Running DC relays and lamps from a DC supply that is well regulated (Regulated or pretty solid non-regulated):
Relays have either a current spec or a voltage/resistance spec. Use Ohms to get current. (In many systems, a "24V" relay is run on an UN-regulated supply that may rise to 30V, which may be inside the relay's rating, but you have to compute the current at the higher voltage.)
Incandescent lamps have a running current. But when cold, they eat MUCH more current. They heat-up a lot faster than transformers, so with an unregulated supply you can let the cold-start surge "overload" the transformer since the lamp will be hot and normal-current before the transformer is slightly warm.
This "overload" may be OK on regulators: the lamp warm-up is short, and the regulator will protect itself. Warm-up could be slower, but not enough to see. Still, I would like to see a regulator max rating 3 to 10 times the lamps' hot-current, to be sure it won't stall in a low-voltage high-current situation.
SO the DC running current is the sum of all the relay and lamp currents. In some cases you may "know" that not all relays and lamps can ever be on at once: an 8-input selector will only have 1 of 8 relays and lamps live at once. And in theory, it may be silly to have more than half your Phase switches flipped (but stuff happens). Obviously your Phantom relays/lamps could be all on or all off depending on the gig.
The regulator must have ample heatsinking for this maximum total running current at the expected high-line input voltage and worst-case temperatures. It should probably have a peak rating of the total relay current plus 5 times the total lamp current, just to be sure.
As a good safe rule of thumb, the AC current rating of the transformer needs to be twice the maximum DC current. And if the load might be maxed-out for an hour, round-up and leave plenty of air around the core, or bolt the transformer feet to heavy chassis.
4 x Relays @ 20.8mA
3 x Relays @ 20mA
Relay current is 83.2+60= 144mA
7 x T1 24v Lamp @ 20mA
ASSuming incandescent lamps:
Lamp running current is 140mA
Lamp cold-start current could be over 1,000mA. A 7824 can handle this.
Total running current is 144+140= 284mA.
I would figure relays and lamps could vary 20%. They probably won't, but to be safe: 340mA.
A 24V regulator needs (rule of thumb) a 24VAC transformer which will deliver 40V to 30V raw DC on the first cap. A sloppy invalid rule of thumb: use 1,000uFd per Amp of DC current. 330uFd may work. If the transformer is any good, you will get 32VDC-35VDC under load. So the regulator sees 35V-24V= 11V across it, 0.34A through it, 3.75 Watts dissipation. Use at least 4 square inches of heavy aluminum.
The AC current and heating effect in the transformer is 1.6-1.8 times the DC current. Round to 2. The transformer must be rated for 2*0.34A or 0.7 Amps AC.
The tranny VA rating is 24V and 0.7A or 17VA. A 20VA or 24VA unit meets spec. A 35 or 50VA unit does not cost a lot more, and will run cooler under constant load.
> I am paralleling the transformer output windings
Identical windings on the same core can be paralleled BUT: connect one end together, put a voltmeter across the other ends, carefully connect line voltage (or a beefy signal generator st to 50-60Hz). If truly parallel, the voltmeter says zero (less than a Volt). If anti-parallel, the voltmeter says 48V. If you connected it this way, it would short itself and smoke.
> I am driving this from a 24v 24VA transformer.... Assuming this means that there is 1 amp available per winding
Not, this is the TOTAL rating. (It is really the core size and cooling area.) You can wire it parallel 24VAC 1AmpsAC, or series 48VDC 0.5AmpsAC. Or you can use just one 24V winding at about 18VA or 24V 0.75A. But you can not get more than 24VA total without sag and heat.
And the maximum well-regulated DC power from a 24VA core is only about 12 DC Watts.
> Does this mean that I have enough current for at least 4 of these systems
e.g. 303.2 x 4 = 1.1128A and the transformer provides 2 Amps paralleled
No. 25VA is actually just a hair big for ONE of these systems.
Each channel eats 8 watt of regulated DC. Each channel needs 16VA of core. Total requirement is 64VA, and always round-up to 100VA.
DC current is over 1Amp which means the common 1N4004 diodes will NOT live long: rectifier math is complictated but rects are cheap and failure is ugly, use at least 3A rectifiers and I would like 5A or 10A for a 1.1ADC supply.
BTW: try a 220 or 470 resistor in series with the lamps. Brightness will be a little down, but life will go WAY up.
