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Bridge Diodes use
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mprather(at)spro.net
Guest

Posted: Wed Jun 11, 2008 8:41 am Post subject: Bridge Diodes use

I think this is the point that nobody was making..  Vf is not the knee

voltage.  Bob White sent me a link to a Fairchild datasheet that showed

the I-V curve for one of their devices.  Forward biasing conduction got

going around 0.6V - 0.7V, but at the rated current the voltage drop was up

around 1.0V - 1.1V.  Eric, had you mentioned this early in the discussion,

I think we could have avoided a bunch of this churning...



http://www.fairchildsemi.com/ds/1N/1N4007.pdf



I still haven't seen where B&C stated that the voltage drop would be half

by running in parallel..  Certainly the voltage drop is reduced by running

the diode lower on the I-V curve for a given buss current..  But not by

half obviously.  Maybe an estimate would be halfway between the published

Vf and the knee voltage.

Regards,



Matt



 	  Quote: 	
		   The Vf of a diode is a strongly influenced by current. A diode can be

0.7
	


 	  Quote: 	
		   Vf at zero Amps, and 1.0 Vf at 10A and 1.2 Vf at 12.5. For this you have

to read the data sheet. What?! Radio Shack doesn't supply one?! Then
	


just

 	  Quote: 	
		   guess and hope you're right. Double up on the life insurance, too.

  "In times of rapid change, experience could be your worst enemy."

      --Jean Paul Getty



 --------

 Eric M. Jones

 www.PerihelionDesign.com

 113 Brentwood Drive

 Southbridge, MA 01550

 (508) 764-2072

 emjones(at)charter.net





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=187361#187361

Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Wed Jun 11, 2008 9:20 am Post subject: Re: Bridge Diodes use

 	  Quote: 	
		  I think this is the point that nobody was making.. Vf is not the knee voltage. Bob White sent me a link to a Fairchild datasheet that showed the I-V curve for one of their devices. Forward biasing conduction got going around 0.6V - 0.7V, but at the rated current the voltage drop was up around 1.0V - 1.1V. Eric, had you mentioned this early in the discussion, I think we could have avoided a bunch of this churning... 

http://www.fairchildsemi.com/ds/1N/1N4007.pdf 

I still haven't seen where B&C stated that the voltage drop would be half 

by running in parallel.. Certainly the voltage drop is reduced by running 

the diode lower on the I-V curve for a given buss current.. But not by 

half obviously. Maybe an estimate would be halfway between the published Vf and the knee voltage. 

Regards, Matt 	


 

Matt, et al.



See: http://www.bandc.biz/Diode_Installation.pdf



I don't claim that B & C ever stated that diodes' Vf could be summed like parallel mosfets (etc.). But I saw the 0.6 Vf on their schematic as a very suspicious indicator that somebody thought so, and that this should be attended to.



Yes, Matt, maybe I should have clarified some points earlier. But it was 100 degrees yesterday, fer chis'sakes. Thanks for your sage input. (Ahem...which you should have said earlier....)

_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

echristley(at)nc.rr.com
Guest

Posted: Wed Jun 11, 2008 9:41 am Post subject: Bridge Diodes use

Eric M. Jones wrote:

 	  Quote: 	
		   The Vf of a diode is a strongly influenced by current. A diode can be 0.7 Vf at zero Amps, and 1.0 Vf at 10A and 1.2 Vf at 12.5. For this you have to read the data sheet. What?! Radio Shack doesn't supply one?! Then just guess and hope you're right. Double up on the life insurance, too.



   


	


Wow!  Hyperbole and fearmongering.  All in the same post.



Are you sure you don't work for those 'news" organizations that tell us 

how we're killing all our children with minute traces of pesticides in 

our apples?  You could have a career there, you know.



You're trying to sell a product with minor improvements in areas that 

don't matter.  Your diode uses half the power.  We get it.  The only 

time that would possibly mean anything is in a power out situation.  In 

that situation, it will be removed from the circuit with a switch.  The 

rest of the time, the RadioShack solution is wasting 12W of heat.  That 

is   0.016 horsepower [international], which will mean the difference 

between hitting the trees versus warbird type climb out (NOT!).  If a 

system is so loaded that 12W makes a difference during normal operations 

the plane won't be flying much anyway.  The alternator/generator will be 

constantly burning up.   You state your solution doesn't weigh as much.  

OK.  Some guys obsess over every ounce.   More power to them.  If I 

needed 100 of these, it might make a difference.  The call out is for 

exactly one in most cases.  The draft in the closed hangar when I weigh 

my plane will have more effect on the finish weight.



