Low current control of triac speed controller? - Page 1
Forum section: electronics / projects

author: PP3331
author_rank: Contributor
timestamp: December 12, 2018, 09:49:00 pm
content: Hi. I'm using a commercial triac speed controller to control a universal motor that pulls considerable amperage (where "considerable" is no more than 15 amps at 117 volts). I would like to modify the controller so that it will cut all current to the motor, via a relatively small low current micro switch and associated wiring. However, I'm unsure how much current actually flows on the "control" side of the unit (e.g., in series with the control potentiometer) and can not easily measure it without complete preemptive disassembly. Is there any rule of thumb for how much current is actually flowing on that side of the circuit, as opposed to the "hot" side of the triac that is in series with the output load (in this case, the motor)? Thanks!

author: Circlotron
author_rank: Super Contributor
timestamp: December 13, 2018, 04:36:10 am
content: You could use the microswitch to short gate to MT1 or A1 on the triac. Only about 10-50mA at several volts.Beware!!! These connections are at mains potential so are a safety risk!

author: PP3331
author_rank: Contributor
timestamp: December 13, 2018, 05:04:54 am
content: Quote from: Circlotron on December 13, 2018, 04:36:10 amPerfect. Thanks!

author: PP3331
author_rank: Contributor
timestamp: December 15, 2018, 04:17:48 pm
content: What kind of wire gauge would I need to be using in this case? The micro switch itself is only capable of dealing with small currents. I'm assuming that in practice the low current capacity switch and thin wire would work, assuming this wiring or switch never contacted ground? Thanks.

author: Circlotron
author_rank: Super Contributor
timestamp: December 16, 2018, 12:34:48 am
content: For this low current even a single strand e.g. 0.2mm would be enough. Thicker is just fine of course. Just make sure the insulation on the wire is mains rated. That is important.

author: PP3331
author_rank: Contributor
timestamp: December 16, 2018, 12:45:46 am
content: Quote from: Circlotron on December 16, 2018, 12:34:48 amYeah, so this pretty much means using conventional mains wiring to get that insulation, and hope that the micro switch meets its rating. I definitely don't want a surprise jumping to ground. Thanks.

author: PP3331
author_rank: Contributor
timestamp: December 16, 2018, 11:56:39 pm
content: Quote from: Circlotron on December 13, 2018, 04:36:10 amHmm. Is there some alternative way the triac could have been wired? A dead short (verified with a meter, between G and A1) has no effect on the triac operations -- the "dimmer" pot continues to control the output as before! And I tested this at the leads of the triac itself. Thanks!

author: PP3331
author_rank: Contributor
timestamp: December 17, 2018, 06:10:53 pm
content: Could I just switch the gate with the micro switch to accomplish what I want? Is significant current flowing through the gate at any time? Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2018, 06:51:36 pm
content: Quote from: PP3331 on December 17, 2018, 06:10:53 pmYes that would work, but remember to treat the connections to the micro-switch with the same respect as anything connected to the mains.

author: PP3331
author_rank: Contributor
timestamp: December 17, 2018, 07:03:21 pm
content: Quote from: Hero999 on December 17, 2018, 06:51:36 pmUnderstood. But the actual current running through the micro switch would be very low, correct? Thanks.

author: Sylvi
author_rank: Regular Contributor
timestamp: December 18, 2018, 02:24:03 am
content: HiI think it should be pointed out that the term "microwsitch" apart from being a brand of switch only refers to the mechanical stroke of the actuator - tiny movement causes the contact to open or close. This in no way reflects on the current rating of the switch. For example, my electric lawn mower has a 15A "microswitch" in its switch housing.In the OE's application, there is no reason to use house wire or a heavy zip cable to connect the microswitch.Doh! It just occurred to me that the suggestion of a microswitch is to have a microcontroller do the switching? In that case, an electronic element such as BJT or mosfet would short the gate of the triac, with this element being placed right by the triac. How this device is in turn controlled might be via optoisolator for safety. The opto would also be mounted wherever the triac is and the input to the opto is then only an isolated LE voltage and current - tiny wires with tiny insulation.

author: PP3331
author_rank: Contributor
timestamp: December 18, 2018, 02:47:34 am
content: Thing is, shorting the gate to A1 does not disable the triac -- and I've verified the pins.

author: Circlotron
author_rank: Super Contributor
timestamp: December 18, 2018, 06:32:07 am
content: Quote from: PP3331 on December 18, 2018, 02:47:34 amI'm more than interested to see how this turns out to be so...