I have a bridge rectifier that is converting 240v AC into approx 358V DC

The rectifier is smoothed by 2 X 10,000uf capacitors

Under full load, I am drawing 3000 watts on the DC side but for some reason the voltage drops down to 280V DC yet the AC side stays at 240

My question is why is my voltage dropping so much? The DC cables are 300 feet long #10 awg wire but the voltage measures the same at both ends +/- 1% so its not lost in the cable. I also checked that all the connectors are tight and making proper contact. Nothing is getting hot either..

The meter I am using the measure with is a "Circuit test DCL-320"

Here are the components I am using: https://www.digikey.ca/product-detail/en/crydom-co/M5060SB400/CC1656-ND/752672

2 X https://www.digikey.ca/product-detail/en/rubycon/400LSU10000MNB90X151/1189-1911-ND/3927506

Thanks for any assistance


  • 1
    \$\begingroup\$ The voltage is dropping because the filtering can't keep up. \$\endgroup\$ Commented Dec 21, 2016 at 1:14
  • \$\begingroup\$ So the simple answer is just to add more capacitors? \$\endgroup\$
    – John
    Commented Dec 21, 2016 at 1:15
  • 1
    \$\begingroup\$ And reduce the resistance. Instead of two large capacitors, use many more smaller capacitors. \$\endgroup\$ Commented Dec 21, 2016 at 1:18
  • \$\begingroup\$ I am having a hard time figuring out just how much capacitance I need for these power levels. I thought 20K would be enough.. When you say many more - how many are you talking about and at what capacitance? \$\endgroup\$
    – John
    Commented Dec 21, 2016 at 1:20
  • 1
    \$\begingroup\$ I haven't done the full math, but the main problem is the ESR of the caps. By using more caps of smaller value you distribute the resistance across all the caps, reducing the R in the RC filter. \$\endgroup\$ Commented Dec 21, 2016 at 1:31

1 Answer 1

  • 358Vac pk No load x 0.707 = 253Vac rms if it is a sine.
  • 3000W at 240V @ 12.6A @ ~20 Ohms linear load

The engine on the generator is capable of sustaining a continuous 3kw load.

This is where people make false assumptions. This only applies to linear loads and a bridge cap is a short circuit of about 0.1 to 1 Ohm not 20 Ohms.

It all comes down to impedance ratio of load/source. For a linear PFC it needs to be a high R Load/Source ratio. Batteries and caps are very low impedance with pulsed current.

Solution : Active PFC. enter image description here

  • \$\begingroup\$ Tony - thanks for the response. I've been running the generator non-stop for over a week now and the generator does not heat up at all. Its only a few degrees above ambient. The engine itself is a 6 hp lister diesel.. thats where I get the 3kw from - the full load of the engine itself - not the generator head it is powering. If I could find a 3kw Active PFC power supply that outputs between 230 & 600V DC at a reasonable cost - it would be the ideal solution. But thus far I have not found one nor do I have any clue on how to build one! \$\endgroup\$
    – John
    Commented Dec 21, 2016 at 1:57
  • \$\begingroup\$ Its a good generator then if not over heating but coupling factor depends on mass of core in kg/kW with high primary inductance. I estimate gen coupling is 0.998 out of 1 to get 280Vdc from 358Vdc no load. Look for 3kW active PFC yet? \$\endgroup\$ Commented Dec 21, 2016 at 1:59
  • \$\begingroup\$ Its a cheap chinese clone of a stamford ST generator head. It has an automatic voltage regulator.. I'm searching for an active PFC solution... \$\endgroup\$
    – John
    Commented Dec 21, 2016 at 2:03
  • \$\begingroup\$ ok now its a shopping issue. \$\endgroup\$ Commented Dec 21, 2016 at 2:06
  • \$\begingroup\$ I have no idea what to buy! or where to look! Ideas? \$\endgroup\$
    – John
    Commented Dec 21, 2016 at 2:09

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