A critique needed

[charlie_r]

While messing around with a few 1W LEDs, I came up with what I hope is a cheap way to add strobes to my bike.

Using a NE555, set up to deliver pulses at this rate:

On time (t1) is 0.00229 seconds.
Off time (t2) is 1.04 seconds.
Duty cycle is 0.22%.
Frequency is 0.96 Hz.

With this schematic:

**broken link removed**

I noted that the 555 gets warm if I add a second output transistor directly off pin 3 with the same resistor value.

What I need to know is if this will work to drive multiple strings of LEDs, and what corrections I would need to make to have it work properly.

**broken link removed**

Thumbnails are clickable.

 

[audioguru]

Your transistors are emitterr-followers so the resistors are not needed and cause the output voltage to be very low.
Emitter-followers waste 0.7V to 0.9V for each transistor and your circuit with many transistors has two transistors in series wasting about 1.5V to 1.7V. You should change the transistors to be common-emitter switches then they will have a low voltage loss.

Why do you have a positive power supply and a negative power supply? Why not use just a positive power supply? How many volts?

What is the forward voltage range of your LEDs (typical voltage and maximum voltage)? What is the current their brightness is rated at? What is their maximum continuous current?

You have the LEDs turned on for only 2.3ms. Durations less than 30ms appear to be dimmed.

 

[charlie_r]

Your transistors are emitterr-followers so the resistors are not needed and cause the output voltage to be very low.
Emitter-followers waste 0.7V to 0.9V for each transistor and your circuit with many transistors has two transistors in series wasting about 1.5V to 1.7V. You should change the transistors to be common-emitter switches then they will have a low voltage loss.

I expected a voltage loss of around 2V, and in this application, it might be needed.

Why do you have a positive power supply and a negative power supply? Why not use just a positive power supply? How many volts?

I am running this from a 12V 8Ah SLA battery. those symbols were just handy. I'm not that good at drawing schematics.

What is the forward voltage range of your LEDs (typical voltage and maximum voltage)? What is the current their brightness is rated at? What is their maximum continuous current?

LEDs are typical 1W buttons on star heat sinks. Vf of 3.2-3.8. Running without current limiters, so they are getting whatever is able to be passed through the transistors. Max current at 0.1% duty cycle is 1000mA (1A). Max sustained current is ~350mA.

You have the LEDs turned on for only 2.3ms. Durations less than 30ms appear to be dimmed.

Running 3 LEDs in series for a V drop of 9.6-11.4, with the extreme short on time is giving me a very bright flash, able to be seen for about 500 meters, tested outside on an overcast day.

The reason I am wanting to use the extra transistors is to lower the load seen by the 555. I tried using several transistors directly from the output, but that warmed up the timer more than what I am comfortable with.

I've used the MPSW01As because that happens to be what I have on hand that will handle more than 500mA. I do have a few P55NF06L power mosfets (able to handle 50A at 60V), but I'm thinking those will pass too much current.

I'm probably wrong about this, but I was thinking that by controlling the base current, I would also be controlling the current through the C-E connection.

If I need to. I can feed the LEDs through an LM317 set up as a 500mA current source, but that may also limit the brightness of the flash.

Right now, I'm just experimenting around with this, before admitting the possibility that I'll have to buy xenon flash to achieve what I want.

 

[Reloadron]

Have you given any thought to using maybe a logic level MOSFET? Assuming you use a fast enough one.

Ron

 

[charlie_r]

By logic level, are you refering to a 5v control circuit? I don't have that available on the trike, only because so far I haven't needed to build a buck converter/power supply for that level. 317s and 7805s tend to get rather hot dropping that much voltage at the currents I need, and from what little experience I have with them, get unstable with intermittent loading. But perhaps I've not set them up properly yet.....even following the datasheets, I note instability in the voltages being output from them.

 

[Reloadron]

No, a logic level MOSFET simply needs 5 volts for full turn on.The gate voltage can exceed 5 volts. They work fine in 12 volt systems (or more). The benifit being able to switch a large current load easily.

Ron

 

[charlie_r]

I didn't know that. Thank you.

I might have to order some, and check it out. Could you give me an example part number?

 

[charlie_r]

I know, google is my friend. Looked it up, and I'm not liking what I'm seeing in reference to logic level mosfet. The common suppliers have not too much stock of through hole TO220. Most appear to be various styles of SMT/SMD/SOT...you get what I mean. I do not have the capability to use anything but through hole.

I've not seen anyone be able to "dead bug" SMT etc components successfully yet, or use without a pcb, so that idea is lost.

I do appreciate the suggestion, though.

 

[audioguru]

You should not use transistors to limit LED current because a transistor has a range of current gain and it changes when their temperature changes. I think you are destroying your LEDs with way too much current. You should reduce the current and increase the on-time.

The output transistors should have an output of maybe 330mA then their base current should be 33mA. A 555 can drive 5 of them and be fairly hot, 6 will make the 555 very hot.

Since the LEDs might be 3.8V then connect only 2 in series and in series with a current-limiting resistor.

 

[Reloadron]

IRL3302 from Allied Elect and other distributors is a common example. However, if you are more comfortable using transistors AG has provided some advice.

Ron

 

[charlie_r]

I appreciate the datasheet link, Reloadron. However, in searching my normal suppliers for that, I find no one stocks it. Same with most other through hole versions. I can only assume that most users of this type of mosfet have gone completely to surface mount. In doing a google for equivalents, the same thing there. all SMT.

Audioguru --

You should reduce the current and increase the on-time.

Increasing the on time defeats the purpose. I do agree with limiting the current though, but not as much as you may be thinking. 800mA limit seems to give me the brightness that I require, without overdriving to destruction.

