Does this Schematic look right? (Firing Electronic Matches)

[NickS]

1) Having the greens shorted out across a bank of 10 when no ematches are put in, how will this effect battery life? Seems it would be really bad on it?


Not great on battery life but I didn't think it was an issue since you are using a large SLA battery and I assumed you only connect power when you are getting ready to go. If power consciousness is on the plate too the design may require some tweaking



you're looking at about 30mA per device. That 1K resistors (R2, R4, R8) limit the current in various paths. (10mA if no ematch in circuit)


I measure about 23mA through Q3 when not firing and 10mA when no ematch is present. I was able to reduce that to 14mA and 7mA respectively by changing R8 to 5k.



2) it doesn't matter whether you place the resistor in series with the anode or cathode. It just has to be in series. You could swap it over if you wanted to.


Roger that.



3) the 1K resistors could dissipate up to 0.144 Watts. You should pick a larger SMD resistor that is rated for higher power. R2, R4, and R8 are affected. One easy option is to get twice as many 500 ohm resistors and place them in series since each will dissipate half as much heat.

I actually calculate it higher than that. My bad, I am sorry and a bit embarrassed for that rookie mistake. The series 500ohm solution is not the best way to go for space savings. If you have to double power handling in a surface mount chip resistor what you want to do is double the value and stack the second on top of the first 2k||2k = 1k @2x power rating. But there are other options. So if all the values remain the same you could get away with changing R4 to a 1206 package of which can handle up to 1/3W.



(I wonder if R1 and R8 couldn't be a higher value? 10K anyone?)

I don't think that will work. The limiting factor in trimming in on final values is the LED. Changing the resistors around to limit currents puts me up against the limits with the LED model I am using. So if you have a part number on that LED it would be most helpful. Without it I can give some suggestions but it may require tweaking. This is a good time for a breadboard and a multi pack of leaded resistors from radioshack.

Here is what I think will work.
(1) Change R2 to 2k 0603 (reduces ready current to ~5mA 50mW
(2) Change R8 to 5k 0603 (reduces current through Q3)
(3) Change R4 to 1.5k 0805 or stacked 3k 0603 (reduces current through R4 8mA 96mW)

But again I emphasize this is really a dial in process that requires knowing more info about the specific LED's used.



1) per channel (300ma), or per full blown unit of 10 (30ma)?

All of my calcs are per channel so multiply by 10 for the whole unit. What you are referencing was 30mA/ch 300mA total but that is old info with the mods above.



3) I will see what I can find for R4 and R8,
R2, will be a 1/4W right on the green Led Anode .

Not anymore see previous comments.

 

[NickS]

See revamped schematic with FP indication.

 

[funman1]

Well the LED's were off some really crappy car audio place, and thus no specs.
But I was thinking last night What if we scrap the LED's
and get some Red / Green ones that change from red to green based on polarity?
So forget the shared Cathode all together...

Let me know what you think of this.
I think I could get away with still using the transistors in a different config and use new LED's.

This would improve Battery life as well, since no direct drains like before
The battery I have in the small box is a 12 V SLA .80 Amp Hours
So yeah battery life is a major concern, since the box might be turned on 30-60 mins before a show, and still have to be able to fire the show.

Let me know what you thgink of these ideas...
~Steve~

 

[NickS]

Well the LED's were off some really crappy car audio place, and thus no specs.
But I was thinking last night What if we scrap the LED's
and get some Red / Green ones that change from red to green based on polarity?
So forget the shared Cathode all together...

So instead of sharing a cathode you want to share the cathode of one with the anode of the other and vice versa. What big advantage to you expect by doing this?


Let me know what you think of this.
I think I could get away with still using the transistors in a different config and use new LED's.

"Whether you think you can or you can't you are usually right"...with enough effort. Are you certain it will be an improvement?



This would improve Battery life as well, since no direct drains like before
The battery I have in the small box is a 12 V SLA .80 Amp Hours
So yeah battery life is a major concern, since the box might be turned on 30-60 mins before a show, and still have to be able to fire the show.


There are no direct drains now. With the mods in my last post The bypass transistor draws about the same current as the LED's consume. That is a pretty small SLA. I am guessing your shows are not very long (maybe 15min?). The circuit as is draws 15mA total/channel in standby with a ematch in place and the green LED on (bypass off) With the ematch out the consumption actually reduces to about 7mA/channel. For all 10 channels that works out to 19% and 9% reduction in battery capacity respectively for the whole system over the space of an hour.

The best way(I think) of reducing consumption is picking low power LED's. If the consumption goes down then all the resistor values can rise meaning we use less. For instance using 1mA LED's could easily reduce consumption to under 2.5mA/channel worst case which your battery could run for 32 hours.

 

[funman1]

Well if the unit sits for 1 hour before the show and we have a 1/2 hout show. That's 1,5 hours on.
While this would be minimal draw the unit also has an LCD with backlight, and an Xbee Pro that's constantly running so that thing draws big

Well Hmmm, Do you have any LED's you would recomend? I went with those because they were hard to find ones with 3 leads and shared Cathode. (back in the orig setup thats what I needed)

I thought elminating Q3 to gnd would improve battery life.
And thought it might be better to use the two lead LED because then when the unit mosfet fires it send gnd to the Red and when you put in the ematch then the LED would get pos for green. Then we'd just have to work out the other side of the LED??

Don't know; you guys are the experts, I know just enough to get myself in trouble

 

[NickS]

Have you verified that you can do a half hour show with that battery?

