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Schottky Diode

 
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Peter(at)sportingaero.com
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PostPosted: Thu Dec 27, 2018 2:45 pm    Post subject: Schottky Diode Reply with quote

Dear Aeroelectric People,

I’m in the process of designing an electrical system for a sailplane and am looking for some advice on selecting a Schottky Diode.

My aim for this system is to avoid the pilot having to manage the battery power during flight. Ideally the batteries should be switched on before take-off and nothing more done until they are switched off after landing and removed for re-charging. It is accepted the batteries will need charging each night.

The batteries cannot be changed because of the provided battery trays. The avionics are primarily powered by 2 x 7Ah gell cells. The equipment specs say the total current drain is 1.7A (including a transponder), therefore it is quite possible to run short of amps after a long flight, a 6 hour flight is not unusual, and longer is definitely possible. I suspect the power numbers are a little on the high side and are also somewhat variable, depending on radio usage, screen brightness and other factors. Until the system is in use its difficult to be sure of the electrical load. There is also a 4Ah battery in the fin for cg management purposes that is usually kept in reserve to ensure a flight can be completed with power in hand – its is rarely possible to predict exactly how long a flight will take. Often it is essential to keep a GPS logger running at all times for competition scoring purposes, it is desirable the glide computer is operating at the end of a flight to help with planning the final glide.

This aircraft also has a get-you-home 2 stroke motor that extends from behind the cockpit to guard against an out-landing, known as a “turbo”. A 15Ah gell cell extends and retracts the engine using an electric spindle drive, it is dive started and has no throttle or generator. I want to make the engine battery available as a back-up to the avionic batteries.

The problem is to design a system that requires minimum management and provides maximum endurance. Connecting all batteries to the bus results in stronger batteries back charging the weaker and wasting power. Installing regular diodes to prevent the back charging drops too much voltage. Having to switch batteries in and out manually is bound to lead to increased workload at the most inconvenient times. I want to run on the main avionic batteries initially, keeping the engine battery to run the engine when needed and the fin battery in reserve. Theoretically Schottky Diodes, with only a 150mv forward voltage, on each battery line will prevent back charging with an acceptable voltage drop.

It is possible to use a pair of DPDT switches to split the avionic system and use 2 batteries to power the 2 halves of the system, but allowing one battery to power all if one depletes before the other. However, if both main batteries can be connected directly to a single bus system management will be more straight forward.

I know little about electronic components, and have only confused myself looking at the various tables. The maximum instantaneous current draw is around 4.7A on the avionics bus (3.2A for the radio plus 1.5A for other services), the engine can pull 8A instantaneously when it is raised. I have only been able to find one device that comes close to meeting what I think I need, https://www.mouser.co.uk/datasheet/2/205/L497-1110523.pdf, but this is quite expensive and is described as a rectifier, not completely sure it is suitable. This https://uk.rs-online.com/web/p/rectifier-diodes-schottky-diodes/1023567/ may be a candidate, but the forward voltage is higher than I would like, it seems that the temperature must be high (over 100*C) to reduce it to my 150mv target.

Does anyone know of a component that may be suitable? Sketch of architecture below.
Am I going to have to accept a small overhead in managing the battery power on a long flight?

Regards, Peter

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PostPosted: Thu Dec 27, 2018 4:35 pm    Post subject: Re: Schottky Diode Reply with quote

I do not think that "back charging" is going to be an issue. There would have to be a large voltage difference between batteries before one would charge the other. Just connect the two 7ah batteries in parallel and let them both discharge simultaneously. A load will draw current from whichever battery has the highest voltage.
Consider using discrete, through hole, diodes which cost 51 cents each.
https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Schottky-Diodes-Rectifiers/_/N-ax1mjZ1yzvvqx?P=1z0z63xZ1yuoc72Z1yuoc5bZ1yuoc76&Ns=Pricing%7c0
Or dual diodes are available in a TO-220 case for $1.50
https://www.mouser.com/ProductDetail/Diodes-Incorporated/SBR10U40CT?qs=sGAEpiMZZMtQ8nqTKtFS%2fNX0wTwLhSQZv372G1bq%2fbs%3d
Insulate the mounting tab from aircraft ground.
The stated forward voltage drop is for maximum current. The voltage drop will be less with less current.


