40-50A power supply - Page 1
Forum section: electronics / general

author: shadowless
author_rank: Regular Contributor
timestamp: August 25, 2011, 10:09:49 pm
content: I am hunting for a 1-20v 40-50A supply at the moment.Wonder why some of the old HP 6261B are so huge and bulky but the new stuff such as the MCM 60A supply so light:http://www.mcmelectronics.com/product/72-7655&t=1

author: IanB
author_rank: Super Contributor
timestamp: August 25, 2011, 10:13:08 pm
content: The light ones are switch mode...

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 26, 2011, 12:02:03 am
content: That's true , I have build a 3-30V 30A PSU and still have it, the transformer is huge.Originally I made it so to feed 4 X MRF317 600V FM .The eight transistors in it, can handle 7A its one (56A).

author: shadowless
author_rank: Regular Contributor
timestamp: August 26, 2011, 12:19:36 am
content: What is switch mode?

author: Hypernova
author_rank: Unknown
timestamp: August 26, 2011, 12:35:12 am
content: Quote from: shadowless on August 26, 2011, 12:19:36 amA good start:http://en.wikipedia.org/wiki/Switch_mode_power_supplyIn short: efficient but noisy.

author: shadowless
author_rank: Regular Contributor
timestamp: August 26, 2011, 02:42:19 am
content: Thanks for the link.How noisy does it get, can it be used to power a brushless motor+ drive setup and achieve stable rpm?I heard a lot of complaint from the ham users about noise from these switch mode supply, i suppose this is the reason.Does anyone still use the big and heavy linear regulator?

author: Psi
author_rank: Super Contributor
timestamp: August 26, 2011, 03:35:12 am
content: Quote from: shadowless on August 26, 2011, 02:42:19 amIt's only really an issue for Audio/RF gear or areas where noise on the power supply has a negative effect on the performance of the device. Even in those cases you can use a switchmode supply if you want/need to. You just have to be very careful to design it so it produces very low ripple (not an easy thing to do).If you were powering a light with a switchmode supply then a tiny amount of ripple in the brightness of the light/led isn't going to be noticeable at all.If on the other hand you have a microphone preamp or anything that is amplifying a tiny signal to a large one then you don't want any ripple at all, as that will get amplified along with the signal.To answer your question though, a motor will work fine on a switchmode supply, you wouldn't notice any negative mechanical effect from a bit of supply ripple on the motor. You might get a little bit more EMI from it though, but it's nothing to be concerned about unless the motor is HUGE. Lots of motors are driven using PWM which is essentially switchmode without any filtering what so ever.

author: IanB
author_rank: Super Contributor
timestamp: August 26, 2011, 04:00:58 am
content: How variable do you need the output voltage to be? I am looking at a stack of discarded ATX computer power supplies right now, and they have outputs like 5 V 20 A, 3.3 V 15 A, 12 V 5 A. They are small and light.You can put the power supplies in series to get different voltages, for example the 5 V output of three boxes in series would give 15 V 20 A.(Apparently you cannotput ATX power supplies in series. The outputs are not isolated.) Bigger ones have more current, up to 60 A or more.

author: Psi
author_rank: Super Contributor
timestamp: August 26, 2011, 04:04:37 am
content: Quote from: IanB on August 26, 2011, 04:00:58 amUnfortunately you can't put PC power supplies in series without first disconnected earth from the negative side.And if you try and do that they get really noisy with horrible outputs.Though it does depend what you plan to use the voltage for, horribly noisy outputs might be ok for some things.

author: IanB
author_rank: Super Contributor
timestamp: August 26, 2011, 04:10:48 am
content: Quote from: Psi on August 26, 2011, 04:04:37 amI didn't know that. The outputs then are not isolated?

author: Psi
author_rank: Super Contributor
timestamp: August 26, 2011, 04:12:51 am
content: Quote from: IanB on August 26, 2011, 04:10:48 amcorrect, all the black negative wires are mains earthed.

author: Zero999
author_rank: Super Contributor
timestamp: August 26, 2011, 04:01:06 pm
content: Disconnecting the earth will probably also pose an electric shock risk as there probably won't be adequate insulation from the mains which is one of the reasons why it's earthed in the first place.

author: Kozmyk
author_rank: Regular Contributor
timestamp: August 27, 2011, 11:58:15 pm
content: For some time now RC modellers have been using Compaq HP DL580 Proliant G3 Server PSUs ESP114/HP114A to power their higher powered LiPo battery chargers.Other Server PSUs are available, some with higher current outputs still but the 114s are cheap and still plentiful.The problem has been that the chargers have limited output at 6s/25.2V and above when they are fed from 12V/13.8V supplies.The output of these PSUs is not normally floating.Running these HP114As in series used to involve lifting the mains ground from one of them and isolating it's case from the other.Not a satisfactory arrangement even for those who felt safe around electricity.People tried to see if they could isolate the output by removing various components and/or adding capacitors where resistor once stood.DC isolation could be achieved but a mains fault could still make the case go live.Along came xygax/Steve who discovered that you could break the print on the PCB at a certain point and separate the -ve output from circuit/case ground.Most modellers just ignore the 5V and 3.3V outputs but they are there to be used if you need them.the grounded series method:http://www.rcheliaddict.co.uk/batteries-chargers/60761-i-need-power-supply-1000w-higher-what-you-got.htmlthe early work:http://www.rcgroups.com/forums/showthread.php?t=1170784&page=25I'm in the process of adapting a pair myself, IKEA box and all. ;-)

