Help me with a voltage problem...(semi OT)

[ntshane]

I was going to post this in the Off topic forum, but this seemed more appropriate.

The problem: I am using three 9v batteries in series to run 27v to a solenoid. This works, but since 9v batteries have such a low mah rating (something like 150mah), they die VERY fast.

I wanted to switch to using AA rechargeables, since they have a much higher mah rating (something like 1300mah), and they would last much longer. The only problem is, to do this, I need to wire 22.5 of them in series. This will not be practical for my application. I remebered that stun-guns and other devices take a relitively small voltage and turn it into many thousands of volts. The solenoid will operate anywhere between 24 and 27 volts AC or DC.

Is there some sort of component/setup (an inverter?) that I can run in this setup to turn a smaller number of AA cells (5 or fewer, preferably) into a higher voltage?

Please let me know if you have any ideas, and where I can find the parts. Or perhaps you have another solution?

Thanks all
-shane

Oh, and in case you were wondering, yes, it is for a spud gun.

 

[Superdoopercooper]

yes... there are various things you can do... the easist would be to use HOUSEHOLD voltage, and step it down. This can be done fairly cheaply.

The other option is to use some small DC voltage, and used a DC-to-DC boost converter to bump up to 27V. I could probably design you one using a 555 timer chip, an inductor, a transistor, diode, and a large capacitor, but i'm not sure how well it would work. That is a big power boost... from say 6V to say 27V.

How much current do you need? Does your Solenoid really need 27V? You could get two small sealed lead-acid batteries at 12V a piece, and wire them in series... for 24V. The downside.... a bit heavy... and generally it takes 12-20 hourse to charge... but w00000t... they can have up to 7Ah... yes... 7000mAh... and not be so restrictive as you couldn't carry them around without too much effort.

Let me know... which way you wanna go... I'll direct you in the approriate direction.

 

[ntshane]

Thanks coop,

Well, it does need to be very portable. My setup now is this: a small project box (2"x2"x4" approx.) with the 9v batts and a safety toggle switch and a momentary switch. I used to have a cool led that showed when it was armed, but somehow I thought that it might be sucking juice even when it was un-armed, so I took it out.

Anyway,

I wanted something that would fit inside the same or similar project box. The gun itself is already relatively heavy (3' section of 3" pvc + solenoid valve + 3' section of 1.5" pvc) so I want this component to be light. Yes, the ideal way to do it would be to have two small 12v cells wired in series. Should work, but I'm not sure. The gun gets pumped to 80-100psi, and if there isn't enough juice it won't fire. I guess the ideal situation would be to figure out how to use a non-electrical system to actuate the solenoid. I could use one of this guy's link ) 'supah valves' but I'm not trying to spend $100 bucks on this setup.

If I could bump up the voltage of a few small rechargeables (aa's should work) , then that would be great.

--shane

 

[ntshane]

Here's something similar to what I have: link

Maybe that will make more sense. The setup he's got at the bottom is cool too, but I have no idea how it works. It replaces the electric solenoid with a pneumatic trigger. Pretty cool.

-shane

 

[Superdoopercooper]

You can get Ni-MH rechargable 9V... but they have only aout 150mAh rating.

It was a good idea to get rid of the "arming light"... it did suck power... esp. if you didn't have it set up right (a resistor in series to limit the current properly).

The Sealed led Acids are sort of big... 5lbs each. This evening or weekend I'll see if I can find something better. In the mean time... I'll draw up a basic circuit that may get you going if you are prototype savvy guy. Luckily the current is small, and the components are relatively cheap.

EDIT: how does a spud gun work? What gives it the power to launch? A spring? or air compressor? I'm not in "the know" on these things.

 

[ntshane]

Coop, Right on! Thanks!

Okay, first, here's a pic: gun

That's the first pneumatic cannon I ever built. The bottom pipe is the chamber, the top is the barrell, the green thing on the left is the solenoid sprinkler valve, and the black box tie strapped to the side is the project box with the batts/switches i told you about.

