On Mon, Aug 28, 2017 at 5:52 PM, Rob Jarratt <robert.jarr...@ntlworld.com> wrote:
> I have posted some information along with a rough schematic here: > https://rjarratt.wordpress.com/2017/08/28/decstation-5000-model-240-h7878-possible-fault-found/. > Other information is in the previous post: > https://robs-old-computers.com/2017/08/27/decstation-5000-model-240-h7878-power-supply-failure/. > > Interesting that Tony mentions a Triac. There is indeed a triac. I am using > 220/240V input > (being in the UK), would it be expected to double the voltage in this case to > 325V? That is absolutely the classic circuit. I assume the AC terminals of the bridge rectifier come from the mains input, possibly via fuse,switch,filter. To understand it, draw the bridge as 4 separate diodes. Now assume the triac is turned off (open-crcuit) and it's just a standard bridge rectifier circuit. Then consider the triac turned on (effectively shorted). You have the classic half-wave voltage doubler using the 2 capacitors and 2 of the diodes. The other 2 diodes end up in parallel with the capacitors (remember the triac acts a short circuit now) and are always reverse-biased so don't do anything. For 230V mains input, I would expect the triac to be turned off. So it's a full wave rectifier with 230V AC in. Giving around 325V out. I suspect those capacitors! -tony
