Post-turnoff cooling

[Squalish]

Plan: Use two 120mm computer case fans, in a push/pull going over/under the bulb area. Use another going into the seperate LCD chamber, where the air is forced by the fresnels to go over the LCD itself.

Use a 12v wallwart and an array of 10 supercaps(2 parallel arrays of 5 serial 10F 2.5v caps), for a total of 50F @ 12.5V. If I'm not mistaken, this would equal something like 600 watt-seconds. The fans would probably be sucking down somewhere between 6-2 watts, depending on working voltage. This would cool the bulb after it has been turned off.

[Guy Grotke]

That huge cap will look just like a dead short to your wall-wart. Can you say "blown fuse"?

I've been thinking about maybe a 555 timer-based fan shut-off. I keep leaving my fan running all night. Maybe I just need a LOUDER fan, so I can't fall asleep with it still running!

[Squalish]

+ diode protection (which I assume is adequate here)

[Guy Grotke]

You will need to add some resistance so you don't try to pull too much current out of the power supply. But with that much capacitance, it will then take a long time to get the voltage up high enough to run the fans. If the PS can supply twice the current it takes to run the fans, then it would the ratio between charge and discharge time would be 1:2.

I think you would need a power supply that can source 20 to 100 times the current the fans use, just so it can charge the caps quick enough for startup! Not a very practical design.

How about a design where the fan power supply sends current to the fans directly, and the lamp power circuit sends direct current to a relay with a large cap in parallel. When you shut off the lamp power, the current to the relay gets turned off. As long as the cap holds the relay closed, it keeps the AC going to the fan power supply. When the relay opens, then the fan power supply is turned off.

[unit24]

You could always go the route that PC modders have used:

Auto shutoff controllers

Timed like this $19.00

or mod a thermal monitor like this to keep a fan running if the monitoring probe is above a certain temp.

For those of you who like really getting down & dirty, there is also an article here showing how to build your own thermally controlled fan that will kick on at a preset temp and cut out when the temp goes below a set temp.


Currently I am evaluating a mod using option #2

[Guy Grotke]

You create a problem if you use the wrong temperature probe. If it is a thermister embedded in epoxy, then it could get too hot next to a MH lamp, and melt or burn. High temperature probes are true thermocouples. They can take very high temps, but you need a very good DC preamp to get a reading, since they put out a few millivolts per degree.

I think maybe the best type of temperature control for a MH projector fan is a 1940-technology bimetallic strip that closes a switch contact when it gets hot and bends. Very difficult to overheat, and pretty much fool-proof. You can put it right next to the lamp inside the light engine box. You just need to protect the wire insulation, if that is exposed to the light.

[davemcmahon]

Hey just use a AC fan and get a plug in timer. In britain these are used to have dishwasher's etc come on at night and off a couple of hours later. Or if using a DC fan with 'brick power supply (Laptop universal PSU's are good) just plug the brick into the timer.

[Guy Grotke]

I think most people would like the fan(s) to run for about 5 minutes more after the lamp goes off, and then shut off automatically. That is not possible to do with a plugin timer.

[davemcmahon]

My timer has 15-min pins good enough?? cost about a £5 (approx $10)

[rth]

Here's how I decided to handle the cooldown timer on my projector. I already had the parts sitting around but if you don't then you can expect to spend about 8 bucks on the relay and a couple bucks on the wall switch and cover plate. The FET can be sampled free from Fairchild, though any old N-channel FET should work if you have one. I used the relay because I wanted to use my wall switch and it has but one pole, but if you buy or have a 2-pole switch capable of carrying 5A at 120 Vac then you could eliminate the relay and wire everything directly to the switch. In either case, be very careful when wiring this thing since you don't want your 120 V and 12 V lines mingling

When the wall switch is turned off, the capacitor C1 will hold the FET channel open until resistor R1 discharges it, which takes a certain amount of time. In this case, the 2200 uF and 43 Kohm combo yields a 3 minute 5 second discharge time, so the fan will run for this amount of time after shutoff. Raising either the cap value or the resistor value will yield a longer cooldown period, if required. Vice versa is also true. I had that cap in front of me at the time but you can use a smaller one (470 uF, for example) and compensate by using a larger resistor value (220K or so).

I would suggest going with a DPDT switch instead of the relay, I was mainly looking for a reason to put that relay and spare wall switch to use