Will a 70amp Isolating Diode handle 140 amp if I do the following...

[fuss]

I have a 70 amp isolating diode connected to my house battery and no other. I use the isolater solely so that no power is at the alternator when everything is off, for safety reasons. This way the always live cable runs are very short and the alternator can never be accidently disconnected from the battery.

This means that one of the diodes is not used.

If I bridge this to the other diode then the current can flow from the alternator through both theoretically.

Would this then mean that the isolating diode could handle 140a instead of the rated 70 amp?

Thanks
John

 

[lw395]

Technically, not really.
The diodes will not share perfectly so one will take more current than the other.
Also at 140A, heat dissipation is an issue.
I would not expect to run a splitter diode anywhere near the nominal rating of the actual diodes on a yacht.
Diodes of very big nominal rating are not expensive (look at www.rswww.com) but be aware that most need to be a) cooled, and b) de-rated for any rise in temperature.

As a vague guess, paralleling the second diode might give you 50% more rated current, but I would see it as a way of getting a bit more margin ( and slightly less voltage drop) if you were putting nearly the rated current of one diode through the pair.

Hope that helps?

 

[halcyon]

Would this then mean that the isolating diode could handle 140a instead of the rated 70 amp?

No.

Assumming it's a 70 amp blocking diode the heatsink is only good for 70 amp.

So assumming 1 volt diode drop, at 70 amp = 70 x 1 or 70 watt, heatsink 1watt per degree C rise, = 70C + ambient say 25C= total temp of heatsink 105C.

If you pass 140 amp through it, = 140 x 1 = 140 watt, thus 140 degree C heatsink + ambient 25C, giving a total 165 degree C way to hot for diodes.

Why this odd arrangement, with a relay system the engine/alternator is peminently connected to the engine battery. So you have to turn off the engine battery. to loose alternator feed.

Charging through the diode, you are loosing charge amps via heat ( 3-5 amp ), and depending on alternator type/set-up a lot of house battery capacity, upto 30/40%.

Brian

 

[fuss]

Thanks for the very helpful replies so far.

If you pass 140 amp through it, = 140 x 1 = 140 watt, thus 140 degree C heatsink + ambient 25C, giving a total 165 degree C way to hot for diodes.

But surely the 140amp is not going through one diode, it is a 2 pole device, so the amps should be shared. It has cooling fins and is made by Sterling.

Why this odd arrangement, with a relay system the engine/alternator is peminently connected to the engine battery. So you have to turn off the engine battery. to loose alternator feed.

Engine is 12v but my house bank is 24v, so 24v alternator output goes to house batteries 4 meters away. With an isolating diode power comes from the 24v alternator to the isolating diode then to the house bank but when all is off then the power from the house bank stops at the isolating diode and does not reach the alternator. This is important for safety. With all master switches off then, with this setup there is also no power at the alternator in the engine room.

Charging through the diode, you are loosing charge amps via heat ( 3-5 amp ), and depending on alternator type/set-up a lot of house battery capacity, upto 30/40%.

Yes I understand that, but it is also important to me that there is no power in the engine room when all master switches are off and also that the alternator to battery connection cannot be accidentially disconnected using a master switch (which someone will always do eventually)

 

[halcyon]

But surely the 140amp is not going through one diode, it is a 2 pole device, so the amps should be shared. It has cooling fins and is made by Sterling.

The diode pack are normally rated for total amps, so a 70 amp diode pack will carry a total of 70 amp, not 70 amp per output.


Look at the size of a 70 amp and 140 amp diode pack.

Brian

 

[Danny]

I have a 70 amp isolating diode connected to my house battery and no other. I use the isolater solely so that no power is at the alternator when everything is off, for safety reasons. This way the always live cable runs are very short and the alternator can never be accidently disconnected from the battery.

This means that one of the diodes is not used.

If I bridge this to the other diode then the current can flow from the alternator through both theoretically.

Would this then mean that the isolating diode could handle 140a instead of the rated 70 amp?

Thanks
John

John, why do you need to do this? Are you changing your existing alternator for a more powerful one?

If each diode is rated at 70A and the heatsink is rated to dissipate the heat from two forward-biased diodes each passing 70A then I think you should be able to use them in parallel and handle the best part of 140A (assuming they are identical types, of course).

