higher current switching with MOSFET?

[evandude]

I haven't really delved into the realm of transistors very much in the past, but I have been struggling a bit lately with this problem...
I am trying to switch a 6 volt, 3 watt solenoid. I got an IRF510 MOSFET, and i can't seem to get the solenoid to actuate with it.
by just attaching the solenoid leads across a 9v battery directly, with a DMM between, i read somewhere around 350 mA of current when the solenoid is actuated, probably much higher when it first switches.
when i put the mosfet in there, and switch it by applying 9v to the gate, the solenoid doesn't actuate, but there is about 175 mA of current flowing through it, so the mosfet is at least switching to some degree.
the mosfet is rated to around 16 amps intermittent, 4 amps continuous current, so i know i'm not overloading it.
i was initially using 5v through a resistor (10k, 1k, 330 ohm, i tried several) to switch the mosfet...

my setup is just the solenoid, battery, and mosfet source/drain in series, and a wire from the + battery terminal to the gate to switch it with. pretty simple.
is there something I'm doing wrong?

 

[ljcox]

higher current switching with MOSFET

You said you have a 6 volt, 3 watt solenoid but you only measured 350 mA with a 9 V battery.

So there is something wrong. 6 volt, 3 watt implies a current of 500 mA. and a coil resistance of 12 Ohm.

I don't have the data for the IRF510 MOSFET so I don't know what its threshold is (however, 9 Volt should suffice). You need to apply a voltage to the gate in excess of the maximum threshold (but less than the maximum allowable gate voltage).

It appears to me, based on the info you have supplied, that your 9v battery is not able to supply the 750 mA. If you read 350 mA when the battery is connected across the solenoid, then this implies that the battery voltage has dropped to about 4.2 Volt.

A 9 Volt battery should deliver about 750 mA through the solenoid, so you need a battery capable of supplying this current. I suggest you read the battery voltage under load, ie. when connected to the solenoid.

It may also explain why the FET is not turning on fully, ie. if the 9v battery drops then there may not be enough gate voltage to turn the FET on fully.

You can prove this by using a separate battery for the gate.

You should also have a diode across the solenoid (cathode to the +9 Volt end asssuming it is an N channel FET) in order to suppress the back EMF when the MOSFET is turned off. Otherwise, you could damage the FET. So you may have damaged it already. So I suggest you connect the diode before you do any more measuring.

I also assume that you connected the solenoid between the +9 Volt and the Drain. If it was connected between the Source and the negative end of the battery, it will not work properly.

Len

 

[evandude]

hey, thanks for the reply... your advice has helped me a lot. I checked the voltage, but even when it was fully on, the battery voltage only dropped to about 8.5 volts (it's a 9.6v NiMH, runs around 9.0 volts unloaded on the current charge)

I did have the solenoid between the source and ground, as opposed to the drain and +9, but I switched it...

anyway, it is now switching properly, running around 310 mA after switching... My guess is that when it actually switches, it pulls a whole lot more current, but all I have is a DMM so I can't really measure that.

I am in my second year of Electrical Engineering in college, and it amazes me that we still haven't even touched upon transistors yet...

 

[Nigel Goodwin]

I am in my second year of Electrical Engineering in college, and it amazes me that we still haven't even touched upon transistors yet...

Transistors aren't really 'Electrical devices', they are 'Electronic devices', is your course Electronics as well as Electrics?.

I did TV servicing at college, which was all Electronics - the Electrical courses (at that time) didn't touch on Electronics at all. It was all big 'nasty' stuff in Electrics - we were accidently sent to the wrong room with the wrong lecturer for a number of lessons (until I complained, when it all got sorted out - my actual complaint was "I'm not doing that!"). In the Electrical lab (during these 'wrong' lessons) we were taught about phase shift - using a resistor and capacitor - but they came on wheels and were about three feet tall! - we powered them from live bare brass three phase (440V) terminals on the wall!.

 

[bogdanfirst]

well....i am a bit curious....
why use a 10A FET when you only draw 0.5A? a 1A transistor could do the job even better....it's also much smaller.....anyway, i still tink that there is something wrong with your selenoid....if it is 6V/3W, then at 9V it should definetelly draw more than 0.5A!!!!

 

[evandude]

well, i don't really know what to tell you... i'm going to try hooking the solenoid up to a real power supply sometime tonight, maybe see if it will draw some more current... but as I said, i imagine it uses a lot more current when it actuates than when it's holding.

the reason i'm using this mosfet is because it's what i had laying around.

I will try screwing with it some more tonight if I get around to it.

 

[ljcox]

You said "anyway, it is now switching properly, running around 310 mA after switching... My guess is that when it actually switches, it pulls a whole lot more current, but all I have is a DMM so I can't really measure that"

The solenoid has inductance, so the current will be 0 at the moment the switch closes and then it will grow expontentially to the level determined by the total resistance in the circuit.

As I said in my initial note, and as bogdanfirst also said, there is still something wrong. If the battery drops to 8.5 Volt under load, then the current should be 8.5/12 = 708 mA (neglecting the drain/source drop which should be small). So if you are reading only 310 mA, then either the FET is not fully on or your battery voltage is not what you claim.

I suggest you measure the voltage acxross the drain/source. It should be only about 0.1 Volt or thereabouts.

In general, if something does not work properly, measure all of the voltages and currents and use Ohm's law to check whether these measurements make sense.

Len

 

[evandude]

well i just did some more messing around with it... with my DMM set on the mA range, i read 300-something milliamps, but if i put it on the amps range i'm getting 1.7 amps!
i did the voltage testing again also, and the battery seems to be dropping to about 7.5 volts, then rising back to about 8.5 or 9 normally...

so my guess is that my meter was just reading low due to it being in the mA range... but then again. almost 2 amps seems quite high as well, especially from a 9v battery.

my dmm claims to be rated to only 400mA... it has a 10A jack, but when i put the lead in that, it doesn't read anything...
i give up, i guess. the fact that the current is about the same when using the FET as it is when just running wires directly from the battery to solenoid, makes me confident enough about it, so i'm gonna live with it i guess.

 

[ljcox]

Measuring current

Another way to measure current is to put a small value resistor in series and measure the voltage dropped across it. Then calculate the current using Ohm's law, ie. I = V/R.

In your case, I would use a 1 Ohm resistor. Thus the voltmater would read 300 mV if the current is 300 mA. This would give you an independent current measurement to compare with that measured directly.

Len

 

[Phasor]

my dmm claims to be rated to only 400mA... it has a 10A jack, but when i put the lead in that, it doesn't read anything...

Does the circuit operate with the lead in the 10A jack? If not, check the fuse in your DMM.

 

[ukeee]

MOSFETS do draw fairly large currents when they are switching (only on the switching transients). This is because they have internal capcaitances that need to be charged before the voltage can change and

I = C dV/dt

so the greater the voltage change and the faster you require the voltage to switch the more gate current is needed. If you have a look at the transistor datasheet the manufacturer will list these capacitances, from these you can work out the gate current required to switch the transistor at a certain speed. The gate resistor is then normally used to determine the gate current however if you are using a battery this may be determined by the battery. The large currents drawn are only for a very short period so if you are using a battery and having problems switching the MOSFET placing a capacitor in parallel with the battery may help.