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 Post subject: More Amperge?
PostPosted: 23 Feb 2018, 09:05 
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Joined: 23 Feb 2018, 08:59
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This is my first post here but I figured this would be a good place to start!

I have a power supply that has 8 regulated 9v dc outputs rated at 100ma each. I need more amperage per output. The schematic looks pretty simple. AC from the transformer to a rectifier, some capacitors and a transistor. LM78L09ACZ. This transistor is rated at 9v 100ma. I would assume that is where the 100ma limit is coming from. Would it be possible to replace the transistor with another one with a higher amperage? Like the NJM78M09FA-ND which is rate 9v 500ma. Any ideas? See attached schematic for reference.


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 Post subject: Re: More Amperge?
PostPosted: 23 Feb 2018, 19:58 
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Joined: 06 Apr 2009, 10:08
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That's not a transistor; it's a voltage regulator. The LM78Lxx series are fixed at 100mA output current. See here: http://www.ti.com/product/LM78L.

You need two things, a bigger power transformer, and a voltage regulator rated for higher loads. You might also need higher rated diode bridges. Your best bet is a more modern adjustable regulator and resistors to set the output voltages.

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 Post subject: Re: More Amperge?
PostPosted: 27 Feb 2018, 05:11 
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Joined: 23 Feb 2017, 02:02
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I would recomend a lm7809 with resonable heat sink for higher current outputs.


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 Post subject: Re: More Amperge?
PostPosted: 27 Feb 2018, 05:39 
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Thanks for the advice! A bigger transformer? I don't know exactly what's in there now. It has some sort of donut shaped torrodial transformer. And the diode rectifiers are all rated at 1amp. Would I need different diodes if I'm only trying to get 500ma?

And I do have some 7809s.


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 Post subject: Re: More Amperge?
PostPosted: 27 Feb 2018, 20:24 
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dafortier wrote:
Would I need different diodes if I'm only trying to get 500ma?
Yes. Your diodes need to be rated higher. For a 1/2A dc with 470µf on the bridge output, the conduction angle will be small and the current spike large. I would estimate that you should probably have 3A to 5A rated diodes.

Also, in the capacitively loaded full wave bridge configuration, you can only draw about 60% of the rated current off the transformer. So if the transformer is rated for 1A AC, you can only draw about 600mA of DC current before you overload the transformer. So for four 500mA DC channels you need a total of 2A of DC current. For one transformer to drive all four it needs to have an AC current rating of 2A/0.6 ≈ 3.34A.

The real issue here is that you haven't clearly stated with what you want to end up.

    Do you still want four channels?
    Do all four need to provide 500mA of current?
    Does one transformer drive all four channels?
    How is the rest of the supply wired?

All these things matter. And we haven't even discussed the thermal issues with the voltage regulators.

I suggest you think a little about exactly what you want to do and then see if you can articulate your desires more clearly. That will make it easier for us to help you out.

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