# What resistor wattage with rectifier and capacitor?

I have tested a classic full bridge rectifier:

simulate this circuit – Schematic created using CircuitLab

I haven't yet connected the load (a relay switch whose coil has 90 ohms).
The result was that the R1 burned.

I guess I need a higher wattage R1, how do I know what it should handle?
Any more caveats I should consider?

The operating voltage of the coil is 12V-15.6V.

• Any more caveats I should consider? — Do not connect this circuit to mains. It's a death trap. – Janka Oct 18 at 2:45
• Wait, are you working on the mains?! Do you know what you are doing?? That's dangerous, man! – Sim Son Oct 18 at 2:46
• It is. Because mains voltage is top notch destructive if you don't know what you are doing. And you don't. – Janka Oct 18 at 2:56
• you are lucky that the resistor acted as a fuse before the capacitor exploded ... the voltage on the capacitor could have gone to about 360 V – jsotola Oct 18 at 2:59
• People are telling you to stop because mistakes in your circuit indicate you do not know anywhere near enough to do this project and not get hurt. Your circuit clearly doesn't do what you think it does and indicates the knowledge gap is too big for us to fill it such that you can work safely. You almost presented us with a gun, pointed at the trigger and told us know this is the safety then asked us what else do you need to know to use the gun. Instead of trying to help you only halfway and have you go ahead and hurt yourself, we are just telling you to stop. – DKNguyen Oct 18 at 3:08

First the safety: Responsible comments on your post are mainly concerned for your safety because to control the hazard you must first identify and evaluate the risk. It takes only 30mA to provoke respiratory paralysis. You can easily shock yourself, there is no galvanic isolation. If you are working on a breadboard (breadboards are not all made to work on such voltage since the distance between contacts depends on the voltage) you may easily touch a high voltage node or even while moving or carrying it you will touch contacts since usualy from the bottom the isolation is too bad it is only an adhesive paper that isolate the contacts.

Then the circuit:

• there is nothing to limit the voltage at the output. So it is not only the resistor overheating, the capacitor (25V) will reach a certain voltage and may explode. At least a Zener diode is needed to limit the output voltage.
• when you apply the mains AC 220V first the capacitor initially discharched will be shorted and the resistor will be as if directly connected to 220V and will dissipate around 30W.
• the solution is not to replace the resistor with a giant powerful one. It is a bad design and consuming energy for nothing, just to drop the volatge from 220v to 12V.

Usually if the power is too low we use a capacitor (we add also a high value resistor in parallel to the capacitor) in series with a resistor of low value just to reduce the transient current on the zener and we add a zener parallel to the load. In your case the current is a little more than 130mA. But the circuit still not safe. Nothing to isolate the high voltage. You defenitly need assistance in an equipped lab when working on it.

• Thanks. I don't have such huge resistor anyway. But I still want to learn from it, so does the fist point mean that the voltage would keep increasing up to 360V or roughly 12V*1.5=18V? Good point about it being inefficient, I thought transistors always consume power unnecessarily. What do you mean nothing to isolate high voltage? Is that just an assumption how I would do it? – Daniel Katz Oct 18 at 4:25
• @DanielKatz look at the schematic that you posted ... touching any part of that circuit can easily result in your death ... there is no safe point – jsotola Oct 18 at 4:40
• @DanielKatz the main concern and what is alarming in your circuit is that the risk is all along the circuit from the input to the output. There is no safe region. We cannot say this part is dangerous and the rest is safe. All is risky. – Paul Ghobril Oct 18 at 4:48

There is a very standard way to do what you are trying to do. It doesn't involve designing any circuitry. You get a 240V to 12V transformer. You select a relay that is rated to operate from 12V AC input on the coil. You turn off mains power while connecting the primary side of the transformer. The secondary side of the transformer (12V side) is relatively safe. But make sure you avoid touching anything on the primary side. There is more to it than this, but those are just the broad strokes.

The circuit you have shown is dangerous in the sense that the entire thing must be treated the same way you would treat a bare mains wire. The shock hazard is high. Shocks can be lethal. So you can't work on it casually the way you would with a low voltage circuit on a bench.

Be careful and have fun.