How much Amperage is compromised by a 4mV drop.

BiduleOhm

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Well, 100 mOhms for 20 m of 6 mm² wire sounds about right.

Why do you think you should see 6 mV?
 

Hedges

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Can these be safely projected in a linear upscale fashion to calculate losses @ 32V & 30A?

Power losses aren't linear with current, but go as the square of current.
Power is Volts x Amps
Voltage drop across a (linear) resistance (could be wire or relay contacts) is Amps x Ohms
Power dissipated in that wire is therefore Amps x Amps x Ohms

Power lost as a percentage of total power would be inversely proportional to system voltage if current stayed the same. But, higher system voltage means lower system current for the same power. So we like a 48V system at 1/4 the current compared to a 12V system.

If things don't heat up much resistance stays constant. If your wire heats up from 25C to 125C, resistance will increase 40% (0.4%/degree C for copper). So hot spots can get into runaway. Above a certain temperature oxidation will become a problem, and then you really see charred connections.
 

Bhupinder

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Well, 100 mOhms for 20 m of 6 mm² wire sounds about right.

Why do you think you should see 6 mV?

Because here RCinFL states that it should be ~6mV.
20 meters of 6 sq mm of copper (between #9 and #10 gauge solid) with 1 amp should have 0.06 ohms with 0.06v drop
20 meters of 6 mm X 6 mm square of copper (36 mm sq, bit larger then #2 gauge solid) with 1 amp should have 0.01 ohms with 0.01v drop.

Should measure wire with four point method, Force known current at ends of wire and measure inside ends directly on wire with voltmeter contacting wires at specific distance separation. Do not measure voltage drop at current connection terminals as measurement will be impacted by current contact point resistance. This is exactly the same way a current shunt works.

Relay contacts have resistance (that will likely degrade over use life). Not specified as voltage drop unless stated at a particular current.

1596512077534.png
Also chart from a random website shows that voltage drop/ Internal resistsnce of 1 Meter 6mm sq wire should be around 3mV.
3mV X 20 ~ should be around 60mV.
Its still double in actual readings, but yes I did got lost in Volt/mVolt/Ohm/Meter conversions :LOL:.

Disclaimer (Okey, here it comes):
I'm a simple IT guy with telecom background and electrical calculations are just cross domain talks - but somehow once an extra- terrestrial subject is now getting interesting :ROFLMAO:. All thanks to highly talented & supportive enthusiasts on this forum(y). Thanks to everyone around!
 
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BiduleOhm

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You also need to take your meter accuracy into account, and it may even be a bit off.

And 20 m is for a single wire? because if you have 20 m of dual wires you shorted at one end and then measure between the two at the other end then that's 40 m, not 20.
 

Bhupinder

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You also need to take your meter accuracy into account, and it may even be a bit off.

And 20 m is for a single wire? because if you have 20 m of dual wires you shorted at one end and then measure between the two at the other end then that's 40 m, not 20.
Ahhh - it's a fluke so kind of accurate mostly.
Measurement was for 20 M Singe wire - checking it for the complete loop doubles the reading as expected.
I could say - may be the quality of copper/ tin coating/ number of strands etc. could factor into the overall IR reading.
 

BiduleOhm

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I did the math just to be sure and I found 57.6 mOhm for 20 m of 6 mm² copper wire.

So that's 57.6 mV for 1 A.

How did you measure the current?
 

BiduleOhm

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Yep but the tin is extremely thin, it's basically negligible here, and if anything it would lower the resistance a bit.

And 6 mm² is 6 mm², stranded or not.

Either the meter has a problem, or the wire isn't copper or 6 mm², or it's an user error but something isn't right.
 

Bhupinder

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Would revisit test conditions soon, with other cables lying around.
Lets see if its cable specific issue/ DMM thing or just human error cheeped-in somewhere.
Well have been busy with the DIY SSR. Am thinking on the following lines, could you suggest what kind of FETs would be the best to use in this case:
1596850352677.png
Am thinking of putting a Capacitor bank before charge hits those mosfets to normalize any spikes etc and then switch them with Mosfets.
Have 2 options to choose the right FET:
1) 75V, 120Amp NMOS
2) 200V, 30Amp NMOS
Which ones should be a stable install?
 