You can't really see it in the pic, but the chamber has a schrader (tire) valve on it at the end. The chamber gets filled with compressed air to around 80 or 100 psi. The solenoid sprinkler valve keeps the air in the chamber until I 'press the button.' Basically, you ram a potato (pear, apple, gerbil, whatever...) down the barrel, and when the button is pressed, the air in the chamber is immediately released and it propels the potato out of the barrel. This particular gun is fairly powerful, but the one I am using now....whoa momma.
Anyway, it's alot of fun, and if you've ever tried the hair-spray fired spudguns, it's a big step up, and alot less hassle + alot more power.

Anyway, as you can see, whatever I use has to be fairly light/portable and not cause the gun to become more unwieldy than it allready is.

-shane

 

[Superdoopercooper]

That gun is INSANE. Wow...!!!!!!!
I'll work on the circuit/looking for the info. I'll get back to you in the next couple of days. Either PM me, or email me at the address in my profile if I forget... give me a day or two to dig something up before you "remind" me, tho.
thanks.

 

[ntshane]

Sounds cool!

That gun is nothing compared to what alot of the other crazies around have.

-shane

 

[Factor5]

Hehe.... what a coincedence..... i too am designing a spud gun.... if you will notice my post in HT about the power VS arc length..... well that is the ignition for my Acetalyne/oxygen gun..... hopefully it'll fire well.. and wont explode.... it shouldnt explode.... i'd hate to have 1/4 inch stainless steel flying at me, even though i'm over 100 feet away while firing.... anyway... i'm rambling... Hope your project goes well Shane, PM me.... we should have a contest

Edit: I should prolly check my spelling

 

[schmedy]

Easiest way for you to step up your voltage is a transformer, easy to get, small, and cheap. Easy go 10+:1 up.

 

[Superdoopercooper]

d00d... you can't use a transformer to step up **DC** voltage. Sorry... just ain't gonna happen.

 

[Superdoopercooper]

Shane,

Ok... here is what I found so far:

You can make a boost converter using the following chip from Texas Instruments - http://www-s.ti.com/sc/psheets/slus216/slus216.pdf. This is an inexpensive chip that contains the active control functionality of a BOOST converter (a DC-DC converter that steps up the input voltage). It will need some other external components, such as resistors, capacitors, a transistor and an inductor. Total cost would hopefully be less than $20, just as a quick guess... you can probably get some for stuff for free by ordering samples.

This is basically the circuit I was going to have you make, but with the control functionality a more rudimentary design which would require even more work.

Unfortunately, there really is no other way to boost DC voltage. This way will allow you to use 3 - 4 NiMH cells... I would suggest 4.... and I would also suggest "C" sized batteries. These days, for some reason, C and D batteries have basically the same mAh rating, even though the D could really have 2x as much because of the size. That's how it was in the past (i.e. 5 years ago... before "D" batteries quit being used as much). Definitely dont get anything smaller than AA... but C's will give you at least 50% more "shoot" time.

I believe that the effective mAh rating will be as follows: You are boosting say 4.8V to 27. that is a factor of 27/4.8 ~= 6. If you're drawing 50mA on the output, then you'll need to draw 50mA * 6 = 300mA on the input (power-in = power-out) However, there is a 75% max efficiency with this converter.... so, factoring that in... you have 300mA/efficiency = 300/.75 = 400mA. So, even though you may have 2500mAh batteries, they won't last as long as you think they will (i.e. when compared with 150mAh 9V cells...they wont get 2500/150mAh ~= 20x the "shoot time"... just a warning). So, if I'm doing my "calculations" right (I'm taking some educated guesses... it's been 4 years since I had my DC-DC converter class), your C batteries will @ 2500mAh will ahve an effective (E) mAh (to make a comparison to the 9V) of 2500 / 6 * 0.75 = 312.5 EmAh.

