Variable Regulated DC power supply type and protection advice sought

[BVH]

I've now fried a couple of switch mode Mastech 30V, 50A regulated variable DC bench supplies. The first one when I was making some connections by just touching wires together and probably caused a spike? On the advice of a member here, on the second one, 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 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. I want something in the neighborhood of 30V & 50-60 Amps, which, I know will require 240V input which is not an issue and I know it will be over $700. It needs to be variable and to be able to run in both constant voltage and constant current modes. It doesn't matter if its somewhat big and heavy as I can sit it on the floor. I found a U.S. made Lambda 40V-60A for $1200 but I'm gun-shy to get another one and end up frying it, too.

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

2. How to protect it.

[TedTheLed]

would this help? http://power-topics.blogspot.com/200...onverters.html

[BVH]

I'm not knowledgeable enough to fully comprehend what is being shown but it's showing DC to DC converters whereas I'm using AC to DC Converters.

[Norm]

The sort of supply your talking about has always been known as linear in the Amateur Radio world, be prepared for boat anchor weight, my last 40A commercial built supply weighed about 35kg.

I now use two Powertech (Manson) 30A supplies..

I do miss the old supply but for my use the switching supplies are far more practical.

Norm

[Mr Happy]

The advantage of linear supplies is that they generally provide much more "finesse" than switched mode supplies. They generally have very good regulation, very low noise on the output, and very precise control. Mostly they are found with lower currents as they get big, expensive and heavy very quickly when you ask for high current outputs. There are some "hybrid" supplies that have a switching pre-regulator and a linear final stage. They can be smaller and more efficient while keeping most of the advantages.

I think the problem you are having with your ballasts is that you are feeding bad stuff back into the power supply. Power supplies are generally intended to feed power out of their terminals into the load. They generally don't support having power fed back into their terminals from the load, which may happen with awkward kinds of inductive or capacitive load. Good supplies incorporate some protection against reverse power, but you can presumably exceed the rating of that protection if you try hard enough.

If you want some good detailed answers from experts, you might like to go over to the EEVBLOG forums (Google it) and discuss your problems there. They are populated with some professional electronics engineers (and some idiots too), but with discernment you can filter out the idiots Power supplies are a perennial subject of discussion.

[BVH]

TY Mr. Happy. I made a post over there.

[Mr Happy]

Also, a few general pointers. Good practice with power supplies is not to connect up wires while the power supply is energized. Usually you should connect up the device under test first, and then switch on the output from the supply. Higher quality supplies have an "output on/off" switch to support this. Also, if you are uncertain about the thing you are testing, you should set a current limit somewhere safe, and then slowly wind up the output voltage from zero, while carefully watching what is happening. It is a good idea not to apply full power at the design voltage at switch on.

So: set the current control to a limit no higher than the current you need; lower if you are testing something that might have a fault. Set the voltage to zero. Connect up the device under test. Attach meters or other instruments to the device under test if appropriate. Power up the supply. Slowly increase the voltage, while watching like a hawk for any signs of things going wrong. If all is good, continue winding up the voltage to the operating value.

[TedTheLed]

slightly off t but, Ive been using a 30v 3 amp dc power supply to charge batteries for years, and I use it much the way you describe, limit the amps, slowly dial in the voltage until there's a responce on the needle, then set it a hair above that and leave it. When the voltage limit is reached it stops.. it kind of takes practice which is why I am really tempted to get the exact same unit ( an "EZ" ?) but with DIGITAL READOUT , so I can just dial in values (ie. multiples of 1.4 ) and forget it.

question is, could I benefit from this here linear dc supply you're talkin about ? sounds like a goodun and I require only 4 or 5 amps...
?

[BVH]

Here's the diode I was advised to use. Is it what I should be using? I cut the center leg off and soldered the output cable across both outer legs. Other end of output cable soldered to mounting tab.

[Mr Happy]

question is, could I benefit from this here linear dc supply you're talkin about ? sounds like a goodun and I require only 4 or 5 amps...
?

A linear supply is basically the standard, traditional kind of laboratory bench power supply. It consists of a big transformer, a rectifier, some smoothing capacitors, and some voltage and current regulating circuits. The supply you have may well be a linear supply.

But for battery charging you don't need a linear supply. A cheaper switching supply should work just fine. It will be smaller, lighter, cheaper and cooler running. I have the Extech 382260 and it works quite nicely for me.

[Mr Happy]

Here's the diode I was advised to use. Is it what I should be using? I cut the center leg off and soldered the output cable across both outer legs. Other end of output cable soldered to mounting tab.

That diode has a reverse breakdown voltage of only 120 V. I suspect you could get 100's or 1000's of volts from big inductors. I doubt it would offer enough protection in worst case scenarios.

What you probably need is back EMF protection, which would be a diode connected across the ballast so as to short out and bypass any high voltage spikes before they can reach the power supply.

[BVH]

Can that be accomplished by physically and permanently installing the diode directly across the large output lugs of the supply instead of at the load input? I see a large component connected directly across many of the PS's I've been looking and wondered what it was.

[TedTheLed]

wow cheap digital meter power supplies! thanks for the link Mr Happy...( ha, thats what a friend calls his, uknow.. )

[BringerOfLight]

That diode has a reverse breakdown voltage of only 120 V. I suspect you could get 100's or 1000's of volts from big inductors. I doubt it would offer enough protection in worst case scenarios.

The 120V seem to be the point, it looks like he is using the diode as TVS diode (TVS = transient-voltage-suppression). I would use one actually intended for that purpose with well-defined breakdown voltage, fast response time and higher power rating (that V60120C is only rated for 0.5A reverse current). For Vishay, look at:
http://www.vishay.com/diodes/protection-tvs-esd/

[BVH]

Thank You Bringer. There's a bunch on that page and I don't quite understand the column titles as far as current and Voltage ratings. Could I just go for ones that have the highest ratings? What do I lose by going as high as I can versus something lower? Is there a higher Voltage loss, cost of the device or? Which columns do I need to be looking at for unwanted high transient Voltage protection?

[BringerOfLight]

The purpose of the TVS diode is to start conducting when the voltage is too high (to protect your power supply from over voltage). So you should choose a reverse standoff voltage slightly higher than the maximum voltage you want out of the power supply, e.g. for a 30V power supply a 1N6285A or 5KP33A.

Datasheet for 1N6285A:
http://www.onsemi.com/pub/Collateral/1N6267A-D.PDF

In terms of current/power rating, if you choose too little, the diode may go up in smoke if you have a serious overvoltage condition. Given the high currents you are working with, I would choose a pretty beefy one or put a couple in parallel (they will share the load pretty well), e.g. a single 5KP33A or 2 1N6285A.

In normal operation, the diode will consume almost no power (conduct a few microamps), it will only burn off the overvoltage spikes.

[BVH]

I'm considering a used Sorensen 40V/75A variable. Would 4 of 1N6288A, G in parallel be sufficient? Do they need to be heat sinked and if so, how is that done given they are plastic body diodes? If I made 2 each, 1/8" thick, x 1" long x 1/2" wide brass or copper metal bars and soldered the diodes in parallel across the bars and then mounted the bars to a larger sink, does enough heat get sinked thru the diodes leads? And finally, does the diode package go across the output terminals or in the positive output cable. Thank you very much for all this good info.

I looked at some other devices from that main page and see that there are some devices that look like they are already packages of multiple diodes probably in some sort of case/enclosure. I just don't know enough to know if one of those would be what I need instead of making my own?