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FrozenGate by Avery

Timer for Duty Cycle

M

M7CC

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Hi all, first post...

I'm putting together a small AC powered line laser to achieve that "liquid sky" effect. Most of the modules out there (<100mW, <$100) have a duty cycle. Is there some kind of timing circuit I could place between the power supply and module to handle this automatically (i.e. 60 sec on, 5 sec off and repeat until turned off)?

Ideally it would be something off the shelf -- surely this has come up before (for both laser and non-laser purposes). I know it could be designed from scratch but I'd rather not reinvent the wheel.
 





Are you in the US? They are available is just about any dept store. They are used for timing kitchen appliances or whatever else you can think of to plug into them. They run $5-$20 depending on what features you want, but they're smallest time resolution is on the order of 1-5mins. Not seconds. Just look in the electrical accessories section next to the power strips.
 
M7CC said:
I'm putting together a small AC powered line laser to achieve that "liquid sky" effect. Most of the modules out there (<100mW, <$100) have a duty cycle. Is there some kind of timing circuit I could place between the power supply and module to handle this automatically (i.e. 60 sec on, 5 sec off and repeat until turned off)?
Click to expand...

I have been pondering something very similar to this for quite some time. Although you're using A/C to power the laser, the laser is STILL subject to duty times. I've been wondering how small one could make such a circuit. I've been wishing some of my more expensive laser pointers had a built-in circuit to cut off the laser before I operate too long and cook it.

It would be really great to make it more idiot proof. The other night I was attempting to trick a street light into shutting off by shining my strongest laser pointer at the daylight sensor connected with it, and after some time, I realized I had the laser on way longer than I should have. :(

Calling all driver designers and builders: Is this hard to do? Would we want to do this? I'm imagining some configurable circuit using a tiny IC and a tiny resister. (dare to dream)
 
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I am & I am familiar with that kind of timer. They usually only let you run 1-5 cycles per day (in fact I just bought one for something else). This would be over 1000/day if left on a 60sec/5sec cycle. More likely it would be on for an hour or two.

I figured surely there is something small & simple that can handle this. Does everyone out there just use a giant heat sink? Watch the clock & turn off your lasers manually? Just let them burn out?

So I guess another question would be -- can anyone suggest a green laser module, 50-100mW that can be fitted with a line lens (I have one from AixiZ w/ screw on cap), has a long duty cycle and costs <$100?

This is the one I was looking at but it only goes for 60sec. www.rayfoss.com
 
Hope you guys don't mind me posting my findings. This might be the one. Configurable on and off times from 200ms - 200min. Time is set by two potentiometers (which could get tedious). Input 3-12VDC. I think it only has 1000mA max current (checking on that). Ships from Hong Kong for $12.

About 2cm long -- wouldn't fit in a small pointer but could be rigged to just about anything else.

0 2S 200min Automatic Repeat Time Cycle Setting Delay Timer Module New | eBay
 
Duty cycle is copy pasta nonsense that Chinese vendors pass between each other. Especially 60s/5s which is 92% duty cycle - pretty much the same thing as no duty cycle at all. You can't trust that specification; you have to make one yourself.

Rule of thumb: if the module becomes hot to the touch, turn it off. It's that simple.
 
Cyparagon said:
Rule of thumb: if the module becomes hot to the touch, turn it off. It's that simple.
Click to expand...

Good to know. I don't need to worry about the propagation time of the heat from the diode to the host?

I mean, is it possible for the diode to cook and be ruined BEFORE the host, or the tip of the host, becomes noticeably warm/hot?
 
pschlosser said:
Good to know. I don't need to worry about the propagation time of the heat from the diode to the host?
Click to expand...

You mean the thermal resistance? No. Most of them have direct contact with the case, and since you're only dissipating a watt or two at most, the difference in temp is minimal. If it dies when the case is still cold, it was gonna die soon anyway.
 
Agree that Duty Cycle (when/if provided) is probably not that scientific. But neither is - let go when it gets warm. My laser pens get fairly warm after keeping them on for a while, mainly due to the heat of my hand applying enough pressure to keep the button pushed in. Plus it's hard to say when warm or hot is too warm or too hot.

Makes sense that if it's all metal between the diode and whatever your hand is holding there won't be much thermal resistance. But, getting back to my original point, I'm trying to build a hands free device.

I guess it boils down to this. Trying to figure out when to turn off any laser before it burns out is probably hit or miss which means you will probably fry one or two (which gets expensive). In the end if you can't keep it on all the time you will have to figure out what the duty cycle is or get into heat sinking.