BTW in the unlikely event of an isolation diode going open circuit while the engine is running then your alternator is immediately "accidently disconnected from the battery"!

 

[lw395]

If each diode is rated at 70A and the heatsink is rated to dissipate the heat from two forward-biased diodes each passing 70A then I think you should be able to use them in parallel and handle the best part of 140A (assuming they are identical types, of course).

"!

Unfortunately silicon diodes have a negative temperature coefficient of Vf, so if one gets slightly warmer than the other, it will tend to pass more current and get warmer still. Hence they do not share current well. De-rating the second diode by 50% might be a good starting point, giving you perhaps 100A. But great care is needed to keep the resistance (and inductance) equal between the two.
You would be better advised to fit a single diode adequate for the task.
Also if you look at the heatsink that typically forms part of a diode blocking module, then compare it with heatsinks of known degC/W, you will realise that a lot of airflow is needed to keep the temperature down if you are flowing 100+ Amps for any length of time.
I think the normal yacht splitters are designed to cope with a charging current that drops to a lower level before the diodes overheat, particularly if the unit is mounted out of any useful airflow.

 

[Danny]

Understand about temperature coefficient (used to use ntc thermistors in the distant past) hence the proviso about "the best part of...".

If the diodes are not actually a pair in the same physical package then it would probably be a good idea to mount them close together on the h/s.

I was interested to know why the OP wanted to do this. If he doesn't expect his alternator to deliver more current but just wants the reassurance of running the diode well below its rating then I think what he proposes might be a good idea. Presumably it would also give some protection to the alternator if one of the diodes failed o/c while charging? Maybe this failure mode is too unlikely to worry about?

I'm sure you are also right about the assumption that the current will decrease quickly while charging.

The OP says he's charging a 24V bank so even 70A is no mean charging current unless they are huge batteries.

 

[fuss]

Thanks again for all the replies.

I was interested to know why the OP wanted to do this. If he doesn't expect his alternator to deliver more current but just wants the reassurance of running the diode well below its rating then I think what he proposes might be a good idea. Presumably it would also give some protection to the alternator if one of the diodes failed o/c while charging? Maybe this failure mode is too unlikely to worry about?

I want to change the alternator from 70 amps to 100 amps. With the 70a alternator and the 70a isolating diode with the diodes connected , I always thought they were not working at their maximum rating, rather at 35a each. They got a little hot but not too hot.

I thought that as long as I bridge the 2 diodes then each should do half the work. I take IW395s point though that they wont share equally. But then 100 amps is not 140 so maybe this is ok.

I never experienced any isolating diodes failing, did anyone?

 

[William_H]

Alternators

Any alternator running at it's full rating will get pretty hot. Fortunately they usually if ever run at full output for a short period if charging batteries. A larger alternator gives a more robust alternator at ordinary current so hopefully more reliability. I tend to think as others do that the diodes between the alternator and the battery are superfluous. It should be OK to connect the batt +ve directly to the alternator output terminals. A large fuse at the battery could provide some protection from short circuit but really it should be possible to physically construct and install this one wire to have adequate protection from short circuit to negative. The diodes in the alternator will then stop any current flow further back. IMHO olewill

 

[Maine Sail]

I have a 70 amp isolating diode connected to my house battery and no other. I use the isolater solely so that no power is at the alternator when everything is off, for safety reasons. This way the always live cable runs are very short and the alternator can never be accidently disconnected from the battery.

I don't see why you consider this such a safety issue? Thousands upon thousands of vessels have live alt feeds with the batt switch off. If you are concerned about safety you could simply fuse the alt lead. Mine is fused on both ends as is my bank.

As for the concern about frying the diodes simply run the alt output directly to the house bank with fuses then no one can fry the diodes..

This means that one of the diodes is not used.

If I bridge this to the other diode then the current can flow from the alternator through both theoretically.

Would this then mean that the isolating diode could handle 140a instead of the rated 70 amp?

Thanks
John

Personally I don't like diodes, for a number of reasons, and in today's world they are really not as needed as they were years ago.

I'd vote for direct alt wire to the house bank and fuses for protection.