Re: Sorta OT: Adding soft-start to table saw. I found a better E-Mail address for a technical question on the Soft Start for Power Tools device and got a very nice, thorough response. I thought I'd share it here in case any one is interested. ***** Hello Phil, This question has been asked before and we have had. TPS61175 3-A High-Voltage Boost Converter With Soft-Start and Programmable Switching Frequency datasheet (Rev. D) TPS61175PWP Errata.

I'm modernising an old bench saw, and I want to replace the switch it has with a safer one, so that the tool will not start if the power is disconnected and then reconnected. The usual way to do this with these sort of tools is a magnetic switch, basically a relay that has the coil connected to the output. The disadvantage of mechanical relays is that they can be accidentally triggered by a whack with a bit of lumber, and the cost. Industrial safety switches are too expensive for my home workshop. So I figured out this circuit using. Power is off until the start button is pressed, which then 'closes' the output side of the SSR.

This energises the input side when the start button is released, so that it will latch on until the stop button is pressed. If power is interupted the SSR will switch off so that the machine won't start again until the user pushes the start switch. – Schematic created using Just mentally replace the mechanical relay with an AC-AC SSR in the circuit above. My question is: will running the output from the SSR back to the input work? It would on a mechanical relay, but I don't know if an SSR will work exactly the same. Also, will I need a varistor on this kind of SSR, or is one built in? EDIT So this is the new circuit I propose.

It has the start and stop buttons controlling the relay from a low voltage source, which has the benefit that I don't need to run mains wires around the cast-iron body of the saw to the switches, and I can add an indicator lamp to show that the power is on. Do NOT use Fotek solid-state relays from eBay or Amazon. All of them are counterfeit. See this Underwriter's Laboratories warning notice: Not one picture of a Fotek relay on eBay or Amazon matches UL's picture of an approved relay.

Most of the fakes fail at much lower current levels than their markings would indicate, and sometimes they fail in the ON state. The proper answer is a motor starter with contactor, but those are expensive. Consider something like this 30 amp 'Relay in a Box' unit: These come in an electrical box with UL approval, which is good around a saw where you need to protect electrical parts from sawdust.

I know there are soft start modules available, but i want to build my own. I googled around for schematics, but no real luck. I know of the series resistor/relay type of soft start, I'm not looking for that. I want to have a module that you can plug a 1. Rockstar Hindi Movie Songs Free Download Zip more. 3kW electric tool/motor into.

Because I want to keep the tool's power switch and control, but get the soft start every time. I made some back-of-the-envelope calculations for a permanent series resistor to limit the peak current. It doesn't look too good - too much power dissipated in normal use(after start), too much voltage drop. Considered using a series PTC, but I guess after the first start, it would get very hot, and the second start would be hard, on full power. Consider the tool an angle grinder, so you have an idea of the usage cycle.

As far as I can understand, the initial current surge is because the coil of the electric motor acts initially as a resistor, before there is a magnetic field to limit the current through it. So trying to limit the current peak by adding a series inductor will most likely do nothing. I'm trying to avoid complicated current sensing, time controlled thyristor circuit.

Or at least is seems complicated now. The inrush current is due to the lack of back EMF. An induction motor is really a transformer with a moving secondary. With the rotor stationary, you only see the resistance and the leakage inductance, which are both small. Once it spins up you have the back EMF generated by the rotating secondary. A permanently inline inductor would indeed limit the current, but would also limit the torque, so I wouldn't really recommend it.

A proper soft start should, as the name implies, only work when starting, and do basically nothing while running. For that you need a switching element. Either simple passive switch like an NTC (not a PTC!), a resistor/inductor + relay, or (the best solution), a triac with a controller that gradually increases the conduction angle. The simple solutions might be fine for something like a power transformer, but for your application I think you need the more intelligent controller. I am not an expert on motor control by any means, but I don't think it needs to be that complicated. You are driving a normal AC motor, not a BLDC motor, and the motor is capable of handling the full inrush.

Also, you will presumably always start the motor at no load, so you don't have to handle widely varying conditions. I don't see why you couldn't get away with a circuit that just detects startup and increases the conduction angle to 100% smoothly over a fixed number of line cycles. You might need to tweak them minimum conduction angle and the ramp-up time to get optimal behavior for your tool, but other than sensing start/stop, you don't need to do any fancy sensing, and even if you don't have the 'optimal' ramp-up, you know the motor can take it. Again, don't take my word for it: do some research on soft-start motor control or hope one of the guys here who knows more about power electronics chimes in -- but this seems pretty straightforward to me. Obviously all the normal considerations about not working on mains voltages unless you know what you are doing apply. Thank you for your replies.

OK, seems I have to roll up my sleeves, build a circuit and pop some fuses in the process. I found a schematic for variable speed. So how do I modify it into a soft start? So far I get this: on every half cycle, C1 gets charged faster or slower depending on R1 value. When the voltage on C1 is greater than ~30V, the diac breaks down, and conducts a current into the gate of the triac, witch remains on until the next zero crossing.

What I don't get: - why use the diac and not directly connect the triac to R1-C1? - R2-C2 is supposed to protect the triac from fast voltage rise when in turns off.

Is this necessary only when driving inductive loads? Now for modifying it to serve my soft start purpose: How about a diode in series with a fixed high enough value of R1. This should allow C1 to slowly charge, but never discharge, therefore increasing the voltage across it, and thus the conduction angle to 100%. Then maybe a 10Mohm across C1 to discharge it when the circuit is off. OK, answers: - the diac is there to prevent premature ignition of the triac.

Especially if the triac is what is called a 'sensitive gate' one, the triggering current may be miniscule. The diac holds the current off until the RC circuit voltage is above the diac breakdown, and this creates a well defined ignition threshold for the triac. - The second RC circuit across the triac is there to protect from excessive dv/dt across the main terminals. The reason is this: if the voltage across the main terminals rises very fast, the gate in some cases cannot float/follow at the same rate and is 'left behind'. This is equivalent to a gate voltage pulse and will cause spurious triggering. You definitely don't want that.

You should check from the device datasheet what the value is for max dv/dt and design accordingly. I am afraid your control scheme is too simple and won't work.

Ash Maurya Running Lean Pdf Writer. You see, a triac is a kind of SCR or thyristor, and with these devices the voltage control is based on the phase angle of mains voltage. This in turn means that you need to generate the firing signal for each half-wave individually. In your scheme there will initially be no conduction until the diac threshold is achieved. Then the triac will fire. However, the control voltage in the cap will maintain its level (and in fact rise further) so when the ongoing half-wave ends and the next begins, the diac is already conducting, firing the triac at 0 phase angle i.e. Full voltage.

This will continue indefinitely. So, what you have created is not a soft start, but a delayed hard start.

Referring to your circuit, the proper way of course is to decrease the value of the adjustable resistor until full voltage is achieved. The cap must discharge and recharge for each mains half-sine wave separately and this creates the relative complexity of the soft starter circuits.

If you google some commercial implementations e.g. From my employer back in the day, ABB, you will see that they are 'proper' gadgets, not just a couple of components soldered together. You will of course manage with less, subject to your specific requirements, but there will be some designing to do.