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Old 1st September 2007, 06:10 AM   #1
OzMikeH is offline OzMikeH  Australia
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Default Cheap 12V ultra low impedance supply.

T-amps like battery supplies, unfortunately I don't. I don't want to replace lead acid batteries every couple of years and I don't like the idea of having a hyrogen producing container of acid in my living room. Accidentally leaving such a device switched on and ruining the battery appears to be very common.

I have an old linear regulated 8 amp 12 volt power supply already. The caps are getting tired in it but it still works well apart from the hum when loaded. There are enormous low ESR car audio capacitors (of dubious benefit) available very cheaply. I was thinking of using one of these silly capacitors across ye olde power supply with short cables to the T-amps.

I have parts on hand for a simple soft starter using a 12V lamp to limit inrush current. A relay will close to bypass the lamp after the cap is charged on a simple resistor-capacitor timer. A second pole on the same relay connects the load to the capacitor.

It's much cheaper than a battery and charger. It appears a few people have considered this path, has anyone gone ahead and built something like this?
Any thoughts or suggestions?
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Old 1st September 2007, 08:58 AM   #2
AndrewT is online now AndrewT  Scotland
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Hi,
the lamp has the wrong characteristic for a good current limiting start-up.
A negative coefficient Thermistor is much better and using your same time delayed relay to bypass the residual hot/warm resistance is even better. The time delay could be 2S to 10S depending on capacitance and resistance.

You can also use a time delayed bypassed resistor on the transformer input to reduce it's start-up current, this transformer delay could be between 200mS and 1S.
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Old 1st September 2007, 09:51 AM   #3
Stuey is offline Stuey  Australia
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Hi Mike,

I believe those large car audio capacitors (if you're referring to those 1 Farad jobs and the like) are a container full of smaller paralleled caps.

If you're not, please ignore...

Stuey
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Old 1st September 2007, 10:23 AM   #4
OzMikeH is offline OzMikeH  Australia
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That's exactly what I was talking about. I can buy a 1 farad one for less than a 7A/hr battery and charger.
It would be great if it was made from many smaller caps, I could then disassemble and use something slightly smaller than ridiculous. $ per uF it's cheaper than buying a box of capacitors.
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Old 1st September 2007, 11:20 AM   #5
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Hum when loaded? Why not fix it instead of using band-aids like capacitors on the output? It probably is the main filter capacitor that has dried up.
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Old 2nd September 2007, 12:22 AM   #6
OzMikeH is offline OzMikeH  Australia
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It almost certainly is and I probably will replace the main filter caps. The T-amps will only draw an average of 600mA together but the intention is a low noise supply that looks like a battery to the T-amps. The hum when loaded is with 7 amps.
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