Hi there,
This is my first post here, aiming at showing some fun stuff, and getting some help - yep, I have some issues.
The following image shows the ugly design that I am aiming at improving:
It looks ugly but it works well enough for me. The following picture shows the plane flying, and the 3 cylinder radial engine.
But I would like to add three functionalities (3 current sensors, 3 thermometers, and voltage regulators) because I wish to troubleshoot the engine. He (it) is moody. Sometimes, the glow plug filament is broken. Sometimes, it is not warm enough, or the opposite. Sometimes, the engine is too hot (mixture lean ?). Most of the time, one of the cylinders doesn't want to cooperate, and it is hard to know, even though I am now familiar with its roaring. Hence, I am willing to do my own clean PCB design. This is a first for me. I also get some motivation because that effort would also benefit the in-line twin engine that tracts a P51 mustang and the radial 9 a friend of mine just got. Another motivation to get a PCB done is to redo my ugly on-board glow driver : you might have notice the hot glue blob on the DC/DC converter in the above picture. The capacitors got unsoldered after about 50 flights. Now is a good time to invest some time in this project.
So, I am giving a shot to the following PCB design project :
A preliminary schematic draft is attached as a pdf file.
The idea is simple :
1. an attiny converts the receiver PWM in ON/OFF (0V or 5V) signals
2. that triggers a relay to power up the 3 glow plugs (~3A each)
3. an ATMEGA 328 is electrically independant and does the clever work :
- 3.a - measure each cylinder temperature
- 3.b - measure each glow plug current
- 3.c - report the 3 glow plug currents on a 3 color LED
- 3.d - report the 3 cylinder temperatures on a screen (on board / hot plug, don't know yet)
Here is the idea behind the current measurement.
When the cylinder is OFF, the glow plug current is about ~2.5A.
When the cylinder is ON, the glow plug current is about ~2.3A.
When the glow plug filament is broken, the current is exactly 0A.
--> so I can make sure this when my engine is moody.
Here is the idea behind the temperature measurement.
- I can double check if a particular cylinder is ON or OFF (warm or cold)
- I can make sure the fuel/air mixture is about right ( warm=lean , cold = rich)
Here is the idea behind the UART and ISP connection
- I use quite randomly expensive OS F plug (4 strokes) and very cheap hobbyking 2 strokes glow (it works fine for a third the price). So I need to adjust the firmware to account for this.
- I will need to toubleshoot before it works.
Here are the ideas behind using regulator :
- LiFe (safer than LiPo) battery are by far lighter for the same energy storage, but their voltage is about 3.3V, or some multiple. So I would need to get the voltage down to 1.2V - 1.5V, depending on the glow plug brand / batch.
- Three glow plug is up to 9 A @ 1.5V. The DC/DC converter is specified for 6A. I should be pulling about 7-9A through it (never measured it actually). It is warm but it works. There is a potentiometer to adjust the output voltage : that is necessary : some plugs likes 1.2V, some other 1.5V, and I like switching in between the cheapo and the expensive one.
So far so good, but I am stuck :
I have hard time to find a simple switched regulator that gives me 9A @ 1.2V-1.5V adujstable from a 3.3V or 6.6V battery. Would you recommend some ?
- The LM1771 sounds great because it outputs low voltage (down to 0.8V) from 2.8V-5.5V (1S LiFe is 3.3V +/- 0.3V). The TI datasheet section 8.2.3 indicates it can delivers up to 5A @ 3.3V from a 5V battery.
--> Can I get 9A @ 1.2-1.5V ?
--> Can I put a couple of regulators in parallel ?
--> Should I simply harve 3 regulators, one for each glow plug ?
--> Can I simply design the circuit with figure 6 BOM : sounds complicated to me (2 FET, 3 caps, 1 inductor, 1 resistor, 1 pot).
- The LM2576 datasheet suggests my life would be easier with this guy. Indeed, when I look at figure 33 of the TI datasheet, I readily find the circuit I need, and it is simple (2 caps, 1 diode, 1 inductor, 1 pot). It is less efficient but simpler : I like it. But it outputs max 3A. I would need 3 either in parallel, or one on each glow plug.
--> Can I connect this guy in parallel ?
- The LM338 is linear, even less efficient, but that is fine with me. Doubling the LiFe battery size is not a problem on that plane. According to the datasheet page 9 (1.2V–25V Adjustable Regulator figure), the circuit is fairly simple (2 caps, 1 potar). That's sweet for my PCB area, soldering skills, though I guess I can handle more. But the may point here is that I may be able to two of these regulators in parallel to output up to 10A.
-->But can I put such a regulator (linear) in parallel ?
Many thanks for your hints for theses issues. I hope there are quite interesting to you.
May be I should get these various guys on a breadboard, and see how it goes. Would that be the path moving forward ?
- kikinou
PS : english is not my native language, so I welcome notification on sentences that should be clarified.