http://www.koolflightsystems.com/ultimatebec.htmhttp://www.koolflightsystems.com/ultimatebec.htm
this will handle up to 3amps continuous and on the better versions 6v
Note that most commercial BECs are designed for RC planes that generally do not have 100 Amp electric motors. Our motors and cables tend to create a bit more radiated and conducted interference than RC planes motors!
I was trying to uses a 30A RC plane speedo to power the compressor but it kept cutting out. Also tried it on 24V set to half speed for 12V compressors but the BEC shut down at 18V so I cut it off. Although it worked on its own it would not work with the drive connected and had terrible interferance. Could have been something to do with not having a voltreg on it and powering the 5V side from the Rx battery. The speedo was rated at 6-12 Cells but the only limiting factor is the BEC, beyond that it will work upto a max of 30V where it would try and clamp the supply.
I still want a 12V compressor to work off a 24V battery that can be controlled from the Rx & a pressure switch. So Im currently building something. Surly someone else outthere must have already done this. If so how did you do it?
I was planning a 24v compressor driven system before I discovered that the motors/gear ratio made the robot uncontrollable so Ive since cut down to 12V.
However, when I was using 24V the method I was going to use to solve the fried compressor problem was to use two in series, hopefully giving a faster recharge time and definitely giving 12V per compressor.
Of course, this wont be an option if youve not got the space or weight which, having seen the interior of Big Nipper, seems likely.
My brother was lightening it last night while I try and fixup the electrics. Therere may be space but we dont have the weight. He lost 1.5kg, only another 2kg left. Mind you when we started the base plate on its own was nearly 20kg! And it doesnt help when the valve is nearly as big as the ram.
Using the IBC controller I few things came up. First was we bolted the heatsink to the chasis. As it turns out the heat sink is connected to the battery +ve. Big flashes when a ground lead touched the chasis. Problem solved by using insulating kits for the MOSFET that didnt have them fitted. It also turned out that the weapon channels can only switch upto 12V as there is a voltage clamp on it and I wanted to connected it to the 24V solinod directly as it could supply enough current. Actually I connected it anyway as the instructions didnt give a max voltage limit and the valve kept clicking every time the battery was connected and I thought the IBC was not working. It turned out be clamping it to 12V so I had 12V across the coil when off and 24V when on. This was easily fixed by altering the clamp to the battery voltage.
Back on subject. A range check with the BEC built into the IBC controller gave no noticeable reduction in range from using a 4.8V Rx battery. Although the IBC doesnt have opto-isolators on the Rx wires. Range was alot further than big nipper. That is without the Compressor running.
Mark,
If you want to drive a 12V solenoid from 24V without it overheating, try just chopping the 24v supply to it at 50% duty cycle. The average voltage will then be 12V and the solenoid will not overheat. This assumes that the solenoid is already rated for continuous 12V operation (check that - some are rated only for a few seconds, or require a chopped/AC supply anyway).
The chopping frequency must be high enough so that the solenoid doesnt chatter on and off of course - a few kilohertz should be OK (less than 20kHz may give you problems with audible screaming of the solenoid - thats up to you!).
You can use a MOSFET or BJT to perform the chopping. If you need a circuit I can draw one, but you may be able to work that out yourself.
Congratulations on getting the range with a BEC - I consider you lucky with that!
Paul, The solenoid is 24V but the compressor has a 12V motor. Same thing just need 10A at 12V but thats exactly what Ive now done. Got a PIC12C508A to get the signal from the RX and convert it to a PWM output through a MOSFET to the compressor motor. It also has a auto zero upon powerup for the RC input and a softstart function to reduce EMI from the motor. The pressure switch is then connected through an LED to another input of the PIC to light when upto pressure.
The PWM freq is only 2.08kHz but with 32 steps (only first 16 used upto 50% duty cycle) gives a smooth startup. This gives me 7uS for the PWM rountine and 8uS inbetween to read the Rx input with 16uS resolution. Now that was a challenge.
It seems to work ok so far.
Mark, well done - that sounds good. Do you hear any audible noise from the 2kHz chopped drive?
At low speeds yes. The output waveform shows clearly its in the discontinous mode but above around 10% the compressor is so noisy you cant tell anyway and it starts to enter the continuos mode. After its recieved 50 pulses within +/- 32uS it takes this as the zero position and theres a little subroutine that then sweeps from 200Hz to 1kHz and back with a 3% duty cycle to create an audiable tone to let me know its received this signal and is ready to turn on. To my suprise this actually moves the motor slightly but as its a compressor it doest matter.
If you would like a copy of the source code/circuit I can Email it to you.
Could I have a copy? Im just interested to see how you measure servo pulses. I did a similar thing a while ago, on the PIC16F871 but I should be able to follow the PIC12 code. EMail address is on my profile.
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