simple "one shot"

[loppydog]

hi guys, I am new here and i am hoping to get some guidance. I am trying to wire some guns for my arcade cabinet. inside the plastice housing there is a 12 volt soleniod that acts as the "recoil". I need a "one shot" circuit so when I pull the trigger the soleniod fire just once even if the trigger is held down. I have come up with a circuit here:
**broken link removed**
here is a pic of the insides of my gun:
**broken link removed**
This circuit works great, but I need to know if it is safe and or efficient. Is there a better easier way to achieve the same result? I do not know much about capacitors, is it bad to have them being charged for extended periods of time? Also, my AC adaptor say its output is 12 volts DC, but I measure 19 volts at the leads. Is this bad for my 16 volt cap? Should I find a real 12 volt power supply? And for my final question.....The capacitor I am using has barely enough charge to fire the soleniod. Is there any way to slighly increase the force of the soleniod without jumping up to the next size CAP? Maybe by adding a resistor or something. Anyway, thanks in advance for any suggestions!

 

[crutschow]

The circuit is safe and reasonably (50%) efficient.

You can leave the capacitor charged for an extended period. They operate that way all the time in the power supplies inside any line powered electronics.

Yes, electrolytic caps are generally intolerant of over-voltage, so it may fail prematurely at the higher voltage. Either use a higher voltage cap or a supply voltage no higher that 16V.

A resistor will not help to fire the solenoid. You need a larger cap (or two caps in parallel) for that.

 

[loppydog]

two caps in parallel...Why didnt I think of that?lol That is a good idea, so if I use two of the same caps, will it double the capacity? Or is there some other formula?

 

[EN0]

two caps in parallel...Why didnt I think of that?lol That is a good idea, so if I use two of the same caps, will it double the capacity? Or is there some other formula?

Hi,

When you connect caps in parallel, they simply add together. Thus, for two caps:

[LATEX]C_T = C_A + C_B + ...etc...[/LATEX]

If Za = 10µF and Zb = 47µF, then:

[LATEX]C_T = 10uF + 47uF = 57uF[/LATEX]

All you do is add them together.

 

[loppydog]

Ok guys thanks for the help, I just finished wiring up two 2200uf caps parallel, now it seems to work lots better. I tried to connect a true 12 volt power supply and the solenoid barely even moved . So i just went back to the 19 volt power supply, I hope I dont fry anything. I have one more question tho.....I am using a micro switch from radio shack, and I am not 100% sure it will hold up to 19 volts, here is the link let me know if you think it is OK.

http://www.radioshack.com/product/index.jsp?productId=2049718

It says 250 volt but that doesnt seem right to me, I have seen the contacts inside the switch and they are TINY!
Also, sometimes the microswitch appears to get "stuck". The circuit stops functioning until I bang the gun a bit, then it works again for a while. Do you think this is due to the contacts sticking together? I am thinking these switches are mostly used for 0 voltage or the negative side of a circuit.

After further review...it is definitely the switch getting stuck. I checked the continuity between the posts and it is getting stuck in the normally closed postion, or the position where it is charging the capacitor. Any ideas on what might be causing this? I think I may have to go back to the drawing board with my design if this problem persists. My original design was based around a relay and the switch would have been connected the negative side of the relay. Maybe someone could let me know if this kind of design would work better.

 

[EN0]

Hi,

You can calculate the voltage on the cap at any given time via these equations:

Charging Capacitor:

[LATEX]V(t) = E(1 - \ell^{-\frac{t}{RC}})[/LATEX]

Discharging Capacitor:

[LATEX]V(RC) = E(1 - \ell^{-1})[/LATEX]

 

[loppydog]

Thanks ENO, but I am pretty bad with equations. I am not sure if you are trying to help me find the proper capacitor for a 12 volt power supply. If so I think I am missing a major part of the puzzle....I know nothing about the solenoid. I would need to know how many amps it draws correct? Anyway, the 19 volt supply with about 4400uf cap seems to be about right. I also found a cap that is rated for higher volts, so I should be good there as well. But this switch is becoming the real problem. After a little research I found people having trouble with "contact welding". I think this might be what is happening to me. The last time it got stuck I carefully took apart the switch and tried to press the plunger and watch the contacts. It took alot more pressure than normal to get the contacts apart, then it was fine after that. Also, it sparks when making contact, I am sure this is normal but I thought I would mention it.

