Post by thread:[EE:] Switching 250A @ 12v

Hi Everyone, I've got a little project where I need to control a hydraulic power-pack with a PIC. The manufacturers tell me that the hydraulic unit can consume 250A max when it's running at full pressure. It's running off a 12v car battery. It will only operate for (max) 1 minute, twice a day. It should also only use about 100A as the load it's moving is quite small compared to it's maximum. It has solenoid valves which consume 4A at 12v. I need to control this unit completely from my PIC, by that I mean the (possible) 250A going in, and the solenoids. The solenoids I can happily do with a relay or a power MOSFET, but I'm having trouble finding something that can switch the 250A easily. Is it wise to parallel-up power MOSFET's? I've found some (IRFP3703) which are rated at 209A and 30V, but wasn't sure if they let their magic smoke out when they're paralleled.. Cheers, Tim

At 03:15 PM 3/16/2006 +0000, you wrote: {Quote hidden}>Hi Everyone, > >I've got a little project where I need to control a hydraulic power-pack >with a PIC. > >The manufacturers tell me that the hydraulic unit can consume 250A max >when it's running at full pressure. It's running off a 12v car battery. >It will only operate for (max) 1 minute, twice a day. It should also >only use about 100A as the load it's moving is quite small compared to >it's maximum. It has solenoid valves which consume 4A at 12v. > >I need to control this unit completely from my PIC, by that I mean the >(possible) 250A going in, and the solenoids. > >The solenoids I can happily do with a relay or a power MOSFET, but I'm >having trouble finding something that can switch the 250A easily. > >Is it wise to parallel-up power MOSFET's? I've found some (IRFP3703) >which are rated at 209A and 30V, but wasn't sure if they let their magic >smoke out when they're paralleled.. Tim: Have you carefully read Note 6 on the IRFP3703 data sheet? Why don't you use a contactor or relay? At two operations per day it will never wear out, and you'll probably get less voltage drop. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com ->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff

> Is it wise to parallel-up power MOSFET's? I've found some (IRFP3703) > which are rated at 209A and 30V, but wasn't sure if they let their > magic > smoke out when they're paralleled.. MOSFETS tend to parallel well, unlike their bipolar cousins, due to PTC affects. Adding some spreading resistance doesn't hurt. With currents of that magnitude it's probably enough to arrange the wiring so each FET has some series wiring resistance not shared by the others. eg rather than taking wiring X-A_B_C where ABC ae FETS, use XA, XB, XC, XD. Each lead has resistance which helps balance MOSFET loads. If you parallel several you can test the assembly at say 150A so a single FET could handle it and then measure voltages and currents for each FET to see how well they do share. As always at such current levels YMMV :-) RM

On Thu, 2006-03-16 at 10:44 -0500, Spehro Pefhany wrote: > Have you carefully read Note 6 on the IRFP3703 data sheet? I have now... You'd have thought they'd mention that a bit earlier on... > Why don't you use a contactor or relay? At two operations per day it will > never wear out, and you'll probably get less voltage drop. I'd prefer to use relay's yes, but I can't find a supplier of high enough current ones at reasonable prices. I've found some 180A automotive relay's though, wonder if I can parallel them...? Cheers Spehro, Tim

> MOSFETS tend to parallel well, unlike their bipolar >cousins, due to PTC affects. Adding some spreading >resistance doesn't hurt. With currents of that >magnitude it's probably enough to arrange the wiring >so each FET has some series wiring resistance not >shared by the others. eg rather than taking wiring >X-A_B_C where ABC ae FETS, use XA, XB, XC, XD. Each >lead has resistance which helps balance MOSFET loads. > >If you parallel several you can test the assembly at >say 150A so a single FET could handle it and then >measure voltages and currents for each FET to see how >well they do share. > >As always at such current levels YMMV :-) My understanding is that it pays to have separate low value resistors in series with each gate, going back to a low impedance drive. This helps stop miller effect capacitance from interacting between FETs and makes the turn off/on times between FETS happen more equally. About 10 ohms per gate seems to be the norm.

Well, it seems my salesman neglected to tell me there's a solenoid on the powerpack itself for switching power to the pump. The unit's just arrived and I see no point in switching the main feed to the pump when that's what the solenoid's for. The solenoid's coil is showing up at 6.2ohms, so it's only a couple of amps at 12v. Quite easy with bog standard relays or FETs... Thanks very much chaps, Tim

Just be sure to use a TVS across the solenoid coil, it will help protect your mosfet. P6KE18CA is what I like to use across the coil (12V). Regards, Bob On 3/16/06, Tim ODriscoll <.....timKILLspam@spam@stokepogesway.net> wrote: {Quote hidden}> > > Well, it seems my salesman neglected to tell me there's a solenoid on > the powerpack itself for switching power to the pump. The unit's just > arrived and I see no point in switching the main feed to the pump when > that's what the solenoid's for. The solenoid's coil is showing up at > 6.2ohms, so it's only a couple of amps at 12v. Quite easy with bog > standard relays or FETs... > > Thanks very much chaps, > > Tim > > -

On Fri, 2006-03-17 at 20:33 +0200, Peter wrote: > What's the inrush current for that pack ? 250A draw working may mean > 1500A inrush. And when you release it, it will load dump into the > switch. I'm not sure what the inrush current is, and the specs don't mention it. They just have a graph showing current and flow rate against pressure. >From the nature of the system though, surely the inrush current wouldn't be as high as the peak current as the motor has to build up to it's higher pressure first? Don't know for sure on that one though... > Is this the first time you deal with a 250A switch ? Yes ... Can you tell? :-) I've decided to go with the built-on solenoid control instead of trying to interrupt the main motor's power. The solenoid needs only a few amps, so it's quite a nice job for a relay. This hydraulic unit can be mounted right next to it's battery, so any 'problems' will be localised. Besides, I can always fit a smoke alarm in the same enclosure..... The discussions on heatsinking on this thread are most interesting too. Cheers, Tim

On Sat, 18 Mar 2006, Tim ODriscoll wrote: > I'm not sure what the inrush current is, and the specs don't mention it. There will be a maximum current draw when the pack is energized. If you use the same battery to power other things (like the control computer) then likely bad things will happen. > I've decided to go with the built-on solenoid control instead of trying > to interrupt the main motor's power. The solenoid needs only a few amps, > so it's quite a nice job for a relay. This hydraulic unit can be mounted > right next to it's battery, so any 'problems' will be localised. > Besides, I can always fit a smoke alarm in the same enclosure..... This is not really about smoke. The load dump and induced voltages from such currents can glitch your computer even if only conducted through ground wiring or chassis (or solenoid wires). By the way long wires make excellent radiators and conductors for RFI/EMP. Peter