Help me choose a transistor for this circuit

Audioguru

Joined Dec 20, 2007
11,248
I think PWM dimmed LEDs will be like using a strobe light. Moving objects in the movie will jerk along and wheels will stop or appear to rotate backwards.
 

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campeck

Joined Sep 5, 2009
194
Well luckily this whole thing will be confined in a thick opaque plastic sphere. (The reason I need so many Lumens) So the light is hardly shining on anything. You will only see the colors when the sphere is in the shot. So the only issue will be if the sphere itself appears to flicker. I just set a test date with them on Monday so I will report back then.

I will put together a couple of 555 circuits from a couple kilohertz to 1 megahertz with some MOSFETS. They switch that fast right? 1uS?
 

Wendy

Joined Mar 24, 2008
23,415
Yes, but I wouldn't go that high frequency. 100Khz is going to be difficult enough, but it would work OK. Personally I think 10Khz would work well, but AG might be right about the strobe effect. 10Khz works out to 0.1ms or 100µs, take your pick.
 

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campeck

Joined Sep 5, 2009
194
Ok so I will try 10Khz, 50Khz, and 100Khz.

Bill, being the resident 555 expert, what is the best way to get as close to 0% duty cycle and as close to 100% duty cycle as possible with one pot and without changing the base frequency too much? Digipots are made in a wide range and I will be using one of those.

or perhaps something besides a 555?
 

marshallf3

Joined Jul 26, 2010
2,358
Ok so I will try 10Khz, 50Khz, and 100Khz.

Bill, being the resident 555 expert, what is the best way to get as close to 0% duty cycle and as close to 100% duty cycle as possible with one pot and without changing the base frequency too much? Digipots are made in a wide range and I will be using one of those.

or perhaps something besides a 555?
555s require a different circuit if you want to adjust between 0 - 50% DC.
 

Wendy

Joined Mar 24, 2008
23,415


Then add a simple buffer after for the comparator. Remove Q1, R7, and the LED, leave R8.



You don't have to use a 555, a hysteresis oscillator will use any Schmitt Trigger. A 555 has a very predictable amplitude for the triangle wave (always 1/3 of the Vcc), which is an advantage. A LM393 is a dual comparator version, which should work well for this application.

With tweaking on R4-R6 you should get 0% to 100% easily. You may want to use a darlington on the top transistor on the buffer, you can't lower R8 from 10KΩ much, and it will affect the drive to the top transistors.
 

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campeck

Joined Sep 5, 2009
194
Very nice Bill.

I get everything that is going on except the two transistors with emitters together.

The comparator is pulled up and when on goes low correct? I have a LM339 quad comparator. With what I understand of electronics I can't see how the two transistors are supposed to work. when one is on the other is off preventing current flow no?
 

Audioguru

Joined Dec 20, 2007
11,248
Your vision's response to brightness is logarithmic, not linear (disregarding the automatic-gain-control produced by the iris) so that you can see in moonlight and in sunlight. So a 2% PWM drive to an LED will be brighter than you think.

My Chaser circuits use PWM for the dimming and with the pulse width so narrow that I can't see it on my 'scope the LED is glowing very visibly.

The complementary emitter-follower transistors provide lot of current up and down to quickly charge and discharge the high capacitance of the gate of the Mosfet. The comparator has a very small pullup current by the 10k resistor and only a few mA of pull-down current from the output of the low power comparator. Without the complementary emitter-follower transistors the Mosfet will be slowly ramping which produces a lot of heat.
 

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campeck

Joined Sep 5, 2009
194
Ok. So when the comparator is off and pulled up high. The top transistor lets voltage through to the MOSFET. And when pulled down to ground the MOSFET discharges through the bottom one?

The 555 and digi-pot (R5) will be on a 5v supply. Should the two transistors Vcc be 5v or 15v? Probably 15v right? Also should I not use Logic Level MOSFETS in this case?

Also is the gate resistor necessary?

Thanks again everyone.
 

Audioguru

Joined Dec 20, 2007
11,248
Your 555 must use a 12V supply so that the gate voltage is +10V.
If you use the 555 plus a comparator and complementary transistors then they should also have a 12V supply.

If you use logic-level Mosfets then the 555 with a 5V supply will have a max voltage of only 3.5V which is too low to properly drive a logic-level Mosfet.
The 555 plus comparator plus complementary transistors all using a 5V supply will drive a logic-level Mosfet pretty well.

The gate resistor for a Mosfet is 10 ohms to 100 ohms and stops the Mosfet from ringing or oscillating at a VHF frequency. The resistor must be at the gate pin so that there is very low inductance in series with the gate.
 

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campeck

Joined Sep 5, 2009
194
Your 555 must use a 12V supply so that the gate voltage is +10V.
If you use the 555 plus a comparator and complementary transistors then they should also have a 12V supply.

