When to use a relay - Page 1
Forum section: electronics / beginners

author: Jane
author_rank: Frequent Contributor
timestamp: September 12, 2015, 08:03:20 pm
content: I would like to build a device that will use a ceramic heater. So I am going to use a PID controller Altec PC410 with a solid state relay (SSR )to control the ceramic heater. So far I have thought that I mustalwaysuse a SSR if I use a ceramic heater. Today, however,I have disassembled a device that uses REX-C100 PID controler to control a ceramic heater but there was NO relay.How can I decide when I must use a relay and when it is not necessary to use a relay?Thank you

author: obiwanjacobi
author_rank: Super Contributor
timestamp: September 13, 2015, 05:39:57 am
content: To my (hobbyist) knowledge a solid state relay is merely a switching transistor (MOSFET,TRIAC etc) with some support components. Not sure how they do isolation and if there is any isolation in a SSR - one of the major benifits of using an old electro-magnetic/mechanical relay. The device that did not have a SSR must do the swicthing itself.How to choose? I would go with the packaged SSR or even an old relay. Probably makes designing your circuit simpler.

author: jitter
author_rank: Frequent Contributor
timestamp: September 13, 2015, 06:07:57 am
content: Quote from: Jane on September 12, 2015, 08:03:20 pmIn that case, why does the manual state that pins 4 and 5 have the relay output with normally open contacts that can handle 250 VAC up to 1 A (confusingly, in specs it says 3 A)?Might this so called relay be made with discrete components to form a SSR?Edit: d'oh! There seem to be different versions, only one of which has a relay output... Your's might have a different type of output, depending on the suffixes to the model number (see second attachment).

author: SeanB
author_rank: Super Contributor
timestamp: September 13, 2015, 08:15:15 am
content: Your controller will most likely have a relay inside with the 2 contacts connected to the rear. If you order the optional outputs then you will get them as depicted, the board inside will have the appropriate modifications for them installed instead of the relay.The SSR driver simply removes the relay and gives you the coil drive from the relay as a 24V drive for most common industrial SSR units, which operate with an input on the control of 3024V at 30mA.Trigger output will have a small daughter board with an opto triac on it and the associated 3 resistors and a RC snubber, to provide the control side of a SSR inside the controller and drive to use an external BT157 triac as a control device.The difference in current is that the alarm relays are typically very small, designed for a limited current and a limited operation cycle, as they typically operate once per power cycle of the machine or less to drive a separate alarm or control circuit. The output relay has to be a better larger unit, which has to handle a moderate current to drive a larger contactor ( thus the 3A rating, though you want the contactor it drives to have a 10VA or less coil or a VDR clamp for a coil under 100VA) for a large number of switching cycles, where it might do 5 cycles per minute in heating for a few years in controlling a heater oven. It has to have a a lifetime of at least 1 million mechanical cycles at rated load.Standard though on all I get off the shelf is a relay output, so either you use an external contactor to drive the heater ( and in setup do limit the cycle time so it does not try to switch once a second, try 10 seconds or so for a heater using a relay) as the built in relay will only handle a heater that draws less than 3A, which would be 500W in countries with 220VAC mains and 300W in the USA.I have converted units with relay output to drive a SSR, simply by removing the relay and using 2 47R resistors ( they were conveniently to hand, you can use 2 wire links instead) to link the coil connections to the output pads, and then using the SSR externally, and marking the case with the appropriate changed part number with permanent marker.