The Radio Shack rectifier has been recommended, and presumably used by 

builders, for years.  It has proven adequate to the tasks asked of it.  

It was chosen for its price, availability, and ease of construction.  If 

it there had been any instance of them causing harm, financial or 

physical, someone somewhere would have said something about it.  Enough 

with the smoke, mirrors and demagogy.  You stated the benefits of your 

solution.  Some see validity to your claims and buy your products, just 

like some people swear by mystery chemical fuel additives.  Most took a 

look and responded with a big "Ho-hum".  To suggest at this point that a 

Radio Shack rectifier would require someone to need more life insurance 

in an attempt to sell more product is at best ridiculous, and at worst 

downright scandalous.

mprather(at)spro.net
Guest

Posted: Wed Jun 11, 2008 10:06 am Post subject: Bridge Diodes use

 	  Quote: 	
		   

 <emjones(at)charter.net>




	


snip



 	  Quote: 	
		   Yes, Matt, maybe I should have clarified some points earlier. But it was

 100 degrees yesterday, fer chis'sakes. Thanks for your sage input.

 (Ahem...which you should have said earlier....)



Glad I can bust your chops a bit..    Stay cool...
	


 	  Quote: 	
		   --------

 Eric M. Jones

 www.PerihelionDesign.com

 113 Brentwood Drive

 Southbridge, MA 01550

 (508) 764-2072

 emjones(at)charter.net




	




Regards,



Matt-

prtrotter

Joined: 19 Jan 2007
Posts: 14

Posted: Wed Jun 11, 2008 11:58 am Post subject: Bridge Diodes use

All this banter has been very interesting C but I think some people are missing part of the point here E 6nbsp; The power dissipation is only one issue in diodes and the lessor important issue in my opinion E 6nbsp; It is easy to deal with heat dissipation with the proper heat sink E 6nbsp; What concerns me is the voltage drop across a normal diode E 6nbsp; I don 7t to want to lose 1 E2 volts if the alternative is to lose less by using a Schotky diode that costs only pennies more E 6nbsp; This is especially critical if your alternator fails and you are on battery alone E 6nbsp; This voltage drop could be the difference between keeping things running properly and not E 6nbsp; Yes C you can parallel the diode with a switch as a direct E-bus feed C but I don 7t want that voltage drop in normal operations either E

  6nbsp;

 Just because something has worked well for years does not mean that newer products that perform better are not available today E 6nbsp; While the FWB is a nice package for mounting and wiring C it is no longer the best product for the application E 6nbsp; If we were to continue to think this way C there would be no innovation and we would all be running steam gauges rather than EFIS systems E 6nbsp; In all these discussions C I have never seen anybody make a statement that a plain diode is the best solution C only that it is adequate E 6nbsp; Adequate in not good enough for me if a better solution is available E 6nbsp; It is somewhat contradictory to install a 21st centrury flight instument system 6nbsp;connected to 6nbsp;a 1950 7s technology power system E

  6nbsp;

 Paul Trotter

 RV-8

 Finishing the Wiring

 



---

retasker(at)optonline.net
Guest

Posted: Wed Jun 11, 2008 1:34 pm Post subject: Bridge Diodes use

I suspect that it wasn't that at all.  I suspect it was just someone 

writing the installation document that "understood" that diodes have 

about 0.6V drop.  It is quite obvious that whomever wrote this didn't 

know a whole lot about ratings and power dissipation.  They didn't know 

that there is no such thing as a "12W" heatsink without knowing a few 

more parameters.



On the other hand, kudos for trying to educate users!



Dick Tasker



Eric M. Jones wrote:

 	  Quote: 	
		   

 Matt, et al.



 See: http://www.bandc.biz/Diode_Installation.pdf



 I don't claim that B & C ever stated that diodes' Vf could be summed like parallel mosfets (etc.). But I saw the 0.6 Vf on their schematic as a very suspicious indicator that somebody thought so, and that this should be attended to.



 Yes, Matt, maybe I should have clarified some points earlier. But it was 100 degrees yesterday, fer chis'sakes. Thanks for your sage input. (Ahem...which you should have said earlier....)



 --------

 Eric M. Jones

 www.PerihelionDesign.com

 113 Brentwood Drive

 Southbridge, MA 01550

 (508) 764-2072

 emjones(at)charter.net





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=187390#187390

echristley(at)nc.rr.com
Guest

Posted: Thu Jun 12, 2008 7:12 am Post subject: Bridge Diodes use

ptrotter(at)optonline.net wrote:

 	  Quote: 	
		     Yes, you can parallel the diode with a switch as a direct E-bus feed, but I don't want that voltage drop in normal operations either.

