Soft Start circuit - Page 1
Forum section: electronics / beginners

author: couillebleue
author_rank: Newbie
timestamp: June 24, 2020, 03:57:00 pm
content: Hi,I've been wanting to make a soft start for my table saw which I believe is a universal motor like most tools. So I've come across this circuit onwww.homemade-circuit.com. I thought it was simple enough for me to use.The question I have is why the 100 µF/25V cap after the rectifier is only 25V rated ?, shouldn't be more like 250V ?From what is understand the 2µF/400V is just gonna restrain the current coming through the main so you don't blow the rectifier. The voltage at the output of the rectifier is still gonna be something like 170V assuming no current.thanks

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 24, 2020, 07:38:10 pm
content: Most of the voltage is going to be dropped across the 2uF cap in front of the rectifier. Also notice that that little timing circuit is also driving the 12 VDC relay coil, so the voltage in that part of the circuit is not expected to be more than about 12 V.

author: Cubdriver
author_rank: Unknown
timestamp: June 24, 2020, 07:58:29 pm
content: The 2uF cap is a cheap-and-dirty voltage dropper for the power supply that drives the relay. The power supply should never be operating without a load - when the power is first applied, it will be charging the 100uF cap at the base of the transistor through the 4.7k resistor, and as soon as the transistor turns on it will be feeding the relay coil. That should keep the current flow through the 2uF cap to the point where it limits the voltage fed to the 25V filter cap.Here is a redrawn version of that section of the schematic to hopefully make things a bit clearer:-Pat<edit - reworded for clarity>

author: Benta
author_rank: Super Contributor
timestamp: June 24, 2020, 08:12:23 pm
content: That circuit is extremely nasty and incomplete. The idea is apparently to limit voltage to the motor with the triac stage until voltage has built up on the 100 uF caps to trigger the relay and short out the triac for supplying full voltage to the motor.Mains connected capacitor drop circuits are very dangerous and should be avoided if at all possible. Ifreallyneeded they should be designed correctly for safety.This is apparently designed by a clueless amateur. Throw it in the garbage can before you kill yourself.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 25, 2020, 07:54:17 am
content: That cap can be protected with 18-20-22V Zener (1-2W rated)

author: Benta
author_rank: Super Contributor
timestamp: June 25, 2020, 01:48:11 pm
content: Quote from: Vovk_Z on June 25, 2020, 07:54:17 amThat doesn't solve the other problems with the circuit.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 25, 2020, 11:41:23 pm
content: Of course there are improvements to be made, but if the circuit does what it is intended to do and the chance of failure or injury is anything less than 100% then there is nothing ‘wrong’ with the circuit.

author: bin_liu
author_rank: Contributor
timestamp: June 26, 2020, 01:16:19 am
content: 2uf capacitor lacks discharge resistance;C1 is missing a Zener diode;If your system has frequent start/stop actions, you also need to consider the discharge of C2;The charging time of C2 from 0V to 0.7V is short. If a Zener diode is connected in series with the base of the transistor, a smaller C2 can be used to achieve a longer delay time.Remember, when you upload any circuit diagram and need to communicate with everyone, you must add the serial number to facilitate the discussion

author: Benta
author_rank: Super Contributor
timestamp: June 26, 2020, 08:32:36 am
content: Quote from: Ground_Loop on June 25, 2020, 11:41:23 pmI'd rather turn that around and say if the chance of failure or injury is higher than 0%, then there's a lot wrong with the circuit.Just to mention a couple of points:There's no current limiting resistor to the circuit. If it's turned on at the voltage crest: Wham! You can probably say good bye to the 1N4007s, what burns next is anyone's guess.There's no bleed resistor across the series cap. If you pull the plug and accidentally touch the prongs, you will potentially receive the discharge of a couple of 100 volts from a 2.2 uF cap. No fun, I assure you.And so on.Designed by an amateur and potentially lethal.

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2020, 11:30:25 am
content: It's too complicated. Use a timer relay, bypassing a series resistor.

author: Benta
author_rank: Super Contributor
timestamp: June 27, 2020, 04:17:13 pm
content: Quote from: Zero999 on June 27, 2020, 11:30:25 amA much better idea, and it won't kill anyone.