oldmanbiker

Thanks for the info. I've been searching for a 33v mov, but there doesn't seem to be any. Here's one that is close, a 30vac mov. Open circuit voltage on the transformer is 26 vac. V47ZA3 Max applied voltage: 30 V Acrms, 38 V DC Max energy, 2ms: 4.5 J Clamping voltage: 93 V Varistor v0.1mA: 47 (42-45) V

I can't put it on the AC side of the transformer, because it is in the attic somewhere. Exactly where, I'm not sure. Anyway, I read somewhere that protection components are best placed on the board, right where the power enters. The idea is that inductive surges from lightning, etc. are also handled. In my case, there is maybe 50 feet of cable between the transformer and the circuit board.

I had a recent power failure that blew out routers, hard wired smoke detectors, etc. The most painful was a pic-based motor controller that runs off of 24vac. Now that I've replaced everything, I would like to protect the motor controller. From what I've read, I could use opposing Zeners on the controller board where the ac input comes in. I also read that Zeners are slow, and that MOVs are faster. But MOVs are imprecise on voltages, they have a range where they begin kick in. Does this suggest the best protection is opposing Zeners and MOV, with a series fuse? In any case, what is the normal way to determine the correct voltage for the protection components? Surely it must be significantly higher than 24v to prevent false trips. Thanks

By the way, most people seem to refer to a transistor in switching mode as "saturated", is there a more correct term for that?

You're right, the datasheets for NTE semiconductors aren't like others I've seen. Anyway, I found out that NTE2343 cross references to TIP102. Luckily, fairchild has plots for the tip102. I hope that has the necessary info ;D http://www.fairchildsemi.com/ds/TI/TIP102.pdf

indulis: I'm a newbie. Newbie + static sensitive parts = poof ;) audioguru: I could not get a tip120, I ended up with an NTE2343. Also a darlington npn, supposed to similar and handles more power. http://www.nteinc.com/specs/2300to2399/NTE2343.html Any difference in the calcs? Is there a link to a tutorial on how to do the calcs? Thanks

Thanks the info, audioguru. I plan on using a readily available led, which means Radio Shack :o http://www.radioshack.com/product/index.jsp?productId=2062553 This led does need a current limiting resistor. It is Vf 2.25, 90mA. If the darlington turns on at 1mA, will 50mA thru the base cause any problems?

Hi, I need to blink an array of 78 white leds. I figure that I need to switch about 4.0 volts at 1.6 amps. I want to take a $1.00 blinky led as a signal source and drive a TIP120 with it. When using a TIP120 as a switch, can I just wire it in series with the LED array ground connection? Also, I figure that the darlington in the TIP120 is high gain, so I should be able to saturate it with the blinky led. Will connecting the blinky led from V+ to the base do it? Thanks