AC - DC across Ecap/ Res - Page 1
Forum section: electronics / beginners

author: mengfei
author_rank: Regular Contributor
timestamp: May 22, 2015, 02:59:06 am
content: hello,this is the first time I saw this kind of circuit in the rectifier section where both AC ~ ~ in Diode just comes from a single AC(120V) line then the other AC line acts as neutral while the + of the Diode is supply & - of Diode is GND.The AC & DC shunts across an Ecap & a 100ohm resistor - wont this circuit short out, please see attached file.thanks

author: SeanB
author_rank: Super Contributor
timestamp: May 22, 2015, 04:35:08 am
content: Standard voltage doubler circuit. You will find 2 wire links that can be changed to an alternate location to switch the diode into the other mains lead ( and disconnect the connection to the centre of the capacitors) for this to be used on 220VAC supplies. This is used to get a rectified supply of around 300VDC for the switching regulator following it.If you have an alternate circuit with power factor correction this will not be there, the circuit will take any mains voltage from 80-265VAC and charge the capacitor to 400VDC while drawing current in phase with the mains voltage.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: May 22, 2015, 05:40:53 am
content: Those resistors would make a lot more sense if they were 100KOhm - I think that's a misprint. 100 ohm resistors would be dissipating hundreds of watts.Their function is to bleed off the charge in the main capacitors to make them safe soon after unplugging.

author: mengfei
author_rank: Regular Contributor
timestamp: May 22, 2015, 05:47:01 am
content: Thanks @SeanB, so this is a standard connection from AC to rectifier? It's the first kind I saw even while searching the net.@Paul, this is what worries me since I'm the one making the PCB for the circuit. this is a 58V 5A charger, so if we calculate the the power needed for the resistor would P=I x E applicable?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: May 22, 2015, 05:56:43 am
content: You mean P = I * E? If so yes. And since E = I * R, P = E^2 / R. If the capacitors run at 170V, a 100 ohm resistor would have to dissipate 170^2 / 100 = 225 watts. With 100Ks that would be more like 0.225 watts, which is about right for a 1/2W resistor.It's very unusual to run diodes in parallel like in the schematic for no good reason. But if you think about it, you can easily add a switch to this circuit to make it run on 220V by disconnecting your green line from between the capacitors and feeding it into the bridge. Naturally you'd disconnect the red line from the side of the bridge you attached the green line to. At that the resistors balance the voltage across the capacitors so they split the rectified 220v evenly.

author: mengfei
author_rank: Regular Contributor
timestamp: May 22, 2015, 06:02:24 am
content: would there be a difference in the formula since one side of the "Ecap/ Res" is AC and the other is DC?Or would it reference to GND making the resistors 200Ohms?

author: Paul Moir
author_rank: Frequent Contributor
timestamp: May 22, 2015, 06:08:35 am
content: I think you could answer that yourself, just simplify the circuit. Imagine if R46, EC1A and C2 weren't there (from EC1's and R45's perspective, they really aren't relevant), and the left hand diodes of the bridge weren't there. It's a simple half wave rectifier then right?

author: mengfei
author_rank: Regular Contributor
timestamp: May 22, 2015, 06:25:59 am
content: OK thanks, I will verify this with the EE before finalizing the PCB design.thanks again for all your inputs.