Hey all, Thought I'd ask here as my electronics knowledge is REALLY limited. I'm looking to a high current pot or a way of varying a high current motor speed. Input voltage = 9.6v Amps: up to 35A (if possible I'd like to accommodate for up to 50A draw) From what I understand and can figure out at my voltage and amps I will simply burn out a normal pot. The question is, how can I make my motor speed variable (probably want to vary the voltage from ~7.2v ish > 9.6v) in this instance? Is there such a thing as a high current pot or is there some other way of driving this motor at a variable speed? Any help that anyone can give is much appreciated.
Cheers. Luckily I wont be needing the H-bridge as I only need to run one way. Time to lookup building PWM stuff then!
So the only really useful article I've been able to locate so far is actually an old article I remembered from here on bit: http://www.bit-tech.net/modding/2001/12/03/pwm_fan_controller/1 Thing is I don't know if this could take the levels of sustained draw that I would need or how I can improve it to make it take what I need. Any thoughts?
From the little I know about this, I think a PWM setup controlling high current mosfets would do the trick. Don't forget that when an electric motor is under heavy load, the current usage will be many times higher... So what is this project? And is the motor from a remote control car?
Ok, I just looked at Mouser for high current/powered pots, didn't find any. I then looked for high current rheostat and found them: http://www.mouser.com/Passive-Components/Potentiometers-Rheostats/Rheostats/_/N-5g9j?P=1z0vki6 Seems as they exist, but price is too much.
The idea is that I want to create a variable rate of fire module for an Airsoft gun. To vary the ROF the motor has to run slower which means less voltage essentially. The motor is indeed RC car type. From what I've found/read peak draw caused by the motor on startup is up to about 35A (though my batteies top at 25A anyway), and the sustained draw once it gets going is around 16A, hence wanting to be able to cope with particularly high power to account for spikes during burst fire. I'll be running it from a 9.6v, maybe even 10.8v to allow a suitable range motor speed to be acieved. There is one other gun out there that has this control in it (A&K M60) but I've not been able to find a schematic for the controller they use, other wise I would simply copy it I think a MOSFET type control is probably going to do what I need but not 100% sure how I do it!
Pot isn't the right way of changing any motor speeds (the resistance of the motor varies -> the voltage will vary as well) and 25/35 A is wayyyy out of the pot-league. (9,6 V - 7,2 V * 35 A = 84 W of wasted power and produced heat on the pot. Not what you want on a portable device ) If the motor is an RC-motor, why not use a factory-made RC-speed controller? I'm not 100% sure how you can make one work without the receiver/radio, but I think it would be easier than trying to build a new controller from scratch.
Well, while I should have been out in the sunshine, I was instead designing you a custom motor controller from scratch. Because I don't like doing things the easy way. <IMG removed> Points of interest: The TO-220 STP30NF10 will need heatsinking. D2 doesn't need to be anything particularly fancy, but if you're having to buy the parts you'll probably have a couple of 1N4001s left over so you might as well multipurpose it and use it for D1 and D2 although it's overkill. V+ refers to the battery's positive wire while +5V refers to pin 3 (VO) of the 7805. The 7805 won't need a heatsink. Sorry about the trace routing around Pin3/7 of the 555... it's pretty lame... stupid pinout. Don't get confused and join the overlapping traces on the actual circuit. 0.25W resistors will be fine. And last, but not least, I'm pretty sure this should work. Almost positive in fact . In thery anyway, you'll be able to handle ~30A at 10V with a range of 63%~98% duty cycle.
Muchos thankies. I'll have a butchers at making one up on a bread board or similar to start with, and let you know how it goes.
<img removed> Behold... version 2! (C4 changed to a lower capacitance to increase the frequency) Yeah, I was a bit bored... and still didn't want to go outside so I breadboarded the bad boy. Yeah the meter's a bit hard to read and you can't actually see the dimming of the LED in these pictures, but meh... I'm lazy. The values aren't exact because I don't have the exact parts the circuit requires... but it's close enough. And before anyone says anything, I know I haven't calibrated that scope in ages. And ignore the stuff on the right hand side of the breadboard, that's just a little buck converter I'm working on. Edit: OK, one more. This one includes the LM7805 voltage regulator so the whole thing is run off a 9v battery. 6v-9v adjustable power supply. So the principle is certainly sound. Edit (again): Right, so I said to myself I wouldn't make another post in this thread until later on but I can't seem to leave it alone. What can I say, I just really ****ing love motor controllers electronics. So the way you work out the minimum duty cycle is (R2+R3)/R1 so in my design it's 4.8/8.2=0.59 (59%). That's 5.7v from a 9.6v supply. I know you said you wanted 7.2v, but I was thinking when you have a PC case fan, a good range of adjustment is 5v~12v so 7.2v~9.5v seems a little small. And it's better to have too much range but not much precision in adjustments compared to too little range and no way to adjust beyond it. However, if you're going to be breadboarding this yourself, I may as well explain how to change the range. To increase the minimum voltage (i.e make your control more precise but over a smaller range) you'd swap R1 for a lower value. 6K8 (6800 Ohms) gives you a range of 6.78v~9.5v for example. Closer to the 7.2v you wanted originally. Oh, and always buy more components that you need. You never know what may go wrong. Plus if you get a selection of resistors, you can have a little mess around.
