I've been looking at them for a possible future motor controller with current and thermal limiting and no requirement for HIP chips.
http://www.irf.com/product-info/hexfet/ips.html


We've used the IPS0551T extensively and they've been almost indestructible. We weren't driving them at high frequencies so we skipped using a FET driver and they turn on/off just fine (in about 100usec.). Each device easily handled 20A continuously without a heat sink (Super TO220 pkg, 4oz. copper on PCB).

The only problem we had is that you need to match your surge current levels with their specs very carefully. We needed to use twice as many of them as we thought (when considering the steady-state current thru each device) due to the very high surge current levels of our application. When the surge-current spec was exceeded, the device turns off and you need to cycle power to the device to get it to reset.

I think we paid about US$6 each in 50pc. lots.

Infineon and ST Microelectronics also make intelligent/smart FET power switches but IR's IPS0551T had the highest surge current rating and lowest Rds-on of all the devices out there.
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Post Fri Aug 27, 2004 2:14 pm 
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Spockie-Tech
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Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia


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quote:
Originally posted by JohnMuchow:
We've used the IPS0551T extensively and they've been almost indestructible.


I like this Laughing. is that like "almost unsinkable" Wink - still, they should be less blow-upa-able than a normal Fet.


quote:
When the surge-current spec was exceeded, the device turns off and you need to cycle power to the device to get it to reset.


How about using the micro driver to periodically pulse the gate drive off and on again once every half second or so to reset it if the current limit triggers ?


quote:
Infineon and ST Microelectronics also make intelligent/smart FET power switches but IR's IPS0551T had the highest surge current rating and lowest Rds-on of all the devices out there.


Thats what I noticed. The operating voltage is a little lower than I would like (40v) which might make 36v operation a little close to the wind, but I dont see any other devices with in built limiting and a higher operating voltage out there.
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Post Fri Aug 27, 2004 2:47 pm 
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JohnMuchow



Joined: 25 Aug 2004
Posts: 21
Location: New York City, USA


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quote:
Originally posted by Spockie-Tech:
I like this Laughing. is that like "almost unsinkable" Wink - still, they should be less blow-upa-able than a normal Fet.


Absolutely. We short-circuited them, overheated them, and subjected them to 100V Vds (they just turn on to protect themselves) without problems. They will blow up if you keep overheating them without allowing the device to cool sufficiently or try to turn it on/off at too high a frequency. The App Notes address both of these failure modes though.



quote:
How about using the micro driver to periodically pulse the gate drive off and on again once every half second or so to reset it if the current limit triggers ?


Dropping the gate voltage won't reset it, you need to bring Vds to 0V (if I remember correctly).



quote:
Thats what I noticed. The operating voltage is a little lower than I would like (40v) which might make 36v operation a little close to the wind, but I dont see any other devices with in built limiting and a higher operating voltage out there.


You might not be able to use them with 30-cell (36V) packs as the IPS0551T turns on if the Vds exceeds 42V (with the batch we had) and a freshly charged 36V pack can exceed that. But, if 36V was your max, no problems.
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Post Fri Aug 27, 2004 3:00 pm 
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timmeh
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Joined: 20 Jul 2004
Posts: 2523
Location: Victoria


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VROOOOMMMM way over my head lol Laughing

Anywayz how long do you thing before some 100amps+ fet switches will be avalable to buy?
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Post Tue Aug 31, 2004 7:54 pm 
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timmeh
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Joined: 20 Jul 2004
Posts: 2523
Location: Victoria


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@VALEN

Do you make small relay switches that can be used for controlling a flip input on the ibc? Or eaven a very small fet switch with like only i fett enoughf to controlle the ibc's flip input or ground it with a relay?
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Post Mon Nov 29, 2004 9:17 pm 
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bigjimmy



Joined: 18 Jun 2004
Posts: 40
Location: New South Wales


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Is there a schematic or something for these fet switches yet?

I don't know a lot about how fets work and all that yet but it wouldnt be that hard to make one with like a picaxe 08 micro? (correct me if i'm wrong)

Post Sat Dec 04, 2004 8:50 pm 
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bigjimmy



Joined: 18 Jun 2004
Posts: 40
Location: New South Wales


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Does anyone have any of these IPS0551T MOSFETs? They sound really good for playing around with.
Would anyone like to sell me some of these Fets?
Name your price.

Post Wed Dec 08, 2004 11:42 am 
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Ajax
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Joined: 17 Jun 2004
Posts: 298
Location: Sydney


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I tryed to bye some but there were none in Australia and I did't want to bye 100 + just yet.

I was able to get 3 IPS5551T FET's in stead. (Sorry they are for a speed controller for my weapon.)

There a hight side Fet so some of the cct is going to be a bit different.
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Post Thu Dec 09, 2004 12:08 am 
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Waddy the phoenix



Joined: 16 Jun 2004
Posts: 971
Location: sydney


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@sam just go down to your local dicksmith and buy a cheap mercuary switch (looks like an led but when turned upside down the mercuary falls down and exposes the terminals its a rather simple construction you just have to mount it in such a way that it wont turn around through the match and activate the flip control
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Post Thu Dec 09, 2004 9:36 am 
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bigjimmy



Joined: 18 Jun 2004
Posts: 40
Location: New South Wales


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Is there any other mosfets like that that dont need a HIP chip to drive them, or is there an alternative to HIP chips? (a transistor???) I just hate looking for the designs for a high current controllerv and finding out I have to use a HIP chip. They seem really confusing and I havent found anything on how to use them or whatever.

Post Fri Dec 10, 2004 2:10 pm 
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colin



Joined: 16 Jun 2004
Posts: 102


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um.. I think the HIP chip just simplifies the implementation of the H-bridge, so i don't think you can get around it....could be wrong though

Post Fri Dec 10, 2004 6:32 pm 
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