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Today I received a preheat bed AOYUE-AO853A. I was really eager to plug it in an test it, ignoring the voltage input rating. It worked for a second making a strange noise, but then some smoke started coming out.

I turned it off, unplugged, I left the capacitors to drain and finally I attempted opening it after it cooled. It turned out the transformer was rated for 110V 60Hz. enter image description here

I have two questions:

  1. Is there any chance that the components beyond the transformer have fried? From a visual inspection, everything looked ok on the single PCB this device has.

  2. What transformer should I use to replace this one to be able to power the device with standard European 220V?

Update: I noticed that the board is not only hooked to DC (from the transformer), but also to AC.

Here's a picture: enter image description here

  • 1 is the heating element
  • 2 is the AC in
  • 3 is the DC in

Note that the capacitor at the right of the DC in is rated at 25V and all the other capacitors are rated 16V

enter image description here

As required by a top user, I posted a picture of the wiring for the heating element. It seems it's wired in series already.

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    \$\begingroup\$ A 220V to 9V transformer rated at 8W or more. As it says on the label. \$\endgroup\$ – user_1818839 Sep 21 '20 at 10:21
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    \$\begingroup\$ The heating element is probably rated at 110 V, so you might need to change that for one rated at the same power but 240 V. If you are lucky, it uses two elements in parallel, so you could rewire them to be in series. \$\endgroup\$ – Andrew Morton Sep 21 '20 at 10:59
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    \$\begingroup\$ whats next to '3'? the type of diode and whats the rating of C18? \$\endgroup\$ – schnedan Sep 21 '20 at 10:59
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    \$\begingroup\$ It's just an arrow. C18 is rated 25V. The rest of the capacitors are 16V \$\endgroup\$ – bem22 Sep 21 '20 at 11:04
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    \$\begingroup\$ @AndrewMorton it looks like there are two elements. Thank you for that \$\endgroup\$ – bem22 Sep 21 '20 at 11:11
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This device is basically a temperature controller with a sensor (thermocouple or thermistor most likely), a control circuit, and a triac-controlled mains heater. The unit is powered from a control transformer.

Such a transformer will output considerably less than double output voltage if the input voltage is doubled, because they're run fairly close to saturation. That's why the transformer failed, because it saturated. Given the nature of the other components and ratings, there's a fairly good chance that the rest of the circuitry is okay. So replacing the transformer with a 110VAC:9VAC 8VA or more transformer could well restore it to functionality, but it would still be a 110VAC unit. Which doesn't solve your problem unless you intend to use a voltage converter to power the unit.


As far as converting the unit- the transformer can be substituted for one with a 220VAC primary voltage. However, the heater(s), as @Andrew noted, are a more serious problem. At 4x the rated power they will burn out in short order. If there is only one pair of wires to the heater itself you're probably scuppered unless you want to try to wait for a 220VAC spare part to come.

The BTA20 triac and (probably) MOCxx opto-triac drive circuit may or may not be appropriate for 240VAC. I would guess they are okay, but I would also check them carefully.

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  • \$\begingroup\$ There is a 220v version of the product as well, likely having several differences in off-board components though they might(?) have tried to keep the PCB itself the same. But even if it were the same part, the board may not have entirely survived... \$\endgroup\$ – Chris Stratton Sep 21 '20 at 15:48
  • \$\begingroup\$ @ChrisStratton Yes, I would have done that. Price differences for 400V vs. 600V parts are pretty minimal to nonexistent. \$\endgroup\$ – Spehro Pefhany Sep 21 '20 at 15:56
  • \$\begingroup\$ yeah C10 looks like a 25V part, and the LM7805 can withstand over 30V, everything else is probably protected by that. 3021 is written on the board next to the opto, if MOC3021 is the part fitted then that's good. \$\endgroup\$ – Jasen Sep 22 '20 at 12:42
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  1. Yes, it might be damaged because as there was double the voltage into the transformer, there was double the voltage out from the transformer too.

  2. With a transformer that has identical output rating but has 240V 50Hz input. Note that the transformer rating reads on the label: 8 Watts, 9 Volts. Just as a disclaimer: if you don't know how to work with mains voltage circuits safely, leave the modifying to a competent service technician.

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  • \$\begingroup\$ Thank you very much! I have updated my question with some more information about the board \$\endgroup\$ – bem22 Sep 21 '20 at 10:46
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    \$\begingroup\$ There won't be DC coming in from transformer. It's 9V AC. It will have peak value of about 12V. When it was connected to double the mains, the peak voltage exceeded 25V. After rectification it might be slightly below 25V at the capacitor. \$\endgroup\$ – Justme Sep 21 '20 at 11:24
  • \$\begingroup\$ you wont get double voltage out of an over-driven transformer the core saturates and that will limit the output voltage somewhat. \$\endgroup\$ – Jasen Sep 22 '20 at 12:28
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when a 110V transformer converts to 9V, we can assume if you connect 220V, it has produced 18V...

So it's upon you to check what was after the transformer and if there was a rectifier, capacitor and stuff which was rated eg. 16 or 25V.

Hard to tell if, but I suppose possibilities are high you fried more than the transformer

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  • \$\begingroup\$ Thanks for this, with your guidance I added some more information to the question! \$\endgroup\$ – bem22 Sep 21 '20 at 10:56
  • \$\begingroup\$ your info about the capacitors doesn't sound to bad,... guess the 16V ones are after the regulator. And I assume the regulator will be rated for more than 18V. \$\endgroup\$ – schnedan Sep 21 '20 at 19:25
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The windings ratio would be approx 2:1 so the output AC from the secondary winding was likely to have been 18V before the transformer expired. If you didn't blow any electrolytic caps i.e. a big mess, discharge and smoke and you can't see any obvious damage you may have got away with it. Bear in mind that even if you do replace the transformer the PSU board may have devices with ratings consistent with a 120V supply e.g. smoothing caps.

If you're very lucky you might get away with just changing the transformer. It's worth a try. As mentioned you'll need a 240V transformer with a 9V secondary rated >8W.

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