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  #1  
Old 08-05-2008, 11:36 AM
anurag anurag is offline
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Exclamation 120 Volt D.C. and 5 or 10 Amp. Variable Power Supply

Hello members of allaboutcircuit.com forum.i am in urgent requirement of design of 120 Volt D.C. and 5 or 10 Amp. Variable (0 to 120V) Power Supply. but i couldnt figure out which components i should use for this , i searched various websites but i could not get anyone fulfilling the criteria.may i use multiple circuits of lm 317(or any other power ics) in series to get this higher voltage. i already have a 0 to 64volt Variable 5 Amp dual power supply and i test it with load by connecting both the supplies in series just like a cell or transformer it works well, i communicated to the customer service of the supplier of the power supply in this regard but they told that it is harmful, and any damage will not be considered in warranty, hence i stop the practise.kindly suggest your opinionThanksanurag
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Old 08-05-2008, 03:52 PM
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Tahmid Tahmid is offline
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Hi Anurag,

I think you want a linear variable power supply with 0-120v dc and 10A power rating. It is not very difficult but very tricky. For that, you require a large transformer (2000va rating minimum), one bridge rectifier-gbpc3506(35A, 600v) with huge heatsink, 15000uF 250v(minimum) and 1uF non polar 250v(minimum) capacitors, one lm317 with heatsink, 10 nos. mjl21194 (250v 16a 200w each) npn pass transistors, with huge heatsinks, to increase the current capability, a potentiometer, some decoupling capacitors, few resistors, optional led indicators etc. You must provide appropriate fuses at the input and output.
If you require the schematic, I can provide you.
Thanks.
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Old 08-06-2008, 08:59 AM
anurag anurag is offline
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Thank you Tahmid pls give me schematic
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Old 08-09-2008, 07:07 AM
RmACK RmACK is offline
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Does it have to be a linear supply? For such high power it's going to make a lot of heat!
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Old 08-09-2008, 07:24 AM
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An LM317 would not really be suitable for such high voltage. It's limited to 40V.
A Texas Instruments TL783 goes from 1.2v up to 125V out, 800mA. Power dissipation would be very high at low output voltages. You would need a huge heatsink.

Better to have a switching or buck/boost preregulator and a linear postregulator. You'd save a lot on your air conditioning bill.
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Old 08-09-2008, 07:22 PM
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Hi Anurag,

In my mentioned circuit, LM 317 will be used only to trigger the pass transistors and to obtain the regulated voltage and hence it is safe and sound for your requirement. In LM 317 the input and output difference is 40v, not 0 to 40v. So, you can get your required voltage and amperage with the help of a LM317 and other parts as I have mentioned. I will provide you the circuit tomorrow. Linear circuit is easier to make in comparison with smps. So, you can try. You can try with TL783 also,which I did not use.
Thanks.
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Old 08-09-2008, 07:42 PM
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Quote:
Originally Posted by Tahmid View Post
Hi Anurag,

In my mentioned circuit, LM 317 will be used only to trigger the pass transistors and to obtain the regulated voltage and hence it is safe and sound for your requirement. In LM 317 the input and output difference is 40v, not 0 to 40v. So, you can get your required voltage and amperage with the help of a LM317 and other parts as I have mentioned. I will provide you the circuit tomorrow. Linear circuit is easier to make in comparison with smps. So, you can try. You can try with TL783 also,which I did not use.
Thanks.
Well, the fact is that your LM317 still has to withstand more than 120V in order to regulate. The pass transistor works more like an emitter follower, and thus doesn't prevent the LM317 from burning. It is just not safe to submit a LM317 to a voltage greater than 40V, and I guess it will burn.

Plus, you still need a huge heatsink attached to your pass transistor, since your pass transistor is doing all the work. You have a better chance on controlling the voltage by the means of pulse with modulation, because a transistor in its active region dissipates a lot more heat than a transistor used as a switch. A switched-mode power supply would be your best bet, both in terms of cost, weight, volume and dissipated heat (and, of course, efficiency).

You can see the advantages of a switched-mode power supply, when compared to a linear one, on this wikipedia article:
http://en.wikipedia.org/wiki/Switched-mode_power_supply
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Old 08-09-2008, 10:25 PM
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Hi,
I actually don't use linear power supply. I made and use smps power supply. But making SMPS power supply is not easy and hence I put the suggestion to Anurag, whom I think a newcomer in this field.
LM 317 will not be burned as it will not consume more than few miliA current, just to trigger the pass transistors and almost all the current will pass through the pass transistors bypassing LM317 and as long as input output difference is 40 volt,LM317 is safe(with minimum current).
This is a suggestion and a practical one. One can try and learn valuable lesson. Regarding heat sink- linear curcuits require huge heatsink and anybody trying with linear power supply should cope up with it.

For Anurag,
I will provide the circuit as early as possible. You can try and if you are confident, you can try smps route also. Linear power supply is bulky, weighty, big with huge heatsink etc.and costly but very reliable and also more lasting than smps.

Thanks.
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Old 08-10-2008, 07:10 PM
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Quote:
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...
LM 317 will not be burned as it will not consume more than few miliA current, just to trigger the pass transistors and almost all the current will pass through the pass transistors bypassing LM317 and as long as input output difference is 40 volt,LM317 is safe(with minimum current).
...
Not burned because of excess of current, but because of voltage excess between the input and ground pins.
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Old 08-10-2008, 09:53 PM
RmACK RmACK is offline
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Let's see his schematic before criticising too harshly, he may have a clever way to ensure that the 40V rating is not exceeded. Hear the guy out he said the schematic would be up soon...
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