Using a capacitor across the energizing coil to give time delay to a relay - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: September 09, 2012, 11:54:06 pm
content: I am trying to create a soft start circuit for a HV power supply. I have a 12 volt coil, 30 amp relay that will short out the dropper resistor to the HV transformer after a certain amount of time for the HV supply capacitors to charge has elapsed, and then send full power to it. I am testing the relay energizing circuit away from the psu. Supplying the relay energizing coil, which is rated 12 volt and measures at 16.5 ohm resistance across the coil, so circa 0.7 amp current draw, off my bench power supply at 12 volts, the relay energizes almost instantly. Adding a 2,200uF 25 volt working electrolytic across the energizing coil pins seemed to make no noticeable difference at all. I then tried paralleling another 2,200uF electrolytic with the first. Still no noticeable difference. What sort of delay SHOULD these impose? My one concern is they are possibly not true relays, but rather contactors, but surely the caps should still impose a delay? Should it be a noticeable delay, or are we talking milliseconds here? I am trying to duplicate this circuit, but using a 12 volt relay and feeding it from the on board 12 volt supply for the other relays in the amp, but feeding it in a similar manner to the attached circuit is OK, too. I don't quite follow why one side of the relay in the schematic is grounded though, maybe it's because it's a US sourced circuit and their mains supply isn't live and neutral like here in the UK. I am told the schematic gives over one second delay before the relay closes. Thanks.

author: David_AVD
author_rank: Super Contributor
timestamp: September 10, 2012, 12:31:26 am
content: To get any reasonable delay, you'd usually use an R/C circuit with a driver transistor to switch the relay.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 10, 2012, 03:23:01 am
content: Henry occasionally has a "brain-fart" when they are drawing up their circuits.Logically,the other side of the relay coil should go to neutral.Note how the coil of RY701 next door to it goes to earth--I think they accidentally copied the same connection.I believe the kind of relay they used was based on the old Post Office type,but with a heavy duty microswitch operated by the actuator,instead of the normal finger type contacts.(I'm not sure if the 240v switches they use have enough current capacity,so you might need to use an ordinary P.O. relay switching a contactor or similar)These P.O. relays have (nominally) 2000 ohm coils,so t= CR =4.4s for the caps Henry use.They will give a delay both on operate,& release----empirically the release one is longer.If your 12 volt supply comes on with the same contactor closure,you can use that to drive the time delay relay ,but if it is already on,you need to switch it with the contactor.My suggestion in that case, is to ignore the existing 12v supply,& connect a small transformer with its primary switched by the contactor,then connect the time delay relay circuitry to its secondary.US 240v supplies are obtained from a transformer out on the pole,which has a 240v centre-tapped secondary.They can use it to supply two separate 120v circuits,with the centre tap as neutral (connected to earth at the power inlet to the house),or a single 240v circuit.As when they are using this type of circuit,there is no current flow in the neutral line,this is often used with a 3 conductor cord on the appliance-Hot1,Hot 2 & Earth.This is largely compatible with the Australian/NZ.UK/Europe type single phase 230V systems,except:-For safety,the US 240v circuit requires fuses in both legs,so that a short to chassis/earth will blow a fuse,no matter which side is shorted.(This is against regulations in Australia,where you should not fuse the neutral line)If any circuitry ,like noise filters in a receiver or similar,is connected between both legs of the circuit & earth,the US devices only need to be rated comfortably in excess of 120v,whereas in the single phase 230v case,only one half of the filter has substantial voltages on it,but must be rated at well in excess of 230V.Insulation between each leg & earth needs to be similarly rated.(Although neutral is normally at a similar voltage to earth,occasionally power outlets are reverse wired,so it is common practice to treat both active & neutral as "hot" when providing insulation.)

author: Chris Wilson
author_rank: Super Contributor
timestamp: September 10, 2012, 11:39:42 am
content: I have decided to go high tech and just buy a DIN mount timer relay, at least it can be used for other things and they are pretty cheap these days. I get adjustable time delays, and a wide operating voltage, with 8 amp switching to control the 30 Amp soft start resistor bridging contactor. Thanks everyone!

author: Unknown
author_rank: Unknown
timestamp: September 11, 2012, 12:06:13 pm
content: you should use a timer relay, and you get a good delay....http://www.lightcontrol.net/a4135.htmor better something like this with a base, so in future you will buy only the base and use the relay for 2 projects...http://www.lightcontrol.net/a4180.htmOr, if you want something cheap you can also consider a transistor + a 555 timer circuit, If you consider this, remember to put the freewheeling diode or you transistor just blown up...