how to make Regulated power supply of 0 - 48V 5Amp using 24-0-24 V 5Amp center t - Page 1
Forum section: electronics / beginners

author: VickySalunkhe
author_rank: Regular Contributor
timestamp: September 17, 2017, 03:57:31 pm
content: hello i am new to the world of electronics fieldi was trying to build an regulated power supply using24-0-24 V 5 Amp Center tapped transformer input is 230 V ACTrying to get 48V 5Amp as an outputi was not able to conclude which components should be used and how to determine which components will be helpfuli have attached an Circuit diagram for 12-0-12 V 1 Amp center tapped transformer but i am not able to conclude which components to be changed as per my requirementThanks your help will be really appreciated

author: Kleinstein
author_rank: Super Contributor
timestamp: September 17, 2017, 04:11:15 pm
content: Any power supply, especially a linear one has power loss. So there is no way the 48 V 5 A transformer is possible to deliver 48 V DC at 5 A without overloading the transformer. If done in the usual way, one can get up to about 3 A of output current. This already is a lot of possible power loss, even with using both windings as two transformer taps. Rectifying 24 V AC one can expect around 32 V DC with some ripple. So for a short and 3 A current already would be around 90 W of power turned into heat at the heat sink.If one wants higher current it would likely take a kind of switched mode regulator - however this is even more difficult.Such a high power regulator is more like too much for the beginning. So for a first test I would prefer something like a 2x12 V 2 A transformer.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2017, 05:09:10 pm
content: The LM317 is limited to just 1.5A. It will work up to higher currents, with an additional pass transistor. Unfortunately additional components are required to provide short circuit protection. The circuit will still withstand short current spikes though, so that might not be an issue.The maximum input-output voltage differential rating of the LM317 is 40V, so it's possible to drop 66.5V (what you'll have on a bridge rectifier, with 48VAC in) to 48VDC, but there will be no short circuit protection. What's worse is, if the LM317's current limiting does kick in, say a large capacitor is connected to its output, causing a sudden current surge, it may be zapped, by the high differential voltage. Another issue is that at higher input-output voltages, the LM317's safe operating area protecting kicks in, limiting the current further. Additional transistors can be added to keep the voltage across the LM317 below 40V.

author: stj
author_rank: Super Contributor
timestamp: September 17, 2017, 05:26:15 pm
content: when your 48v is rectified & smoothed it will be upto 1.6x the AC voltage depending on mains frequency.so over 70vtake that into account when selecting capacitors etc.or get ready for a big wet bang - and your silicon will be toast too.

author: IanB
author_rank: Super Contributor
timestamp: September 17, 2017, 05:33:08 pm
content: Quote from: VickySalunkhe on September 17, 2017, 03:57:31 pmI think this project is much too difficult for someone who is new to the field and has not done anything like this before. A 48 volt 5 amp regulated linear power supply is very difficult to make even for professionals, and would cost several hundred dollars if purchased. Please consider that you cannot make one and try a less ambitious project instead.You can comfortably make a 24 V power supply that delivers about 0.5 A using common components like the LM317. Maybe you should try that first?

author: Ian.M
author_rank: Super Contributor
timestamp: September 17, 2017, 06:13:02 pm
content: Depending on the headroom required by the regulators, and the voltage drop across the bridge rectifier diodes, a 24V RMS secondary could give up to 20V regulated DC out at a DC current of 60% of the secondary's RMS current rating. Depending on the transformer regulation, the unloaded unregulated voltage could be as high as 37V, possibly a bit higher for a really crappy (poor regulation) transformer, and if the mains supply voltage is significantly higher than nominal it could rise to 41V.The center tap (C.T.) of a winding feeding a bridge rectifier is at half the unregulated DC output voltage from that bridge rectifier, so if it is grounded, it splits the output into equal positive and negative rails, though the reservoir caps should also be split so they are between Bridge+ and C.T, and C.T. and Bridge- respectively.A bench supply with dual outputs, positive and negative is more useful than a single output supply of double the voltage, and if you want the full voltage you simply wire the load across the +out and -out. Getting the peak unloaded unregulated voltage under 40V makes the regulator design many times easier.Therefore I'd start off with making a dual output +/- 1.2V-18V supply using a LM317 for the positive rail and a LM337 for the negative rail. Given good heatsinking it should be able to supply up to 1A. If the transformer's unloaded rectified and smoothed peak DC output voltage is above +/- 36V (with respect to the C.T.), it will need preregulators to limit the voltage to the adjustable regulators.Then start working on a design for a +/- 0-15V supply with adjustable current limiting, say 0-500mA, with tracking positive and negative outputs, that you can test running off the +/-20V supply you've built, and when you've got that working, figure out how to boost it to +/- 0-20V, 0-3A, replacing the LM317/LM337 regulators to use the full rating of the transformer. Warning: this will be *TOUGH* even after you've gained a lot of experience with the LM317/LM337 supply.

author: stj
author_rank: Super Contributor
timestamp: September 17, 2017, 06:22:07 pm
content: Quote from: Ian.M on September 17, 2017, 06:13:02 pmi KNOW you didnt post that from experience.i dont know where you got it from though.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2017, 07:16:03 pm
content: Quote from: stj on September 17, 2017, 06:22:07 pmSeems reasonable to me. I've done a simulation with a 24V transformer with 20% voltage regulation and the mains at 90% of the nominal value.With a 1.5A load, the minimum ripple valley is just over 23V, giving a maximum stable output voltage of 20V, using a standard linear regulator, such as the LM317, which needs 3V of headroom.EDIT:The RMS current is 2.25A, ignoring the turn on surge, but I only specified the transformer for 2A! The transformer needs to be rated to 50% more current, than the load.

author: Ian.M
author_rank: Super Contributor
timestamp: September 17, 2017, 10:03:15 pm
content: Yes, if you assume the mains supply is 10% low, the transformer is nominally 24V RMS, 120 VA, 15% regulation, draw a load current of 3A (60% of the secondary RMS rating) and use a reservoir capacitor of 1500uF per Amp of DC current, but that capacitor is 20% under its nominal value, with 3A rated silicon diodes in a bridge rectifier, you get about 23V on the ripple troughs, which gives 3V headroom above 20V. Parameterised sim with .measure statements attached.Of course you can make more optimistic assumptions, but it wouldn't be sound engineering design if you did that. You can also buy a few more volts headroom by increasing the reservoir capacitance, but even a factor of ten increase only gets you 4.5V headroom, and that extra 1.5V almost certainly could have been got cheaper by speccing a slightly higher output voltage transformer.However that brings its own set of problems if the mains supply is above nominal and the PSU is unloaded. e.g. at 10mA load current and 10% high mains supply, the simulation of the same transformer, bridge and reservoir capacitor gives 41.4V peak on the capacitor, so if you don't preregulate, the odds are you'll blow the <****> out of your regulator if you use a LM317 and, starting with no load, short its output to ground.