discharging

HI

I did you a voltage divider. , I have about 4 volt to the base. The battery supplying the coil is a 12 v battery and I am barely getting 5 volt impulses from the coil. i am not doing any Em kick back as you mentionned.

ken

i am not sure if i am following you here. the last project i did like that consisted of keeping the coil at saturation and then generate a narrow pulse to allow the kick back discharge to take place!, this means inverting your signal from the 555 or tailoring the components to generate a narrow off pulse for the EM Kick back. P.S. you should think about a small cap on the Collector to ground so to not allow high voltage to appear at the transistor. also, the 555 does not go to the - rail completely if memory serves, this means you need a voltage divider at the base.

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Are you measuring the voltage with a scope?

Duty cycle of the 555?

DC resistance of the coil and inductance? or what kind of coil.

If you are using a scope to measure voltage . . . the inductor may not be saturating fully before you take the pulse away. It takes time. With the inductance and resistance of the coil you can calculate the time constant and have some idea if your pulse width is great enough.

If you are pulling lots of current through the 2N3055 it may not saturate without a lot of base drive current - the 3055 doesn't have a lot of gain

From my old Texas Instruments "The Power Semiconductor Data Book"

F hfe 20 KHZ minimum

Static forward current transfer ratio between 5 at 10 amps collector current and 20 at 4 amps (minimums)

Collector Emitter Saturation Voltage Ib = 400 milliamps drive, Ic 4 amps collector current = 1.1 volt Ib = 3.3 amps , Ic 10 amps = 8.0 volts!

So, if you're sinking lots of current don't expect a 3055 to pull to ground and even if you're not, the 555 can only supply so much base drive current to the 2N3055

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hi

what I have tried doing is putting some voltage (from 6 to 40 v) to the base. with the collector connected to the a simple coil ndn the coild to another 12 v supply, still, there is nothing going through the emitter. i used a 2n3055, I have a few of those, so I do not think they are deffective. I just want the coil to discharge. It does discharge if i connect it diretly to the battery, then I get an eletromagnet, this is what i want to do woth the transistor.

ken

Directly with no resistor ? That may be a problem for startes.

Graham

You doubtless have a resistor between the output of the 555 and base of the transistor, you've calculated how low you can go and still have the 555 survive the experience or how high and still expect the 3055 to saturate. Right?

Another trick that may come in handy - bypassing the current limiting base drive resistor with a capacitor to increase the rise time and dump a little more "get moving" energy into the transistor's base. Size would be best determined by some experimenting.

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The 555 doesn't have enough power. Use the 555 to drive a smaller transistor that will then drive the 2N3055. Do a gain calculation to find out how much current you need into the base of the 2n3055. How much collector current will you need the

2n3055 to conduct -- in other words, how much current do you need to flow through the coil? Look up the gain of the 2n3055 at that current level. If minimum gain at that collector current is say 5, then divide coil current by 5; that is the current you need to drive the base of the 2n3055 with. Bung in a transistor between the 555 and the 2n3055, and use a resistor to control current into the base of the 2n3055.

How do you know ( you mean current anyway ) ? The OP hasn't specified his load. The 555 has a reasonably healthy output current.

This is called a darlington arrangement.

Or use a mosfet.

Graham

Lots of options here. i was not sure to try an intermediate transistor. i try that and the capacitor too. Also anotehr thing. if hook up the coil to the battery I get an electo magnet. if I put a 50 ohm resistance in series or even an higher resistance i get nothing. measured the resistance of the coil. its 50 ohms. the resistance get very hot, but no electro magnet.. why

ken

The capacitor 'thing' is just for 'high speed circuits' it's irrelevant here.

Yes. That's normal.

Your measurements sound like they are in error then. You should still get a weaker magnetic field with the sereies resistor. The resistor gets hot because it's dissipating power.

