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Author Topic: Does anyone have good examples of micro to solid and non relay? Need rugged.  (Read 782 times)
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jnz
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« on: December 02, 2014, 01:35:30 01:35 »

I have a project coming up that is a little outside of what I'm used to (application specific medium speed digital comm). I need to put about 12 30A relays on a board and drive them with a micro. I have no idea what the user will do to screw this up (wire the outputs right to ground I assume at the least), I have a 2-3V ARM micro while most of the out and inputs are 14V, and if any of the relays go down, there is a chance I could leave someone's stranded somewhere they don't want to be (not acceptable).

1. I'm almost inclined to use traditional relays that anyone can find at a gas station in case that part goes down (and they do). The opposite side of this is I'm considering solid state relays, although 30A-40A at 10-18V is going to be pushing it there I'm sure.

2. I'll need a driver for each relay, followed by a relay, and a fuse. This is easy, but on the driver from the micro, I'd like something as robust as possible, considering a redundant driver on each line.

3. Protection. I can not fry a micro. I can not fry the PCB traces. The user will try and screw me. I'd like to protect against shorts and over-current mainly. I don't really care about quality of the line as it's mostly DC motors, solenoids, lamps, and other "dumb" things.


Does anyone have comments or suggestions? A reference or anything that they can point me to? I'd really appreciate it.
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solutions
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« Reply #1 on: December 02, 2014, 02:11:20 02:11 »

Lamps don't leave anyone stranded.

Nor do "users screw you" to leave themselves stranded - they screw themselves, which is unlikely and, if they do, they deserve to die anyway.

It sounds like you are putting 12, 30A relays in parallel for a motor disconnect on on an electric motorbike. Yes or no?

You have to be very specific about what you are doing or you are wasting your time trolling for a solution to a different problem someone is solving than what you had in mind.
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cadence
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« Reply #2 on: December 02, 2014, 02:42:08 02:42 »

I've found solid state (AC) relays to be very robust when driven directly from a microcontroller line, taking less than 5mA at 5V - way less than the 25mA a PIC micro port (for example) can source or sink.

Even 12x 5mA = 60mA shouldn't stress most microcontrollers beyond the maximum total VSS/VDD current, though I'm not too familiar with ARM specs and I've never driven one from 2-3V.

These (DC) SSRs look like they could be a good starting point: http://www.aliexpress.com/store/product/40DD-SSR-input-3-32VDC-load-5-250VDC-DC-single-phase-DC-solid-state-relay/613696_876284515.html

There are 10A, 20A etc versions available too.
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solutions
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« Reply #3 on: December 02, 2014, 02:48:25 02:48 »

Sure there are.

But the on, "contact", resistance is not specified.

Their recommendation - use thermal grease and pray, LOL
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cadence
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« Reply #4 on: December 02, 2014, 03:46:35 15:46 »

Output 'on' voltage drop <= 1.5V
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jnz
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« Reply #5 on: December 02, 2014, 06:12:22 18:12 »

Nor do "users screw you" to leave themselves stranded - they screw themselves, which is unlikely and, if they do, they deserve to die anyway.

Yea... It doesn't sound like you've ever been the guy answering the tech support line Wink

It sounds like you are putting 12, 30A relays in parallel for a motor disconnect on on an electric motorbike. Yes or no?
No, the relays will not be in parallel at all, at least, they aren't supposed to be! I have a micro controlled box that will act like a smart high side driver array, I'm not certain where it will wind up. Fleet vehicles mainly I bet, maybe some consumer, could find it's way into bikes and other places, I'm not sure. Basically, I don't know what the user is going to do, so I'm looking for "fool-proof" options.
[/quote]

Quote from: cadence
That is a good starting point, thanks. But I'm thinking solid state might not work. After looking them up from this link and my own googling, I can't find something that isn't in an industrial package screw terminal or specialized socket. These are far too large, and expensive. I'm actually leaning towards 30A automotive relays because if one ever does burn out they can be replaced in a socket easily. The user may also appreciate the familiarity, and ability to determine when they are being triggered by sight/sound for diagnostics. It sort of doesn't seem like solid state relays are there yet.

The idea is that my relay array is to the PCB if possible (if I can make robust enough traces). I can't have wires flailing around this thing, the idea was to provide a nice waterproof harness that the user pinned out as needed. The initial plan was to support the 12 relays, drivers, micro, in a 9"x5" or so package.