   

Why?  I can understand you being bothered by the idea that something is 
	


just wasting power.  Great.  Fix that.  That's what's beautiful about 

getting away from the FAA that would dictate what you can do.  But after 

swapping out the FWB, don't bother giving a big sigh of relief and 

proclaiming that you just saved someone's life.  All you did was scratch 

and itch, not cure world hunger.

 	  Quote: 	
		   Just because something has worked well for years does not mean that newer products that perform better are not available today.  While the FWB is a nice package for mounting and wiring, it is no longer the best product for the application.  If we were to continue to think this way, there would be no innovation and we would all be running steam gauges rather than EFIS systems.  In all these discussions, I have never seen anybody make a statement that a plain diode is the best solution, only that it is adequate.  Adequate in not good enough for me if a better solution is available.  It is somewhat contradictory to install a 21st centrury flight instument system connected to a 1950's technology power system.

   

You'll need to define "best".  The BEST I know how to do would be to 
	


save up $10K to have someone else paint my airplane.  That would be the 

BEST.  But it ain't gonna happen.  I'll be painting it in my garage, 

with my $50 Harbor Freight HVLP gun, and with chemicals that are not 

guaranteed to kill my family and neighbors.  That is adequate.  

Sufficient.  Gets the job done.  Then I'll go flying.  (The $50 gun did 

an excellent job with the Polyfiber chemicals on my airplanes belly and 

control surfaces, BTW. The only problem was the MEK destroyed the can's 

seal.)



The requirement for change isn't to prove the old method inadequate.  

The requirement is to prove the new method superior.  "Superior" 

includes not just minor esoteric operating benefits, but acquisition, 

build complexity, cost, and whatever else someone might be concerned 

about.  I say the EFIS is better because it is lighter, more reliable, 

easier to build into the plane, easier to source (one piece vs several) 

and cheaper.  I'll be buying a Dynon very soon now (I've installed their 

mounting fixture, but I've been waiting till the last minute to buy the 

actual unit). Whether you agree with my analysis or not, I have defined 

what I consider to be "better".



OTOH, I see no measurable benefit to the Schottky diode for this 

application.  The argument is that it conserves power in use, but that 

power consumption doesn't rank worthy of merit.  I also see having to 

purchase from a seperate source being an additional headache.  I see the 

more complex build requirements being more of a headache. While the 

extra expense is marginal, it is also pointless.  The Schottky diode 

does not meet my criteria for being better.  If the Schottky were widely 

available in a package like the FWB it might be different, but with the 

information I have I declare the FWB is most definitely the best product 

for the application.



If you feel the arguments presented for Schottky diodes make it a better 

solution, GREAT!!  Go buy you a bagful if it suits you.  But realize 

that many of us don't see it that way, and declaring that we should 

consider doubling our life insurance is fearmongering, disingenuous 

bull.   There are NO safety issues with the FWB solution.

rhdudley1(at)bellsouth.ne
Guest

Posted: Thu Jun 12, 2008 8:31 am Post subject: Bridge Diodes use

Hear Hear!! Ernest.

There is an old expression: "Perfection is the enemy of good enough". Though 

one man's perfection is another's absurd extravagance.

Regards,

Richard Dudley

RV-6A flying with Z-11

---

nuckolls.bob(at)cox.net
Guest

Posted: Thu Jun 12, 2008 3:18 pm Post subject: Bridge Diodes use

At 06:28 PM 6/10/2008 +0200, you wrote:

 	  Quote: 	
		  > >On B & C Diode Installation sheet they state that "Heatsink must 

> dissipate 0.6 X Amps = Watts. Example For 20 Amp Essential Bus, 0.6 X 20 

> = 12 Watt Heatsink".

In my experience, I have never come across a 12W heatsink. Nor a 1W or 

1000W heatsink.

A heatsink is chosen appropriate to the temperature rise above ambient the 

casing of the device can tolerate, or what the spec-sheet says. A single 

machined-bolt head can dissipate 12W without any additional cooling, 

however only when it reaches 200 degrees C (thumbsuck). If you can only 

tolerate a 1degC increase in temperature, the heatsink required to 

dissipate 12W will be the size of a large desk.

So, a specsheet specifies the heatsinking requirements in [degrees above 

ambient]/[Watt], or specifies the maximum allowable temperature. So 

consider an ambient of 23degC, a maximum of 100degC, and we have 

calculated that the device will need to dissipate 10W. That gives us a 

requirement for a 7.7deg/Watt heatsink, which can be ordered accordingly.

