# Stupid question 300amp Diode



## rmay635703 (Oct 23, 2008)

I need a 24v + diode that can handle 300amps, given the crap I have gotten in the mail I am uncertain what source to get such a bugger from.

Any recommendations on a brand/source that will actually work? I know of various 60ampish bridges but haven't located an inexpensive solution in a higher amperage range

If your wondering what its for its to block a lithium and lead pack from each other, I've got the voltages matched up nicely so the lithium can't bottom unless I overdischarge the lead.

Cheers
Ryan


----------



## kennybobby (Aug 10, 2012)

How about a gated diode like an SCR ? A schematic is worth a thousand pictures...


----------



## PStechPaul (May 1, 2012)

Here is a 300A 60V diode pair for about $40:
http://www.mouser.com/ProductDetail...=sGAEpiMZZMutXGli8Ay4kMI5P2t23muv6TUQFjZR6EQ=

You might do even better with MOSFETs. Here is a 300A 60V device for about $2.66 each in quantity 10:
http://www.mouser.com/ProductDetail...=sGAEpiMZZMshyDBzk1/Wi6rYPvSui93I15hBu/C0rfY=

It will probably take several in parallel to carry the full 300A. The body diode will have a drop of about 1 volt so 100 amps will be about 100 watts will take a good heatsink. If you switch it on, it has 1.9 mOhms so 100 amps will drop about 200 mV and only dissipate about 20 watts.


----------



## rmay635703 (Oct 23, 2008)

PStechPaul said:


> Here is a 300A 60V diode pair for about $40:
> http://www.mouser.com/ProductDetail...=sGAEpiMZZMutXGli8Ay4kMI5P2t23muv6TUQFjZR6EQ=
> 
> It will probably take several in parallel to carry the full 300A. The body diode will have a drop of about 1 volt so 100 amps will be about 100 watts will take a good heatsink. If you switch it on, it has 1.9 mOhms so 100 amps will drop about 200 mV and only dissipate about 20 watts.


Hmm, I didn't realize all diodes had 1v drop, I figured a low volt unit would be less, also at what duty cycle is that diode "300amps", many things I have got will only allow a peak about half of their rating and steady at even less.

Hmm, how would I peg an SCR or mosfet full on without any significant circuitry?

Add a duty cycle and I could delimit the output from one battery or another, interesting, could make a poor mans charger that way.

Anyway, If its real easy that would make for a nice electronic F/R, I would use a contactor to go forward and SCR to go reverse


----------



## Siwastaja (Aug 1, 2012)

Low voltage diodes indeed have less drop than high voltage diodes, but that 0.3V drop cited for schottky is still for practically zero current.

Diodes do NOT have a constant voltage drop as often stated, it depends on current. It's just logarithmic instead of linear as with resistors. So at 300 amperes, you practically cannot get the the drop below 1 volt. What you can do is to oversize the diode or parallel multiple units to reduce the voltage drop but this way has diminishing returns. 

In your application, quick switching is not needed and hence a large MOSFET could be used as an active diode rather easily. You just need an accurate voltage sense over the MOSFET so that you can disable the gate when the voltage reverses. See "active rectification" or "synchronous rectification".


----------



## Ziggythewiz (May 16, 2010)

rmay635703 said:


> If your wondering what its for its to block a lithium and lead pack from each other, I've got the voltages matched up nicely so the lithium can't bottom unless I overdischarge the lead.


Why isolate them? I've been running lead with lithium for over a year with no issues.


----------



## rmay635703 (Oct 23, 2008)

Ziggythewiz said:


> Why isolate them? I've been running lead with lithium for over a year with no issues.


Different charge profile and resting voltages in my specific circumstance.

The way it will work for me is the lithium will provide a bit of a speed boost for a while then sag into the lead relm.

the lead batteries "bottom" is higher than the lithium bottom thus protecting them.


----------



## PStechPaul (May 1, 2012)

SCRs, bipolar transistors, and IGBTs will generally have voltage drop of 1 volt or more at useful current levels for your application. MOSFETs have a characteristic resistance which can be low enough for 100-300 mV drop at the current you need. Any of these devices can be controlled with 5-20 VDC and only a few mA (except for bipolar transistors and large SCRs). For MOSFETs and IGBTs the gate current is low enough that you could use a 9V battery to switch on, and an optoisolator or even a small relay so you can use the vehicle's accessory battery or other control voltage source.

You may also need to consider what to do when charging the batteries, and how to handle regeneration if you have that with your setup.


----------



## Ziggythewiz (May 16, 2010)

rmay635703 said:


> Different charge profile and resting voltages in my specific circumstance.
> 
> The way it will work for me is the lithium will provide a bit of a speed boost for a while then sag into the lead relm.
> 
> the lead batteries "bottom" is higher than the lithium bottom thus protecting them.


A programmable voltmeter can isolate them during the part of the charge curve where the profiles are different.

How are you planning on using your diode(s)? Post schematic plz.


----------

