How to use 120V transformer in 220V? Series input diode?

Hi,
I recently found a small 120VAC 60Hz 1.2W input and 1.2V 150mA output rated linear transformer power supply intended to charge 1.2V battery. Here we have 220V mains and someone already burned the fuse by using this in 220V.

Roughly 120V soldering iron work in 220V with a series diode. What about the diode in this power supply?

A diode to reduce AC voltage only works well with a resistive load.
It likely won't work with (and probably will damage) your supply.

crutschow said:

A diode to reduce AC voltage only works well with a resistive load.
It likely won't work with (and probably will damage) your supply.

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Iron core linear transformer won't work as resistive load? It's not SMPS supply.

Willen said:

Iron core linear transformer won't work as resistive load?

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Obviously not, as it's not resistive - placing a diode in series means you're feeding DC to the transformer - not a good thing.

Nigel Goodwin said:

Obviously not, as it's not resistive - placing a diode in series means you're feeding DC to the transformer - not a good thing.

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It's going to be half-wave AC - sort of pulsed DC. I think that's going to be even worst than straight DC.

the DC component will cause excessive heating of the primary. the AC from the secondary will NOT be a clean sine wave. either get a step down transformer of the appropriate VA rating, or you can use a dual voltage power transformer primary from a piece of hifi gear wired with the input across both primary windings, and the 120V tapped off the center connection. make sure you don't have a load on the secondary. also the common connection for the two should be the neutral. in this way you can use an old dual-voltage transformer as an autotransformer.

Hey Willen how are you doing?

Really if I were you and since your power requirement is so low I would just start with a 220/240 VAC to 110/120 VAC step down transformer. That's what I did with most of my US Spec things while I lived in Europe (Italy). Such transformers were real common and not expensive anywhere in Europe so you should be able to find one in your back yard too or at least somewhat locally.

Ron

Personally, I'd just buy a charger from ebay - the're approx US$2. **broken link removed** a 4 gang one.

Mike.

Willen said:

Iron core linear transformer won't work as resistive load?

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Nope.
A transformer requires an AC signal with no significant DC component to avoid saturation of the magnetic core.
Half-wave rectification generates a pulsating DC value, which has a large DC component.
Applying such a signal to a transformer will result is it going "poof" in short order.