I suppose 30A mechanical relays are fine, but I still need a smaller solid relay or transistor setup to drive those, and that'll need to be rugged as well. Like I said, I'm definitely outside of my wheelhouse here.

But any other comments or recommendations would be super helpful!


edit: Ok, I found one SSR that might work for size and constraints, but it's entirely a gamble, I'd really have to abuse them in testing to consider them reliable. http://www.songle.com/pdf/20085271539341001.pdf But, it does have a 5v "coil" and 30A in a PCB appropriate package.
« Last Edit: December 02, 2014, 06:39:30 18:39 by jnz » Logged
Ichan
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« Reply #6 on: December 02, 2014, 07:37:33 19:37 »

The relay on pdf link is a mechanical relay (?)

I may choose that kind of relay but with contact pins on the top, so the high current is not going to pcb - fuse can be added somewhere else such as on the wire.

As it will be used on automotive environment and you do not want to fry the mcu then pay attention on the voltage regulator for the mcu and the rest, expect at least 36V load dump voltage and reversed polarity abuse.

-ichan
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jnz
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« Reply #7 on: December 03, 2014, 01:31:35 01:31 »

The relay on pdf link is a mechanical relay (?)
I may choose that kind of relay but with contact pins on the top, so the high current is not going to pcb - fuse can be added somewhere else such as on the wire.
As it will be used on automotive environment and you do not want to fry the mcu then pay attention on the voltage regulator for the mcu and the rest, expect at least 36V load dump voltage and reversed polarity abuse.

Ha, yea, linked to a mechanical. My bad. Songle's site is impossible for me to navigate. At some point I got switched from SSR.

Reverse bias is fine, and I can protect the MCU itself. But driving the mechanical relay I'll need protected drivers, and if I can I'll want to make those redundant, onwards to the replaceable relays in sockets.

I thought about keeping the high power lines off the PCB, but that makes this almost impossible to package for production. I'll likely end up with the a relay socket on the PCB with thick ass lines/plane for the power/ground. I am slightly concerned about it still burning at the terminal however.

I have to look up a medium-side driver for the micro to relay I suppose
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solutions
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« Reply #8 on: December 03, 2014, 10:14:23 10:14 »

Output 'on' voltage drop <= 1.5V

I missed that one. Thanks

45W of loss in EACH relay...x12....540W of contact losses.

I'd consider using a p-channel MOSFET - may actually be a lot cheaper as well
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jnz
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« Reply #9 on: December 04, 2014, 12:50:41 00:50 »

45W of loss in EACH relay...x12....540W of contact losses.

Where seen? And also, I wonder how I could reliably measure that loss?
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robotai
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« Reply #10 on: December 04, 2014, 09:07:28 09:07 »

I think it's based on the simple calculation 1.5v x 30A = 45W for each solid relay. I do agree MOSFET is a good candidate because their ON resistant may lower to couple mini ohms.
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jnz
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« Reply #11 on: December 04, 2014, 06:22:30 18:22 »

right right right... Yea, it'll be extremely rare that these things hit 30A ever, let alone at 12 at once. I know I still need to design for that, but not really worried about it.

More worried about 12 ARM to relay drivers at this second... need to research more
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solutions
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« Reply #12 on: December 05, 2014, 01:57:01 13:57 »

"Drivers"?

12 n-channel "logic level" MOSFETs will turn the relays on and off
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cadence
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« Reply #13 on: December 05, 2014, 03:11:34 15:11 »

As a reference to solution's suggestion take a look at this very basic article:

http://bildr.org/2012/03/rfp30n06le-arduino/
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Checksum8
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« Reply #14 on: December 05, 2014, 04:17:16 16:17 »

I like to use a FOD3180 or similar in projects like yours. It can drive a FET or a relay, and being opto-isolated takes care of some noise and level shifting.

https://www.fairchildsemi.com/datasheets/FO/FOD3180.pdf
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Gallymimu
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« Reply #15 on: December 09, 2014, 11:45:50 23:45 »

Like other's have said.

if you are driving the relays use low side N channel mosfets, make sure to have a free wheeling diode across the coil of the relay to avoid inductive kick taking out your drive transistor

If you want solid state, and this is unidirectional, low side NFET or high side PFET (higher resistance and higher cost than N) either should work fine.

SSRs are overkill unless you need isolation or something.
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