Hi all.
I'm using a ZLDO1117 3.3v regulator to drive 2, 3V 1W solenoid valves, and I can't get them to stop oscillating. I apply logic high to the MOSFET and the solenoids first click on, and then start oscillating. The higher the input voltage, the faster the oscillation. I ultimately need to drive them from a 12V power supply, but right now anything over 5 causes oscillation.
here is my schematic... any suggestions for how I could improve it to make these operate smoothly?
Sounds like the regulator is going in/out of thermal shutdown. How big is the copper fill / heat sink you are using?
Quick math.
3V, 1W solenoid draws about 333mA. So you're drawing 666mA with two of them.
5V In
At 5V in, 3V out, 2V across the regulator. Power drop of the regulator is 2V times 666mA is 1.332 Watts.
According to datasheet a 5mm-sq PCB copper fill will yield a junction to tab thermal resistance on the order of 73-100°C/W. (This is why your heatsink/copper fill is important. This might be a "best case.")
Which means the junction temperature is between 95°C and 130°C. Datasheet says regulator starts shutting down at 150°C, which is probably why you seem to be able to run at 5V.
12V In
At 12V, the numbers turn into:
Regualtor Drop: 9V, 666mA, 5.4Watts
Junction Temperature: 394°C - 540°C. (Might explain why the regulator is shutting down.)
You may need a physically larger regulator, more copper for heat sink, or different solenoids.
WOW!
thank you for the great response. I really appreciate the clear explanation. I hope that is the case. I was making my boards as small as possible, but I will try a larger heat pad and maybe a sink if that doesn't work.
cheers!
obucklin:
I ultimately need to drive them from a 12V power supply.
Connect the two solenoids and an 18ohm/2watt resistor in series, directly to 12volt.
Or a 3watt+ cement resistor if they are going to be "on" most of the time.
No regulator needed.
Leo..
Wawa:
No regulator needed.
Quite so.
Solenoids do not need regulators.
And if they only ever operate at the same time, put them in series.
If you are switching them with a transistor, put the "kickback" diode across the series combination of resistor and solenoid. This will make them switch faster.
To be precise, switch off faster - the switch on time is automatically faster with a series resistor, although
the mechanical delay may dominate anyway.
Hi,
What is the rating of your 12V supply. That is how many Amps?
Make sure the two capacitors either side of the regulator are as close to the regulator as possible.
Tom..... 
In general a 12V main supply would imply using 12V solenoids. Is there a reason for choosing
3V solenoids?
MarkT:
To be precise, switch off faster - the switch on time is automatically faster with a series resistor, although
the mechanical delay may dominate anyway.
OK, so I was lazy and didn't feel like incorporating that in my reply. So shoot me. 
MarkT:
In general a 12V main supply would imply using 12V solenoids. Is there a reason for choosing
3V solenoids?
they are solenoid valves, which are quite expensive, and these were loaned to me. I want to run them from the same power source as the motors which are all 12V.
These are all good suggestions. Thanks everyone!
DC-DC converter might be more sensible then, LM2596 one from eBay are dirt cheap and several amps.
obucklin:
they are solenoid valves, which are quite expensive, and these were loaned to me. I want to run them from the same power source as the motors which are all 12V.
Then either use a switchmode down-regulator module, or calculate the resistor values. The latter will be much easier.
