Power Supply Recommendations 24VAC(rms) to 24VDC - Page 1
Forum section: electronics / projects

author: appalachia12
author_rank: Newbie
timestamp: July 16, 2012, 11:22:38 pm
content: I'm looking to build a 24VAC (rms) to 24VDC @ <1.5A max continuous power supply. Any suggestions on the best topology to minimize cost and space requirements?

author: digsys
author_rank: Unknown
timestamp: July 17, 2012, 12:43:53 am
content: QuoteYou have a 24VAC supply already? You want it regulated? Current limited? Adjustable? What ballpark costs? How much space?We'll need more info.Welcome to the forums

author: appalachia12
author_rank: Newbie
timestamp: July 17, 2012, 06:42:26 am
content: Source is output of a 24VAC transformer. Need fixed output 24VDC with VOUT(min)=18VDC. Current limit is not necessary just fused. Main load is a bank of 24VDC relays. Secondary load is input of a step down 5V supply. Thanks.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 17, 2012, 07:41:16 am
content: Use a bridge rectifier off the 24AC transformer.I think you may have problems with operating the relays with a raw full-wave rectified supply.It still tends to look like AC to a DC relay,so you will probably need to filter it a bit.With a filter capacitor,(or capacitors),you will get a bit more than 24V DC off load,but that shouldn't bother the relays,so you don't need a regulator.Another point:If it is just a 24DC supply for relays which are separately switched,you should be OK,but if you want them all (or some ) to operate on switch on,& release on switch off,remember that,if you do the on/off switching at the transformer primary,(or secondary before the filter cap),you will have both an operate delay,& a release delay,as the cap/caps,charge & discharge respectively.How well it will do the job depends mainly on the transformers ratings,but 1.5A should be easy enough.

author: David_AVD
author_rank: Super Contributor
timestamp: July 17, 2012, 09:12:32 am
content: Your 24V AC will give you approx 33V DC after full wave rectification and some filtering (capacitor).To get back down to your 24V DC, you could use a 6v8 zener diode in series with each relay coil (cathode to +33V).If you know the relay coil current, you can work out the dissipation in the zener diode and chose an appropriate wattage using Ohms Law.You may also find that the lightly loaded transformer results in a higher than 24V AC secondary so make sure you measure and take it into account.

author: appalachia12
author_rank: Newbie
timestamp: July 17, 2012, 01:00:11 pm
content: Thanks for the suggestions. The relays should work ok on the bridge rectified supply with some smoothing. However the input to the <10W output step down converter only takes up to 26v recommended and 30v max.

author: Poe
author_rank: Frequent Contributor
timestamp: July 17, 2012, 09:24:26 pm
content: A good transformer datasheet will note the output regulation (no load to full). This will be an important bit of the puzzle.A typical laminated bobbin 20Vac 36VA transformer with no load and 25% regulation will output ~25Vrms (~35Vpeak). The output drops to ~19Vrms at full load. Toroid transformers typically offer better regulation (~5%), but are higher cost.If you're going with an inexpensive transformer (poor regulation) for cost reasons, maybe use a linear regulator set at 25V in front of the SMPS.Industrial 24 relays are fairly forgiving. Use the pickup and drop-out voltages to determine maximum supply voltage ripple.Best of luck!

author: vk6zgo
author_rank: Super Contributor
timestamp: July 18, 2012, 02:45:44 am
content: "24V" relays are often used on nominal 24V supplies which can be around 28V.With your transformer supply,when they operate,the voltage will probably drop to around nominal,anyway.As Poe said,maybe a linear regulator for the input to the 5V supply.

author: appalachia12
author_rank: Newbie
timestamp: July 18, 2012, 09:30:29 am
content: The minimum operating voltage of the relays is 18v. What would I set the linear regulator for to optimize:smoothing cap sizeheat dissipation in the linear regulatorefficiency of the smpsconsidering it has to operate on the rectified & smoothed output over the possible voltage range from when no relays are operating to all relays are operating?I could also use the center tap of the 24vac transformer to output a split supply 16vdc & 33vdc but then I would need 2 banks of smoothing caps.