20 bps Blowback? [And lack of real world testing]

[nerobro]

Well, 314519 bought an m3 frame to put on his ~rental~ pirhana. We won the guns at the CCP first timers tourny ;-) (the illinois locals should know this event)

the idea was to make an ultumate sleeper gun. running screw in n2, stock barrel, a beaten up pirhana body... and well you get the idea. One of the first things we tried was to run the gun f/a at 20bps. It failed. In fact, on the pirhana it was originally installed on there were all kinds of problems with recock and the like.... Sooo the frame passed into my hands.

I have a first gen Dragun. It's a full body, fatty striker, vertical feed, spyder clone. After installing the frame, and doing a little playing around, I soon discovered that the carbon zinc batterys I had been using on my loaders (because they are cheap and I can replace them... as often as needed) just couldn't give the juice the frame needed to fire more than 12 bps.

After talking to the M3 rep at the chicago open I went off and bought GOOD 9volt batterys. There was a change in preformance, the gun was able to reliably drop teh sear at 13-14 bps. Anything beyond that was hit or miss, and often depended on how long the gun was left to sit to recharge the capacitor.

The gun contains a 6800uF cap. it's pretty hefty, and has pleanty of juice to drop the solinoid sear, once. And given a good battery, it can even hold up at pretty serious rof's. Now My mind went back to the classes I took with AGD, and that alkeline 9volts can only supply something liek 350ma. Well we hooked the frame up to a bench power supply at 9v, and watched the ammeter. The frame wants a little more than 500ma at 20bps. When you hook up a voltmeeter to the alkeline battery at 20bps the battery voltage drops to sub 6 volts.

Either this was planned by M3, ignored by M3, or the frame was only tested on a bench power supply. :-) So here we are, left with a frame that's neutered by it's own power source. It seems kingman has seen the light and they are shipping 9.6volt nimh batterys with their frames. 314159 and I are going to be trying a 9.6v pack of our own to help fix the issue...... going to nicad will hopefully help keep the voltage up, and they should be able to supply the 500ma the frame wants. *crosses fingers*

Now, what we don't have is a number on just how fast the gun cycles. it cycles better than 20bps (the 5 or so shots you can get off at 20 confirms that) And what can be done to power the frame... internally. the battery pack we're going to be going with will be external. (maybe it can't be done... *looks at the emag* If only that much thought went into the m3 frame)

314159 will post the real data later. Including pics from the o-scope...... I've heard of 9.6v nicads the size of a 9 volt.. but will they dump the .5 amp that the frame wants?

[Azemoth]

Considering I know jack, nor squat of electronics. well...no more than the average Joe, I wish you luck, and hope that it works out for ya man! I can't wait to see a final product!

[314159]

=/ still have half a roll of film left after thoes pics...

/me needs a digital camera

when the frame was powered by a fresh, new, 9v courtesy of nerobro, i hooked up my multimeter to measure the current the gun was using off of the 9v battery.... 400mA.

in one of my classes, i was told that if i expect to get near 9 volts out of the battery, i should only plan on using 100mA of current from it.

my next thought was to measure the voltage on the battery when it was going 20bps, the voltage was fluxuating around 1.5v. in hindsight i should of used an o scope to get a better reading because of the fluxuating voltages, but it was late, and the caffiene was running low.

taking 400mA out of a 9v is the equilivant of punching it in the stomach, and getting a couple good kicks in while it is on the ground. according to the maximum power therom, you get the maximum power out of a power source when the internal resistance of the battery = the resistance of the load. without getting into the analysys of a changing load with resistive, load, and capacitance properties. it can also be said that you get maximum power out of a source, when the voltage drop off of the internal resistance of the battery = the voltage drop off of the load.

bottom line it would appear that the m3 frame is not getting the bang for it's buck off of battery usage.

[314159]

oh, the above data was recorded WITH THE BACKLIGHT LCD SCREEN REMOVED... so it consumes more power in normal operation than previously stated than in the #'s above O_o.

i need to toy with the m3 frame on the bench supply and see what i can get away with slipping it voltage and current wise to get away with it cycling 20bps.

if i can get away with powering it off of 2 9v in paralell, i will adapt a board that i am making for another popular paintball gun to power the m3 frame. this will let me shove everything in the frame and make it neat, even if it does mean the loss of the display =[.

the o scope pics just show the curve of the voltage drop across the solenoid. which when the frame is firing, is a series circuit with the battery, cap, solenoid, and an electronic switch (probally a FET). to get a better idea what is going on, i need to take some more measurements.

anyone ever hear of a company called honjet? they are the makers of the microcontroller that powers the frame, they don't have a website that i can see, and the suppliers that i usually go through don't carry their parts.

