I was thinking more along the lines of using 4 of them in an H bridge with suitable MOSFET drivers... Relays are a little low tech and high current ones would be very bulky and expensive :LOriginally Posted by Max
Good point Nick, hadn't spotted that... Just stick a TZ85 on each pole then and job's a goodun :P (Kidding.. though could be an interesting experiment as to what a TZ can do :L)
No, these parts will not carry anywhere near the current stated - its a theoretical maximum in perfect lab conditions. Using the numbers in the spec sheet, one of these FETs can carry 25A without a heatsink. With a reasonably sized heatsink, six of these will carry 150A comfortably but the spec sheet doesn't state the maximum current that the case pins will handle. In many small to medium FETs the pins will melt between 50 and 75A so with a small surface mount pack like this, its not going to be higher than that. You can get much better cost per amp with the IRFP4004 FET at only £22.10 for ten.
Its also very hard to drive a bank of FETs - it takes an experienced electrical engineer and plenty of trial & error to make a reliable ESC.
Last edited by overkill; 17th June 2015 at 22:46.
The Botbitz guys tested the TZ85a at some crazy current like 500A, but only for a short pulse. Modern FETs have amazing performance but there are no free lunches when it comes to long term power handling
BTW: I believe that Sewer Snake use pairs of the Victor 885 ESC on it's Ampflow motors. They are only rated at 120A each and SS is a very reliable bot. Nowadays a pair of the Robot Power Vypre controllers might be a better and cheaper choice
I do know a fair bit about ESC design (all my robots use homemade ESCs. And yes, I will agree with you on the lots of trial and error front!). I wouldn't expect those FETs to handle 500A in real conditions, but I'm not seeing where you're getting quite such low current figures from (though those FETs don't have the clearest of datasheets)..
I do agree though it is generally better to use many smaller FETs than one big one; my first HW ESC used 32 x 100A FETs ('400A' per channel), which worked nicely until it caught fire due to a driver fault.. Making ESCs that work is easy once you know how... making ones that keep working seems to be the real trick.. The latest version is holding up well so far at least :L
I have a clever little spreadsheet that I copied from someone who actually knows what they are doing. Based on the thermal resistance from the junction to the case and from the case to a heatsink and calculating the RDS at a realistic 40 deg. C ambient temperature, the sheet shows the current handling without a heatsink and the thermal resistance of the heatsink needed at various current ratings. I have modelled several other common FETs in the sheet and it has proven to be quite accurate. I had to guess the thermal resistance from the junction to air, so the power handling could be higher.
I am not dissing that IXYS fet - it has some great specifications but it's still not a magic bullet for a HW controller and you will still need a bucket of them. Its a pity that they don't state the current handling of the pins; with a custom package like that its hard to tell. Keep in mind that this is a HUGE surface mount part at 32 x 25mm so an array of them will take up plenty of space, which is why I still prefer the TO247 case FETs.
Ah ok, fair enough. I currently use 7 pin AUIRF2804S in my HW ESC, mainly as they are cheap, have a good package current capabilities (240A), and have a nice low on resistance to help keep things cool. Works nicely, but I'm only running NPC-T64s at 24v so nothing as serious as Ampflows. My plan for my next ESC is to run a few of these in parallel, though they are quite large so could be more of a challenge.. May look into other packages as the whole lot needs to be printed on a 100mmx100mm board to get a good price from the pcb printers I use in China :P
Last edited by Rapidrory; 18th June 2015 at 00:22.
Haven't been here for a while so catching up from when I posted mantis video. I did have a full build diary of pics from mantis, did I give you a disk Mario of all the photos. Their was loads! Maybe I have a copy around, is the hydraulic motor not available in uk, I cant remember its spec but I think it was for a agricultural application.
On a somewhat related note, has anyone tried using these 800W scooter motors as Heavy drive motors? Having seen the 300W versions used to great effect in feathers, I'd have thought the 800W version could be quite good, given that they are readily available and are only £50. However I've never heard of anyone use them. Is this cause no one's tried, or has someone tried and they turned out to be unsuitable?
I remember seeing someone use them on a partially built heavyweight back in, I think, 2006. The robot never got finished and I don't think the guy ever made it to events, it was just a build thread on the old Technobots forum as they used to sell those motors. It did get to a driving stage but I can't remember how well it performed.
Not a particularly helpful post and I'll leave it to others with a better idea as to their suitability, but for something that seems to have similar specs to a Bosch 750 but for much cheaper, there must be a reason for their lack of widespread use in heavyweights other than people not having discovered them yet.
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