Trying not to blow up a 555 timer

BobRoss · Feb 20, 2013

I've got a working prototype of the schematic below for a blinking tail light using a 555 timer and two LEDs as a voltage limiter for the 555 circuit (LEDs 1 and 2). All LEDs are flashing when the switch is on, and two of them stay on without flashing when it is off, which is what I wanted. This will run off of a 3W dynamo hub.

The circuit is a very basic astable monovibrator circuit where the flashing rate of the LEDs is driven by R1 and C1. Of course, there is a rectifier between the dynamo and the circuit.

I'm not sure about the current limits for the 555 - I'm using the CMOS version TLC555CP (http://pdf1.alldatasheet.com/datasheet-pdf/view/28913/TI/TLC555CP.html)

Am I going to fry the thing running it on a dynamo hub? As shown, There would be about 300 mA (I think - that may be incorrect) available through the switch at high speeds. Which parameter from the data sheet should I be looking at?

I've considered running another resistor between the switch and the R/V+ pins (#s 4 and 8 on the pinout), but that seems inefficient. Is there a better way to limit the current (if I actually need to do that)?

Steve K · Feb 20, 2013

okay... let's try to address one question at a time:

1. "am I going to fry the thing?"
Let's look at the datasheet, which is the keeper of all wisdom regarding the LMC555:
http://www.ti.com/lit/ds/symlink/lmc555.pdf (I feel better getting datasheets from the manufacturer directly)
Page 3 has a section called "absolute maximum ratings". This is where you learn that applying 15v across the V+ to ground pins will most likely destroy your IC. Looking at your circuit, it appears that the two LEDs on the right will clamp the voltage to ??? about 6v? Depends on the Vf of the LEDs and the current going through them.
Page 4 of the datasheet shows that T.I. tests most of the parameters down to 1.2v Vdd and up to a Vdd of 12v, so your 12v Vdd should be just fine.
Having said that, it wouldn't hurt to put 0.1uF across the V+ to ground pins of the LMC555, just as a measure of good practice. You'll see this done in most/all of the schematics in the datasheet.

I'm not sure what else is going on in the circuit, but .... can I assume that the power coming into this is full wave rectified? I'd be tempted to add a diode in series with S1 in addition to having a 0.1uF cap from V+ to ground. This will prevent the 555 from powering down everytime the dynamo's AC wave cycles (or is C2 holding up the power so the 555 doesn't see any significant voltage ripple?)

You might want to check that your schematic is correct, because I think you are missing a resistor from the 555's timing network, and the far side of R1 should be hooked up to the discharge pin instead of the output. Refer to page 7 of the datasheet.
edit: are you trying to run this as an astable oscillator or monostable? If monostable, what are you triggering it with? The typical thing to do would be to run it astable.

ianfield · Feb 20, 2013

BobRoss said:
I've got a working prototype of the schematic below for a blinking tail light using a 555 timer and two LEDs as a voltage limiter for the 555 circuit (LEDs 1 and 2). All LEDs are flashing when the switch is on, and two of them stay on without flashing when it is off, which is what I wanted. This will run off of a 3W dynamo hub.

The circuit is a very basic astable monovibrator circuit where the flashing rate of the LEDs is driven by R1 and C1. Of course, there is a rectifier between the dynamo and the circuit.

I'm not sure about the current limits for the 555 - I'm using the CMOS version TLC555CP (http://pdf1.alldatasheet.com/datasheet-pdf/view/28913/TI/TLC555CP.html)

Am I going to fry the thing running it on a dynamo hub? As shown, There would be about 300 mA (I think - that may be incorrect) available through the switch at high speeds. Which parameter from the data sheet should I be looking at?

I've considered running another resistor between the switch and the R/V+ pins (#s 4 and 8 on the pinout), but that seems inefficient. Is there a better way to limit the current (if I actually need to

The series pair of LEDs (assuming high power types) will clamp Vcc to about 6.8V (for white LEDs) - which is "only just" adequate Vgs-thr for some MOSFETs.

If the LEDs connected to the MOSFET drain are red; the only current limiting will be the RDS-on of the MOSFET, so they'll clamp Vcc during their on time at less than the Vf of the 2 white ones.

If you had more realistic current limiting for the LEDs, Vcc could rise above the 16 - 18V rating (depending on manufacturer) of a bipolar 555.

If you allowed Vcc to rise to about 9 - 12V, the ability to supply current at that high voltage would be less - so you could be able to get away with a TL431 programmable zener/shunt regulator, but to be on the safe side I'd go for using it to drive a power transistor shunt - most TL431 manufacturers have online appnotes that show how to do it.

As long as the LEDs survive direct connection to the rectified output of your dynamo - in the present form they'll shunt the voltage to what the 555 can take - but it might not work quite exactly as you'd planned.

BobRoss · Feb 20, 2013

Steve K said:
I'm not sure what else is going on in the circuit, but .... can I assume that the power coming into this is full wave rectified? I'd be tempted to add a diode in series with S1 in addition to having a 0.1uF cap from V+ to ground. This will prevent the 555 from powering down everytime the dynamo's AC wave cycles (or is C2 holding up the power so the 555 doesn't see any significant voltage ripple?)

You might want to check that your schematic is correct, because I think you are missing a resistor from the 555's timing network, and the far side of R1 should be hooked up to the discharge pin instead of the output. Refer to page 7 of the datasheet.
edit: are you trying to run this as an astable oscillator or monostable? If monostable, what are you triggering it with? The typical thing to do would be to run it astable.

I could have been more clear - I dashed the diagram off in a hurry... Yes, the power coming in is full wave rectified.

This diagram is for a rear blinky light that runs in series with two white leds on the front of the bike (not shown in the diagram)

C2 holds up the power well, provided it is large enough. I've been using 1500 uF electrolytics, which seem to work fine. I'll take your advice on the diode and other cap, though.

This circuit diagram is a hysteretic oscillator (which works as an astable monovibrator - from here http://openbookproject.net/electricCircuits/Exper/EXP_8.html#xtocid179702) and I have it exactly as diagramed - it works on a 5V supply as well as just hand-spinning a dynamo.

I get that the 555 has voltage limits, but I was asking about current limits. I'm guessing that what I'm doing is ok. I'm used to being concerned about over-current because of working with LEDs, but I'm guessing that's not an issue here.

BobRoss · Feb 20, 2013

ianfield said:
The series pair of LEDs (assuming high power types) will clamp Vcc to about 6.8V (for white LEDs) - which is "only just" adequate Vgs-thr for some MOSFETs.

If the LEDs connected to the MOSFET drain are red; the only current limiting will be the RDS-on of the MOSFET, so they'll clamp Vcc during their on time at less than the Vf of the 2 white ones.

Sweet. This is helpful. That's the exact setup - 2 white LEDs to hold this voltage. I've tried a single white LED with a CMOS version of the 555 (min 2V Vdd) and it worked beautifully in prototype. I'll watch out for the VGS of the MOSFET, though.

Trying not to blow up a 555 timer

BobRoss

Newly Enlightened

Steve K

Flashlight Enthusiast

ianfield

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BobRoss

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BobRoss

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