I'm thinking of putting together a simple DC motor speed control circuit that consists of just 2 resistors, 1 diode and a small MOSFET. As the components involved are quite small and the whole thing is intented for a mini quadcopter I thought rather than putting them down on a circuit board I would just solder the connections as necessary and then just hold them together using some kind of non conductive glue. I already have a hot glue gun and a quick google search seems to suggest that the glue it uses is non conductive.
Is this generally a good idea?
I had a look at the datasheet for the MOSFET (PMV31XN) but I can't really tell what kind of temperatures it would reach during normal/heavy operation....to see if it would melt the glue! I think transistors are meant to get hot aren't they? That's why some have heat sinks? As this is a logic level MOSFET though, on a SMD (SOT-23) package, is it still going to get very hot?
I've seen references to other types of glue (epoxy, ca, foam, etc). Can anyone suggest a better alternative for the circuit I'm trying to build?
Is the transistor switching your load, or operating in its linear range? If it's just a PWM switch, running at reasonably low frequencies (xxkHz) then it probably stays cool. You might run it for a bit before gluing everything together to determine if you need a heatsink though.
Hot glue is fine. I've used it before to do just this sort of thing.
Hot melt glue softens at a very low temperature (in a car in summer for instance),
so don't expect it to work for any circuit consuming significant power (something
like 50mW might be an upper limit at a guess).
The plus side is that it will readily melt again if you unsolder the parts (don't
breathe the fumes), so some re-work is feasible (epoxy glue wouldn't allow this).
vaspoul:
2. I had a look at the datasheet for the MOSFET (PMV31XN) but I can't really tell what kind of temperatures it would reach during normal/heavy operation....to see if it would melt the glue! I think transistors are meant to get hot aren't they? That's why some have heat sinks? As this is a logic level MOSFET though, on a SMD (SOT-23) package, is it still going to get very hot?
But you need to figure this out. Any transistor that gets hot enough to melt glue won't last very long. At the very least build a prototype and put your finger on it. If it burns, revise the design.
You can get special high temp hot glue in hardware stores that will work better if you expect a fair bit of heat, the stuff gets way more fluid and burns way, way more on your skin though so watch out.
fungus:
Any transistor that gets hot enough to melt glue won't last very long.
Not so true, hot melt glue softens by 60C(*), semiconductor devices routinely rated to
125 or 150C where the glue is totally liquid - hence my warning about low power
circuits only. Reliability at 125C is indeed reduced, but say at 70C (my laptop CPU is
at 68C currently) semiconductors are perfectly happy.
(*) Melted into car upholstery in an English summer day, never use the glue to fix your car(!)
Of course not all hot-melt glue is the same, different brands will have different softening
points, but until you can characterise the stuff you use its best to assume it will be soft
by 60C.
A handy use for hot melt glue is making little plug-in connectors from snap-off male
headers - solder on your leads then coat all the soldering in glue and you have an
insulated (and mechanically stronger) little plug. Works well for plugging into breadboards
and Arduinos.