If you use logic-level Mosfets then the 555 with a 5V supply will have a max voltage of only 3.5V which is too low to properly drive a logic-level Mosfet.
The 555 plus comparator plus complementary transistors all using a 5V supply will drive a logic-level Mosfet pretty well.

The gate resistor for a Mosfet is 10 ohms to 100 ohms and stops the Mosfet from ringing or oscillating at a VHF frequency. The resistor must be at the gate pin so that there is very low inductance in series with the gate.
So 5v on CMOS 555, 5v on comparator, 5v on complimentary transistors (what should I use for these?)

Will the 5v comparator not switch a 12v signal on the complimentary transistors?
Is it because they are emitter followers? (still need to read up about those more...)

So 5v for everything though should work well? Darlington for the top one and a small signal one for the bottom one? I have numerous 2n2222 and 2n3904 type ones. not sure if those are PNP though. But I have PNP ones.
 

Audioguru

Joined Dec 20, 2007
11,248
So 5v on CMOS 555, 5v on comparator, 5v on complimentary transistors (what should I use for these?)

Will the 5v comparator not switch a 12v signal on the complimentary transistors?
The complementary transistors are emitter-followers that do not have voltage gain. The 5V p-p from the comparator will produce a high of 4.3V and a low of 0.8V at the output of the transistors.

So 5V for everything though should work well?
Yes, if you use a logic-level Mosfet.

Darlington for the top one and a small signal one for the bottom one?
DO NOT use darlington transistors. A 2N3904 is the NPN and a 2N3906 is the PNP.
 

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campeck

Joined Sep 5, 2009
194
Ok.

I got Bill's circuit completed and everything works good up until the complimentary transistors. I tried a 10k pullup and a 1k pullup. I also used a 100 ohm and a 22 ohm gate resistor. Not sure whats wrong. Oh and I'm using an IRF510 which apparently is logic level. I even tested this and it had turned on by 5v
Any ideas?
 
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Audioguru

Joined Dec 20, 2007
11,248
I got Bill's circuit completed and everything works good up until the complimentary transistors. I tried a 10k pullup and a 1k pullup. I also used a 100 ohm and a 22 ohm gate resistor. Not sure whats wrong. Oh and I'm using an IRF510 which apparently is logic level. I even tested this and it had turned on by 5v. Any ideas?
Simply look at its datasheet. The IRF510 is an old ordinary Mosfet with poor performance. It is not logic level because it is spec'd with a gate voltage of 10V. Some might turn on a little with a gate voltage of 5V.
 

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campeck

Joined Sep 5, 2009
194
EDIT

never mind. I hooked the transistors up wrong. So I hooked them up right and still nothing. So I replaced them and now the circuit works. works great. And the O-scope shows all the signals perfectly. I am having a problem of it jumping to 100% when it's at about 95% I think I need to tweak the 10k resistors on either side of the adjustable ones.
 

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campeck

Joined Sep 5, 2009
194
Simply look at its datasheet. The IRF510 is an old ordinary Mosfet with poor performance. It is not logic level because it is spec'd with a gate voltage of 10V. Some might turn on a little with a gate voltage of 5V.
Is the Gate to Threshold Voltage not the spec I need to look at?
http://www.eecs.berkeley.edu/~boser/courses/40/labs/docs/IRF510.pdf

This says min 2.0v max 4.0v
And when I tested it increasing it past 5v did not brighten the LED anymore.

oh...casualties of my testing include two transistors and a .1uf cap that I didn't think was polarized and I let the magic smoke out. lol
 
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Audioguru

Joined Dec 20, 2007
11,248
The threshold voltage of most Mosfets (also your old IRF510) is when it conducts only 0.00025A so it is almost turned off. The datasheet does not show how much gate voltage makes it conduct only 0.7A but shows how much is its on-resistance when the gate is 10V.

Most Mosfets conduct 75A (but your old IRF510 conducts a minimum of 5.6A when its gate is 10V). Your LED needs only 0.7A so some (not all) ordinary mosfets will work when the gate is 5V.
 

Thread Starter

campeck

Joined Sep 5, 2009
194
Well Since I am pulsing the LED I ramped up its current to 1A with a 3.9ohm 10W resistor and increased the TL555, TS339, and emitter follower BJT's to 7v to test reliability of the circuit. Things are getting hot but probably not as hot as if I had gone linear. I also hoooked up a fan to my 7v rail and the O-scope was showing a LOT of noise so I put a 470uF cap on the rail and no more noise! (should I put a diode across the rail to for the fan since its an inductive load?) Also, are you referring to Fig 8 in the datasheet?

The O-scope sure is a darned handy tool!

I'm going to upload some O-scope pics and a schematic. There is some ringing in my power supply rail. I got rid of most of it with some 220uf and 450uf caps but There is still some.
 
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