I understand the calculations relating to dissipation of heat, and IIRC a 

Schottky diode, with a forward bias voltage of 0.2V rather than the 

0.6~0.7V of a regular diode, dissipates 1/3 the energy for a given 

current. However, there are always other considerations that have not been 

touched in this discussion (such as thermal robustness, vibration 

tolerance, ease of mounting), some of which are the same for both Schottky 

diodes and for normal ones, and others which will draw very clear lines in 

the sand precluding one or both from certain applications.

   Very good! You have nailed the physics of heat-sink

   sizing. Any size heatsink will dissipate the energies of

   ANY size heat source . . . the question to be asked and

   answered is "hot hot does it get" over and above ambient

   while being stoked at that energy level -AND- environmental

   temperature.

   B&C's advice was ill worded but I don't work for them

   any more. At the same time, I was hoping someone

   to rise to the task of illuminating the physics of

   the matter. As I write these words, there's a Schottky

   device in the chamber getting it's thermal portrait

   taken on the same heatsink that gave us a portrait of

   a common power rectifier earlier this week. I'll have

   comparable DATA to share in the not too distant future.

   My hat's off to you sir.

   Bob . . .

        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

nuckolls.bob(at)cox.net
Guest

Posted: Thu Jun 12, 2008 3:25 pm Post subject: Bridge Diodes use

At 06:05 PM 6/10/2008 -0600, you wrote:

 	  Quote: 	
		  



Eric,



Am loath to get between you and Bob when discussing electrons but don't 

understand what you said about bridge diode heat dissipation.  If one 

diode is running 20 amps with a forward voltage of 0.6 then it generates 

0.6 x 20 = 12 watts of heat.

If two diodes in parallel are carrying the same 20 amps then the current 

is split between the two diodes, with each having more or less half the 

load with a total of 20 amps between them.  So each diode is generating 

about half of 12 watts with a total generation of still only 12 watts not 24.



What am I missing?


	


   You'll never be 'stuck' between us my friend.

   Your question is very much on point because until

   a posting pointed out a little detail about

   "thermal resistance" ratings of heat dissipation

   systems, there was no way that anyone should be

   expected understand what was correct and/or incorrect

   about any of the advice.



   Know that a Bob's Shop Notes is being crafted with

   real data that will make the physics clear and give

   you the tools with which to make a design decision

   about your system. There is nothing inherently evil

   or good about either device. Both technologies

   have and are producing satisfactory performance when

   used in a manner that does not violate their

   performance envelopes.



   The smoke and mirrors will be dealt with.



   Bob . . .



        ----------------------------------------)

        ( . . .  a long habit of not thinking   )

        ( a thing wrong, gives it a superficial )

        ( appearance of being right . . .       )

        (                                       )

        (                  -Thomas Paine 1776-  )

        ----------------------------------------

mprather(at)spro.net
Guest

Posted: Thu Jun 12, 2008 3:54 pm Post subject: Bridge Diodes use

One final thought about this..  If we keep our E-buses on a diet, the Vf

will be lower, and the heat loss through whatever diode you choose will be

less.  These obese E-buses are killing us!!  Hah.  

Matt-



 	  Quote: 	
		   

 <emjones(at)charter.net>

> I think this is the point that nobody was making.. Vf is not the knee

> voltage. Bob White sent me a link to a Fairchild datasheet that showed

> the I-V curve for one of their devices. Forward biasing conduction got

> going around 0.6V - 0.7V, but at the rated current the voltage drop was

> up around 1.0V - 1.1V. Eric, had you mentioned this early in the

> discussion, I think we could have avoided a bunch of this churning...

> http://www.fairchildsemi.com/ds/1N/1N4007.pdf

> I still haven't seen where B&C stated that the voltage drop would be

> half

> by running in parallel.. Certainly the voltage drop is reduced by

> running

> the diode lower on the I-V curve for a given buss current.. But not by

> half obviously. Maybe an estimate would be halfway between the published

> Vf and the knee voltage.

> Regards, Matt

 Matt, et al.



 See: http://www.bandc.biz/Diode_Installation.pdf



 I don't claim that B & C ever stated that diodes' Vf could be summed like

 parallel mosfets (etc.). But I saw the 0.6 Vf on their schematic as a very

 suspicious indicator that somebody thought so, and that this should be

 attended to.



 Yes, Matt, maybe I should have clarified some points earlier. But it was

 100 degrees yesterday, fer chis'sakes. Thanks for your sage input.

 (Ahem...which you should have said earlier....)



 --------

 Eric M. Jones

 www.PerihelionDesign.com

 113 Brentwood Drive

 Southbridge, MA 01550

 (508) 764-2072

 emjones(at)charter.net





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=187390#187390

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