I looked for some break-before-make DPDT toggle switches at my local Radio
Shack today, but didn't see any specifically labeled as such. The store's
tech guy didn't seem to know what I was talking about. They had 3-position
DPDTs (on-off-on) and 2-position DPDTs (on-off). I picked up a 2-position
(part #275-666 if that means anything to you). Is there any way I can
determine whether it is break-before-make (I'm guessing that means, in this
application, that the 120V AC circuit is turned off before the capacitor
draining circuit is turned on). The RS tech told me that the switch should
be okay, but I'd rather get your opinion.
Also, since I got a couple of warnings about electrocution (which I do
appreciate, BTW), would y'all mind commenting on my plans for wiring this
switch? It has 6 pins on the back (see below), 1 and 4 are at the bottom
(off position), 2 and 5 are in the middle, 3 and 6 are at the top (on
position). When the switch is OFF, I get continuity between pins 1 and 2,
and also between pins 4 and 5. In the ON position, I get continuity between
pins 3 and 2, and also between pins 6 and 5. I was planning on wiring the
120V AC through 6 & 5, and the capacitor drain through pins 1 and 2, as
follows:
ON
6 3
5 2
4 1
OFF
Transformer:
*120 VAC (black wire) to Fuse to switch pin 6 (upper left pin, ON position)
*Transformer Lead-1 to pin 5 (middle left pin)
*Transformer Lead-2 to 120 VAC (white wire)
Capacitor Drain:
*Capacitor +lead to 100-ohm resistor to switch pin 1 (lower right pin, OFF
position)
*Switch pin 2 (middle right pin) to DC ground
I did a test run with this wiring on a prototyping board, but with 25VDC
instead of 120VAC, and it worked perfectly. Is there anything wrong with
this set-up?
Thanks for your time on this!!
-Jonathan
John Fields said:
I recently built a variable regulated power supply (see
www.meridianelectronics.ca/circ/vps.htm). It works great.
When I turn the unit off, though, I still read a voltage for 5 or 6 seconds,
and the LED stays lit for the same amount of time. I believe that happens
because the 2200uF storage capacitor is discharging over this time period,
supplying current both to the voltage regulator and to the LED.
Is there a way to short the capacitor so that it discharges more rapidly
when I turn the unit off? I think I could use a normally open push-button
switch to short the capacitor -- I would simply press it after turning the
unit off. I really don't like that idea, though. I would rather build
something into the circuit that would automatically drain the capacitor once
the power was switched off. Any ideas?
---
Yes. First off, it is definitely _not_ a good idea to short the cap with
just a switch, since the only thing limiting the current through the
switch would be the ESR of the cap and the inductance and resistance of
the circuit, all of which would be very low. Because of that the
possibility exists that you could weld the switch contacts closed and
not know it, and then the next time you switched the supply on you'd
start blowing fuses, best case.
What I'd do would be to replace the ON-OFF switch with a DPDT switch
wired to short the cap in the OFF position, like this:
ON OFF
120AC>--[FUSE]--->\ <-- -->\ <--O----+------>TO EVERYTHING ON
\----------------\ | THE INPUT SIDE
O O | OF THE REGULATOR
| | |
| | [2200µF]
[XFMR PRI] [100R] |
| | |
| | |
120ac>--------------+ +----------+------>DC GROUND
Notice that I placed the fuse _before_ the switch, which is the right
way to do it. Your schematic shows it after the switch, which will
leave the switch hot when the fuse blows. Not a good thing.
The 100 ohm resistor will be discharging the cap at about 1/2 amp if
it's charged up to 50V, but only for a short time, so something like a 1
watt resistor would be fine. I just tried it with a 100 ohm 1/2 watter
and a 3300µF cap charged up to about 40V, and I could feel the resistor
warm up a little, so even that ought to be OK, but I'd use a 1 watter
anyway.
If you decide to try this, make sure you get a break-before-make switch.
Good luck, and BE CAREFUL.