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Heat Sink necessary?

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jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Tue Jul 06, 2010 10:07 am Post subject: Heat Sink necessary?

Using a 25a bridge rectifier for dual power sources for EFI. The injectors might pull 10a, worst case. For normal ops, 6a.

Is heat sinking necessary for the rectifer?



Thanks,

J

longg(at)pjm.com
Guest

Posted: Tue Jul 06, 2010 11:14 am Post subject: Heat Sink necessary?

Jon,



You are mixing supply and demand (but that's ok). Normally it is

suggested a heat sink be used for a sink load of 20A or more. Since your

hardware is bigger than your load you could try it without. If it is

continuous load you may want to monitor the rectifier temperature to be

sure you're in limits. The manufacturer should publish that.



If you're at all concerned about temperature, just add a heat sink. If

the heat sink sets you back wait till you see the price at the gas pump.

Wow! The poor man's option is to mount it on a piece phenolic block

which you'll find does little for heat dissipation but is virtually

indestructible should your diode take to glowing in flight. You can cut

some grooves in the top of the block to allow air to travel underneath

the rectifier. You can also buy one of those computer fans and a frame

for about a $1.00 to point in the general direction. I have a variety

pack of those stick on thermometers that lets one visualize surface

temps. Could be handy for something like this. Most of this is much more

fuss than just installing the heat sink.



Hey, what kind of injectors pull 10A? I suppose you mean the ignition

module.



Enjoy,



Glenn E. Long



--

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jul 06, 2010 12:08 pm Post subject: Heat Sink necessary?

At 01:07 PM 7/6/2010, you wrote:

 	  Quote: 	
		  



Using a 25a bridge rectifier for dual power sources for EFI. The 

injectors might pull 10a, worst case. For normal ops, 6a.

Is heat sinking necessary for the rectifer?


	


   yes . . . but simply mounting it to a metallic

   surface on the airplane will be sufficient.

   10A on any one leg of the device will dissipate

   about 7 watts of heat that is easily managed by

   most metal surface mounting situations.



   Doing in-situ temperature measurements on the

   molded bridge rectifier is VERY difficult. There's

   no thermally integral metal surface available for

   sampling. You'd need to bury a tiny-wire thermocouple

   in the mounting under the rectifier assembly.

   You can measure the temperature of the sheet metal

   directly opposite the rectifier and make some

   assumptions about thermal resistance of the

   mounting . . . but it's more fuss than you'd

   probably want to experience. 10A doesn't present

   much of a challenge to a device with a metal

   heat-sink interface. Use a little 'heat sink'

   grease under it when mounting.



http://www.radioshack.com/product/index.jsp?productId=2102858



   Bob . . .

jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Wed Jul 07, 2010 8:29 am Post subject: Re: Heat Sink necessary?

Glen and Bob, Thanks.



I used a .063 alum washer for a stand-off from my composite structure which allows air to the metal backside of the rectifier. I will add a larger surface area piece of alum w/recommended grease.



The injectors are 55#/hr and my 6 cyl engine, with a bsfc of .55 lbs./hr/hp, will have a max fuel consumption of 132 lbs/hr so these injectors will only see a 40% duty cycle. At 75% they draw 9W each or 54 W,  so my normal ops should draw about 2.5 amps. So my initial guess of 6a is too high. The 10 amp circuit recommended by the EFI mfgr is to cover all the bases including low voltage and higher power than my engine makes.

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 07, 2010 8:49 am Post subject: Heat Sink necessary?

At 11:29 AM 7/7/2010, you wrote:

 	  Quote: 	
		  



Glen and Bob, Thanks.



I used a .063 alum washer for a stand-off from my composite 

structure which allows air to the metal backside of the rectifier. I 

will add a larger surface area piece of alum w/recommended grease.


	


   Okay, recommend something on the order of 10 square inches

   of .062 aluminum. If you wanted to make it "tighter" on

   real-estate, you could bend the edges up so that the

   diode still sits on a flat spot, but the overall width

   is reduced.