[sniper1rfa]

see if AGD will get you an e-mag battery case, then use a 9.6 volt nicad or NiMH for R/c cars. that should be able to do it, my car pulls about .4 amps when im running the car full speed (no load, wheels in the air.)

_or_ for a sleeper effect tape it to the side of he gun with something OTHER than duct tape or zip ties (those are too professional...)

[314159]

a 8 cell 9.6v 1000mah rc car battery would not fit in the emag battery holder. =[

i have a handful of 1.2v nimh batteries with solder tabs that can kick out about 400mA at their suggeted discharge level that i could also use. i have a place in mind that would not add relatively any footprint to the marker and balance well if i have to resort to a battery pack. right now i want to see if i can get by with 2 9 volts and keep everything in the grip frame with a custom circut board i'll whip up.

[hitech]

Originally posted by nerobro
I've heard of 9.6v nicads the size of a 9 volt.. but will they dump the .5 amp that the frame wants?

Don't use nicads. Try the NiMh. They make them in as "9 volt" replacement. Be carefull, there are two different ones, 8.4 and 9.6 volts. The emag uses NiMh batteries (18 volts worth). Tom said he used the NiMh because of the current draw; the emag pulls 3 amps. :eek:

[314159]

other than storage capacity / mass, i can't think of any reasons to use nimh over nicad

nimh are about 2 bucks a cell from digikey, nicads are cheaper =]

[nerobro]

Originally posted by hitech


Don't use nicads. Try the NiMh. They make them in as "9 volt" replacement. Be carefull, there are two different ones, 8.4 and 9.6 volts. The emag uses NiMh batteries (18 volts worth). Tom said he used the NiMh because of the current draw; the emag pulls 3 amps. :eek:

Actually.. Nimh have a higher internal resistance and will give a lower voltage. They suck at high discharge rates. So nicad it is.. at least with a 9 volt pack. In the case of the emag, when you pull 3 amps you may only see 12 volts under load. with a nicad you won't get teh voltage drop.

[sniper1rfa]

i thought NiMH's could take the hit better..?
though i [i]do[/] know NiCd can handle huge current. could the emag battery case hold an 8 cell battery of AAA cells, instead of AA, maybe (probably) even quad-A? and if you used a quad-A pack, it would still last longer than two 9 volts.

[Butterfingers]

NiMHs handle current load alot better than alkies. Alkalines output less than 1C NiMH batts 4-5 C NiCD 10+ C.

Given the capacity of the NiMH IMHO it is the best choice.

[nerobro]

Those fractions of capacity are a bit misleading. An alkeline 9 volt will hold somethign on the order of 500mah.... Where a simmilar size nimh.. will hold exactly what the emag holds (isn't that 230mah?) And then the nicad will also hold 110mah.

so... giving up less than 1/c will give us the 350ma that we saw during testing.

giving up 4/c will give us nearly 1000ma.... Hmm... sounds good. but we'd still see voltage drop.

with the nicad we're talking nearly the same 1 amp.... but we'll see less voltage drop. Given the nature of nicads. I suppose testing is in order..... (this is also backed up by the people who know small batterys the best.. the park flyer crowd. Same size pack, nimh versus nicad, and nicad will give you less voltage drop, even given the extra cell they normally install in the nimh to make up for the slightly lower average voltage)

[AGD]

Guys,

It is missleading to measure the amperage draw under fire. THe problem is you are measuring the AVERAGE amperage. If you show 500 ma and the solenoid has 50% duty cycle it is actually 1 amp draw.

I sugest you measure the resistance of the solenoid and then to the math with 9 volts to figure the amps. You will probably be surprised at the results.

Funny how all roads lead to a bigger battery pack.....


AGD

[Butterfingers]

Yes the true measure of a batts output is mWh. The E-mag batt is 650 mah. But that only tells you about the capacity. If the same batt was made out of alkaline the batt would have more than twice as much MAH but it would not provide enough current to trip the e-mag sear reliably.

[nerobro]

Originally posted by AGD
It is missleading to measure the amperage draw under fire. THe problem is you are measuring the AVERAGE amperage. If you show 500 ma and the solenoid has 50% duty cycle it is actually 1 amp draw.


