Looking at your schematic in reply #7, it shows the mosfet drain and source terminals the wrong way round. But I think you must have wired them up the right way round, otherwise the solenoids would be permanently on.What you need to do to get that circuit working is:1. Connect a pulldown resistor between the gate terminal and the source terminal of each mosfet. 470 ohms is about the right value.2. Do away with R4, you don't need it because you are supplying the optocouplers with only 7.5V and it will prevent enough voltage getting to the mosfets gates (once you have added the pulldown resistors). Connect the collectors of the opto isolators direct to +7.5V instead.One problem with using opto isolators with mosfets is that opto isolators turn off quite slowly. This in turn means that the mosfets they are connected to turn off slowly, which causes them to dissipate more power. Probably not a problem if you are just turning the solenoids on and off, but a problem if you want to use PWM.
What I still dont understand is how doesnt power from one optocoupler go through the pulldown resistor onto negative lead and then leak in on other pulldown resistor and activate the other FET.
Quote from: Grumpy_Mike on Dec 16, 2012, 12:57 amI mean:-The emitters of the optos should be connected to ground, the -ve of your 24V source and the FET gates should be connected to the collector of the FETs.Oh, now I get it (I read the sentence wrong). Do you mean like this?
I mean:-The emitters of the optos should be connected to ground, the -ve of your 24V source and the FET gates should be connected to the collector of the FETs.
Not at all like that. You are grounding your gates; there is no chance those FETs will turn on.
Also, there is no such thing as a "collector" of a FET. There are collectors on BJTs, and on phototransistors.
I would try a 1 kOhm pull-down and see how that goes.
As long as you don't drive these guys at PWM rates (just on/off,) that'll probably be fine.
I did not know what exactly grumpy_mike meant with collector, so I looked on Wiki and saw that its the drain.
QuoteI did not know what exactly grumpy_mike meant with collector, so I looked on Wiki and saw that its the drain.Wehe I said the collector I meant the collector. There is only one component with a collector and that is the output of the opto isolator.If you are determine to miss represent what I said I am out of here.
... and the FET gates should be connected to the collector of the FETs.
All FETs have source, drain, and gate terminals that correspond roughly to the emitter, collector, and base of BJTs.
And what could I do to also allow PWM? Cooler or some better component?
QuoteAnd what could I do to also allow PWM? Cooler or some better component?The danger with your pull-down resistor being as low as 470 ohm is that the optocoupler may not be able to pull the gate high enough to fully turn on the MOSFET. There are two concerns with driving MOSFETs for high power:1) Get a high enough gate voltage to drive it all the way on. This is often a voltage that's twice the rated "Vgs threshold" voltage -- 10V is not uncommon. 7.5V can do it for many devices, too.2) Get enough current to turn it on quickly. You want ideally several amperes for a handful of nanoseconds in the really high-power cases. Working with milliamperes means it will take much longer to turn on the device, which is still often OK, as long as your duty cycle is long (so, no PWM in that case.)The problem is that the photo transistors aren't high-current drivers. If you want to drive heavy loads with PWM, you want a dedicated MOSFET gate driver circuit, such as the International Rectifier series: http://www.irf.com/product-info/cic/fsgatedriverics.html or ST microelectronics: http://www.digikey.com/product-detail/en/TD351IN/497-4440-5-ND/725331Note that those drivers, in turn, ONLY allow PWM; they don't work well with prolonged 100% on cycles, because of the way the gate boost capacitor works.Sadly, most of these chips are going obsolete, because power switching is going all surface mount and integrated controllers now. Something like the FAN7390N would make a nice driver chip, too.The driver chips can replace both your opto coupler and your pull-down resistor.
The danger with your pull-down resistor being as low as 470 ohm is that the optocoupler may not be able to pull the gate high enough to fully turn on the MOSFET.
You want ideally several amperes for a handful of nanoseconds in the really high-power cases.
Note that those drivers, in turn, ONLY allow PWM; they don't work well with prolonged 100% on cycles, because of the way the gate boost capacitor works.
that allowed Gate -> Source current was 62 A.
But isn't lower resistor -> more current -> better switching? Please, correct my logic. Or did you mean low current?
If drivers cant work 100% on cycles and optocouplers are too slow for PWM, is there any way to have both, PWM and 100% on cycles?
First, I saw that h-bridges have capacitors on them to smooth out voltage spikes caused by the motor. Its usually 10uF caps. From what I have seen, none of the solenoid driver examples for Arduino I find online have them. Is there any special reason for that or is it left out to leave the example circuit as simple as possible?
Second thing, I want to get rid of the additional power supply (7.5 V) because its very impractial for such a simple circuit to require two adaptors. With Arduino, they need three: 5V, 7.5V and 24V. Here I bump onto a problem: The max gate voltage for FET is +- 20 V (and this goes for most FETs I find in stores), so I cant directly use the 24V power supply for switching. The optoisolators have max collector-emmiter voltage 80, so they dont have a problem with this. What can I do to be able to use the power supply for solenoid also for switching the FET?
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