Graham

For the sake of argument I'll assume you have a twelve volt battery driving the coil. So you need 12/50 or 0.24 amps collector current (coil current). .05 amps base current to the 2N3055 ought to get you by, which your

555 can probably deliver. I seem to remember seeing 0.2 amps source current in datasheets for typical NE555. Here's my guess what's happening: The base-emitter diode of the 2N3055 is forming a shunt to ground, overloading the output of the 555. You should put a base resistor in there and see if that eliminates the problem. Try a 100 ohm, one watt resistor. That will give you .07 amps to the 2N3055 base if your 555 output is 7 volts as you stated. Ohm's law says power=v^2/r=49/100 or about half a watt. I suggested using a one watt resistor to stay within the power rating. Also if you're driving the coil with less than 12 volts, this resistor value will still work.

yes , its about 90Hz

there is a minuscule difference with even the smallest resistance. The point I was getting at as soon as i add resistance to this network the electro magnet is not considered operational if we consider operational the ability to repulse a weak ceramic magnet.

I added a TIP41c NPN transistor between the timer and the 3055 but no difference. thats why earlier I put a 6 v battery directly to the base of the 3055 and no effect. I was hoping to activate a dc current in the electro magnet .

ken

I tried a 100Ohm resistor in betwe the output of the timer and he base and nothing changed. swapped the 3055 for another.. no change. This I thought to be a very simple design is giving me a headache

K

i tried using just the TIP41C instead of the 3055. to my surprise it do not heat up, but I get a very week voltage in the coil now, far from connecting it directly to the 12v battery. This time the setup was; battery -coil- transistor- ground and timer to the base

ken

Try connecting the coil in the emmiter circuit of you 3055. Not from the collector to 12 supply. Use the resistor in the base circuit of 3055 to limit the base drive, and connect the collector of 3055 to +12 volt - emmiter to coil and coil to ground. Also add a diode collector to emmiter of

3055 reversed bias. This may help drive the coil a little better. JTT.

Use PB's suggestion of using a power MOSFET, its voltage driven instead of current driven like a bipolar transistor so you can just connect the O/P of the 555 directly to the MOSFET gate without any other components to complicate matters.

......................and connect a reverse shunt diode across B/E to protect it from back emf!

I added the diode across thebase and emitter, I got good results, switching the coil around to the emitter did not help. I am getting some power on the electro magnet but just around6 v, not the

11.5v I was anticipating

ken

somewhere you state that the resistance of the coil is 50 ohms - well if you put a 50 ohm in series with a 50 ohm coil half the voltage will drop across the coil and half across the resistor. Your total current with 100 ohms total resistance should be 120 milliamps and close to a watt of power in the resistor - plenty to cause heating.

Your current with no resistor should be .24 with a power dissipation of close to three watts in the coil causing it to heat (and depending on its physical size and construction it may take awhile to heat noticeably)

As for the electromagnet effect: with the resistor you cut the current in half and you cut the voltage across the coil in half. You drop the effective "ampere turns" by a factor of four so the magnet should be 1/4 as strong as it was with no resistor.

It doesn't sound all that hard to do even with a 555 and a 3055

So something is obviously wrong . . .

What became of the 2 KHZ you were initially driving it with? Is/was that a square wave? Is your coil very large?

I had a little 555 circuit driving a car ignition coil. The coil resistance was ~3 ohms. I used a TIP 120 to drive the coil and it worked reasonably well (made sparks). I forget the duty cycle, but the frequency was adjustable and duty cycle fixed (I think - this was several years ago)

This shouldn't be a big problem unless the inductance of the coil is very high - and then it may be hard to pump any real energy into it at

2 KHZ with only 12 volts.

And when dealing with MOSFET's and inductors, like someone suggested, it takes some savvy to protect the mosfet from the inductive kick to keep from destroying them. You need a relatively high voltage part and some high speed diodes or zeners to protect them - it would be a good idea to read up on their characteristics first.

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