Updating ematch ignition circuit for altimeter

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profmason

Member
Here is my current thought on 1S altimeter ignition using 3.3V TTL logic. VBat is a 1S Lipo which ranges from 4.2V-3.2V.

When the low impedance FIRE1 pin is pulled high, the NPN (100V 5A) transistor allows current to flow between VBat and GND. The 1K resistor on the base of Q1 provides ~3.3ma which given the gain of the Darlington will allow ~3.3A of current to flow through the ematch at ~(VBat - .3V) so should provide plenty of kick for the ematch (The ones I use work fine at 2V, 2.5A)

When there is continuity across Ematch1, the opto is powered through the 1K resistor. The opto drops about 1.3V, so we have ~2mA through the ematch all the time. When the opto turns on, current can flow from VBat through the 1K, opto and schottky diode to the CONT1 digital input which has high impedance(~1MOhm) The 1K is not really necessary, just there in case some bounce on startup sets the digital CONT pin to low resistance. The idea is that the NPN in the opto and the schottky give about .4V CE drop + .5V Schottky diode drop, which drops the voltage of the Battery by 0.9V. Fully charged the battery is 4.2V - .9 = 3.3V, discharged the battery is 3.2V - .9 = 2.3V both of which are within the range of 3.3V TTL high.

Finally, when the charge fires, the opto shuts off, but the capacitor C10 holds the CONT1 input high for about 5s +- based on the time constant.
Comments: I had thought about doing this with a P-Channel mosfet, but was concerned about firing on power up as the pins float down at power up for some 10s of milliseconds.

A couple of things I have noted:
1. The use of the opto seems silly here, since I have tied the same voltage rail to both ends! I had first tied the NPN of the opto to the 3.3V rail which made sense but then started to worry that the voltage drops across the diode and the NPN would pull the voltage below the TTL high threshold.
2. It seems like I could eliminate the opto and do this:

3. I thought about doing a voltage divider, but then the diode isn't there to block the cap discharge which holds the continuity input high.
4. I think the TIP122 equivalent is overkill and would like to go do to a smaller SOT223 NPN part which will do 2A.

Any thought on favorite ways to do this?

Voyager1

Well-Known Member
TRF Supporter
Personally, I would use an N-MOSFET switch here. However, I’m confused as to why you need to hold the continuity high after the ignition. You definitely don’t require the optocoupler. I would just use a suitable voltage divider for the continuity sense.

Perhaps I’m missing something with your intentions here.

jderimig

If you have intermittent continuity what is going to discharge your capacitor to detect that?

profmason

Member
1. N mosfet switch. I am using a voltage rail of a 1S battery. (3.2-4.2V) and the logic level is nominally 3.3V but due to the LDO, actually 3-3.3V. The mosfet switches on based on the Vgs threshold being reached and the source voltage in this arrangement changes between off (0V) and on (Vin). This means you can’t switch the gate to Vin, you need a different voltage rail that is higher than Vin by at least the mosfets Vgs threshold (At least a few volts!). I don't see how I can use a N channel without some shenanigans. I could use a p-channel, but it would be conducting until the micro booted and pulled it high. I have used n-channels for switching with a 2S supply. If I am not thinking about this correctly, please let me know!

2. "If you have intermittent continuity what is going to discharge your capacitor to detect that? " DOH. that makes good sense. I have filtered out intermittent continuity problems. Eliminate the cap and use software to only detect continuity until fire is initiated.

3. Suitable voltage divider. If I am eliminating the charge hold cap, then I don't need the diode and could use a voltage divider. It is cheaper and more compact.

I updated the NPN to a SOT223 part so that I can shrink the whole thing down. Thanks for the help. Any more input is appreciated.

Voyager1

Well-Known Member
TRF Supporter
If you have intermittent continuity what is going to discharge your capacitor to detect that?
Very good point.

