Advice on purchasing a 30-40VDC, 50-60 Amp Variable Reg. supply - Page 1
Forum section: electronics / beginners

author: BVH
author_rank: Contributor
timestamp: June 03, 2012, 11:28:45 pm
content: Well I certainly fit this section. Just a little background. My hobby is acquiring and playing with high power military and non-military searchlights. I had a WWII General Electric 60" Carbon Arc "grand Opening" light. I have a few 1KW and 2.2KW Army tank and helicopter searchlights. I also have smaller 100 to 600 Watt Short Arc lights I use a variable regulated power supply to run. All of these ballasted lights are from 12V to 28VDC input. I am NOT an electronics expert, probably not even a novice for that matter but I am fairly knowledgeable in general electrical. I can read schematics and identify many electronic components. I can replace components on a board.OK, now my issue. In the last three years I've fried a couple of Mastech 30V, 50A regulated variable DC bench supplies. The first one when I was making some ballast input connections by just touching wires together and probably caused a spike? I was powering a flashlight ballast. On the advice of a member at a different forum, on the second Mastec, I installed a Schottky diode in the + output cable. I was recently testing a defective 15 Watt ballast which flickered heavily and before I could disconnect it, poof went my PS. In both cases the displays died and in the latest case, it still produces Voltage that is variable but won't sustain a load.I've been told by some that a switch mode DC PS (which I'm told my Mastecs are) is not really the best PS to use for running HID ballasts - which is my primary use. I don't remember what they're called but it was suggested that something like an "SDR"? the older, bigger, heavier supply would be better. I don't think "Linear" was the word they used but I'm not sure. I want something in the neighborhood of 30V & 70 Amps, which, I know will require 240V input which is not an issue and will be relatively expensive. The Mastecs were about $450 each. I would like to be able to power two of my helicopter lights at one time (started separately) which requires 28V and 32A (total) while running but as much as 2 to 3 times the current when igniting and during boost phase. It doesn't matter if its somewhat big and heavy. I found some Sorensen 40-75's and a Lambda 40-60 but I'm gun-shy to get another one and end up frying it, too. I don't specifically need a variable but I want one. Who knows what I might need different voltages for in the future.Lastly, I was told that there needs to be two "diodes" used to protect against reverse power flow and "something else" (again, I was not fast enough to absorb everything said). I heard the term "blocking diode" but it looks like something that is used in solar power systems?So I'm seeking advice on:1. What type (and brand, if important) of Variable Regulated PS to buy for running ballasted loads2. How to protect it.This is the diode I was advised to use in the mastec.

author: free_electron
author_rank: Super Contributor
timestamp: June 04, 2012, 03:51:00 am
content: Most likely the power supplies die because of massive load dumps.These lights are made to run off batterie. Batteries can absorb energy. Power supplies cant.Putting a diode in series will probably fry that diode too.Try putting a fast heavy shottky in parallel with the power supply output . The cathode connected to + and the anode connected to ground. If there is a loaddump the diodes will absorb it.

author: electroguy
author_rank: Regular Contributor
timestamp: June 04, 2012, 05:11:25 am
content: To do the "blocking" diode setup you are talking about, you would want quite a few diodes in parallel, and you would also have each one mounted to heatsinks, as you would be dissipating 30 to 80 watts in just the diodes.According to your description, the diode you have was connected in blocking mode (only on the + terminal) and must have been extremely hot if you didn't have it on a heatsink. Did you fry it?

author: digsys
author_rank: Unknown
timestamp: June 04, 2012, 07:01:32 am
content: I would approach it differently, a method I've used many times.Firstly I doubt you need a precison regulated supply for what you are driving. If NOT -Find a 240V 10A VARIAC = ~2400W, this shouldn't be hard to source 2nd hand / cheapish.Feed that into a 240V --> ~24VAC Transformer. It may be a bit harder to findsomething around 70A+ but see what you can come up with. 24VAC is a very verypopular voltage in industrial systems. DO NOT bypass the transformer, it's yourisolation and safety !! Then you make up a bridge with say 100A diodes, WELL heatsinked,and add say a total of 50,000uF 35-50V electrolytics. This will give you reaonable ripple.Put app 50A of 40-60V Polyfuses in series with the output. You can use eg 5x10A in parrallel,poly's love being in parrallel. You can add a 36V 1.5KE+ Transorb across the Filter capacitorsto help against spikes. A really BIG spike will just trip the polyfuses.Now you simply DIAL UP whatever DC voltage you need using the Variac, slowly.That dual diode you show has NO hope, they need to be really BIG ASS ones.NOTE: This is a potentially dangerous P/Supply, so don't attept it unless you arecapable or have a friend who is.

author: Jeff1946
author_rank: Regular Contributor
timestamp: June 05, 2012, 02:49:53 am
content: Maybe you could use and arcwelder to power it