 	  Quote: 	
		  The injectors are 55#/hr and my 6 cyl engine, with a bsfc of .55 

lbs./hr/hp, will have a max fuel consumption of 132 lbs/hr so these 

injectors will only see a 40% duty cycle. At 75% they draw 9W each 

or 54 W,  so my normal ops should draw about 2.5 amps. So my initial 

guess of 6a is too high. The 10 amp circuit recommended by the EFI 

mfgr is to cover all the bases including low voltage and higher 

power than my engine makes.


	


    Hmmmm . . . maybe you don't need to do anything with

    the diode. Wait until you're able to run the engine

    and take some measurements of your own. It sounds

    like it might be just fine with zero-heat sink.

   Bob . . .

klehman(at)albedo.net
Guest

Posted: Wed Jul 07, 2010 2:48 pm Post subject: Heat Sink necessary?

It is sometimes good to imagine what can go wrong. Generally high 

impedance injectors run around 12 ohms resistance. If one or more 

injector driver fails on, the engine will likely keep running unless 

subsequently a fuse pops or a diode fails due to the extra current. I'd 

consider making sure the diode had some heat sinking and was capable of 

feeding all six injectors at 100% duty cycle which is likely 7+ amps at 

14+ volts.



Ken



Robert L. Nuckolls, III wrote:

 	  Quote: 	
		   

 <nuckolls.bob(at)aeroelectric.com>

 

 At 11:29 AM 7/7/2010, you wrote:

> 

> <jonlaury(at)impulse.net>

>

> Glen and Bob, Thanks.

>

> I used a .063 alum washer for a stand-off from my composite structure 

> which allows air to the metal backside of the rectifier. I will add a 

> larger surface area piece of alum w/recommended grease.

 

   Okay, recommend something on the order of 10 square inches

   of .062 aluminum. If you wanted to make it "tighter" on

   real-estate, you could bend the edges up so that the

   diode still sits on a flat spot, but the overall width

   is reduced.

 

> The injectors are 55#/hr and my 6 cyl engine, with a bsfc of .55 

> lbs./hr/hp, will have a max fuel consumption of 132 lbs/hr so these 

> injectors will only see a 40% duty cycle. At 75% they draw 9W each or 

> 54 W,  so my normal ops should draw about 2.5 amps. So my initial 

> guess of 6a is too high. The 10 amp circuit recommended by the EFI 

> mfgr is to cover all the bases including low voltage and higher power 

> than my engine makes.

 

    Hmmmm . . . maybe you don't need to do anything with

    the diode. Wait until you're able to run the engine

    and take some measurements of your own. It sounds

    like it might be just fine with zero-heat sink.

 

 

   Bob . . .

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jul 07, 2010 6:29 pm Post subject: Heat Sink necessary?

At 05:47 PM 7/7/2010, you wrote:

 	  Quote: 	
		  



It is sometimes good to imagine what can go wrong. Generally high 

impedance injectors run around 12 ohms resistance. If one or more 

injector driver fails on, the engine will likely keep running unless 

subsequently a fuse pops or a diode fails due to the extra current. 

I'd consider making sure the diode had some heat sinking and was 

capable of feeding all six injectors at 100% duty cycle which is 

likely 7+ amps at 14+ volts.


	


   Would that not be a multiple failure scenario?  . . .

   and would the engine run with 5 of the 6 injectors

   turned on hard?



   Bob . . .

klehman(at)albedo.net
Guest

Posted: Thu Jul 08, 2010 3:37 am Post subject: Heat Sink necessary?

Normally injectors are sized such that full on only represents about 30% 

extra fuel over full power. (sized primarily to give better idle 

control) So yes in most cases an engine will run at aircraft cruise 

power or higher with all injectors locked full on. I would consider one 

or more locked on injectors a single failure. Older ecu's batch fire 

injectors and share drivers. But yes you'd need to go into ecu and 

wiring specifics to determine whether a single failure could lock more 

than one injector on. 100% extra fuel (twice normal flow) on my subaru 

makes the exhaust system quite hot but does not stop the engine. Rare 

but injectors do occasionally fail on. A minor concern admittedly.

