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I want use this transformer to power Arduino which will control 8 5V relay. I'm overwhelmed by variety of transformers. Here are few points/question I have in my mind.

  1. I need small transformer as I've to mount it on PCB.
  2. I/P voltage 230 VAC, O/P 9 VAC.
  3. I'll use IN4007 as rectifier. Is this a right choice or IN4001 does better?
  4. I will use an LM7805 to step down DC.
  5. I've narrowed down my search to EE13 transformer but I don't have any idea what function is performed by it's 10 Pins. All I know is 2 pin for input and another 2 pin for output. EE13 is a high frequency transformer. Do high frequency brings any benefit in this case or low frequency transformer would fine? Is there any better substitute for EE13?
  6. A supplier asked for these info: Inductance, IDC, RDC, withstand voltage. Here is my go: Inductance: Don't know, IDC = 150 mA, RDC = Not sure, Withstand Voltage = 260 VAC. Will you please help me quoting Inductance and RDC?
  7. I would use a 10 uF electrolyte capacitor and a .01 uF ceramic capacitor to filter the noise and ripples. Am I correct on this?
  8. I will use a 2N2222 to turn on the PCB mountable sugar cube 5V relays.
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closed as off-topic by Bimpelrekkie, DerStrom8, PeterJ, winny, uint128_t Sep 17 '17 at 1:55

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  • \$\begingroup\$ Why take all this trouble just to make 5 V for some relays ? Using a transformer diodes, LM7805. Sure you can do that but there's a much cheaper and simpler solution. Just use a USB phone charger that can supply 5 V, 1A. There is no reason to build a home-made linear supply for this. What you intend to do is what we did 30 years ago because there were no better (and cheap) alternatives. But now there are. \$\endgroup\$ – Bimpelrekkie Sep 14 '17 at 13:35
  • \$\begingroup\$ And since you're basically asking what transformer to use this turns into a shopping question and those are off topic. \$\endgroup\$ – Bimpelrekkie Sep 14 '17 at 13:36
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    \$\begingroup\$ These questions would take a book to answer to the level of understanding you need - especially if you have to ask about connecting a high frequency transformer to live mains (for safety's sake, DONT!). Buy a power supply instead \$\endgroup\$ – Brian Drummond Sep 14 '17 at 13:37
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    \$\begingroup\$ There are also ready-made mains power supplies that you can mount on a PCB, see: uk.farnell.com/c/power-line-protection/power-supplies/… These are basically the same as an external adapter in such a form factor that it can be soldered onto a PCB. These might be more expensive than an external adapter as these are not mass-produced consumer products. \$\endgroup\$ – Bimpelrekkie Sep 14 '17 at 14:50
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    \$\begingroup\$ @winny Sorry about that, however, as far as I know I type and see only one space before ! and also ? So I do not understand what I can do any different. \$\endgroup\$ – Bimpelrekkie Sep 15 '17 at 11:28
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Regarding your list of points:

  1. You have (most likely) 50Hz 230VAC. You need a power transformer that has a ferrous core, not a ferrite high frequency type. The rating for 600mA output current (see below) would be at least 2A RMS for half wave or 1A RMS for full wave (4 diodes).

  2. Either diode will work. Read the datasheet to understand the difference (mostly PIV rating). 9VAC will give about 12V peak with a half-wave rectifier. The 7805 needs about 7V to regulate, so low line voltage and p-p ripple must account for less than 5V.

  3. How did you "narrow down" the search to "EE13", which is a core size? You need to pick the transformer based on rated RMS output current, which is related the maximum DC current you require (presumably for all 8 relays powered at once, plus anything else that draws current from the regulated or unregulated rails). See above. There are off-the-shelf transformers from Hammond, Tamura, Triad etc. Check out the datasheets.

  4. Inductance is not normally specified on a 50/60Hz transformer. You will not be able to use a HF transformer without a switching power supply circuit. The equivalent is magnetization current, but you should not have to worry too much about that

  5. Not enough information to be 100% sure, but it is likely way, way too low. You have to calculate the input capacitor to filter the ripple to perhaps 1V-2V, which, if each relay draws 75mA (just a guess) you need 600mA so the capacitor will have to be 6-12,000uF for half-wave or half that for full wave. You should also have a capacitor at the output of the 7805. 100uF/10V to 470uF/10V would be fine for that.

  6. Fine, but don't forget the flyback diode across each coil. Read the datasheets for the transistors and relays.

You could just forget about the above and buy a 230VAC->5VDC wall plug adapter. It's easy to get 1.5A or 2ADC which would easily cover your requirement and lessen the chances of you killing yourself with this particular project.

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    \$\begingroup\$ +1 For the killing yourself argument for not doing this. \$\endgroup\$ – Bimpelrekkie Sep 14 '17 at 13:38
  • \$\begingroup\$ @Bimpelrekkie There's still whatever is connected to those relay contacts so I think it just lowers the odds. \$\endgroup\$ – Spehro Pefhany Sep 14 '17 at 13:40
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    \$\begingroup\$ Well, one takes what one can get ;-) \$\endgroup\$ – Bimpelrekkie Sep 14 '17 at 13:47
  • \$\begingroup\$ Thanks for the answer. Really appreciate it. All relays are not going to be operated simultaneously. Only one relay works at a certain time. So 1 relay draws 80 mA + Arduino draws 20 mA. I've in mind using diodes between relay and transistor. Please see my design drive.google.com/open?id=0B2_4lKrJug3jWk5kOU5wQ3AxYjQ Please let me know if anything wrong here. Again I'm not clear about which transformer to use. \$\endgroup\$ – Tamal Biswas Sep 14 '17 at 16:08
  • \$\begingroup\$ @TamalBiswas Here's something important. You must use a transformer rated for 50Hz ~240V primary. Do not use a high-frequency transformer. Here's the thing: you'll find that 50Hz transformers are much bigger than you want. You really don't want to do this yourself: your lack of knowledge will kill you!! Find a nice small pre-built switching converter designed to go on your board or better yet, use a "wall-wart" and save space on your board. This is really the safest way to go! \$\endgroup\$ – DoxyLover Sep 14 '17 at 17:42

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