Upgrading the FET's on the 18m2 High Power Board (CHI035)

[jensmith25]

Thanks Jeremy.

 

[Jeremy Harris]

I can understand why the grain-of-wheat bulbs are still used. It's undoubtedly a lot easier to get realistic lighting effects with them than it is with LEDs. I suspect there would be a lot of fine tuning needed to get small warm white LEDs looking like miniature light bulbs, particularly in terms of getting rid of directionality.

Personally I think that, with upgraded FETs on the high power board, the unit will be massively over-rated for the task, and should be very reliable. I'd certainly have no worries with using it, and a lot of the stuff I've designed in the past has ended up in aircraft, or on vehicles, where failure tends to have more serious consequences.

 

[jensmith25]

Thanks Jeremy. I presume you mean the auto-protected MOSFET suggested by fernando or similar? It looks to be suitable with 3v threshold voltage and 0.028ohm Rdson but at £3 each is expensive!

http://uk.farnell.com/stmicroelectronics/vnp35n07-e/mosfet-omnifet-70v-35a-to-220/dp/1739428

 

[jensmith25]

I found this one at Rapid which is much cheaper @ £1.27 each. They both seem to have a Input threshold voltage of 0.8-3v max. With the 5v voltage regulator on the high power board is that going to be too high? Rdson 0.05ohm.

https://www.rapidonline.com/pdf/47-0394.pdf

For the pin labelling is 'Input' the same as 'G' ?
Drain and source are the same on both the auto-protected and not.

 

[fernando_g]

Indeed the "input" is the gate.

They call it input because this device is a sophisticated IC with a large Mosfet inside.
Actually the input is an I/O pin because it can also signal back a protection condition.
Check the datasheet.

 

[jensmith25]

Indeed the "input" is the gate.

They call it input because this device is a sophisticated IC with a large Mosfet inside.
Actually the input is an I/O pin because it can also signal back a protection condition.
Check the datasheet.

I'm having trouble understanding the datasheet. It doesn't come easy to those of us with limited electronics knowledge which is why I ask for help on here.
They're not exactly easy reading!

 

[Jeremy Harris]

TBH, I'd not bother with anything too sophisticated. Those 50p STP FETs I linked to earlier are such an absolutely massive over-kill for the job that it's likely that all the wiring in the doll's house would go up in smoke long before a FET failed. The protected devices are very nice, but all you're doing is switching a few tiny light bulbs, not something super-safety critical. Stick a slow blow fuse in the 12V supply line that's around double the max working current and that should be more than enough protection for something as simple as this. A slow blow fuse will take surges of many times its rating for a short period, so will easily deal with the bulb inrush current peak, but will protect all the wiring etc perfectly well, as it takes a lot longer for wires to get hot than it does for even a slow blow fuse to blow.

 

[jensmith25]

Thanks Jeremy. The Stp36nf06l.

I might have a temporary solution for the fuse. Dolls houses commonly use socket strips and these are fitted with a fuse. My customer is using a socket strip for some of the bulbs already. If I power the PCB through the socket strip (tested myself just now and no adverse reactions caused with multiple switch on) then I assume it will be protected by the fuse. They are 2amp fuses.

http://www.craft-products.com/de074-12-socket-connector-strip-for-dolls-house-lighting

For a longer term solution, I looked but there's a lot of different types of fuses available. I'm assuming 2amp would be ok as dolls houses rarely go above this and an inline fuse holder would seem to be the ideal solution. Would a 20mm, glass fuse https://www.rapidonline.com/siba-70-001-35-3-15a-3-15a-20x5mm-slowblow-glass-fuse-26-1068 with something like this be suitable? https://www.rapidonline.com/sci-r3-48-white-20mm-in-line-fuseholder-5a-250v-26-1586

 

[Jeremy Harris]

Those 20mm fuses are ideal, compact, easy to buy in small current ratings, cheap, and plenty of protection for this job.

 

[jensmith25]

Thanks Jeremy.

 

[Jeremy Harris]

Hope we get to see a photo of the finished doll's house, with all the lights on!

I remember building one for my little sister when she was very young, but I used ordinary torch bulbs which didn't look that good. She's thirteen years younger than me, so I'd have been around 17 or 18 at the time and she's now over 50..............

 

[jensmith25]

I'll do my best if my customer sends me a photo!

I've ordered the other parts so should get those tomorrow.

 

[premelec]

I prefer positive temperature coefficient thermistors [PTC] for fault current limiting as they 'reset' on cooling rather than straight thermal fuses which must then be replaced - and replaced over and over when trying to find a fault... The PTC is slower acting but in this sort of situation with pretty low power they are usually ok. Anyhow glad your project is progressing...

 

[jensmith25]

Thanks. Hopefully there shouldn't be any faults as the unit is already tested, it's just a failsafe in this case.