Transformer Sizing / Rating

[youngun]

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.

 

[Phil Allison]

** Groper alert !!

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?

** 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 !

http://au.farnell.com/jsp/search2/b...earch_001&Ntt=438+4600&Ntx=&_requestid=389799



I assume you will use a 5 volt regulator IC ?



....... Phil

 

[youngun]

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?

 

[Rheilly Phoull]

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?

Why not use a 3 terminal reg for similar cost ?

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

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

 

[youngun]

Thanks again.

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

 

[Phil Allison]

"youngun"

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.

** 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.

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?

** 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

 

[Peter Bennett]

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.

Hammond has a nice info sheet to aid in sizing transformers for
various rectifier configurations - see
http://www.hammondmfg.com/pdf/5c007.pdf

 

[youngun]

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

Ian.

 

[Fred Bear]

Zenner: 5v @ 1w

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

Do you see any problems with the above?

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