Question about a Triac circuit - Page 1
Forum section: electronics / beginners

author: Yamin
author_rank: Frequent Contributor
timestamp: January 19, 2019, 07:45:09 pm
content: Hi, I'm going through a broken vacuum cleaner - tracing the circuit for self improvement purpose etc.. I was hoping that you could help me out with the speed control circuit.1. Does the load always have to be connected in series with the triac?2. As far as I'm aware, with an inductive load there needs to be a snubber circuit, but it just looks like a normal light dimming circuit.3. At first I drew the circuit and I assumed the live side to be neutral and the neutral to be live - the schematic looked a bit odd so I double checked and did some research online and drew it the right way round. But the problem is with the correct schematic the on/off switch is on the neutral side! wouldn't that be a problem because even with the switch off most of the parts of the circuit would still be live :S . Would that be considered as bad design? I know it doesn't effect the working of the unit and with everything screwed shut it doesn't pose a shock risk but still....(also I'm still assuming that changing the "polarities" is correct but not sure as the wire color also doesn't give it away - are the capacitors and the resistors on the right order in the voltage divider?).4. Can I use this control circuit with another similar size motor? In other words how does the load determine the rating/type of components to be used. The time control part (RC) should work with any load right?5. I have attached the the two drawings that I have done. I would be really grateful if you could explain the working of it (mostly about why these rated components are used and how it effects the function of the circuit).Thanks so much

author: max_torque
author_rank: Super Contributor
timestamp: January 19, 2019, 07:53:49 pm
content: For "internal" circuits, ie circuits where everything is contained within an enclosure that prevents any external interaction to those circuit, then it is unimportant what side of the load the switch is placed. In all cases, the operator would not be expected to have to open the unit, and any service technician would be expected to suitably isolate the supply before they opened it.If a load is "remote" to a switch, and therefore there is a much higher chance of un-intentional or accidental interaction with the wiring to that load, then it's more usual to find the switch upstream of the load.In the case of your cleaner, once you open the case then are potentially exposed to high voltage, unless measures have been taken to prevent contact with conductive parts at high voltage with specific shielding, potting or similar.

author: Benta
author_rank: Super Contributor
timestamp: January 19, 2019, 08:14:45 pm
content: 1: Yes2: Probably mounted at the motor3: Don't know the mains sockets/plugs in your area, but for a vacuum cleaner you can normally insert the plug any way you want.4: Sure, if it's the same type motor (probably universal).5: Standard dimmer type Triac control with RC delay and Diac.

author: soldar
author_rank: Super Contributor
timestamp: January 19, 2019, 10:31:19 pm
content: Correct is wrong and wrong is correct.The capacitors charge through the resistors until the diac reaches climax and fires the triac.

author: Zero999
author_rank: Super Contributor
timestamp: January 19, 2019, 11:07:48 pm
content: Yes, the first schematic, marked right is incorrect and the second schematic, marked wrong is right.Generally the switch should be in series with the live conductor, but if there are only two wires, no earth, then it doesn't make that much difference, as the purpose of the power switch isn't to protect against shock anyway.

author: Yamin
author_rank: Frequent Contributor
timestamp: January 20, 2019, 03:56:42 pm
content: So always the triac's gate should be across the capacitors (is that how you are determining the first schematic is wrong)? and also some follow up questions - how does the Triac "turn off" at the point of zero crossing?Also I just checked the data sheet of the Triac and its a 'snubberless version' I didn't know before today that it existed.And also how can I check whether a diac is damaged or not using a mulitmeter?Thanks so much

author: jmelson
author_rank: Super Contributor
timestamp: January 20, 2019, 04:00:47 pm
content: Quote from: Yamin on January 20, 2019, 03:56:42 pmWith the DIAC out of circuit, it should read infinity Ohms on most DVMs. It should take 5 - 25 V to make it trigger either way, and most DVMs only provide a fraction of a Volt in Ohms mode.Jon

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2019, 06:39:29 pm
content: Quote from: Yamin on January 20, 2019, 03:56:42 pmA TRIAC turns on when the voltage between the gate and MT1 exceeds about 0.6V ion either direction.This circuit doesn't do zero crossing. The TRIAC is turned on at a set part of the mains cycle. Look up phase control. The TRIAC turns off, when the current flowing through it falls close to zero.There's no simple way to test a DIAC with just a multimeter. You need a variable power supply which can go up to about 40V. Connect the DIAC to the power supply, in series with a 10k resistor. Set the power supply to around 20V and the voltage across the resistor will be near zero. Increase the power supply voltage and the DIAC should turn on at around 32V, leaving about 31V across the resistor (the DIAC will drop a volt or so). Repeat the test with both polarities.