Re: Newbie Questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Stephen Conner by way of Terry Fritz <teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>

At 11:21 24/04/03 -0600, you wrote:
>Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
><jtf-at-dejazzd-dot-com>
>
>Struck my first spark last night. Yippe! Little 1/4 inch spark from a 555 
>driving a HEI
>coil. Similar to:
>http://www.geocities-dot-com/CapeCanaveral/Lab/5322/coildrv.gif
>Now for some questions:
>
>1.) The current meter on the 12 Vdc power supply didn't move. Heat sink on 
>the NPN
>transistor didn't get warm. Curious to know the current draw in the 
>primary circuit.
>DMM gave a null reading across a 0.03 ohm sense resistor in the emitter 
>leg. Using a
>50mVolt analog meter movement, calculated the current to be 0.83 Amps. Why 
>did the
>DMM give a null reading? Too high a frequency, plus a square wave, for the 
>meter?

There should be no problem doing this. The DMM will read the average value. 
Maybe your meter is playing up or something?

>I
>would have assumed that without a current limiting resistor, the primary 
>of the HEI coil
>would have drawn more current. The calculated 0.83 Amps was an "average", 
>would
>the inrush current be higher: in the order of several amps? The circuit 
>diagram
>indicates 6 Amp supply. Why the disparity?

It looks like you have the 'On' time set very low. If the transistor is 
only on for a very short time, the current in the coil won't have time to 
build up to a decent level. Increase the 'On' time by increasing the 
resistor between pins 2 and 7 of the 555. Also try increasing the rep rate, 
by decreasing the resistor between Vcc and pin 7. Also there is a 
possibility that your 555 is being prematurely turned off as soon as it 
goes on, by noise spikes in the circuit. So make sure you have a decoupling 
capacitor on the chip power supply and also on the control voltage pin.

Once you have increased the on time, if you're still not satisfied, you can 
boost the supply voltage.


>2.) Next step is to double the supply voltage to 24 Vdc ( two gel cells in 
>series). Does
>the base biasing resistor, for the NPN transistor, need to change with a 
>higher supply
>voltage? The goal is to increase the supply voltage to the coil primary, 
>and hopefully
>get a higher secondary spark. Each time I increase the primary supply 
>voltage, will the
>base bias resistor need to change?

The base bias resistor is probably too big even for 12 volts. As a rule of 
thumb we say that the base current should be 1/10 of the collector current. 
So if you're putting 6 amps through the collector, the base should be 
getting 0.6 amps. That would mean using a 20 ohm resistor, assuming the 555 
pin 3 could even supply .6 amp, which it can't. So you ought to put an 
extra emitter follower in.


>Thanks in advance. These are elementary questions for this list.
>
>John
>