Voltage Regulator Question

I have gotten by without using the caps a few times, but ocasionally the regulator will oscillate which can cause all sorts of strange problems in the down stream circuit as well as overheating the regulator. If the following circuit has a capacitive input filter or bypassing and it is close, meaning a an inch or two, to the regulator then you can dispense with the output filter. You can also dispense with the input capacitor if you are close to other bypassing or filter capacitors.

Add to that the different manufacturers of these three terminal regulators seem to build devices with the same number but very different characteristics when it comes to stiffness and susceptability to oscillation.

If it is for a one off home project thing, try it, it might work. If you are about to go into production, study carefully because the cost of the additional components may be greatly exceeded by the cost of waranty repairs and lost customer good will if problems develop later.

You may get away without the caps - depends on your supply & wiring length from it, and your load & wiring length to it.

Unless it's a very cost- or space- restricted application, it's just as well to put them in anyway. It can save a lot of time wondering why the d**n thing isn't working.

BTW - for images, most reliable way I've found is to save a local copy as a .jpg, and use the image insertion tool (green camera icon) to put it in the post.

yes, you need the caps to prevent the oscillation.

You can use these things as audio amplifiers by AC into the center leg with the speaker as the load with a cap in series. OK for base.

For peace of mind, and the reality that the cost of the caps ($2) is negligible at best. You are better off in the long run to put the caps in.

I use these 3 term regulators all the time and its no biggy. You will want 2 of 0.1uf monolithic caps, a 1uf and a 10 uf electrolytic or tantalum capacitor of the appropriate voltage range, in your case 25volt types will do. Thats 4 caps in total.

Place as close as physically possible a 0.1 uf cap between the input leg and the centre leg and the other 0.1uf between the centre leg and the output leg.

I usually use tantalum caps rather than electrolytics because they occupy much less space than the electrolytics of similiar value. Place the 1uf tantalum with the positive leg on the input lead of the regulator and the negative lead of the 1uf tantalum capacitor on the middle lead of the regulator. Place the 10uf tantalum capacitor positive lead on the output lead of the regulator with the negative lead of the capacitor on the centre lead of the regulator. If you build this on veroboard it will need a piece approxiamately 20mm*10mm. If you are going to ask the regulator to deliver any level of current over 200 mAmps then a heatsink will be required. Doesn't need to be huge, a piece of aluminium flat bar 30mm*20mm*2mm will be sufficient up to 1/2 Amp or if you can mount the regulator onto the metal housing of your project then that is good also. Remember that the mounting tab is directly/electrically connected to the middle lead (earth/ground/negative rail).

Hope that helps.

Cheers

Pez

I think now everithing will be more clearer, ... enjoy.

As Pez said: physically it's better to put the 100nF closer to the device than the 10µF.

Thank you all very much. Wonderful forum guys!

I was, in fact wondering about what current draw would necessitate a sink on the regulator, but was just going to experiment. I don't think I'll be near 200ma but now I can measure the current with my meter when I'm up and running and be able to make a better decision.

The power you've got to get rid of is = voltage across the device x current through it.

@ 200mA, that's (12-5) x 0.2 = 1.4W.

The datasheet may give the °C/W of the package in free air, but as I suspect it's a TO220, designed for mounting on a heatsink, it probably won't.

Anyway, I reckon it'll get pretty warm, but not dangerously so - the smallest TO220 heatsinks are about 30°C/W, so you can probably double that with no heatsink.

I've used a zenier diode in series with the regular to off load some of the power dissipation in the 7805 because of a supply voltage that was too high for the current draw I needed (at 5 volts).

You probably should not drop the input voltage below 7 volts, but the nice thing about the zenier is that it works for a wide range of current (up to its ratings).

You may want to beef up the filtering capacitors. Zeniers may add a little noise.

Another handy trick which costs less than a zener is to put a resistor in series with the input to the regulator. You can calculate the value of the resistor using the desired voltage drop in the resistor by the current you need for the load and the regulator output voltage +3VDC to make sure you don't run the regulator out of head room on the input supply. If you do this, you MUST put the capacitors at the regulator as others have said.

Managing the input voltage this way can get a lot of the power dissipated out of the regulator and reduce the heat sink load.