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Standby switch for bridge rectified power supplies.
Standby switch for bridge rectified power supplies.
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Old 5th September 2007, 07:48 AM   #1
Mush is offline Mush  United States
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Default Standby switch for bridge rectified power supplies.

Hi:

I've searched through the archives and it seems the consensus is that a center tapped power transformer should have its standby switch placed between ground and the center tap.

What about diode bridge rectified supplies? They don't rely on a center tap. Should the standby switch be place between the bridge and ground? Would a switch between the output of the bridge and the first filter cap be acceptable?

I have also thought of using a voltage divider consisting of 20k and 10k resistors that could be bypassed with a dpdt switch for a standby. The tubes would then be allowed to warm up with full bias voltage applied and only 1/3 plate voltage until the switch was flipped. I wonder if this is a good idea, a bad idea or just overkill.

Thanks for all of your help.

Ciao -- Minto
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Old 5th September 2007, 01:16 PM   #2
Tom Bavis is offline Tom Bavis
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The rating of the switch will be a problem... it's not easy to find a switch good for 300VAC, let alone DC. Potter and Brumfield makes an inexpensive relay (~$10 - T92 series) with a 380VAC rating - you must switch both sides of the transformer (or connect the poles in series), NOT the output of the bridge.
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Old 5th September 2007, 01:28 PM   #3
ray_moth is offline ray_moth  Indonesia
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Yup, switch AC not DC. Obviously, if your B+ supply tranny is separate from the heater supply, your delay switch should be in the primary of the B+ tranny; however, if that's not the case, then I'd agree with Tom's suggestion.
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Old 5th September 2007, 08:47 PM   #4
Mush is offline Mush  United States
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Default Voltage divier as standby?

What are your thoughts of using a voltage divider as a standby that could be bypassed with a dpdt switch?

Thanks -- Minto
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Old 7th September 2007, 12:55 AM   #5
ray_moth is offline ray_moth  Indonesia
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A 10k/20k voltage divider is not a very useful arrangement because the amp would be in parallel with the 10k resistor, making the effective division of voltage much less than 1/3. In addition, the 10k resistor would consume more current than necessary just to lower the voltage.

A better way of using a potential divider is to use it to control a MOSFET. A MOSFET allows you to use high value resistors (say 1Meg/470k), since the MOSFET's gate doesn't draw current. Switching the tiny current in the voltage divider, even though it is DC, should be no problem. Use of a MOSFET also opens the door to other possibilities, like voltage regulation, elimination of ripple and soft start (slow ramp-up of voltage instead of sudden turn-on). There are some MOSFET ideas at this link.
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Old 7th September 2007, 05:30 AM   #6
billr is offline billr  New Zealand
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coincidences are amazing at times. I've just ordered a couple of transformers for two amps that I am building.

the secondaries are 6.3/1.5A, 200v/20ma, 10v/5A and 350v/250ma.

This is for a 813 SE amp, the idea is to use the one transformer for everything.

The mains switch will turn on the tranny, obviously,

I am going to put a dual pole switch in the 350V winding. The 350V winding is going to be used with a voltage doubler for the HT. with a secondary HT taken off the junction between the caps for the driver.

200v winding is for bias, I plan to use the powerdrive cct with the FET.

The idea is that the heaters and bias supplies will come up first, then the HT will be applied.

So If you can get a switch, or a relay with a high enough breakdown voltage, I would go for a switch in the AC secondary side.

YMMV

Bill
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Old 7th September 2007, 01:22 PM   #7
Mush is offline Mush  United States
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Default switch center tap and bridge ground

Space is very limited on the chassis I am working on and switching secondary ac lines would involve routing the wires through places where they would probably cause hum problems.

I have the option of being able to switch the unused center tap to ground while lifting the ground point of the bridge rectifier. This would in effect create a full wave in standby condition and reduce the output voltage to half. After the filaments heat up I could switch the bridge back to ground while lifting the center tap and produce full voltage. Is this acceptable?

Thanks very much for your help -- Minto
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Old 7th September 2007, 07:31 PM   #8
billr is offline billr  New Zealand
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Hello

I can't see the point of the running on half the voltages etc.

There are alternatives, get a small relay, with suitable contact ratings etc., then put a small timer on it, that will introduce any delay that you may need.

Or, mechanically connect a button on the front panel, to a push on/off switch using mechanical rods & standoffs.

Or, use the heater suppy to operate a relay that opens the secondary,.

Bill.
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Old 7th September 2007, 07:42 PM   #9
Mush is offline Mush  United States
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I've noticed that even when switching ac, there is a bit of a kick. I was imagined that going to half voltage would minimize this while preventing cathode striping because the bias voltages would be active. Perhaps I am wrong. I will look into spaces that I can fit a relay and timer.

Thanks very much -- Minto
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