author: IanB
author_rank: Super Contributor
timestamp: August 28, 2011, 12:38:19 am
content: Interesting info, 24 V @ 55 A = 1.3 kW, that is one beefy power supply. Better not short that baby out!If a power supply is powerful enough, it might see a short circuit as a load rather than a fault

author: Psi
author_rank: Super Contributor
timestamp: August 28, 2011, 02:09:34 am
content: Quote from: IanB on August 28, 2011, 12:38:19 amYeah, i've seen that with computer power supplies before. Where a case mod has been done and a short accidentally wired in.As soon as it was powered up the wire start smoking and glowing red until it melted away.The power supply was quite happy to do it

author: Zero999
author_rank: Super Contributor
timestamp: August 28, 2011, 04:00:39 pm
content: Quote from: Kozmyk on August 27, 2011, 11:58:15 pmEven that's dangerous as the insulation between the primary and secondary of the transformer may be designed with the assumption that it will still be safe if it fails as the secondary's 0V line is bonded to mains earth. In other words the insulation between the primary and secondary may only be basic insulation, like that between the tip and element on a mains powered soldering iron.

author: IanB
author_rank: Super Contributor
timestamp: August 28, 2011, 05:53:55 pm
content: Quote from: Hero999 on August 28, 2011, 04:00:39 pmIn the article I read that the second, stacked power supply is permanently connected in series with the first power supply inside a case with a common switch, so that the modified supply is never operated by itself in isolation. This means, as I understand it, that the secondary side of the modified supply has a path to mains earth through the first supply which remains unmodified.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2011, 05:42:31 pm
content: Quote from: IanB on August 28, 2011, 05:53:55 pmYes, that looks fine then, it's perfectly safe as long as one of the PSUs is permanently connected to an earthed supply.

author: Unknown
author_rank: Unknown
timestamp: August 29, 2011, 08:32:35 pm
content: What's the impedance of the grounded power supply to ground if it has to sink current? Does it allow enough current to flow so it blows the fuse? What happens if the fuse of the grounded power supply blows first? What's the current sinking capacity of the power supplies? Is it a four quadrant power supply?

author: IanB
author_rank: Super Contributor
timestamp: August 29, 2011, 08:52:34 pm
content: Both power supplies are grounded as normal. The only thing isolated and floating is the transformer secondary side of the second power supply. However, this secondary is permanently connected by 50 A wiring to the secondary of the first power supply, which is grounded at the 0 V rail.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2011, 08:59:16 pm
content: I think alm's point is that if an earth fault develops on the floating supply, the grounded supply connected in series with it may not be able to handle the fault current and could fail open circuit, causing the output voltage to be deadly.I suppose modifying PC PSUs like this is never a good idea.

author: IanB
author_rank: Super Contributor
timestamp: August 29, 2011, 09:22:19 pm
content: Quote from: Hero999 on August 29, 2011, 08:59:16 pmFair point, but it might be thought that there is a vast excess of safety margins in today's world. The supplymightdevelop a fault, and the ground pathmightnot be sufficient to trip the breaker, and the faultmightnot be otherwise noticed, and the usermighttouch the live part while also touching a grounded item with the other hand, and thismightresult in electrocution rather than a painful jolt and recoil. On the other hand, I might get run over by a bus tomorrow.I realize this kind of thinking is not politically correct, but if we try to remove too much risk from the world around us we also forgo many of the benefits that might otherwise be available to us.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 30, 2011, 01:57:50 pm
content: I doubt it is really a problem. Standard PSUs can't sink much current and operate properly, but they should have overvoltage protection that will handle a fault current safely. If you are really worried, stick an overvoltage crowbar across the whole circuit.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 30, 2011, 04:28:03 pm
content: Quote from: ejeffrey on August 30, 2011, 01:57:50 pmI love new wordsCan I see a picture of this overvoltage crowbar ?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 30, 2011, 05:07:34 pm
content: http://lmgtfy.com/?q=overvoltage+crowbar

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2011, 11:04:49 pm
content: Quote from: ejeffrey on August 30, 2011, 01:57:50 pmThat sounds like a good idea regardless.

author: Psi
author_rank: Super Contributor
timestamp: August 31, 2011, 01:44:54 am
content: Did someone say crowbar?

author: shadowless
author_rank: Regular Contributor
timestamp: August 31, 2011, 03:41:22 am
content: Is that a 40A crowbar?

author: IanB
author_rank: Super Contributor
timestamp: August 31, 2011, 03:55:14 am
content: This, kids, is where you learn the importance of the "squared" in I²R...

author: shadowless
author_rank: Regular Contributor
timestamp: August 31, 2011, 04:37:28 am
content: Bad examples for 'kids'. Smoking is bad for health.

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2011, 05:21:31 pm
content: Quote from: shadowless on August 31, 2011, 03:41:22 amIt doesn't have to take 40A continuously, just long enough to blow a fuse or trip a breaker.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 02, 2011, 12:50:39 pm
content: Crowbar circuits usually have insane current ratings. The reason is they clamp fast and hard to 1 volt, so most of the voltage drop (and therefore power dissipation) is somewhere else in your circuit (usually your pass transistors or transformer winding resistance). Plus, they only have to last long enough to blow the fuse. The important thing when designing with a crowbar is to make sure that your circuit can supply several times the maximum operating current for long enough to blow the fuse. If you have a 40 amp supply but it current limits to 41 amps, the fuse won't blow and your pass transistors will probably melt unless you have designed the circuit to supply the full rated current into a short circuit (something that is rare)