I was leaning towards the former & wondered if anyone else had already been down this road. Judging by the number of fancy heat sinks in this forum and no talk about timers -- I may be barking up the wrong tree... Maybe I should just get a bigger chunk of metal with some fins, slap the module in there and call it good.

I should add that you guys have a lot more experience than I do. I'm still curious about how long you all typically run your lasers, how often they burn out, how much you rely on an upgraded heat sink, etc.
 
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If you are looking to protect your diode for overheating, it'd be better to use a thermister to gauge the temperature of the diode. Use that along with a pot, a relay, and some other componenets, and you have a simple circuit that can disconnect power to your driver when the temperature gets too high.
 
Mrcrouse said:
If you are looking to protect your diode for overheating, it'd be better to use a thermister to gauge the temperature of the diode. Use that along with a pot, a relay, and some other componenets, and you have a simple circuit that can disconnect power to your driver when the temperature gets too high.
Click to expand...
Second that.. I was thinking something along those lines.
 
Dumb question, but doesn't putting a switch between the diode and supply cause more harm than just leaving it on? If anything, I'd rather power cycle the supply off and on, if at all.

If you think about this diodes in DVD burners run at 150-200mW, and are in a pretty terrible thermal arrangement. I have a drawer full of reds scavanged from DJ projectors, or bought from fleabay in the 100-200mW range, and mounted in those black rectangular heatsinks. Drop them on any switched 12volt supply and they just run forever. Heat sinks don't get warm either. The ones that have died always seem to do so for reasons other than thermal. None of the constant run 700mA / 445's I've built are given a duty cycle or are pampered either. The Chinese have this thing about duty cycles in their tech notes and I just don't get it.

Maybe I should just get a bigger chunk of metal with some fins, slap the module in there and call it good.
Click to expand...

That's my vote. Unless you're driving the diode beyond where it should be 100mW should be no problem.
 
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Shouldn't be an issue if it's between the supply and the driver. You don't want to do it between the diode and the driver. And I'm always for just making the heatsink bigger, and not worrying about it. Hence my Mace of Doom kits. ;)
 
M7CC said:
Trying to figure out when to turn off any laser before it burns out is probably hit or miss which means you will probably fry one or two (which gets expensive)
Click to expand...

Pointers almost never die from overheating. They do die randomly, but it's practically never because someone left it on too long. If it overheats, you may decrease the life of the diode. It doesn't mean "POP"

What counts as overheating though? That depends on who you ask. Like most semiconductors, they're fine at several tens of degrees over room temp. Most (all?) of them are less efficient when run hotter, but it isn't likely to damage them. Something 110-120F is hot to the touch, but it's fine for semiconductors. 140F is hot enough to burn you, but it's still fine for most semiconductors.

As for you "100mW" module, it's likely got a half watt pump so the entire thing is only dissipating 1.5W, maybe 2. That much heat spread over the entire case isn't enough to heat it to dangerous levels. It's just not much of a concern. Would you put a duty cycle on a 5mm LED if a Chinese retail site suggested it? No, because you know better :)
 
M7CC said:
Agree that Duty Cycle (when/if provided) is probably not that scientific. But neither is - let go when it gets warm. My laser pens get fairly warm after keeping them on for a while, mainly due to the heat of my hand applying enough pressure to keep the button pushed in. Plus it's hard to say when warm or hot is too warm or too hot.

Makes sense that if it's all metal between the diode and whatever your hand is holding there won't be much thermal resistance. But, getting back to my original point, I'm trying to build a hands free device.

I guess it boils down to this. Trying to figure out when to turn off any laser before it burns out is probably hit or miss which means you will probably fry one or two (which gets expensive). In the end if you can't keep it on all the time you will have to figure out what the duty cycle is or get into heat sinking.

I was leaning towards the former & wondered if anyone else had already been down this road. Judging by the number of fancy heat sinks in this forum and no talk about timers -- I may be barking up the wrong tree... Maybe I should just get a bigger chunk of metal with some fins, slap the module in there and call it good.

I should add that you guys have a lot more experience than I do. I'm still curious about how long you all typically run your lasers, how often they burn out, how much you rely on an upgraded heat sink, etc.
Click to expand...

I have a suggestion. Why not use a driver with TTL input and then drive that at 5V with a PWM. You can set the duty cycle to whatever you like, then adjust the frequency to get a pleasant strobe-like effect. Fast enough pulsing will appear to be on constantly, albeit at a lower average power.

Bob
 
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