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Fred S

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I guess you want to build the SSR as a project for fun? Nothing wrong with that! Just checking, because they are quite cheap, especially when rated for 60v or less on the load side.
 

Bhupinder

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I guess you want to build the SSR as a project for fun? Nothing wrong with that! Just checking, because they are quite cheap, especially when rated for 60v or less on the load side.
:LOL: Yes absolutely.
I do like DIY solutions, they add worth to the project and have tons of space to accommodate individual preferences/needs.
And the best part is, with so many DIY heads around it feels safe to try out things!
I always get valuable suggestions from SME(s) here.
 

BiduleOhm

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Why are you limited to the 75 V 120 A or the 200V 30A one?

If it's because you already have some of them then I need the exact model number to check the datasheet ;)

There's a lot more to look at than the title specs of the mosfet (like for example the 200 A one will never handle 200 A under practical conditions...).

Also the drive circuitry on your schematic is basically a single BJT with two of it's pins not connected, not sure where you go with that but you'll definitely need a bit more than that 😁

What will control this? because if it doesn't share the same ground than the SCC input negative (and that's probably not the case...) you'll need some isolation (like a real SSR) so a bit more complexity and cost for the drive circuitry are to be planned.
 
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Bhupinder

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Mosfets are based on Input voltage of SCC ~ 45 V for an open circuit and selection is from whats is readily available here in the market.
If you have something specific in mind, please share but availability becomes an issue generally.

1) 75V, 130A is IRF1407 -> Rds On 7.8mOhms Rja-> 62 °C/W With that I think it should be able to deliver 30-40% of its Amp capacity which should be about 40Amps (Just my guess). Doubling the Mosfets as pair for redundancy and Load distribution.

2) 200V, 30A is IRFP250N -> Rds On 75mOhms Rja-> 40 °C/W This one again with higher IR but lower Rja should be able to deliver around 30%. So would need to put 4 of these to make upto 40A output.

Now for the driver instead of a BJT I am now considering a small logic level Mosfet which should be easily driven with 5V input and no additional resistance (Ideal for a micro controller based triggers). The one I checked is 2N7000. Its Id max (Drain current) is 200mA - which might not be sufficient for the big Mosfets to overcome gate charge quickly, but since there's almost negligible switching required it might do the job in its own time - remains to be seen. This also solves the Issues of common ground or negative - which should be common with BMS/Battery's negative terminal in my opinion.
 

BiduleOhm

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Mosfets are based on Input voltage of SCC ~ 45 V for an open circuit and selection is from whats is readily available here in the market.
If you have something specific in mind, please share but availability becomes an issue generally.

1) 75V, 130A is IRF1407 -> Rds On 7.8mOhms Rja-> 62 °C/W With that I think it should be able to deliver 30-40% of its Amp capacity which should be about 40Amps (Just my guess). Doubling the Mosfets as pair for redundancy and Load distribution.

2) 200V, 30A is IRFP250N -> Rds On 75mOhms Rja-> 40 °C/W This one again with higher IR but lower Rja should be able to deliver around 30%. So would need to put 4 of these to make upto 40A output.

The IRFP250 has a very high Rdson, it would dissipate 67.5 W at 30 A, so yeah, not a good fit.

The IRF1407 is much more reasonable at 7 W but it'll need a decent heatsink. Even with 2 of them they're dissipating 1.8 W each so their Tj will be at 162 °C (for 50 °C ambient) without a heatsink which is far too high, given the low power even a small heatsink would do but you need one.

You would need 3 of them (4 or more is highly recommended) to be able to go without heatsinking (but don't put your fingers on them, they'll still run hot...).

Now for the driver instead of a BJT I am now considering a small logic level Mosfet which should be easily driven with 5V input and no additional resistance (Ideal for a micro controller based triggers). The one I checked is 2N7000. Its Id max (Drain current) is 200mA - which might not be sufficient for the big Mosfets to overcome gate charge quickly, but since there's almost negligible switching required it might do the job in its own time - remains to be seen. This also solves the Issues of common ground or negative - which should be common with BMS/Battery's negative terminal in my opinion.