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nuckolls.bob(at)aeroelect
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PostPosted: Thu Dec 27, 2018 4:40 pm    Post subject: Schottky Diode Reply with quote

I’m in the process of designing an electrical system for
a sailplane and am looking for some advice on selecting a Schottky Diode.

Hi Peter, long time no hear!

My aim for this system is to avoid the pilot having to manage the battery
power during flight. Ideally the batteries should be switched on before
take-off and nothing more done until they are switched off after landing
and removed for re-charging. It is accepted the batteries will need charging
each night.

<snip>


The problem is to design a system that requires minimum management and provides
maximum endurance. Connecting all batteries to the bus results in stronger
batteries back charging the weaker and wasting power.


This is a myth. Batteries cannot charge
batteries. Any number of serviceable
batteries can be connected in parallel
irrespective of capacity. All energy
stored in every battery is available
to the intended task . . . there is
no 'exchange' of energy between batteries.

Installing regular diodes to prevent the back charging drops too much voltage.
Having to switch batteries in and out manually is bound to lead to increased
workload at the most inconvenient times. I want to run on the main avionic
batteries initially, keeping the engine battery to run the engine when needed
and the fin battery in reserve.

A 'reserve' battery seems a confession
as to lack of knowledge of the
total energy needs compared to battery
capacity. The only reason you need
a 'reserve' is to carry out a near-
emergency move because the system went
dark unexpectedly. You should be able
to design the unexpected event . . .

Theoretically Schottky Diodes, with only a 150mv forward voltage, on each
battery line will prevent back charging with an acceptable voltage drop.

Diodes are probably unnecessary . . . pending
further discovery . . .

It is possible to use a pair of DPDT switches to split the avionic system
and use 2 batteries to power the 2 halves of the system, but allowing one
battery to power all if one depletes before the other. However, if both
main batteries can be connected directly to a single bus system management
will be more straight forward.

I'm not so sure than ALL the batteries
cannot be operated in parallel with no
diodes. Further, if capacities of SVLA
batteries prove marginal, have you
considered a lithium system? More capacity
in same volume and weight constraints.

I know little about electronic components, and have only confused myself
looking at the various tables. The maximum instantaneous current draw
is around 4.7A on the avionics bus (3.2A for the radio plus 1.5A for other
services), the engine can pull 8A instantaneously when it is raised.

<snip>

You need to do a load analysis that seeks to
define your ENERGY requirements. A 15 a.h.
battery JUST to raise and lower the engine
seems huge overkill . . . that's a LOT of energy
to supply a need that persists for mere seconds
per flight cycle.

<snip>

Does anyone know of a component that may be suitable? Sketch of architecture below.

Am I going to have to accept a small overhead in managing the battery power on a long flight?

I think your 'cart' is overrunning the horse
. . .

I would advise some further research and
perhaps experiments to ascertain
real energy requirements. I'm 99% certain
that a system can be devised that needs no
diodes and may well offer energy reserves
that far exceed your greatest expectations.

But we need some real NUMBERS first . . .

Can you share an equipment list? Are
these equipment items in-hand? If
so, what's the possibility of powering
these items up on the bench and getting
good current draw numbers? If not on
your constellation of intended equipment,
how about on examples installed on other
aircraft?

How long is an extend/retract cycle on
the engine?


Bob . . .


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ceengland7(at)gmail.com
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PostPosted: Thu Dec 27, 2018 6:15 pm    Post subject: Schottky Diode Reply with quote

On 12/27/2018 4:43 PM, Peter Pengilly wrote:

Quote:
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Dear Aeroelectric People,
 
I’m in the process of designing an electrical system for a sailplane and am looking for some advice on selecting a Schottky Diode.
 
My aim for this system is to avoid the pilot having to manage the battery power during flight. Ideally the batteries should be switched on before take-off and nothing more done until they are switched off after landing and removed for re-charging. It is accepted the batteries will need charging each night.
 
