Breadboard Power Supply

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Re: Breadboard Power Supply

Postby DrNomis » 18 Jan 2011, 03:10

jstbrowsin wrote:Thanks Pokey Pete and Dr Nomis for your input. Most appreciated.
I guess I should have been more specific with my info, the caps that blow are the C1 & C2 and they are rated at 63V, in turn they also blow the 2N3055 T03 transistors that have a huge heatsink. The LM317's are T package and there are no heat sink's attached :oops: but I can remedy this 8) . The filter caps are rated at 75V so I think they should be safe too eh.

Ok so going on your advice I change the LM317T in the neg supply to LM337T and add heat sinks.
Would it also be advisable to double the size of resistance (R1-R2) to slow the current and save the transistors and everything else?

DrNomis wrote:I also noticed that,if the two outputs ate connected to obtain positive and negative supplies,the ground connection will be floating..... :)

Pray do tell how can I remedy this please?



Thanks again for all you help guys :)



75V Caps will be fine as the filter caps,I tend to over-rate the components for the job when I design circuits,makes them more reliable..... :)


The best way to remedy the floating-earth issue,is to use a single Bridge-Rectifier circuit,you have a power transformer in your schematic that has a centre tap,connect this centre tap to ground,as in the schematic I posted,and use the other two terminals to feed the Bridge-Rectifier,the transformer Centre-Tap also becomes the ground reference for the rest of the power supply circuit,note that you then have to use the LM317/337T combination to regulate the supply rails,not two LM317Ts,the LM317/337T Adjustable regulators are rated for a maximum of about 1.5A,so to "boost" the current output capability of the power supply,you can use a 2N3055 and an MJ2955 connected as a series-pass transistor,these transistors are in TO-3 packages and are rated for a maximum collector current of 15A I believe,I'll post a schematic showing how to connect them as series-pass transistors..... :)
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Re: Breadboard Power Supply

Postby DrNomis » 18 Jan 2011, 03:43

Here's the schematic for adding series-pass current boosting transistors,the only downside is that you have to be very careful not to short either the positive or negative outputs directly to ground,otherwise the two series-pass transistors wil self-destruct at the speed of light...ie,instantly.... :)

I must work out some way to incorporate short-circuit protection.... :)
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Re: Breadboard Power Supply

Postby jstbrowsin » 18 Jan 2011, 04:33

Thanks again Dr Nomis for sorting out the floating ground. :D

I do appreciate your design schematic for adding "series-pass current boosting transistors" however is this really necessary for me as 15A seems way to excessive for a breadboard to me anyhow. And am I seeing clearly it looks like there are 3.3Ω 15W resistors on the input of the regulators, or should they really be 5W?


So my list now is as follows:
Change the LM317T in the neg supply to LM337T.
Add heat sinks.
Remove one of the recitfiers and put centre tap in ground position as per schem.

Have I got everything right here? Do I need to do anything else to stabilize the circuit? Am I good to go?

thanks again guys :applause:
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Re: Breadboard Power Supply

Postby DrNomis » 18 Jan 2011, 04:48

jstbrowsin wrote:Thanks again Dr Nomis for sorting out the floating ground. :D

I do appreciate your design schematic for adding "series-pass current boosting transistors" however is this really necessary for me as 15A seems way to excessive for a breadboard to me anyhow. And am I seeing clearly it looks like there are 3.3Ω 15W resistors on the input of the regulators, or should they really be 5W?


So my list now is as follows:
Change the LM317T in the neg supply to LM337T.
Add heat sinks.
Remove one of the recitfiers and put centre tap in ground position as per schem.

Have I got everything right here? Do I need to do anything else to stabilize the circuit? Am I good to go?

thanks again guys :applause:



Yeah,I agree,15A is way too excessive for a breadboard power supply,but,the 15A rating of the series-pass transistors means that you can use a 3 A power transformer and not blow the regulators,I would still include them if I was using a 500mA power transformer too,I was designing for reliability,since the regulators aren't over-dissipating,and the last thing you want is the power supply going faulty just as you're testing a circuit out.... :)


Those 3.3 Ohm resistors are 5 Watt ceramic wire-wound types,they are probably a bit over-rated for the job,but I decided to use them anyway.... :)

Apart from that,your list looks all good to go,feel free to download the schematics I posted,and use them for your power supply,hope it all goes well,any problems feel free to let me know.... :)
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Re: Breadboard Power Supply

Postby jstbrowsin » 18 Jan 2011, 08:11

Ah that makes more sense cheers mate. . .

so I should change my transistors? I have 2 in parallel for each supply oops I guess I should have mentioned that before.
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Re: Breadboard Power Supply

Postby DrNomis » 19 Jan 2011, 07:04

jstbrowsin wrote:Ah that makes more sense cheers mate. . .

so I should change my transistors? I have 2 in parallel for each supply oops I guess I should have mentioned that before.



You really only need 1 MJ2955 for the positive supply rail,and 1 2N3055 for the negative power supply rail,as per the schematic,one of each is ample enough.... :)


The 2N3055 is an NPN Si Bipolar Power Transistor,and the MJ2955 is a PNP Si Bipolar Power Transistors with the same specs as the 2N3055,except that it is a PNP device,of course.... :)
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