Indeed you do. I've seen how overpriced airsoft stuff is. If I get some more info on performance, spacial constraints, connector types etc I would very much consider starting to sell a kit. I can see how using surface mount parts (linear regulator, capacitors, resistors, timer) this could be quite small, but the heatsink needed for a 30A power MOSFET adds some unwanted bulk. I'd like to see how hot it gets and how much room there is inside an airsoft gun.
Depends on the gun really, I'll be using this in a 249 replica to tonnes of space, for the most part the space is either in the fore grip or in the stock, though the actual space allowed in each varies greatly. The 249 has oddles of room which makes it ideal for mods like this. Some people have started making much more complex electronic units (http://extreme-fire.com/) but they cost a fair whack and I think are a little over complex for what I need. Additionally I cannot change on the fly if I use one of their units, hence wanting to DIY if possible. That and the satisfaction of actually piecing something together that works is always nice I would be inclined to say that if you did make them I'm sure you would have customers, though this is a little be more specialist than the more generalised items listed above.
One (or two) final things about the schematic (jeez I hate only thinking about things after publishing): The V+ in the top right hand corner will probably not actually be wired straight to the battery, it'll be the positive wire that in the stock setup would go into the motor (so it's switched by the trigger). So what you'd end up doing for installation is soldering a diode across the motor's terminals and break the black wire coming from the motor and solder the MOSFET across the break. Putting more thought into heatsinking the MOSFET, if you use a TO-220 package (which has a built in metal tab) you might not actually need a sink. I'll be honest, this area or design is not my strong point. I usually just wing it... wire up the circuit and slap some metal on if it's a bit toasty in operation. So that aspect of the design is up to you. I've not actually got any electronics training, I've just taught myself, so some of my methods are a bit odd. I'm going to university in a couple of months to study electronic engineering though, do you think I'm suited for the course? I was bang on the money with imagining people would pay through the nose for this sort of thing. I can imagine a design that replicates the functionality of those which when sold for $85 (well, £55) would earn a tidy profit. A very tidy profit indeed. Design tip: if you're going to have your potentiometer's shaft external to the gun, use one with a nylon shaft (cut it short so it doesn't protrude very much), look into some sort of greased-O-ring-between-the-back-of-the-shaft-and-the-metal-frame arrangement to prevent water getting inside the gun. I don't know if you call it a day when there's a bit of drizzle...? Anyone at a paintball field would say you would . Oh, and maybe keep a spare pot or two to hand and attach it with crimped connectors or something so you can easily swap it out if it gets damaged. Nothing would ruin your day like being unable to shoot until you find a soldering iron. I probably won't though. I don't play airsoft myself, so I wouldn't really be able to test the performance of stuff. I do occasionally perform the odd mod to paintball markers' boards though. That's more my area.
At these kind of currents, I wouldn't drive the MOSFET directly of that 555. Instead I would use a MOSFET driver which will be able to handle the capacitance of the gate. You should choose a MOSFET with a low rds(on), and by using a MOSFET driver, you should be able to get closer to the manufacturer's specified rds(on). You'll also need something a bit bigger than that 1N4001 (Kabooom springs to mind...)
OK, question time. The 1N4001 was really more of a courtesy - the STP30NF10 has a built in source-drain diode. ANyway, how do you properly calculate the transient emf induced by the inductive load? As far as I was aware it was just "much less than the forward voltage"? What's the purpose of using the MOSFET driver? I've never even seen one in use... Is it just to source more current than the 555 can provide to charge the MOSFET's internal capacitance quicker? So that would lower RDS(on) in order to... reduce the amount of heat it's producing, right? What's a good value for RDS(on)? I've always just picked something with a maximum less than 0.04Ohms ish.
After a little bit of reading, here's the latest update. This seems to have become a work in progress rather than knocking out a design one afternoon.
Awesome work as usual CapnPedro I'd like to know how this rate of fire system works Is this your own invention Biosniper? Or are you copying an existing design?