The batteries cannot be changed because of the provided battery trays. The avionics are primarily powered by 2 x 7Ah gell cells. The equipment specs say the total current drain is 1.7A (including a transponder), therefore it is quite possible to run short of amps after a long flight, a 6 hour flight is not unusual, and longer is definitely possible. I suspect the power numbers are a little on the high side and are also somewhat variable, depending on radio usage, screen brightness and other factors. Until the system is in use its difficult to be sure of the electrical load. There is also a 4Ah battery in the fin for cg management purposes that is usually kept in reserve to ensure a flight can be completed with power in hand – its is rarely possible to predict exactly how long a flight will take. Often it is essential to keep a GPS logger running at all times for competition scoring purposes, it is desirable the glide computer is operating at the end of a flight to help with planning the final glide.
 
This aircraft also has a get-you-home 2 stroke motor that extends from behind the cockpit to guard against an out-landing, known as a “turbo”. A 15Ah gell cell extends and retracts the engine using an electric spindle drive, it is dive started and has no throttle or generator. I want to make the engine battery available as a back-up to the avionic batteries.
 
The problem is to design a system that requires minimum management and provides maximum endurance. Connecting all batteries to the bus results in stronger batteries back charging the weaker and wasting power. Installing regular diodes to prevent the back charging drops too much voltage. Having to switch batteries in and out manually is bound to lead to increased workload at the most inconvenient times. I want to run on the main avionic batteries initially, keeping the engine battery to run the engine when needed and the fin battery in reserve. Theoretically Schottky Diodes, with only a 150mv forward voltage, on each battery line will prevent back charging with an acceptable voltage drop.
 
It is possible to use a pair of DPDT switches to split the avionic system and use 2 batteries to power the 2 halves of the system, but allowing one battery to power all if one depletes before the other. However, if both main batteries can be connected directly to a single bus system management will be more straight forward.
 
I know little about electronic components, and have only confused myself looking at the various tables. The maximum instantaneous current draw is around 4.7A on the avionics bus (3.2A for the radio plus 1.5A for other services), the engine can pull 8A instantaneously when it is raised. I have only been able to find one device that comes close to meeting what I think I need, https://www.mouser.co.uk/datasheet/2/205/L497-1110523.pdf, but this is quite expensive and is described as a rectifier, not completely sure it is suitable. This https://uk.rs-online.com/web/p/rectifier-diodes-schottky-diodes/1023567/ may be a candidate, but the forward voltage is higher than I would like, it seems that the temperature must be high (over 100*C) to reduce it to my 150mv target.
 
Does anyone know of a component that may be suitable? Sketch of architecture below.
Am I going to have to accept a small overhead in managing the battery power on a long flight?
 
Regards, Peter
 
 
Hi Peter,

(edit: looks like others have made most of these points while I was 'distracted' with other stuff, but here you go, anyway.)

I'm certainly no expert, but I'll take a swing at a few of the questions.
1. I'd prefer to have a single battery at 14AH, vs 2 (at) 7AH, if it's physically possible to do it. Simpler, and you intend to have a backup anyway.

2. Gell is not the same thing as the currently common SLA, 'starved electrolyte' battery. Gel cells may still be available, but they're a lot more finicky to deal with than a common SLA battery.

3. I have not yet gotten excited about flying a powered a/c with a lithium-iron battery, but have you considered it for your 'total loss' system? Charging the battery outside the a/c removes a significant percentage of risk, and you could carry a *lot* more capacity in a much lighter package (for a price, of course...).

4. Since knowing your exact loads are a bit more critical than a typical a/c, rather than look at mfgr data sheets, consider directly measuring each item's current in actual operation. Most data sheets will show you a conservative number, to be sure the installer doesn't 'short change' the device's needs.