Therefore, you'll get twice the shoot time over 9V cells. If you use AA's, you'll have more like 250 EmAh.

Consequently, you'll have to decide:

1) do I want' to mess with this circuit? If you like to tinker with circuits and EE junk... then go for it, I think it'll be a fun project.

2) does the cost make sense? 3x (might need 4) 7.2V NiMH 9V batteries @ $9 (just a guess) a piece is 4 * 9 = $36. Plus, you'll likely need two chargers since I've never seen a charger that handles more than two 9V batteries at a time. = $30 more. Total ~ $66. This circuit at $20 + 1 charger ($15) + 4 C batteres ($15?) = $50 total. So, you may save some $$ by using the cool circuit. But, you'll spend more than $16 of your time building it.

3) do I want the lightest weight possible? 4 9V batters = 3 ounces or so. 4C batteries + this circuit = 1 pound or so. That is a downside of this circuit.

So, Shane, I know I wrote a lot here.... you need to decide what you want to do. I tried to give you a fairly accurate breakdown of the +'s and -'s of building a circuit or using 9V's. You need to figure out what's right for you. If you chose to go the circuit route, I'll give you as much help as I can in picking the right components and compenent values if you are not familiar with that type of thing. If you chose to go with the 9V's that is cool too, just glad I was able to help.

-COOOOOOOOOOOOOOOOOOOOOOOOP

 

[ntshane]

WOW!

That's alot to chew on. Thanks! Well, I'm not sure what I should do. Another idea that I had was to add another 9v (which will bring me up to 4 x 7.2) and then just add a charging jack to it and charge them in series. You're right though, the 9v's are expensive. I found some on this site: www.onlybatteries.com for like $5

I'll have to give it some more thought, but thanks for the input!

-shane

 

[Superdoopercooper]

If you add a charging jack, it will either have to be your own design (if you want to charge in series @ 4 x 7.2V), unless you already have a true "current source" type of charger.

Best bet is to hook them in PARALLEL during charging... and use a normal 9V (7.2V) NiMH charger. DOWNSIDE: charging will take 4x as long. However, if you don't use this thing everyday... then its not a big deal. Plus, with the 4 x 7.2V, if you really wanted more "shoot time", you could get ** 8 ** or even ** 12 ** 7.2V's and hook them up in 2 (or 3 if you get 12 batteries) parallel strings of 4 batteries each. This would effectively and easily double (triple) your mAh of the system.

Hope this helps you a bit more.

 

[VTHodge]

Just a thought . . . Link Me

When I went looking for this I thought it would be funny, now I am thinking it might actually work! (Don't flame me for how impractical this may be)

 

[Superdoopercooper]

<< Just a thought . . . Link Me When I went looking for this I thought it would be funny, now I am thinking it might actually work! (Don't flame me for how impractical this may be) >>



I'M GOING TO FLAME YOU UNTIL...

wait...actually, that is a NOVEL idea!!! Shane could get 4 (or 5) of those... wire them up in series. Then, he could get a nice, large 10,000uF, 50V capacitor or somehting... and that should give him enough charge storage for a firing of the weapon. heheheheh... Shane, check it out!!!!

 

[ntshane]

Serious?

Hmm, looks neat, but I do some launching at night too. If you have a lake handy, it's a blast to shoot fireworks into it.

-shane

 

[Superdoopercooper]

Dude... I got it... go with the 4x 7.2V NiMH design... and then get the solar cells to do the constant charging/power supplying by day... and then the batteries will hold you over into the evening. In fact... if you had solar cells on there... you could probably just charge the batteries while it's sitting around in your house with the lights on... or sitting in a window during the day while your at work/school. No external power required.

Now I'm just going all over the place with ideal. hehehehe

 

[Jerboy]

<<

Oh, and in case you were wondering, yes, it is for a spud gun. >>



Why not just use BBQ grill igniter?

Another option is using an automotive ignition coil, spark plug, 555 based oscillator and a 12V motor cycle battery.