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Old 27th April 2009, 06:31 AM   #1
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Default Will this work?

Hello all.

I currently run my bench off of two 12V 7Ah lead-acid batteries, but if I'm to use my latest amplifier designs, I might need help keeping them charged. I use the batteries because I don't have a transformer that will supply the required current.

So I thought of this.

The batteries are kept charged constantly, and the trafo is protected by the current limiters. Large transients will be pulled from the batteries, with a limited amount of help from the input transformer.

How will this do?

Click the image to open in full size.

Thank you,
- keantoken
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Old 27th April 2009, 08:59 AM   #2
Elvee is offline Elvee  Belgium
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I don't see how this could work. The MOS transistors will be reversed biased by the charger, and they will be equivalent to a 12ohm resistor in series with a diode. Is this your intention?

And if the reversal is just a mistake, it won't work either: such current sources only work with depletion devices, such as jFETs.
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Old 27th April 2009, 09:12 AM   #3
AndrewT is offline AndrewT  Scotland
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a 317 can give you a 1A CCS.

Are the peaks relatively short term?
This will determine the size of the heatsink on the 317s.
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Old 28th April 2009, 07:12 AM   #4
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Quote:
Originally posted by Elvee
I don't see how this could work. The MOS transistors will be reversed biased by the charger, and they will be equivalent to a 12ohm resistor in series with a diode. Is this your intention?

And if the reversal is just a mistake, it won't work either: such current sources only work with depletion devices, such as jFETs.
I realized my mistake after posting. At any rate, the base concept is to use a CCS as a current limiter so as not to ruin the trafo.

Quote:
Originally posted by AndrewT
a 317 can give you a 1A CCS.

Are the peaks relatively short term?
This will determine the size of the heatsink on the 317s.

I have an LM7915C and an LM1815C, but no 12V regulators.

I could probably make a two-transistor CCS using 2N3055/2955, but the JFET current source is more efficient and concise (I will probably have to use the BJTs since I don't have any JFets).

So I'm assuming that since no one has pointed something out, it is okay for me to have the batteries constantly connected to power, so they'll be forever charged when I need to use them?

- keantoken
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Old 28th April 2009, 08:48 AM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally posted by keantoken


I realized my mistake after posting. At any rate, the base concept is to use a CCS as a current limiter so as not to ruin the trafo.




I have an LM7915C and an LM1815C, but no 12V regulators.

I could probably make a two-transistor CCS using 2N3055/2955, but the JFET current source is more efficient and concise (I will probably have to use the BJTs since I don't have any JFets).

So I'm assuming that since no one has pointed something out, it is okay for me to have the batteries constantly connected to power, so they'll be forever charged when I need to use them?

- keantoken
A pair of 12V 5W bulbs would probably be sufficient as current regulators. They are simpler, cheaper, and more robust than any semiconductor regulator.
You have to make sure the open circuit voltage of your transformer + rectifier cannot rise above 14V, otherwise you risk damaging your batteries in the long term.
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Old 28th April 2009, 08:54 AM   #6
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What about the impedance of the batteries? My reason for preferring the CCS variation is so that the trafo will be allowed to improve the line regulation, without high series impedance. Is this reasonable or do the batteries already have a fairly low series resistance?

Thank you,
- keantoken
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Old 28th April 2009, 10:39 AM   #7
Elvee is offline Elvee  Belgium
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Even a large transformer has a relatively large internal resistance compared to a battery.
And yours is on the smallish side.
Anyway, by definition, any current generator inherently has a high internal resistance.
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Old 5th June 2009, 04:45 PM   #8
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Quote:
Originally posted by Elvee

A pair of 12V 5W bulbs would probably be sufficient as current regulators. They are simpler, cheaper, and more robust than any semiconductor regulator.
You have to make sure the open circuit voltage of your transformer + rectifier cannot rise above 14V, otherwise you risk damaging your batteries in the long term.
Okay, next questions...

Will it damage my batteries if the voltage goes above 14V while charging, or does this limit just apply to float charge?

I think there is something wrong with my setup. At first while I was charging one battery, I found that the other battery was getting warm instead! Following all the wire, there was no logical way this could happen. Anyways, this resulted in the battery getting charged the wrong direction... I don't think it got completely discharged, but now the voltage across it goes to 15V when charging, and the lightbulb does not light up.

Thanks for your help,
- keantoken
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Old 5th June 2009, 05:41 PM   #9
star882 is offline star882  United States
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Lead acid batteries must be voltage charged.
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"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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Old 5th June 2009, 05:43 PM   #10
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Does that mean you have to use a CCS?

I am charging a 12V, 7Ah sealed lead-acid battery.

- keantoken
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