5. The Mouser device you linked is a bit of overkill (200A), and will be a bit of a hassle to mount. The mounting surface is also the cathode, meaning you'd need to isolate it from the air frame and  for safety, from any heatsink you mount it on. The 2nd one you show has an isolated mounting tab. But there are hundreds of choices. Here's one:
https://www.ebay.com/itm/Schottky-Diodes-Rectifiers-2X60-Amp-15-Volt-DIODE-MODULE-ISOTOP/273285163944?epid=1718397917&hash=item3fa110aba8:g:cbkAAOSwgd1axcQL:rk:4:pf:0
Another:
https://www.ebay.com/itm/1-S20SC9M-SCHOTTKY-RECTIFIER-DUAL-COMMON-CATHODE-DIODE-90V-20A-MTO-3P/390680777077?hash=item5af6639175:g:az0AAOxyoahSXtlc:rk:9:pf:0

6. If you can go with one main battery (or parallel the pair of 7As), there'd only be one switch for the 4A backup. The switch for the 15A starting battery you will need, regardless of the rest of the architecture.

Charlie
Virus-free. www.avast.com [url=#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2] [/url]


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nuckolls.bob(at)aeroelect
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PostPosted: Thu Dec 27, 2018 7:03 pm    Post subject: Schottky Diode Reply with quote

Quote:


4. Since knowing your exact loads are a bit more critical than a typical a/c, rather than look at mfgr data sheets, consider directly measuring each item's current in actual operation. Most data sheets will show you a conservative number, to be sure the installer doesn't 'short change' the device's needs.

BINGO!



Bob . . .


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Peter(at)sportingaero.com
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PostPosted: Fri Dec 28, 2018 3:06 pm    Post subject: Schottky Diode Reply with quote

Hello Bob & all, thanks for your replies.

I am reluctant to go to LiPos as I have a ready supply of ‘gelcels’ (or whatever they are really!) I know it could be dumb to avoid spending a couple of hundred ££/$$ on a $100K aircraft, but if I can make the gelcels work I would like to. Getting the system working on the bench is probably a good idea, I have nearly all the equipment to hand so should get something running in a week or two. At present I am inclined to use a plain on-off switch on all the batteries, and to switch the 2 main batteries and the fin battery on together at the start of a flight, I should have a minimum of 8.5 hrs endurance with the engine battery available if required.

A single main battery is not possible due to the battery trays provided by the manufacturer – this is a certified aircraft, but the electrical system is not well defined (no circuit diagram provided as almost all installations are bespoke).

The engine battery is used only to extend the engine into the airflow, run a fuel pump used during starting and to run the control box. Extend/retract cycle times are typically 5 to 7 seconds. The control unit has a 7.5A c/b, so that must be approximately the max current. Yes, it is a large battery for the task, but it fits the battery tray and is a certified system (circuit diagram provided this time!) so I am loathe to mess with it. This is a top level description of the system https://www.schempp-hirth.com/en/individual/engine-options/turbo-sustainer-engine.html.

Once I have measured the load for each item I will report back.

Regards, Peter

The equipment list is (current draw assumes 12v):
Item
Current (mA)
Notes
LX Nav LX9000 glide computer
500
Max brightness
Flap positioner
30
Provides flap angle - always operating
Trig bridge
20
Interfaces to radio
Remote stick
20
Multi-function stick top
V8 electric vario
150
Sensitive rate of climb +much more
Vario audio
50
estimate
Power FLARM Core ***
165
Proximity warning
Oudie (PDA running gliding s/w)
150
estimate - on-board battery
Radio Trig TY91 ***
240
Unknown speaker load
Transponder Trig TT21 ***
420
Likely to be off for some of flight
TOTAL
1745

Entries marked *** have an unpredictable duty-cycle, load may vary

From: owner-aeroelectric-list-server(at)matronics.com <owner-aeroelectric-list-server(at)matronics.com> On Behalf Of Robert L. Nuckolls, III
Sent: 28 December 2018 03:03
To: aeroelectric-list(at)matronics.com
Subject: Re: Schottky Diode
Quote:

4. Since knowing your exact loads are a bit more critical than a typical a/c, rather than look at mfgr data sheets, consider directly measuring each item's current in actual operation. Most data sheets will show you a conservative number, to be sure the installer doesn't 'short change' the device's needs.


BINGO!


Bob . . .


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