Voyager1

Well-Known Member
TRF Supporter
1. N mosfet switch. I am using a voltage rail of a 1S battery. (3.2-4.2V) and the logic level is nominally 3.3V but due to the LDO, actually 3-3.3V. The mosfet switches on based on the Vgs threshold being reached and the source voltage in this arrangement changes between off (0V) and on (Vin). This means you can’t switch the gate to Vin, you need a different voltage rail that is higher than Vin by at least the mosfets Vgs threshold (At least a few volts!). I don't see how I can use a N channel without some shenanigans. I could use a p-channel, but it would be conducting until the micro booted and pulled it high. I have used n-channels for switching with a 2S supply. If I am not thinking about this correctly, please let me know!

2. "If you have intermittent continuity what is going to discharge your capacitor to detect that? " DOH. that makes good sense. I have filtered out intermittent continuity problems. Eliminate the cap and use software to only detect continuity until fire is initiated.

3. Suitable voltage divider. If I am eliminating the charge hold cap, then I don't need the diode and could use a voltage divider. It is cheaper and more compact. View attachment 452103
I updated the NPN to a SOT223 part so that I can shrink the whole thing down. Thanks for the help. Any more input is appreciated.
Use a MOSFET with a lower Vgs threshold, e.g., https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/FQP30N06L.pdf
or

Last edited:

Brian H.

Well-Known Member
N mosfet switch. I am using a voltage rail of a 1S battery. (3.2-4.2V) and the logic level is nominally 3.3V but due to the LDO, actually 3-3.3V. The mosfet switches on based on the Vgs threshold being reached and the source voltage in this arrangement changes between off (0V) and on (Vin). This means you can’t switch the gate to Vin, you need a different voltage rail that is higher than Vin by at least the mosfets Vgs threshold (At least a few volts!). I don't see how I can use a N channel without some shenanigans. I could use a p-channel, but it would be conducting until the micro booted and pulled it high. I have used n-channels for switching with a 2S supply. If I am not thinking about this correctly, please let me know!
Nexperia PSMN5R6-60ylx
Surface mount, 60V, 100A, logic level MosFET.
1.7v gate/source threshold

cerving

Owner, Eggtimer Rocketry
TRF Supporter
The SI2302 is the NFET complement to the SI2301... same price. Eggtimer Rocketry uses them for driving buzzers... a relatively low current application, well suited for a FET in a SOT23 package. Our very first remote switch prototype used one for switching the connected altimeter on/off, after blowing a few of them up in testing we switched to a much more robust FET (NTD4965, with a Vgs(th) of 2.5V max); anything in a SOT23 package is probably not suitable for driving a resistive load from a LiPo without some kind of current limiting in place.

profmason

Member
Thanks cris! Nothing beats your first hand experience saving me from making the same mistakes. I will look for something in a SOT223 package.

Here is the output characteristics of the STN4VF03L

I wondered about needing a transistor to drive the buzzer. I have used 40 Ohm buzzers directly from 5V outputs but they are all through hole. The smd ones seem to be 12 or 16ohm. I was wondering if keeping the 50% square wave duty cycle low would let you run a 16 ohm buzzer within the 10mA output pin current limit, but in retrospect using a small transistor to drive the buzzer makes sense.

I think I will just make up all 4 versions (DPak and SOT223 versions of Darlington, plus the PFet and NFet drive and test them all with ematches. In the PFet version, I will replace the SI2301 with a SOT223 ZXMP4A16G which has the characteristics below.)

I will draft up the PCB tomorrow and get them sent out. Any additional input is appreciated.

Thanks for all the help folks!

profmason

Member
PCBs in to the fabricator and through DFM analysis. Will have assembled boards back in 2-3 weeks and then will update.

Comparing the BOMs:
DPak Darlington $0.48 SOT223 Darlington$0.69
NFet $0.81 PFet$0.68

In quantity 5, the setup and shipping are high etc so the BOM makes only a small difference.

AllDigital

DPak Darlington $0.48 SOT223 Darlington$0.69
NFet $0.81 PFet$0.68