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08-30-2011, 06:13 PM #1
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cybhunter
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Circuit design gone awry (don't know what happen)

A couple week ago I deiced to start building a laser pulse circuit. I ended up having a 555 chip in a 'one-shot mode' shoot a trigger into a NPN transistor to power a 1 watt 808nM diode

The goal was to

A: have a diode deliver 40 J/cm^2 in a pulse
B: have the pulse be variable up-to .25 seconds (for safety)
C: Use a 555 chip and 1 watt 808nM diode

the values I used for the timing circuit
(using: tau=ln(3)RC )
Resistance: 2,312 ohms (for .25 second pulse), variable
pull-up resistor (for pin 2), 10,000 ohms (momentary switched to ground)

Since I was using a 7805 (rated at 1 amp), I thought that by using a 6.6 ohm in-series with the laser should provide a stable 2.2 volt, .454 Amp supply (5volt- 2.0 volts= 3.0 volts= .454Amps*6.6ohms). The laser was attached to the collector of a TIP31 transistor with pin3 going to base

looking up the data sheet for the TIP, the hfe for ~.500 Amps Collector was around 100, so the base current would have to be 4.54mA. Assuming a voltage of 5 on pin 3, I calculated the resistance to 1,100 ohms.

trying to the fire the diode resulted in nothing happening. Checking the values, I was reading 4 volts across the diode leads when triggered and 180mA.

Out of frustration I ended up smashing the project box.

What I don't understand is
1) if the hfe is ~100 at ~500mA (collector), why was it only supplying 180mA (collector)?
2)the purpose of the resistor was to drop the voltage down to 2.0 volts and provide 454 mA, yet why was the potential 4 volts across the diode?

Attached is the schematic

Joe
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08-30-2011, 11:08 PM #2
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lasersbee
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Re: Circuit design gone awry (don't know what happen)

The first thing I would do is change the TIP31 for a HexFet.
The TIP31 will not fully turn 'ON'... a HexFet will...

When you supply 5volts directly to R2 will the output
voltage/current be high enough through the load???

Jerry
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08-30-2011, 11:17 PM #3
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Flaminpyro
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Re: Circuit design gone awry (don't know what happen)

Shouldn't R3 connect back to the 12 volt supply and not the 5 volt supply
I don't think you will be able to draw the current you need to power the 808 if you
go through the 7805 regulator.

BTW what dose key=space mean ?
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Last edited by Flaminpyro; 08-30-2011 at 11:22 PM.

08-30-2011, 11:55 PM #4
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lasersbee
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Re: Circuit design gone awry (don't know what happen)

Quote:
 Originally Posted by Flaminpyro Shouldn't R3 connect back to the 12 volt supply and not the 5 volt supply I don't think you will be able to draw the current you need to power the 808 if you go through the 7805 regulator. BTW what dose key=space mean ?
Good catch FP....
But with it tied to 12Volts then I m sure the TIP31
won't saturate with 5volts on the Base..

Jerry
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Last edited by lasersbee; 08-30-2011 at 11:56 PM. Reason: spelling errors

08-31-2011, 12:27 PM #5
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cybhunter
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Re: Circuit design gone awry (don't know what happen)

Quote:
 Originally Posted by Flaminpyro Shouldn't R3 connect back to the 12 volt supply and not the 5 volt supply I don't think you will be able to draw the current you need to power the 808 if you go through the 7805 regulator. BTW what dose key=space mean ?
That seems odd, considering the 808 is only taking 454mA, the 555 is at most taking 20mA, and the 7805 is rated up to 1A

the key=space just denotes that the space key triggers the push button in multisim

Getting back to the design, I ended up reconfiguring the leads so the current source bypasses the 7805, goes though 20.30 ohms: 12v(VCC)-2.0v(laser)-.7v(NPN drop)=9.3v --> 9.3=(20.39 ohms)*(454mA) and adjusted the pin3 resistance to 730 ohms

I couldn't find a hexfet component in Mutlisim so I left "Tip31" as the transistor

Also attached is the Oscilloscope view (with the 'space bar' push button) viewed across collector to emitter (Channel A) and base to emitter (Channel B)

Joe
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08-31-2011, 02:52 PM #6
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Bluefan
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Re: Circuit design gone awry (don't know what happen)

A transistor should work fine. You measured 4 volts across the diode? Sounds like you got the polarity of the diode wrong.
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08-31-2011, 03:37 PM #7
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cybhunter
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Re: Circuit design gone awry (don't know what happen)

prior to hooking up the diode, I researched the pinout. To confirmed, I ran a continuity check (got 1.8xx), so I hooked it up that way. When the diode was attached to the circuit, the 180mA is way too much for leakage

08-31-2011, 10:50 PM #8
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lasersbee
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Re: Circuit design gone awry (don't know what happen)

I'm not sure why you even need the 5Volt regulator..
A 555 timer will work up to 16 volts...

http://www.national.com/ds/LM/LM555.pdf

BTW.. have you actually built this circuit or is it just
in a simulator ??

Jerry
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Last edited by lasersbee; 08-31-2011 at 10:51 PM. Reason: spelling errors

09-01-2011, 11:14 AM #9
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cybhunter
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Re: Circuit design gone awry (don't know what happen)

Its just out of habit to put in a voltage regulator in. Since I don't the parts I've only ran in simulation. Does anybody see any possible problem with the circuit (sans the 7805 and the tip31) before I create a parts list?

09-01-2011, 01:34 PM #10
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lasersbee
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Re: Circuit design gone awry (don't know what happen)

You may need to tweak R3 depending on the voltage
drop across the actual Laser Diode and U3....

Jerry
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Last edited by lasersbee; 09-01-2011 at 01:35 PM. Reason: Spelling errors

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