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Author Topic: Ultra fast diodes  (Read 267 times)
KB3NOV
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Posts: 57




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« on: March 08, 2007, 05:51:26 PM »

Hi,
   Can ultra fast/hyper etc diodes be used for
NON switching use ?. They are cheaper and easier for my use. I need to make what I call a drop box. 8 diodes in series to take 13.8vdc down in small steps for input to voltage regulators. Would be using 30amp diodes,might be used up to 15a. 4 diodes per 75watt rated heat sink with fan cooling.

                      Thanks and 73,

                      Jennifer
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W3JJH
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« Reply #1 on: March 08, 2007, 06:02:00 PM »

Yes, you can use the diodes that way.  Depending on the current through the diodes and their bulk resistance, you can expect between 5.5 and 8 V of drop across the string.

Unless you already own all the parts and are trying to save money, why not just use a wirewound power resistor (or several in parallel)?
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KB3NOV
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« Reply #2 on: March 08, 2007, 07:01:53 PM »

Hi,
   Thanks W3JJH, you made my day Smiley. I kept seeing switching, and started to think these new type diodes were not suited for old style uses. I checked a graph that showed current/diode temp and they will be nice as I can count on a .6 to .7 volt drop across each.

I had asked about heatsink/voltage regulator rating awhile back and you came up with good stuff. I just did a little test yesterday. I put two 7805 regulators on a delta heat sink drilled for(2)TO-3's rated at 30 watts. I ran each regulator for 5w/10w total. After a 1/2 hour the regulator case temp ran at 110deg F and held. Could keep my pinkie on it for awhile. So my new rule of thumb is without a fan take the watts x 2 1/2 and thats what the heat sink should be. For fan cooled times 2 I would think is good.

I had tried one regulator on a heat sink from a computer power supply. It was about 2x2 inches and was extruded alum. At 5 watts it heated up FAST. I'm amazed how BIG the heat sink has to be to keep things at a reasonable temp.

My next endever is using a computer case with 2 75w heatinks in it. It will have 9 lm317hvt regulators for various B+ voltages. There will be a cutout in the bottom of the case and two computer cooling fans on the top for straight through air flow. The regulators should be nice and cool Smiley.


                          73,

                          Jennifer
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W9GB
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Posts: 2656




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« Reply #3 on: March 08, 2007, 08:14:56 PM »

At least you are learning about thermal deisgn (heat sink area, cooling required, etc.).
I really get upset with many of the thermal designs from presumedly qualified engineering grads.  Actual construction and application is sometimes a better teacher than the formulas alone.

gb
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KB4QAA
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« Reply #4 on: March 09, 2007, 09:20:31 AM »

Just for discussion purposes, why not use resistors in series as classic voltage dividers?

bill.
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WB2WIK
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« Reply #5 on: March 09, 2007, 10:57:44 AM »

>RE: Ultra fast diodes       Reply
by KB4QAA on March 9, 2007    Mail this to a friend!
Just for discussion purposes, why not use resistors in series as classic voltage dividers?<

::I can think of all sorts of reasons not to.  She said her operating current was 15 amps, and she wanted to drop as much as 8V.  That's 120W, which is a mighty big resistor.  Also, if the load current varies, which is typical for most applications, using resistors the voltage drop would vary right along with the load current.  So, there's no correct resistor value, since the value required is determined by the load current.

Using diodes, the forward voltage drop of each device is quite stable over current.  A good 30A rated rectifier has forward voltage drop of about .5V at 1A and maybe .8V at 15A, so the resulting voltage will be stable within 300 mV over a 15:1 change in current.

With resistors, you can't do that.

WB2WIK/6
 
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AA4PB
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« Reply #6 on: March 09, 2007, 04:10:52 PM »

There are good reasons to use the diodes but remember that they also have to be able to disipate the power. If a resistor would disipate 120W in the circuit, so will the diodes. The fact that a diode is rated for 15A doesn't *necessarily* mean that it can disipate the power generated without having it mounted on an approprate heat sink.
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AA4PB
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« Reply #7 on: March 09, 2007, 04:23:47 PM »

Wakefield used to have (maybe still do) a free thermal design handbook. You can add up temperature rise from the junction to ambient air similar to adding the voltage drops across series resistors. As a result you can calculate the heat sink requirements pretty close and eliminate the guess work which often results in overkill. You can also get an IR thermometer pretty reasonable now so you can really measure the heat sink temperature and check your calculations.

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KB3NOV
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Posts: 57




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« Reply #8 on: March 09, 2007, 05:06:19 PM »

Hi,
   Yes, WB2WIK said it perfectly. Why I chose diodes.
Worst case each diode will dissapate 15watts. With 3 thats 45w on a 75w sink. There will be 2 heatsinks and 2 computer power supply fans. I just bought 2 used heat sinks that are rated 140w !. Wow I can put 2 LM338K on one and could run them full power without a fan !.

                     73,

                     Jennifer
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WA9SVD
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Posts: 2198




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« Reply #9 on: March 09, 2007, 08:02:38 PM »

There's almost no such thing as "overkill" when it comes to power devices and heat sinks; it's usually a $$$ factor.  The larger the heatsink, the cooler the device, and the cooler the device, the more reliable and stable it will perform, and have a longer life expectancy.  You can reach a point of diminishing returns, certainly, but I'd rather err on the conservative side.  "Overkill" heatsinking never destroyed a semiconductor.
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