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Practical considerations when using transformer Answered

Hi everybody!

I basically love doing LED experiments and i was replacing many of the night lamps and stuff with simple LED circuits i made. Now i have few questions:

1) I generally use transformers to power them (230v/12v). Now apart from primary and secondary voltages and currents, are there any other ratings which i should consider?

2) I have made a portable night lamp with blue LED's. It has 4 arms in parallel and each arm consists of 3 LED's in series. All the arms are connected finally to a single 100 ohm resistor. Now as at most the whole setup might draw about 50mA of current. I had a transformer rated for about  500mA of secondary current, and so i used it with the setup. Will i have any issues using this? (Although i am sure i won't, but just in case anyway)

3)  Also, the transformer gets hot. How much temperature is in safe operating zone? The setup i used, after an hour, got pretty hot. I could barely hold it for a few seconds and i had to let go. Is it okay if it gets that hot?

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Josehf Murchison
Josehf Murchison

8 years ago

A rectifier and a 30k resistor might be better.
Transformers and the like are hard to keep cool.

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charmquark
charmquark

Reply 8 years ago

Most of my other circuits have the rectifier and, well not 30k but about 1k resistor in series.I placed the arms alternately in antiparallel directions in this circuit, just to check the idea implemented by someone on this site. It's pretty good so far. :)

So no issues with the transformer right? Can i turn a blind eye on this issue? :)

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Josehf Murchison
Josehf Murchison

Reply 8 years ago

no issues with the transformer however you can skip it with a diode to a 30k resistor to the Led.

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charmquark
charmquark

Reply 8 years ago

Yes i will try that out too, thanks!! :)

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Josehf Murchison
Josehf Murchison

Reply 8 years ago

make sure the resistor is the aproprate wattage or it iwll get very hot fast at 230 volts.

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charmquark
charmquark

Reply 8 years ago

Ya i pretty much guessed. But 230v is dangerous stuff, so i think its better off using 12v from the transformer and a 220 ohm resistro for 3 diodes.

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steveastrouk
steveastrouk

8 years ago

Modern transformers are run hot, because it makes them cheaper to make. Finger hot for a few seconds is only about 50C, so no problem.

LEDs need limiting resistors in each series string, or you may well destroy them.

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charmquark
charmquark

Reply 8 years ago

Oh yes, i did place a 1k resistor, so no issues. Thanks for the info!

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steveastrouk
steveastrouk

Reply 8 years ago

1k on 12 V with three blue diodes ? 150 Ohms would give you 20mA in each string, assuming the diodes are 3v forward drop, and you want 20mA.

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charmquark
charmquark

Reply 8 years ago

No not exactly. 1k with actually 1 LED, and i use 220 ohms for 3 led's on 12 v.