I am trying to simulate xenon flashes without getting into the high voltage/high current end of it, due mainly to the exposed nature of what I'm doing. These will be used in all weather conditions, and have to be reliable in extreme vibration. Xenon flashes would highly likely fail in these conditions.

These are going on my delivery trike, as safety measures. If I can wake up a few inattentive drivers (you know the type, cell phone glued to the ear, not paying due attention to what else is on the road) to my presence, I stand less of a chance of becoming a new hood ornament.

This is what the strobes are going on:

**broken link removed**

It is a work in progress, hopefully ready for paint and body next spring. Yes, those are turn signals you see on the front, based on 3mm LEDs.

 

[Reloadron]

I buy the IRL3302 from Allied in a TO220 case design. Hell if you want one I'll mail it to you. Free as in postage paid and all.

Ron

 

[audioguru]

An Xenon bulb is made with high temperature glass. It produces a lot of heat and produces much more light than a 1W LED. The LED must not get too hot or it doesn't work anymore.

What propels your trike?

 

[charlie_r]

My legs propel my trike. It is an AtomicZombie.com loderunner.

To the heat problem: with an extremely short duration, these LEDs are not showing any signs of heating. I've been running a set of 8 --2 per channel-- for the last two hours here beside me, even the pair that are not on stars seem to be staying cold. One of the things I've noted in my research is that you can overdrive LEDs to close to 75% above their constant rating, as long as you have an extremely short pulse duration with a relatively long off period. Ostensibly to give the die time to cool off between pulses.

For the xenon tubes: as stated before, I doubt they would be as reliable as the LEDs, given the conditions they will be subjected to. That, and I'm not too crazy about getting a few kV+mA shots up my pants should something go wrong with them. Would tend to knock me off the seat.

Reloadron, according to the alliedelec.com website, it appears to be an order it in component with a lead time of 8 weeks. or maybe I went to the wrong website? same results going to allied electronics website.

https://www.alliedelec.com/search/s...onSearch=D:IRL3302,Dxm:All,Dxp:3&SearchType=0

 

[audioguru]

The allowed peak current for the LEDs is 1000mA only when the duty cycle is 0.1% and the frequency is fairly high.
But your duty cycle is 0.22% and your frequency is very low so the chip can heat to above its maximum allowed temperature before the case gets warm.

I have some ordinary 30mA LEDs blinking very brightly with a pulse width of 30ms. The low duty cycle saves battery power.

 

[charlie_r]

I guess I'm totally lacking in knowledge pertaining to thermal response. It would seem to me that the chip having most of a second to cool between pulses would tend to allow slightly higher duty cycle. But then again, I do lack any sort of schooling in electronics and physics, especially thermodynamics as it pertains to electronics.

Hanging around forums like this coupled with experimenting with things I have little knowledge of does tend to pound things into this thick skull.

So I will try modifying the circuit, and see what happens.

 

[charlie_r]

Ok, changed from common collector to common emitter. Doesn't seem to make any difference. I still have the same voltage drop across the C-E leads.

Circuit works the same either way.

The only difference I can see is that the base current doesn't get added into the load part of the circuit. Meaning it is added after the load, since the load is placed before the transistor instead of after. Still I can't see any real difference when used in this manner. I can see it being a problem when used in a voltage or current amplifier circuit, but in a switching circuit?

Perhaps one of you guru's could explain this to me?

Tried running the switching transistors directly off the 555 with a bias (dropping, current limiting) resistor. Chip again got rather hot.

Also, I've been running the flasher for 48 hours non-stop, and all LEDs are still flashing as bright as they were when I started.

Yes, I know I am overvolting AND overcurrenting, but with such short duration pulses, it doesn't appear to be making much of a difference to the LED.

 

[audioguru]

Ok, changed from common collector to common emitter. Doesn't seem to make any difference. I still have the same voltage drop across the C-E leads.

The transistor is supposed to turn on. When it turns on then the voltage between its collector and emitter is almost nothing so the entire power supply voltage is fed to the load.
Yours is wrong. Maybe you have the collector and emitter pins connected backwards.

Please post your schematic.

 

[charlie_r]

Ok here are the other permutations I have checked.

The transistors DO turn on.


Common collector with buffer has already been posted here.

Common emitter direct from 555:

**broken link removed**

Heats the 555.

Common Emitter with buffer:


**broken link removed**

555 stays cool.

Common collector direct from 555:


**broken link removed**

Heats the 555.

So, you tell me. what is the difference? I still have the same voltage drop across the transistors. If I was using the transistors in an amplifier it would matter which way I had them. Since I am using this as a switch, it doesn't matter which way I do it. It then becomes a matter of convenience only, insofar as how many wires I have to run from front to back on the trike.

Again, I remind you that this is being used as STROBES, so rapid current/voltage rise does play a part. No, I don't want the added expense of overkill, using enhanced mode mosfets. No, I don't have an oscilloscope to check rise/fall (turn-on/turn-off) times or actual current used. Top of the head guesstimates put the current at around 750mA ~850mA.

 

[audioguru]

Please attach your schematics to your posts HERE, not over at PhotoBucket.

Your transistors do not have enough base current and the LEDs do not have anything limiting their current.

When the transistors are used as emitter-followers then their output voltage is reduced a lot. with a 12.0V supply, the output high of the 555 is about +10V and the emitter-follower has an output of about +9V. If the LEDs are 3.5V white ones then 3 in series need 10.5V but your circuit will not light them.

When the transistors are used as switches then they can switch almost the entire 12V to the LEDs and their current-limiting resistors.