A simplistic HV delay timer

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Hi there,

Following this discussion, where I proposed a simple delay circuit based on a SCR, I have now tested the circuit in reality.
See first pic for the tested circuit.

Note that I won't discuss the necessity or usefulness of such an accessory: this has already been (hotly) debated on numerous occasions.
I simply provide a simple and reliable solution for those feeling a delay is required.

With the values indicated, the resulting delay is 63 seconds, somewhat more than the 43s of the sim.
This is due to component tolerances (the timing cap is an E-cap) and trigger voltage of the actual neon bulb.

Components are pretty standard and non-critical, they just need to be rated for the voltage and current involved.
R5 limits the peak equalizing current between the caps to a safe value for the SCR, it should be a WW or similar pulse-resistant type.
If RI losses are a concern, it could be shunted by a suitable inductor.

The timing resistance is made up of three resistors: R2, 3, 4.
A single large value one could be used instead, but this arrangement is more practical, because the maximum value is under 10 Meg, and the time can be made easily adjustable by using a trimmer of reasonable value in series with R2 or R4.
Diodes D1 and D6 ensure the time is reset each time the circuit is powered off.
If they are omitted, the timing cap will discharge by itself, and the next delay will be dependent on the time the circuit has been off.
This behavior could be desirable, if the electrical time constants more or less match the thermal constants of the tubes involved.

Note that a bleeder is recommended on C1, for safety reasons: left to itself, it will take a long time to discharge to a safe level.

The neon could be replaced by a diac, or an equivalent trigger device. R2 and R4 would need to be adapted.

Note that once fired, the circuit remains completely quiet and inert: it simply drops about 1V from the HV.

Note that this timer is not very stable or accurate, but it is perfectly adequate for the application in mind.

If you build the circuit, be sure to observe elementary HV precautions.

I also include a sim, to make it work you will need the library file and symbol of the Teccor SCR's, available on the Yahoo LTspice group

R6 represents the load, and doesn't have to be physically implemented....


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  • HVdelay2.png
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The sim shows explicitly the parasitic parameters of the transformer: resistance 61Ω, leakage 28mH.
Together, they limit the inrush current to a meager 5A, easily swallowed by the 1N4007's (Ifsm=30A)


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Delay timer

Omron make 230v timer delay relays - I use one to soft start a 1Kva toroid. I bought some with bases for less than £10 from e-bay so probably not worth developing a circuit, etching a PCB, buying in all the components and building this if there is already a cheap, safe, commercial product already available....
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