See schematic below

This circuit is basically a coin gun (except for a different application). It uses a booster module to generate 300V DC which is used to charge up a capacitor. When a button is pressed on a remote control unit, it generates a output on a receiver module (not shown in the schematic), which is used to trigger a thyristor which dumps the charge through a coil. The capacitor then charges up again.

A problem with an earlier iteration (discussed in a previous thread) was that when the current flowed through the coil, the currents and/or voltages were so high that they destroyed the booster module. So to deal with this I put a NC relay on the input line. Now, when the button is pressed, instead of directly triggering the SCR, the signal goes to a ATTINY85, which first puts out a pulse that turns on the relay (isolating the booster module from the rest of the circuit), then another pulse to trigger the SCR.

Which works. However, because I need the operation to be near-silent, I have used a small reed relay (which can be driven directly from the ATTINY). However the relay contacts have a low current rating, and when the capacitor is charging the current inrush can be briefly 4A (possibly higher, momentarily) and this fuses the contacts together. Using a more rugged relay is not an option (because of the noise), so it seems to me I have two options:

1) Limit the inrush current with a thermistor. But I'm not quite sure how these work. From my reading it looks as though if I want to limit the current to 1A, at 300V, I need a 300 ohm thermistor, but most seem to be lower than that. Also I don't know how long they would take to reset (I need mine to reset within a few seconds). Is this a sensible way to go?

2) Use a non-physical switch instead of a relay. I would think a P channel mosfet would do the trick, but do I need one with a D-S rating of 4A and 300V? These seem rare

 

Hi rj,
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• Amateur/homemade rail guns and other high-energy projectile devices

 

Hi rj,
This Thread may fall into the listed Banned Topics.
Moderation.


Restricted topics. The following topics are considered "out of bounds" and will result in your thread being closed without question:

• Any kind of over-unity devices and systems
• Devices designed to electrocute or shock another person
• Jammers intended for actual use in other than legal academic contexts
• Amateur/homemade rail guns and other high-energy projectile devices

I don't think this falls under this restriction. I am not making a high energy projectile device, but a toy that tosses a coin

 

If it's "just tossing a Coin",
then there's no need for ~300-Volts, and there's no need for "silent-operation".

A simple Solenoid and a FET will do the job just fine.
And no setup will be "silent", ( less than the sound of a Relay closing ).
.
.
.

 

Ok let me specify the requirement more explicitly. It has to toss a coin silently or very nearly so. As if by magic.

I know about solenoids thank you

 

Can you momentarily turn off the booster module when the device is triggered?

 

Well the relay disconnects the booster module when the device is triggered. The trouble is not at the trigger point, but when it charges. Momentarily, there is a huge current surge into the capacitor, and that welds the relay contacts together. So I need to limit that inrush current.

 

Well the relay disconnects the booster module when the device is triggered. The trouble is not at the trigger point, but when it charges. Momentarily, there is a huge current surge into the capacitor, and that welds the relay contacts together. So I need to limit that inrush current.

Not my question.
I'm trying to look at ways to eliminate the relay.

You said the boost circuit was destroyed when the SCR was fired, so can you add a circuit to disconnect the power to the boost circuit when fired, which should eliminate the failure.

Or just putting a small resistor in series with the boost circuit output may also keep it from failing.
How fast does C1 need to recharge after a firing?

 

Ah, sorry I get you.
Are you sure the booster module would be safe if it is disconnected from its power supply (8 AA batteries) but left connected to the downstream circuitry?
At the moment it charges to full-enough in about 10 seconds. I wouldn't want it to be much more than that. When you say "small resistor", how small do you think?

 

Are you sure the booster module would be safe if it is disconnected from its power supply (8 AA batteries) but left connected to the downstream circuitry?

Not sure.
I'm assuming the momentary short on the output is the problem, but it could be something else.

At the moment it charges to full-enough in about 10 seconds. I wouldn't want it to be much more than that. When you say "small resistor", how small do you think?

The capacitor will charge to 63% in one time-constant (R x C), to 86% in two time-constants, and to 95% in three time-constants.
So for 10s time to 95% charge, the time-constant should be 3.3s, giving a series resistor value of 3.3s / 400µF = 8.2kΩ.
That would limit the peak charge current to 300V / 8.2k = 37mA.

You might also put a diode from the booster module output to ground (before the resistor), to clamp any reverse voltages directly at the output.