Similarly for multiple coil ignitions. Rare but I've seen single coils 

short out internally and draw continuous current. I'm just suggesting 

that some headroom or safety margin in the sizing and fusing of such 

circuits is a good idea and that would include any diodes.

Ken

Robert L. Nuckolls, III wrote:

 	  Quote: 	

 <nuckolls.bob(at)aeroelectric.com>

 At 05:47 PM 7/7/2010, you wrote:

> 

>

> It is sometimes good to imagine what can go wrong. Generally high 

> impedance injectors run around 12 ohms resistance. If one or more 

> injector driver fails on, the engine will likely keep running unless 

> subsequently a fuse pops or a diode fails due to the extra current. 

> I'd consider making sure the diode had some heat sinking and was 

> capable of feeding all six injectors at 100% duty cycle which is 

> likely 7+ amps at 14+ volts.

   Would that not be a multiple failure scenario?  . . .

   and would the engine run with 5 of the 6 injectors

   turned on hard?

   Bob . . .

jonlaury

Joined: 06 Nov 2006
Posts: 336

Posted: Thu Jul 08, 2010 6:37 am Post subject: Re: Heat Sink necessary?

Ken,

Thank you for your illumination of some issues surrounding EFI. The Simple Digital Systems EFI is the one I'm using. Your comments have explained why SDS said that I might want to consider using a 15 amp circuit. It never made sense to me until I read your comments about sizing for all 6 injectors locking on (100% duty cycle). The bridge diode that I'm using is rated for 25a (at) 200v so there seems to be sufficient headroom.

My injectors are low impedance type and SDS supplies a 3 x 4 x 1/8" aluminum piece with 3 gold anodized resistors??(I'm guessing) that have fins radiating around them. Installation instructions say this piece can get "quite hot at higher duty cycles". 

It seems that the SDS EFI 'wants' to have high impedance injectors. Are there advantages to a high impedance architecture?



John

klehman(at)albedo.net
Guest

Posted: Thu Jul 08, 2010 12:46 pm Post subject: Heat Sink necessary?

John



Port injectors are usually high impedance and they are by far the most 

common injector. They simplify control issues and drive circuits and are 

the only ones that I've played with. Suitable mosfet (transistor) 

switches are very cheap and don't have large heat sinking requirements. 

I allow just over 1 millisec opening delay for mine when calculating and 

the 90 volt spike when I turn them off is not much of a problem.



Large injectors for throttle body injection or very high power engines 

are more likely to be low impedance. Larger injectors need more energy 

to activate them. I have not used them but they are not uncommon in the 

aftermarket. Low impedance ones will open faster I believe but you 

typically throttle back the current to a hold open value. That likely 

extends life but it also lets them close faster. The higher current and 

higher voltage spikes of low impedance injectors complicates things. 

Your power resistors would be there to reduce the hold open current. 

There are other methods like pulse width modulation, flyback circuits, 

etc. There are hardware and software methods of accomplishing all this 

but it is simpler with the high impedance (lower current) injectors.



I'm pleased if I helped a bit. SDS does good work and they likely have 

better explanations of this on their extensive website so definitely 

follow their recommendations.



Ken



jonlaury wrote:

 	  Quote: 	
		   

 <jonlaury(at)impulse.net>

 

 Ken, Thank you for your illumination of some issues surrounding EFI.

 The Simple Digital Systems EFI is the one I'm using. Your comments

 have explained why SDS said that I might want to consider using a 15

 amp circuit. It never made sense to me until I read your comments

 about sizing for all 6 injectors locking on (100% duty cycle). The

 bridge diode that I'm using is rated for 25a (at) 200v so there seems to

 be sufficient headroom. My injectors are low impedance type and SDS

 supplies a 3 x 4 x 1/8" aluminum piece with 3 gold anodized

 resistors??(I'm guessing) that have fins radiating around them.

 Installation instructions say this piece can get "quite hot at higher

 duty cycles". It seems that the SDS EFI 'wants' to have high

 impedance injectors. Are there advantages to a high impedance

 architecture?

 

 John

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