That doesn't solve the isolation problem. What I'd recommend, as switching time is not really a big deal here, is a very simple and easy solution: a small isolated DC/DC converter with a 5 V input and 9 to 12 V output. For example: https://www.mouser.fr/ProductDetail/RECOM-Power/ROE-0512S?qs=wlO1EFRhkBBs39O4z6CwKg== or https://www.mouser.fr/ProductDetail/Cincon/EC1SA02N?qs=a9HUcNGsM2BlT21ZHg8YSQ== or https://www.mouser.fr/ProductDetail/MEAN-WELL/SPU01L-12?qs=5aG0NVq1C4xapU/W8eQMIw==

In addition to that you just need to add a resistor (a few k should be good) between the gate and source (which is the same as saying across the DC/DC converter output) to be sure the mosfets are turned off when they should. And as the converter have a roughly 40 mA no load input current that's too much for a MCU I/O so the 2N7000 is welcome here.

Also, even if not strictly necessary, I would recommend a small (4.7 µF - 10 µF) capacitor on the input of the DC/DC converter. And I would also recommend adding a resistor in series with each mosfet gate.

That's a 4 components solution for the drive circuitry, I don't think we can do better than that :)

Here's a schematic (I used some IRF540 as I didn't had the IRF1407 in my libraries but you get the idea):

schematic.png
 

Bhupinder

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Brilliantly put together.
That Isolated driver voltage is not clear to me, why do we need to have it isolated - couldn't it be part of the same circuitry?
Using common ground, negative reference errors also get cleared out.
Those $3 converters get ridiculously expensive getting here (USD $30 added for international shipping), any alternative ways?;)
Would non-isolated dc-dc converters work?
Or
Can we not just feed +ive 12V of the main circuit, through the drain channel of driver Mosfet?
 
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BiduleOhm

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I highly doubt your MCU will share the SCC negative input as a ground so, yes, it needs the isolation.

It was just an example, you can find similar converters on other websites (digikey, farnell, ...) were shipping will be less expensive or even free when your order is above a certain price (IIRC it's 30$ on farnell, so while you're here you can order components for other projects at the same time). You can also find them on amazon, ebay, ... but then you can't be sure of the quality.

Would non-isolated dc-dc converters work?
Or
Can we not just feed +ive 12V of the main circuit, through the drain channel of driver Mosfet?

Please provide a complete schematic so I can see if there's a ground problem or not ;)
 

Hedges

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Those $3 converters get ridiculously expensive getting here (USD $30 added for international shipping), any alternative ways?;)
Would non-isolated dc-dc converters work?

9V alkaline battery as isolated supply. Relay provides isolated control to enable MOSFETs.
SPDT so either battery charges gate of MOSFET or relay grounds gate.
Zero current draw except to charge gate capacitance when switching - might go 5 years before battery replacement.

I've similarly used a stack of 6, 9V batteries to provide 54V bias of a filament circuit. Not isolated, and batteries only had to supply a milliamp of filament emission current. That was a temporary lab setup until next revision incorporated the bias supply.
 

Bhupinder

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9V alkaline battery as isolated supply. Relay provides isolated control to enable MOSFETs.
SPDT so either battery charges gate of MOSFET or relay grounds gate.
Okey, that could be one of the alternatives but I am still trying to decipher - why do we need an Isolated supply for gate driver?
Can we not use something like this, its a non-isolated boost converter - small on power requirements:
1596998490232.png
 

Hedges

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- why do we need an Isolated supply for gate driver?

Maybe you don't.
A high-side switch usually requires that because it is at an elevated voltage.
Your schematic is switching low-side, the negative side of PV. It shows MOSFET going to SCC, and another terminal of SCC going to battery. Are those two negatives of SCC actually separate nets, which might be at different voltage? Or is it just a wire straight through?

If straight through so your MOSFETs are referenced to battery negative, then you should be able to drive them with a non-isolated signal referenced to battery negative.

Review data sheet, determine what gate voltage is necessary to turn MOSFET on hard enough for the current flow it will see, minimizing power dissipation.
To make it safer, consider a thermostat, thermistor, PTC fuse, or other way to turn gate off if MOSFETs get too hot. It should snap off and remain off with hysteresis.
 
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