Steve Eldridge wrote:
>
>Hello All,
>
>I am nearing completion of a new HV psu. This time, I am doing the job
>properly with due regard to safety and reliability.
>
>My question is : Is there a "tried and trusted" way of introducing a
>dead short-circuit across the supply, preferably repeatedly- apart from
>shorting with the proverbial screwdriver!
>
>I am considering making a well insulated "knife switch", with the earthy
>side to the handle (and the hinge supplemented with copper braid) behind
>a clear plastic screen. Unless there are some better suggestions....?
>
>For background information, the PSU consists of 1550 VAC rectified into
>nominally 60uF- Appx 2200V, Secondary resistance is 22 Ohms. 20x 1N5408
>Bridge and 10x 600uF 300V. Surge limiting resistor is 2x 30 Ohm TV
>dropper resistors in series. (this takes 1 kW from the mains for 15
>seconds before solder starts melting !!). This limits the short-circuit
>current to about 37A.
>
>I have studied all the litrerature available to me, but so far I have
>drawn a blank on this.
It depends why you want to do this. If it's to check that nothing blows
up if the supply is shorted, you only need to do it once or twice and it
doesn't need anything fancy.
For that purpose, you could use a length of heavy insulated wire (no
point in burning up a perfectly good screwdriver) already bonded to
chassis. Also use insulated pliers to handle it, on the principle that
several things would have to fail before putting you at risk. Steve's
comment about goggles is a good idea too - at least do it at full arm's
length.
BTW, 60uF is probably more smoothing capacitance than you need, by a
factor of 2. This is a case where more is not better. The additional
capacitance is mainly storing up energy and potential trouble if you do
have a flashover, and will stress the limiting resistor and the
valve/tube more than strictly necessary.
(Sorry it's a bit late to mention this, but sometimes we get sucked into
choosing electrolytics because they're available, rather than because
they're the right value.)
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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