1. Home
  2. Hobby Electronics Basics
  3. Transformer Sizing / Rating

Transformer Sizing / Rating

My project requires operation of a single relay to control mainly resistive loads up to 5amps ac @ 230v. The main project control circuitry (5v DC) requires very little power (few micro amps idle to

10mA active) - however, the relay coil does require quite a bit of power when the relay coil is energised, details as follows:

- From the datasheet: Relay is 5v coil, non-latching, SPCO, nominal power consumption is 400mW.

I need to select a pcb mount transformer to safely and conservatively provide power for the above control and relay circuitry, but am going round in circles with my understanding and basic calcs as to the size of the transformer I need.... can someone help me with my understanding as follows:

Project Power Requirements: Control power: 10mA Relay coil: 400mW / 5v = 80mA Therefore total project power = 90mA

If my calcs above are correct, then what size transformer do I need?

- Is is good practice to apply a nominal 'margin' of lets say 1.5 * project design power (to allow for safe working of the relay, and some inductive loading)? this would mean I need a transformer with output of

135mA.

Here's the bit I'm stuck on.... calcs above relate to DC power @ 5v, how do I relate that to the Va rating of the transformer, which primary is at 230v?

Thanks.

Reply to
youngun
Loading thread data ...
** Groper alert !!

** The smallest *readily* available PCB trannies were typically 3 or 4 VA. Recently very small ones have become available down to a puny .35VA - I suppose you are considering one of them ?

To reliably get 5 volts DC at 100mA ( 0.5 watts) needs a transformer rated at 1 VA.

Go mad and use a 1.5 VA for luck !

formatting link

I assume you will use a 5 volt regulator IC ?

...... Phil

Reply to
Phil Allison

Thanks Phil.

Actually I made a mistake, I require 9v transformer output. My project requires use of tempory 9v battery as initial power for control ccts and config of EEPROM. Battery will then be removed before connection of

230v mains power. My Vcc will be regulated to 5v.

Transformer: I guess therefore my requirement is now = 9 * 100mA =

0.9A, still using as you suggested a 1.5VA transformer.

For voltage regulation (same for both power sources), I was going to use a simple zener / resistor & smoothing capacitor. I hope that this will give me a stable and reasonable low ripple output. The calcs and values I have selected are against the higher power use when my relay is in action:

Zenner: 5v @ 1w Resistor: 5v / 100ma = 50ohms @ 0.6w Capacitor: 470uF

Do you see any problems with the above?

Reply to
youngun

Why not use a 3 terminal reg for similar cost ?

Cheers ......... Rheilly P

Where theres a will, I want to be in it.

Reply to
Rheilly Phoull

Thanks again.

Yes, I believed these to be more expensive, but they are indeed similarly priced. What smoothing cap do you suggest?

Reply to
youngun

"youngun"

** The 1.5 VA 6 +6 volt trannie has a 40% regulation factor.

So will deliver 8.4 volts AC when lightly loaded = 11.8 volts peak.

** Yep - the zener will have max heat when the relay is NOT energised.

Using a 5 volt reg IC means there is almost no heat dissipated in standby mode.

....... Phil

Reply to
Phil Allison

Hammond has a nice info sheet to aid in sizing transformers for various rectifier configurations - see

formatting link 

--
Peter Bennett VE7CEI 
email: peterbb4 (at) interchange.ubc.ca        
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html 
Newsgroup new user info: http://vancouver-webpages.com/nnq
Reply to
Peter Bennett

Thanks to all for helping me with my project - some very helpful and useful information!

Ian.

Reply to
youngun

Hello youngun, your resistor calculation is not correct. If you use a

9VAC transformer, the rectified voltage will be in the neighbourhood of 11.5 VDC. The voltage across the resistor will be 11.5V - 5V = 6.5VDC. For 100mA current, you need a 65 ohm resistor. 68 ohms would do nicely. Dissipation in the resistor would be 6.5V x 0.1A = 0.65W. A 1W or larger resistor is needed. ... Fred Bear
Reply to
Fred Bear

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.