2

Someone suggested converting a 220v CRT TV to work with 110 work by modding the bridge rectifier stage. He replaced the smoothing cap with two caps in series, then he wired the point between two capacitors back to AC side. I simulated it in LTspice and indeed it generates 220v, green plot, however, positive side of AC source shows a 110v constant DC voltage and negative side shows a strange 220v AC voltage, red plot. LTspice Simulation of Circuit

I am wondering if this is a valid or safe way to boost voltage as it involves feeding DC voltage to AC side?

Terryfi
  • 23
  • 5
  • 1
    What’s the rated power draw of the tv? – Bryan Sep 25 '23 at 02:18
  • @Bryan Certainly, the TV is *not* well-represented as a $50:\text{k}\Omega$ load! ;) So your question is excellent. – periblepsis Sep 25 '23 at 02:21
  • @Bryan it is a 40W to 60W TV small 13 inch – Terryfi Sep 25 '23 at 02:22
  • 1
    @Terryfi So you want to try using something like a 1 k resistor, instead. Simulate that. You are just simulating a 1 W load, or thereabouts. – periblepsis Sep 25 '23 at 02:23
  • @periblepsis I noticed that error too just uploaded the updated simulation – Terryfi Sep 25 '23 at 02:30
  • @Terryfi Are you trying to feed 220 V AC? Or is this somehow to be V DC for the TV? – periblepsis Sep 25 '23 at 02:41
  • @periblepsis this is AC to DC rectifier inside TV, originally the AC source on left was 220V rms we are trying to feed 110V rms and still get the same DC on the right side. I think I did not put the sine AC source correctly based on link the peak to peak should be 340Volts peak-to-peak for 110V rms, any AC experts to advise? – Terryfi Sep 25 '23 at 02:49
  • That arrangement is not at all unusual. I've seen it many times in power supplies intended to be switchable between 220v and 110v. You get higher ripple because each cap is only charged by one half-cycle, but you can compensate for that if necessary by increasing the size of the caps. It might also be kinder to the diodes if you cut the track between the source and D3/D4 and wire that junction point over to the other diode pair to put them in parallel. – brhans Sep 25 '23 at 03:04
  • 1
    @brhans after your confirmation I found another question for switchable supplies here link , best answer explains that when link is connected D2 and D1 will not conduct, so your suggestion of putting diodes in parallel makes sense to distribute the current. – Terryfi Sep 25 '23 at 03:45
  • A TV designed for use in 220V areas will probably use a different video format and channel frequencies than we use in 120V land, so may not be usable in North America. – Peter Bennett Sep 25 '23 at 18:05

1 Answers1

6

This was a common technique. It's fallen out of favour in more recent times due to the introduction of active power factor correction. If you see a power supply with a simple slide switch to select between 120V and 240V then it's probably using this technique (on the other hand, if you see a device using a rotary switch or a movable insert to select voltage, then it's more likely changing taps/winding configuration on a transformer).

In this configuration, two of the diodes take all the current, while two are always inactive, so it might be a good idea to replace them with beefier versions or rewire the two redundant diodes so they are in parallel with the active diodes.

Also if it is desired to maintain the same level of ripple the original rectifier had, larger smoothing capacitors will be needed.

Obviously this power supply does not provide isolation, and the "common mode" voltage on the DC side will depend on what is going on on the AC side, but that applied before it was modified too.

One way this does differ from the behaviour of a bridge is it's asymettery. With a bridge, the common mode voltage on the output would be affected by both poles of the supply. However with this design, the common mode voltage of the output is mostly driven by the pole of the input that is linked through to the output capacitors. If the capacitor center tap is tied to the hot conductor then the common mode voltage will vary through the cycle, if it is tied to the neutral, then it will be mostly stable through the cycle.

Still that isn't hugely relevant in practice. In practice, both the hot and neutral connections from the mains should be treated as dangerous and hence so should the output of the rectifier.

Peter Green
  • 22,193
  • 1
  • 39
  • 82