I have a serious lack of outlets in my garage and that is a problem for a beginner carpenter. I have a light switch by my workbench and was curious if I could change it to a combination outlet + switch?
The Regulated and safest answers is No. The DIY answer is absolutely not.
Running carpenter machinery of even the smallest type will use a good 300Watts? Then to the large saw tables or routers that can be anything between 800Watt and 2000Watts.. It can cause an electrical fire and will void any insurance of any kind in any country.
By building regulation these should be about 1200Watts maximum.. That is 12 100Watt lights bulbs that is easily achieved in 3 rooms with 4 fittings... Also these wires are usually 1.25mm core which is way to small for heavy loads!
You will have to pull a SEPARATE, or find a separate socket that is handled by 1 breaker for this heavy machinery.
It might take a bit of work but this will be the best and safest way to do it. Pulling power from your DB(Distribution Board) you can put a new 15Amp breaker(220Volt) on the supply phase giving you extra 3300Watts just for your heavy tools. You also want to make sure to have earth leakage connected as this is a life saver in many cases!And that is no joke.
You would pull that using a 5mm2 Solid copper core cable. You can run it along the outside of the wall or in tubing but it MUST be in a shielded coating. If you run it within pipes within the walls unshielded wire is fine.
When working on the DB, turn off the Main incoming power, usually a big switch before the DB in newer installations of somewhere outside your house. This can seriously hurt you if you don't.
A DB box should be neatly wired, like this.
Earth leakage, a life saver! It is easy to install however it can be costly, but its worth it!
Cables, this flex cable is commonly used
Cables, a shielded copper core flex cable, for extra safety!
If you really really cannot do it the proper way for whatever reason then you must seriously think about some safety precautions when connecting to your light switch. At least put a breaker 5amp(@220v = 1000Watts) after the light switch stop overload if any and then wire a plug in. Sometimes you can buy Isolated plugs with build in fuses. Try not to exceed 5AMP. This is highly unrecommended though.
Your profile does not indicate your location. ppumkin's answer is clearly influenced by European standards... and his answer is certainly correct, accurate and 'safe'.
But I would propose for North America, in his words: The Regulated and safest answer is No. The DIY answer is maybe.
It is common in North American residences to see lighting and outlets on the same circuit. Arguably in these days of low energy lighting fixtures, we tend to have much unused capacity on our 15A or 20A 120V lighting circuits.
How easy you are able to accomplish this would depend on where the power is fed. Sometimes power is at the light fixture with only two conductors being fed to the switch box; other times, you'll see power enter the switchbox then continue to the light. The later situation would be very handy in your circumstance. You could tap off the power to feed your combination switch/outlet.
Things do get more complicated though. For instance, in a garage, your outlet should be GFCI (not something I've seen on those combo switch/outlets). Also, as ppumkin points out, you could easily run into capacity problems depending on whether you're trying to run a 10W battery charger or a 2200W table saw.
Best advice might be to call an electrician for a quote to go over your options. The carpenter thing is a bug (I've been bitten by it), and you might find yourself wanting a subpanel (DB) in your garage to handle the big iron 220V tools in your future.
Your profile does not indicate where you are from. If you are from the UK, then @ppumkin would be the guy to take advice from. If you are from the USA, I will add my 2 cents!
Short answer is maybe it is possible (I hear so many people cringing now!), truth is back in the 50's & 60's it was kind of accepted. If you have the feed in the switch box, then you can easily add a switch - outlet combination. However as others have warned this is not okay to run heavy tooling with! You should see what other circuits are on the breaker, and if it is the garage only (or lite loaded) you may want to add the outlet for small loads only! That means an occasional hand drill, drop light, etc. I would make the outlet a GFCI type (NEC code requirement for new installs). Some disadvantageous of this are:
I agree with others, if possible the better way is to run a new circuit. Perhaps a split 120-220V circuit if you plan to do a workshop.