Zmanco Posted October 11, 2007 Share Posted October 11, 2007 http://forums.hybridz.org/showthread.php?t=126218 This thread got me thinking about the pros and cons of using resistors vs. the PWM capabilities of MS. In particular, has anyone measured how using a resistor with a low impedance injector changes (if it does) the opening time of the injector compared to using the PWM capabilities of megasquirt? As I understand it, MS doesn't limit current to the injector for some user-defined period and then switches to pulse width modulation (fancy way of saying it quickly switches the voltage on and off to limit the average current level). In theory the unlimited current should allow the injector to open faster, but I wonder if in practice the difference is significant, or even measurable? I ask because I've found that even with the PWM setting of 25% after 1.5 ms using stock turbo injectors on my NA engine, after long periods of WOT at the track I sometimes see resets. I finally dropped the PWM setting down to 20% and think it's ok, but I wonder how much margin is left. I suspect the power transistors in MS that drive the injectors are overheating and causing the resets. If adding the resistors doesn't have much, if any, affect on opening time, I might just add them as insurance. Anyone else experienced this? Quote Link to comment Share on other sites More sharing options...
chrism Posted October 11, 2007 Share Posted October 11, 2007 I don't know but I was thinking that without the resistors which are needed to limit current, everything else in the circuit should start to get a little warmer. If there are transistors in the path, they more than likely will heat up from high current flow. All that current has to find its way back to ground through something. In your case it's probably the transistors. One side of the transistors should be going to ground in order to pull down 12v on the injectors. Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted October 11, 2007 Administrators Share Posted October 11, 2007 As I understand it, MS doesn't limit current to the injector for some user-defined period and then switches to pulse width modulation (fancy way of saying it quickly switches the voltage on and off to limit the average current level). Well, not exactly. PWM and current limiting are two independent actions. Current limiting, with low Z injectors, keeps them from burning up. If you allowed them to draw full current, for more than a short period, they'd fry themselves. Whereas a high Z injector is 'self limiting', so to speak. The advantage of low Z, is a quicker response time... and once they're open, it takes little current to keep them open. A simplified way to restate PWM is injector 'on time'. The longer time period it is 'on', the more fuel it flows, per injection event. Because MS uses a VE table, PWM is hidden in the background. However, the 'pulse-width' is the actual and *end* variable that dictates fuel volume, per 'squirt'. Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted October 11, 2007 Administrators Share Posted October 11, 2007 I don't know but I was thinking that without the resistors which are needed to limit current, everything else in the circuit should start to get a little warmer. If there are transistors in the path, they more than likely will heat up from high current flow. All that current has to find its way back to ground through something. In your case it's probably the transistors. One side of the transistors should be going to ground in order to pull down 12v on the injectors. The current limit function is to protect and properly control low Z injectors... not the drivers. Quote Link to comment Share on other sites More sharing options...
speeder Posted October 11, 2007 Share Posted October 11, 2007 PWM is Megasquirt's way of implementing a "peak and hold" injector driver. All injectors require a big current to open initially (peak), then less current to stay open (hold). The Megasquirt's drivers allow maximum current on the opening ramp for a quick opening time, then reduces the AVERAGE current via PWM to hold the injector open. Use of the PWM rather than resistors will result in better control of injector pulse width in addition to protecting the injectors by limiting current. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 11, 2007 Author Share Posted October 11, 2007 Thanks for the clarifications. My real purpose in asking this question is to learn how adding a current limiting resistor to a low impedance injector will change its opening time. My guess is it will slow down, but by how much? Quote Link to comment Share on other sites More sharing options...
Brad-ManQ45 Posted October 11, 2007 Share Posted October 11, 2007 you could also use the p&h boards that let you run low impedance injectors w/o pwm and resistors... Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted October 11, 2007 Administrators Share Posted October 11, 2007 PWM is Megasquirt's way of implementing a "peak and hold" injector driver. Speeder is right... Damn it Zmanco, please except my apology for misleading you. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 12, 2007 Author Share Posted October 12, 2007 No worries Ron, I already am familiar with PWM and how it's used with Peak and Hold injectors. This thread keeps going down that path while what I"m really trying to learn is how the opening time for a peak/hold injector is affected if a current limiting resistor is used instead of the MS PWM. In other words, pretend I was using a version of MS that didn't support PWM so had to use a resistor (let's say 6 ohm) with low impedance stock turbo injectors. I'm guessing the opening time would be longer. How much though? If it's only 0.5 - 1.0 ms, then it's no big deal, but if it's a lot longer, then that could be a real problem, especially at higher revs. Quote Link to comment Share on other sites More sharing options...
mobythevan Posted October 12, 2007 Share Posted October 12, 2007 what I"m really trying to learn is how the opening time for a peak/hold injector is affected if a current limiting resistor is used instead of the MS PWM Sounds like an easy experminet that z-ya could run if you ask him nice enough Quote Link to comment Share on other sites More sharing options...
speeder Posted October 12, 2007 Share Posted October 12, 2007 Sounds like an easy experminet that z-ya could run if you ask him nice enough That's what I was going to suggest. Whaddaboutit, Pete? Quote Link to comment Share on other sites More sharing options...
