Path: utzoo!attcan!uunet!zephyr.ens.tek.com!tektronix!gvgpsa!gold!grege
From: grege@gold.GVG.TEK.COM (Gregory Ebert)
Newsgroups: sci.electronics
Subject: Re: Switching from 5V to 120V
Keywords: relays or triacs?
Message-ID: <801@gold.GVG.TEK.COM>
Date: 22 Feb 90 17:49:19 GMT
References: <3709@uceng.UC.EDU>
Distribution: na
Organization: Grass Valley Group, Grass Valley, CA
Lines: 53

In article <3709@uceng.UC.EDU> dmartin@uceng.UC.EDU (Dale Martin) writes:
>
>I have designed a circuit using ttl logic chips that activates LEDs.  I would
>like it to activate some 25W bulbs (one at a time).  What is the best approach?
>Could I use relays, or is the switching voltage too low?  How about a transis-
>tor?  Aren't there triacs or something that do just that?  
>ANY suggestions would be appreciated, the more specefic (part #'s and such)
>the better.
>
	There are a few things you can do. First of all, you CAN have common
	ground for the TTL and the 120 VAC bulbs if you are CAUTIOUS. If
	you are willing to hunt around, you can get some opto-triacs. This
	is obviously the safest way to do it. An opto-triac's control side
	is just an LED, so you have that one licked. The other side is
	basically like a relay contact if you run it on AC or pulsed-DC.
	You should put an inductor in series with the light bulb to
	protect the triac from 'di/dt' problems. A 10uH coil will give
	you protection up to a di/dt of ~20A/usec (reasonable) for 120VAC.
	If you use a larger coil, thats OK. Radio Snack sells some $0.79
	coils that are perfect.

	The other way is to use standard SCR's (avail @ Radio Snack).
	The solution has 2 parts : (1) You need to get the +5 volt and 120
	VAC supplies to the same ground, and (2) Drive the lamps. SCR's will
	conduct for only 1/2 cycle (versus full cycle for triacs), so you
	will want to switch to 50W bulbs. The advantage is that SCR's have
	a simpler gate-drive circuit.
	
	The cheapest way is to use the standard transformer/bridge/capacitor
	supply for your +5 volt stuff, and connect it to the neutral side
	of the AC line. Before I get flamed, I want you all to know that
	I have done this for years and I'm still alive. You have to be
	careful about using test equipment such as scopes, etc. As long
	as you dont put 120 VAC a-c-r-o-s-s TTL, all is fine. An SCR's
	gate can be driven by TTL with a 100 ohm resistor in series. If
	your SCR has a lower gate current, say 1-5 mA, use a 1K. The SCR
	anode is connected to one side of the light bulb. The other side
	of the light bulb is connected thru a 10uH coil to the HOT
	side of the AC line. I strongly suggest you use a fuse. The cathode
	of the SCR connects to the ground of the TTL supply AND the neutral
	side of the AC line. If you switch the hot and neutral wires, the
	beastie will still work, but if you accidentally touch the circuit
	'ground' you might get surprised !. One point worth mentioning here
	is that the NEUTRAL wire is physically tied to the earth near your
	KW-hr meter, so you can PRESUMABLY touch it without getting shocked.
	BUT !!! there will be voltage drops throughout your house wiring
	along this line due to resistive and inductive effects, which can
	result in dangerous voltages with respect to ground, so be careful.

	The last, and most expensive, way is to use an isolation transformer
	and hook the rest up as detailed above. Now you dont have to worry
	as much about getting shocked if you always remember to touch only
	one circuit node at a time.