Well, there is a cap in there.... What we did was put the gun on a bench amp meeter, and watched what the guage said. Then we put it on a current limiting power supply to confirm that number. We watched the voltage on the current limited power supply to make sure we weren't out-drawing the power supply. at 500ma, the voltage didn't vary more than .1 volt while fireing 20bps. So.. it would appear that's the minimum that the m3 frame needs to fire 20 bps solidly.



Funny how all roads lead to a bigger battery pack.....


*gives a funny look at the battery pack on the e-mag* After 314159 and I looked at providing power for the electrococker frame that's on his desk, we figured out that the whole idea of using a 9 volt is rediculous. Of course it mystefied us how so many places were using 9 volts.

[mykroft]

I'd go with at least an 1800mAH 9.6V RC pack. It ain't gonna be cheap, figure on $50+ for the pack, and $100+ for a good charger(Most RC packs are 8.4 volts, 9.6V packs are usually cheap Tyco AA packs).

That will give you lots of juice to power the solenoid, and you can just strap it to your tank, oh, and it'll run your Rev too, and a warp if you've got one.

[nerobro]

Methinks the sub-c cell packs are overkill. Serious overkill. even the 1200mah sub-c nicads can dump over 100 amps. Even the 500-700mah cells in the tyco 9.6v packs will be able to supply the 500ma that the frame seems to want to draw.

when we were discussing battery packs for the electrococker we were looking at 120-270mah batterys, because they could easily supply the 500ma that our frame would draw during f/a fire.

During our next test the plan is to use a "tyco" style 9.6v pack, strapped to the n2 tank. The real hope is to get enough of a power pack INSIDE the frame. :-)

I wonder what the frames behavior would be like without the cap in it... maybe then we'd see it's true amp draw like AGD was pointing out. Course.... even at 20bps the solinoid only has a 12% duty cycle, so charging a cap isn't a bad idea. This compares to the emag.. which pulls for 20ms instead of the 6ms that the frame pulls... Er, I just looked at my notes and it says 40ms...

40ms * 20 = 800ms.... eeep. 80% duty cycle at 20bps..

[314159]

on the dragun frame, the pull duration on the solenoid as measured on my storage o scope is 6ms. at 20 balls/sec, the solenoid is operating at a 12% duty cycle.

the resistance of the coil is 1 ohm. this makes for easy math and almost seems like a beginers electronics because amps = volts/ohms. in this case ohms = 1, so volts = amps, so at 9volts, the solenoid is using 9 amps.

here is where it starts to differ from the simple text book problem. lets introduce the internal resistance of the battery in the picture. on one of the slightly used batteries, the internal resistance was measured at 9 ohms. the open circuit voltage of the battery was measured at 8.02v. this gives you enough information to draw up an equilivant circuit for the battery. so if you hooked just the solenoid up to this battry, the current would be about .8A (8.02v / 10ohms).

on the fresh battery, the internal resistance is 6 ohms. 8.35v so if you hooked the solenoid to it, the current would be about 1.2A (8.35v / 7ohms)

if we were to add the capacitor into the picture. the capacitor is rated at 6800uF. on electrolytic capacitors this is a minimum rating, the actual value of the capacitor is guaranteed over this number and the tollerance is usually +100% for this type of capacitor. so the capacatince should be between 6800uF and 13600 uF. as far as getting an actual number i do not have any equiepment to measure the value of the capacitor directly, i can measure it indirictly by charging it to a known voltage, and measureing the time it takes to disipate it's charge through a known resistance. but i would need to unsolder the cap from the board to do this. this is where i am going to stop because there are many other factors that influence, and no matter how accurate of an aproximation you make, it is still an aproximation.

if we make a guess saying that the current/voltage of the 9volt supply is inadiquate. you can rephrase the key question to be, how much voltage/amprage do i need to cycle the gun at 20bps. this is a lot easier to answer. i can just hook the dragun to my benchtop power supply, turn the current limiting knob to not limit current, and adjust the voltage to 9 volts. then i gas up the gun, and dial down the current till the marker does not cycle at 20 balls/second and add a little current to this number for a safety margin to spec out a battery. if the marker will not cycle at 20 balls/second at unlimited current at 9 volts, then the question is flawed and has to be rethinked.

[314159]

finally got some air last night, and threw the gun on the bench supply, and got it to cycle gassed up at 20bps.

thoes 9 volts can only kick out so much power ;)

[CTG]

6 pack of AAs, biggest 9volt bang for the buck.