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Old 31st March 2012, 12:43 PM   #1
tvrgeek is offline tvrgeek  United States
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Default Bench multi-purpose supply

Just wondering if anyone has done something like this:
I have been playing with power amp designs. What is expensive is the chassis. So I thought of some generic design for test and development. Get a big blown amp for the chassis and transformer. Mount my variac inside with a bridge and half the main caps. This gives me a variable output raw DC supply. So for the amp boards, I build a second half filter bank, star ground etc. A few switches and outlets give access to the variac. A cheap meter velcro'd to the front for a DC meter. Use the old heat sinks for the test amps externally. Any further ideas?
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Old 1st April 2012, 08:55 AM   #2
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Some form of DC current limiting , switchable earth lift , volume control , maybe DC offset protection circuit if you intend connecting speakers during testing .
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Old 1st April 2012, 01:09 PM   #3
tvrgeek is offline tvrgeek  United States
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Good idea on switchable lift. A current limiter would be handy. Need to think about that. I was going to use the "disposable" kind, but a safety relay may be really smart. Something simple like sense across the fuse and also have a sense to the amp for DC. Open the power instead of the load.

I have a resistor plate I use for testing. In a perfect world, I'd have an isolation transformer, but no OSHA around here.
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Old 1st April 2012, 01:16 PM   #4
AndrewT is offline AndrewT  Scotland
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Variac feeding a mains outlet socket, for variable Mains AC.
and feeding a small transformer. For LV variable & isolated AC.
and feeding your DC PSU. For variable dual polarity DC supplies.
Mains via bulb tester to mains socket. To test equipment wiring.

DO NOT fit an EARTH LIFT switch.

Instead consider adding a Disconnecting Network with a bypass switch.

The test box chassis must remain permanently connected to PE. No switch, never.
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regards Andrew T.

Last edited by AndrewT; 1st April 2012 at 01:19 PM.
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Old 1st April 2012, 06:10 PM   #5
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Yeah hands up left myself open to misinterpretation again

How about i rephrase it to switchable signal earth lift that only raises the earth seen by the input signal ie :- between chassis ground and star ground as page 360 in Bob Cordells book exept with a shorting switch across the resistor.

Yes never lift the mains safety ground to the chassis or worse still disconnect it .

Last edited by epicyclic; 1st April 2012 at 06:23 PM.
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Old 3rd April 2012, 12:52 AM   #6
Enzo is offline Enzo  United States
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I just got an order of caps today to rebuild my old Heathkit IP-17. it is a basic bench supply.

0-400 B+ variable
0-100 bias supply
6.3VAC and 12.6VAC for heaters.

VOltage and current meters.


I have several old electrophoresis power supplies I got cheap at the local university salvage. Variable DC supply 0-500 on some and as high as 0-1500 on others.
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Old 4th April 2012, 12:36 AM   #7
tvrgeek is offline tvrgeek  United States
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Quote:
Originally Posted by epicyclic View Post
Yeah hands up left myself open to misinterpretation again

How about i rephrase it to switchable signal earth lift that only raises the earth seen by the input signal ie :- between chassis ground and star ground as page 360 in Bob Cordells book exept with a shorting switch across the resistor.

Yes never lift the mains safety ground to the chassis or worse still disconnect it .
Actually sometimes you do, but by never more than about 5 to 10 Ohms depending on if you are having problems feeding line filter garbage back to ground and it needs to be a really big resistor to be safe. Circuit breaker blowing large. Never like lifting circuit ground by 100. I was thinking the output side anyway, not the AC mains safety. An insulated case would be recommended if you wanted to have CSA or UL bless it but they will never see my bench. Come to think of it, a GFCI breaker would be a very smart addition.
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