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Old 3rd March 2013, 12:43 AM   #11
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Could you please define cheap?

A power supply that is capable of adjust its output voltage from 19V to 27V dc providing 20 amps before current limit kicks in, for 20 usd plus shipping is cheap enough?

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Old 3rd March 2013, 01:13 AM   #12
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well, i really cant change the smps, was hoping to drop from 24 to 21 for a few dollars more
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Old 3rd March 2013, 02:43 AM   #13
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Sorry that I can't help any further, I am not aware of a regulated (either linear or switching) power supply solution that can handle 20 amps and cost few dollars.

May I ask what chip are you using?
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Old 3rd March 2013, 05:15 AM   #14
MarianB is offline MarianB  Romania
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If regulated 21V is mandatory and modifying the psu is not an option than the only thing left to do is use another regulator following the psu, but either linear or switching will not be cheap cus of the current needed... still i think switching would be the better option ( and somewhat cheaper than linear ), a buck regulator could be used but it needs about 3-4V Vin-Vout difference to be sure of good regulation so you must make sure the power supply unit can handle the 20A current without voltage drop.

PS: Be more speciffic, what cind of load are you planning to power up with 21V and 20A, with abstract postings u cannot hope to get real help...
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Old 3rd March 2013, 09:29 AM   #15
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I am doing this for a friend, need to check with him to see how much details i can reveal here due to the possibility of this going commercial.
Regulated is not mandatory, but good to have. If there is a larger drop in voltage at full load, i need to know that so that the circuit can be tweaked to handle that situation. Would not like to spend too much, but if it cant be avoided then so be it. Looks like its going to be a tradeoff between regulation and cost.
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Old 3rd March 2013, 10:16 AM   #16
MarianB is offline MarianB  Romania
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Any solution that will reduce the voltage from that 24 you sayd it is available to 21 means regulation, so that will not be a problem, but what is the problem is the current needed, 20A i by me too high for linear solutions cus overload and shorted output protection is mandatory at this current and the latter would imply insane amount of power dissipated on the series pass regulator. The obvious solution i think has to be a switching regulator, but even though it would cost less than linear one, still it will not be cheap, so the tradeoff will have to be the production cost, but not to worry it should not mean it would get one broke...

Now, what you have to provide us with is a test made on that power supply unit giving 24V ( or something like that ) with a resistive load of 20A, you have to make sure that the voltage will not drop under 24V at that current, as i understud it could be tweaked to give higher voltage, if possible than make it so, take is to about 25 or 26V and see how it handles the 20A current, only then can we know if a buck regulator can be possible ( it is the simples and cheapest i think ), otherwise some transformer solution will be needed.
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Old 3rd March 2013, 10:50 AM   #17
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I gave you several viable solutions at Post #5.
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Old 3rd March 2013, 10:58 AM   #18
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by KatieandDad View Post
I gave you several viable solutions at Post #5.
Quote:
Originally Posted by KatieandDad View Post
1. If the SMPS uses a voltage divider and feeds back a sample of the output voltage to regulate its output, all you will need to do is modify the voltage divider...
Modifying the PSu is not an option, it hasd already been sayd.

Quote:
Originally Posted by KatieandDad View Post
2. You could use a few diodes in series with the output...
3V drop needed times 20A equals 60W of power dissipation on the diodes...

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Originally Posted by KatieandDad View Post
3. If the current draw is constant you could simply use a resistor...
Same as above...

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Originally Posted by KatieandDad View Post
4. You could use a simple zener with a transistor as a simple regulator....
Again too much power wasted, and at this high current there is the need of protection from overload, it would need many paralelled power transistors to handle the power.
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Old 3rd March 2013, 10:59 AM   #19
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by KatieandDad View Post
......................
2. You could use a few diodes in series with the output................
Quote:
Originally Posted by KatieandDad View Post
I gave you several viable solutions at Post #5.
I like the diodes solution.
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Old 3rd March 2013, 11:21 AM   #20
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The power supply voltage will drop few millivolts at 20A, from my experience with other meanwell models, they always exceed their specs.

Is this double regulation part of the final product? or only for testing job?
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