GrommitZ Posted October 12, 2007 Share Posted October 12, 2007 Sounds like an easy experminet that z-ya could run if you ask him nice enough I was going to say the same. Quote Link to comment Share on other sites More sharing options...
Tony D Posted October 12, 2007 Share Posted October 12, 2007 This seems to be an issue that comes up many times when using stock injectors without the ballas packs. I still don't see why with stock injectors people have such a resistance to using the stock resistor package in the injector circuit. What is this fascination with PWM and it's fiddling capabilities. KISS was more than a rock band... The low-Z injectors work fine with resistor packages, and idle quality is good. Usually the big honking injectors people buy for high flow apps are high-impedance anyway, and idle quality suffers because they don't really have that crisp a response at shorter pulsewidths...which kind of says something about application selection more than what kind of current limiting you need to employ. Of the startups I've seen, far more people have fried things trying to set up a system without the resistors on low-z injectors. Since Datsun made a nice Resistor Pack for the L... I mean, is weight savings that critical? Of the problems I've had, injector driver issues has not been one of them...and I ran that stock Datsun from the get-go and haven't looked back. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 12, 2007 Author Share Posted October 12, 2007 Tony D, I'm totally open to adding the resistors in, and in fact am leaning towards it. I think I have a heat-related issue with my drivers (I may not have used enough thermal grease). Adding the resistors would be the easiest and probably most reliable way to fix this. Like I said above, I don't yet have an answer to how using resistors on low impedance injectors affects opening time. I suspect it's not a lot of impact, but would prefer to work off fact than supposition. If Z-ya can measure it, that would be great! BTW, I think people are hesitant to use the Datsun resistor pack, at least I am, because I can't easily buy one. But, looks like RS has one that is non-inductive. http://www.radioshack.com/product/index.jsp?productId=2062288&cp=2032058.2032230.2032267&allCount=100&fbn=Type%2FPower+Resistors&f=PAD%2FProduct+Type%2FPower+Resistors&fbc=1&parentPage=family Quote Link to comment Share on other sites More sharing options...
z-ya Posted October 14, 2007 Share Posted October 14, 2007 Rick is right on with the PWM description. The PWM is simple way to limit the current when in the hold state. Look at one of the current plots from my measurements, you can see the PWM current ripple. I have made some measurements with current limiting resistors and the PWM off, and the opening time is less that if the PWM settings were optimized without resistors. I've run injectors in PWM mode on the stimulator for hours at 85% duty cycle, and the output drivers barely get warm (my Wife asks what the hell is that clicking sound is coming from my office!). I think that your injectors are just getting hot on the track. They are fine on the street, right? As the injectors get hot the opening characteristics can change, especially if you are on the hairy edge with your PWM settings. What injectors are you using? Tony, I totally believe in the KISS principal, and I'm not talking about the rock band either Quote Link to comment Share on other sites More sharing options...
Tony D Posted October 15, 2007 Share Posted October 15, 2007 I have made some measurements with current limiting resistors and the PWM off, and the opening time is less that if the PWM settings were optimized without resistors. Wow, that is a suprising bit of information! Sometimes KISS Rocks! Quote Link to comment Share on other sites More sharing options...
z-ya Posted October 15, 2007 Share Posted October 15, 2007 Wow, that is a suprising bit of information! Sometimes KISS Rocks! The majority of the time the opening times are slower though. I think I had one injector that had a slightly faster opening time with a 4 Ohm series resistor. In general though, adding the series resistors makes a low Z injector into a high Z one, slowing the opening time. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 15, 2007 Author Share Posted October 15, 2007 I think that your injectors are just getting hot on the track. They are fine on the street, right? As the injectors get hot the opening characteristics can change, especially if you are on the hairy edge with your PWM settings. What injectors are you using?The problem showed up only on the track. I've had this issue with both the stock turbo and some RC Engineering. In the case of the stock turbo, dropping the PWM setting to 20% after 1.5ms seemed to solve it. With the RC Engineering, I tried dropping below 20% and eventually couldn't get enough fuel and went lean. Also, the RCs are only 225 cc/min and my logs showed I was getting to >85% (which is why I put the stock turbos back in). In both cases there were times at WOT where the power would completely fall off and cause MS resets. I had the laptop next to me and saw that the injector PW was still at a normal reading for WOT, so figured the problem was with the drivers overheating and shutting down, but the engine never came back to power until I took my foot off the gas for a few seconds. I suppose if the impedance of the injectors drops further as they get hot that might explain this? Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 15, 2007 Author Share Posted October 15, 2007 In general though, adding the series resistors makes a low Z injector into a high Z one, slowing the opening time.Have you tested the opening time be for a stock turbo injector with a resistor (say something around 6 ohms)? I'm tempted to try adding the resistors, although with no track days planned until the spring it will be difficult to verify the problem is solved. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted October 16, 2007 Author Share Posted October 16, 2007 I ordered some resistors from Mouser tonight - hopefully they'll be here by the weekend so I can put them in and experiment. I've got a board under the passenger seat with the rest of the MS on it, so shouldn't be too hard to add them. Quote Link to comment Share on other sites More sharing options...
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