 

[crutschow]

It does sound like contact welding. The switch is rated for 5 amps.

You can determine the current through the solenoid by measuring its resistance with an ohmmeter (multimeter). The maximum current would be the cap voltage divided by this resistance.

Edit: The contacts are sticking because there's little to limit the capacitor charging current when you connect it to the supply. If the power supply also has a capacitor, then the current is limited only by the wire resistance between the two caps. Try adding a resistor of about 5-10 ohms or so in series between the power supply and switch. The resistor can be a low power 1/8 or 1/4 W.

 

[loppydog]

Ok, GREAT Info there crutschow, very helpful. I measure 8.4 Ohms across the soleniod. So if the power supply is 19 volts, that would mean that it draws about 2.25 amps correct? I will try the resistor idea tomorrow. If I understand correctly, You are thinking the capacitor is being charged too quickly by the power supply? The resistor will allow the capacitor to fill slower? Would this also mean that the circuit will take more time to "recharge", if so that might become an issue being as I need the circuit the recharge about as fast as you can pull the trigger (within reason). Also, If you think the CAP is charging to fast, would there be any reason to suspect the CAP of releasing its charge to the soleniod too quickly? If so, I guess I would also need a resistor between the cap and the soleniod. I tried to rearrange the circuit to allow the switch to be on the negative side of the circuit to aviod arcing, but I had no luck. Honesly I dont even know, maybe it would still arc anyway. BTW you guys have been a huge help to me!

 

[crutschow]

Your solenoid current calculation is correct.

Yes, with no significant resistance between two capacitors, the current could be many tens of amps when you switch them together when they are at different voltages, which burns and welds the contacts.

The resistance will indeed slow the charge time. The time-constant is R*C where R is the resistor and C is the capacitance. For 10Ω and 4400µ, one time-constant (to charge to 63% of its final value) is 44ms. In three time constants or .132s the cap will charge to about 95% (18V) of the max. value allowing a trigger rate of about 7 times per second.

If you want a faster charge you can lower the resistor value but you wouldn't want to go below 4Ω since that could cause the peak current to exceed 5A. 4Ω would give a charge to 95% in about 50ms for a trigger rate of 20 per second.

 

[loppydog]

wow thanks, this make sense to me now. I dont think I can pull the trigger faster than seven time a second I will go with a 10 Ohm resistor. I forgot to mention that I am planning on swapping out the 2 2200 Caps for a single one with a higher voltage rating but a similar uf rating. So my original drawing is accurate except it is a 35 volt cap @4400uf instead of a 16 volt @2200uf. this should not affect your calculations much correct?

 

[crutschow]

wow thanks, this make sense to me now. I dont think I can pull the trigger faster than seven time a second I will go with a 10 Ohm resistor. I forgot to mention that I am planning on swapping out the 2 2200 Caps for a single one with a higher voltage rating but a similar uf rating. this should not affect your calculations much correct?

Nope. It's just a function of the resistor and total capacitor value.

Regarding the 7 times a second. I didn't include the time to release the trigger to start the charge, so it actually would be some less than that. If you are concerned, just go a lower value, somewhere between 4 and 10 ohms.

 

[loppydog]

OK, I just wanted to post an update. The lowest rated resistor at radioshack was 10 Ohms, so I went with that. Everything seems to be working GREAT now. It does seem to have a hard time keeping up if I press the trigger very quickly. I guess I could parallel two 10 ohm resistors to get 5 ohms, but I dont really want to risk getting the contacts stuck again. Anyway thanks for the help you guys are the best!

 

[crutschow]

5 ohms will keep the peak current below the switch rated current of 5A, so that should still be ok.