transistor to use with propeller
mikea
Posts: 283
Hi, can anyone recommend a transistor or mosfet that is fully turned on by the 3.3 volts of the propeller and can handle at least 12 volts? Right now all i'm doing is switching the coil of a 12 volt auto solenoid. Thanks-mike
Comments
For 12V a darlington transistor is an obvious choice since the 1V drop isn't a problem (heat dissipation might be if really is 10A).
Simple bipolar switching transistors like 2N2222 good to several hundred milliamps, superbeta devices like ZTX851 can handle several amps without needing heatsink.
MOSFETs specified for 3V3 drive are usually surface mount in my experience. SSM3K02T is an example, 0.2ohm at 4V Vgs, 0.25ohm at 2.5V Vgs:
http://uk.farnell.com/toshiba/ssm3k02t-te85l-f/mosfet-n-ch-2-5a-30v-sot23/dp/1714376
Remember to use a free-wheel diode connected across the load as its inductive - this is required.
A very nice MOSFET is the IRF3708
IRF3708 30V 62A 12mO Vth2.0 TO-220
Guaranteed specifications at Vgs at 2.8V of 29 mOhm.
Here are lists of devices I've used:
http://www.redrok.com/electron.htm#nch
Duane J
In that case a 2N2222 or equivalent with 220 ohm base resistor should do I think. And any small diode (even a 1N4148) will handle free-wheeling.
Cheers
Richard
For bipolar transistors you need "Current Limiting" resistors.
For MOSFETs the resistor is, strictly speaking, not required.
However, "Pin Protection" resistors are often used in case there is damage to the MOSFET's gate. Maybe 1K in value.
Also a high value resister, 100K or so, as a pull down to guarantee the MOSFET is off when the pin is set to an input or power is shut off.
Duane J
Ive been using 1k from gate to source while trying to make an h bridge. Still hav'nt resolved the problem yet, but the i/o is 3.3v and i don't think it turned the mosfet on/off all the way. I will switch to 100k and try the irf3708 like you recommended, but just for reference is the 1k alright and only an unecessary power drain? I appreciate the info, i would like to understand these better so i can get past the h bridge hurdle.Thanks-mike
A pull-down on a N MOSFET G-S is a good idea (and a pull UP, also G-S, if you ever drive a P-MOSFET), because the Prop resets Floating-pins. They do not need to be as low as 1K, as they are there to define a floating state. 33K-100K would be fine.
If you are driving a H-bridge, how are the upper-switches handled ?
Some of the simpler designs use a SPCO relay on the upper arms for Direction, and 2 N-FETS on the lower arms for PWM control.
Try this Simple MOSFET H-Bridge.
Duane J
Maybe an app Note to properly run and select a Fet so people learn something
Base is a low impedance input (100 ohms ballpark), it can't float, and secondly a bipolar transistor is current-controlled so it wouldn't matter. MOSFET gates are totally insulated (10^13 ohms is typical for small devices!), and are strictly voltage controlled.
Having said that sometimes a base-emitter resistor is added to speed-up switching-off of a bipolar transistor, it helps to clear the stored charge in the device after it's been conducting large currents.
The prop is reading a r/c receiver and switching a 2n2222 (w/100ohm resistor at the base) to turn on a relay. there are 2 sets of these, one 2n2222 runs a relay coil of 12volt 85 ohm, the second one runs a 12volt relay coil 160ohms. Not sure how to speed the code up to test the frequency idea. The plow method runs in its own cog. I can post the rest of the code, if its needed. Any ideas would be appreciated.Thanks-mike
pub plow dira[5..7]~~ repeat if pulsewidthv>132_400 and pulsewidthv <160_000 'if stick is pushed up then turn on transistor to switch relay 1 outa[5]~~ else outa[5]~ if pulsewidthv<110_400 and pulsewidthv >83_000 'if stick is pushed up then turn on transistor to switch relay 2 outa[6]~~ else outa[6]~The gain is higher at lower frequencies.
The "minimum" thing is saying that the actual Ft will be most likely higher but not lower.
The Prop can't switch a pin at this rate so don't worry about that spec.
Duane J
Spikes from the inductive kickback of the relays?
Do you have snubbing diodes across the relay coils?
What filtering cap are installed?
Would you post the actual complete schematic?
Do you have a scope?
You do have catch diodes on the relays ?
In a situation like this, it pays to divide and conquer.
Change the code to a simple Read-Pin -> Write Pin, which exercises all the hardware, and check you get reliable pin-following.
Measure the voltage across the 2N2222 when it is on, and the prop pin, so you know what a ON measures like.
Any deviation from those values, in more complex code, indicates problems.
Also test without any relay loads, as arcing contact closure can have effects you do not expect.
You don't want to put big caps on the prop pins.
You won't need the wait with this circuit.
Duane J
Timing caps need to be the right value though.
Duane J
That would just slow down the base signal.
as this could easily cause the kind of problems you described.
When the relay it turned on there is a high current flow for a millisec or so before the
current flow through the coil stabilizes. If there is not enough excess supply (current)
then the supply voltage will drop (same kind of time period) causing what we call
"noise" on the supply voltage.
It might be possible that with a small 12 volt supply, that the coil is banging the 12 volt
source hard enough that the regulator is dropping out momentarily.
(it's a stretch, I know, but we don't know what that 12v source is. Could be a 100 ma wall wart?)
Anyway, this is one of the reasons for the big caps in the regulator circuit (input side)