 

[funman1]

Bread boarded,
Currently with 3 leds on, LCD, Xbee, Arduino, Etc..
It will last about 2 to 2.5 hours and still be ok.
Since it only has 10 cues, it will only be "firing" for 10 x .5 seconds (5 seconds total firing) All the rest is Standby / Communicating

 

[NickS]

So you are NOT re-loading it then. Even under the higher 15mA consumption/ channel in standby the battery can hold for over 5 hours. Now after you fire a channel it goes to the lower 7mA draw for the rest of the time. So the math gets slightly more complicated but 5hrs is worst case. Sure you have other stuff running too so before we make any more assumptions perhaps you could measure the current draw of the rest of the circuit in standby mode(LCD, Xbee, Audrino, etc but no LED's). Then we can more accurately decide what concessions need to be made. I found a through hole RED LED that draws only 0.4mA at mouser. The lowest power through hole green LED I could find was 1.8mA so it just about evens out. With those parts I belive we could get the standby draw down to 3.5mA/channel and the already fired draw down to just over 1mA/channel. That would buy you 22hr standby time worst case for just this circuit.

 

[funman1]

No we never switch up durring a show for electric. Hand fire shows are sometimes reloaded but never electric. You just add more ques to an electric system...

OK I may just go with this then as is..
The LED would have to be in the same package..
I don't have room for 2 leds per channel, hench the double color / Tri color leds. (See the front panel Pic to see the room constraints)

Do I need any special resistors then for SMT?
R2 can be a 1/4 watt one right on the Green LED lead and thus not on the board at all.
So R1, R4, R8 would be SMT
Right now on the board I have them as sized (SMT - 0603)

 

[NickS]

Like I said before its hard to say with the LED's you have but lets assume for now they are pretty standard and you are going to use them.

R2 = 2kohm 0603 or leaded if you prefer
R1 = 1kohm 0603 Thick Film 0.1W
R8 = 5kohm 0603 Thick Film 0.1W
R4 = 1.5kohm 0805 Thick Film 0.25W or stacked 3kohm 0603 0.1W

 

[(*steve*)]

R4 = 1.5kohm 0805 Thick Film 0.25W or stacked 3kohm 0603 0.1W

I would generally prefer to use the same size resistor everywhere (wherever possible) as it looks better and is easier to design the board -- but that's marginal.

I would be cautious of assuming that stacking 2 x 0.1W resistors will give you something capable of dissipating 0.2W as you effectively reduce the surface area of the resistors by about 30%

In this case it's probably fine, since the dissipation is only marginally more than 0.1W, but as a general solution, I would give it a miss.

Your layout skill may well make more difference than simply using smaller components (0603 vs. 0805)

 

[NickS]

Aggreed, in practice I only stack parts to verify prototype operation. I never design stacked parts into a new layout but with the max power just under the single part rating I think it will be ok.

I can neither argue with or agree on the matter of using the same size parts throughout. I have cap and res kits in 0603 which is why I default to those sizes for most designs. But I have never considered doubling part count over different footprint based on aesthetic reasons. However, we probably have different definitions of what looks good. I have noticed our full time layout guys like to see all the discrete's lined up in a neat row. But I find that to be sloppy from a signal integrity standpoint(because you have traces running all over the place to accommodate). I think a great looking layout is when it is driven by performance. Utilizing the best parts for the job. Sub-circuit parts jammed together in tight clusters to reduce trace lengths, wrap arounds and layer hoping. And finally short, direct signal layers with good isolation.

I imagine my prefs are based on the fact that I got my start in RF layout and I had my butt kicked repeatedly in the design reviews(over things that seemed trivial at best). Now those techniques have spilled over into how I layout simple less sensitive boards.

 

[funman1]

New parts list look good?

 

[NickS]

Looks good. What is line item 3 for? Breadboarding?

 

[funman1]

Yep thats what it's for

Will place the order tonight...
Having some issues with the layout, thought I had it all worked out, then realized the bottom has to be reversed because I'm looking "through' the board and thus all my bottom SMT was wrong

So I have to rework all that, and its talking longer than I had expected

 

[funman1]

Well got the parts in today and yep...
WORKS EXACTLY how I wanted!!!

Thanks!!!
I did change the Green LED to a 1K because it was just too dim with 2K. I know there's no way you could have known this because there's no specs for the LEDs.

Now if I could just get that bottom side PCB all worked out!

 

[NickS]

Glad it works for you. Check you PCB editor for a layer flip option. In altium I just hit a button and the part flips to the bottom layer accounting for metal layer and pin orientation. It is obviously not critical for the resistors but you do want to get those SOT-23 packages down with the correct pin out.

 

[funman1]

There's no option to flip with this crappy little program You get what you pay for I guess (Free program = few features)

OK Here's the updated PCB
I'm going to tripple check ALL wiring and such before I send this out to Manf.
Let me know what you think...

Is it ok to use the Via's like I did for the SMT stuff?
I can't see any reason why I couldn't, but wanted to check....

Also I have two ways to tie to those, (Thermal Pad OR Solid Pad) The sofftware says thermal pads are better for soldering, but I assume that's when you are using a through hole part, no clue on the SMT stuff with the way I'm using them?

Components

Botom Layer

Top Layer

Ground Plane

Power Plane

Top & Bottom Layer Combined on view

 

[funman1]

I just need to know about those Via's and I will click order on the board.

 

[NickS]

I will look at it later I just don't have the time right now.