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 Post subject: General Power Supply Regulation Question
PostPosted: May Mon 07, 2018 3:04 am 
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I am building a power supply to provide 38V at 5A. Here is my question: If I use a transformer rated at 40V and 15A, do I need to worry about regulation at all? My thought is that if the load varies, the current draw will also vary. Voltage normally drops due to a higher current flowing through the transformer secondary resistance. However, if the transformer is rated at 15A when the max draw will only be 5A, the transformer secondary wire gauge and its resistance will be low enough such that load fluctuations within the 5A range will not result in a voltage drop.


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Mon 07, 2018 3:20 am 
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It would help to know the nature of the load.
Regulation exists not just to prevent voltage sag but also voltage rise. Either can be harmful or destructive depending upon the components used.


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Mon 07, 2018 3:12 pm 
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Rectifying the 40 volts is going to give you a lot more than 38 volts output to begin with. Exactly how do you propose to get the voltage output at 38 volts? Unless the load current is fixed at a constant level and never varies at all, the output voltage is definitely going to change as the load does. Therefore you must have some sort of regulation built into the supply. That's fairly simple to accomplish, but isn't going to work the way you would like it to with that transformer and output voltage combination.

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Mon 07, 2018 3:33 pm 
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Steffen wrote:
I am building a power supply to provide 38V at 5A. Here is my question: If I use a transformer rated at 40V and 15A, do I need to worry about regulation at all? My thought is that if the load varies, the current draw will also vary. Voltage normally drops due to a higher current flowing through the transformer secondary resistance. However, if the transformer is rated at 15A when the max draw will only be 5A, the transformer secondary wire gauge and its resistance will be low enough such that load fluctuations within the 5A range will not result in a voltage drop.

Be careful mixing parameters and [u]ratings/u]
Parameters determine the behavior in a circuit
Ratings relate only to the part reliability and/or lifetime.

That said, there will be a correlation between the current rating of a winding and its effective source impedance. BUT---keep in mind that the source impedance is a function also of the primary.

Consider a simple DC case---a battery driving a resistive load. The "load regulation" is defined as the change in output voltage for a change in current, and is equal simply to the source resistance (The "internal resistance" in the case of a battery)

As already suggested, you can't mix AC and DC voltages. Your 40-volt transformer is speced in terms of RMS voltage. 40V RMS is ~ 56 peak. Assuming 75% rectification efficiency, you'll have ~ 42 volts DC.

You may want to consider a very simple regulator with a pass transistor and a zener diode controlling the base voltage.

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Mon 07, 2018 5:19 pm 
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The load requirements will determine whether or not you need regulation. How tight are the specifications? If you can tolerate voltage swings of a few volts then you probably don't need to worry. If on the other hand you have a requirement plus/ minus 2-3 volts or less then you will need to regulate.


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Mon 07, 2018 5:35 pm 
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Is always good to start with the requirements..:)

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Mon 07, 2018 9:12 pm 
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The "proper" (IMO) way to design a power supply is using the
Power Supply Designer II application from Duncan Amps.
It's free, and does an excellent job.

http://duncanamps.com/psud2/index.html

- Leigh

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Mon 07, 2018 10:11 pm 
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Most of the responses here are valid. When designing any power supply you must first understand your requirements from the power supply. Selecting the proper transformer is just ONE important selection where the output voltage is the critical parameter. Obviously, the transformer must be able to supply the maximum current load with a tolerable temperature rise and output voltage at maximum load at minimum line voltage.

Unfortunately, no one here mentioned the most important basic component of any power supply design which is the output filter capacitor. Your load AND the filter capacitor will determined the average DC output voltage and the output ripple voltage. Again, you must understand your requirements and your acceptable average voltage INCLUDING your maximum output ripple voltage.

Since the OP didn't mentioned any minimum output load I randomly choose 1A as the minimum load, 5A as the maximum and output filter capacitor for comparison, 1000uF and 10000uF. A picture is worth a thousand words, so here it is:

Image Image
1A & 5A load with 1000uF

Image Image
1A & 5A load with 10000uF


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Tue 15, 2018 9:41 pm 
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This is extremely helpful and very in-depth information. I was initially posing a general question, but I see that it was actually a meaningless question in the absence of some specific requirements. Hence, the validity of the response, "always start with the requirements". I will give more serious thought to my requirements, digest the excellent responses and also use the power supply design software suggested.

I think that I was oversimplifying the issue of regulation and was more concerned with voltage dip than with the flip-side - rapid rise in voltage in no-load and low-load conditions. By the way, the supply I am designing is a bench supply for the repair and also operation of a potentially wide range of equipment. I intended to use a variac transformer on the primary of a high current capacity transformer in order to offer an adjustable supply voltage. I figured that if the transformer had a hefty capacity, whatever voltage was set would not dip under a heavy load condition. However, that was an incomplete thought based upon all of your insightful responses.

In summary, let me ponder this more before wasting your time and outstanding expertise. Thank you all for your patience.


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Wed 16, 2018 1:15 am 
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Steffen wrote:
I am building a power supply to provide 38V at 5A.

DC only or AC and DC?


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Thu 17, 2018 12:35 am 
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@Steffen, it sounds like you have to do some homework. If you need a bench power supply "for the repair and also operation of a potentially wide range of equipment " it needs to be much more than a Variac, transformer a bridge rectifier and a filter capacitor.

First, to obtain a variable DC output you don't use a Variac. Any decent bench power supply should provide a Constant Voltage, Constant Current fully adjustable, high efficiency with low output ripple and noise. As far as the maximum output voltage for a bench supply, 38V is too high. To work on most electrical/electronic equipment, 28V DC @ 5A is much more than enough. The reason you want to limit your output voltage to no more than 28-30V DC output is that most PWM ICs are limited to a maximum of 30-35V DC. Your regulator/converter should be a step down PWM (Pulse Width Modulator) type which is very efficient with excellent regulation and low output ripple.

You can get on eBay or AliExpress such a complete step down PWM module (from China) for $5 to $15 and all you have to do is power it with an unregulated DC supply. The exact DC input voltage to these modules is not important as long as at no load the DC voltage doesn't exceeds the maximum input voltage of your converter and under full load the minimum voltage is at least 2-3V above the maximum regulator DC output.

If you can get by with maximum output, let say 28VDC @ 2.5A (70W), at lower output voltage for example 16V, your PWM module will be able to supply 5A output (80W). So in this case your raw DC input supply could be much smaller, only 30V @3A (90W). If you understand this power conversion concept, then the same 30V @3A raw DC input with a 10A converter will supply without any problems 5V @10A (50W) or 12V @7A (84W).


Last edited by upsss on May Thu 17, 2018 6:17 pm, edited 4 times in total.

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Thu 17, 2018 12:17 pm 
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Well, a Variac + Rectifier + Filter is a very good general-purpose solution for many things. As long as you can set the output, you very often do not need regulation. Or...to look at it a different way...your hand on the knob is the regulator.

Range of voltages for a GP supply? I'd want it to handle 32-volt "farm radios".

Did someone mention defining the requirements?...;)

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sat 19, 2018 7:00 pm 
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Understood on all counts. Also, to answer Mike's question: Just DC.

By the way, forgive a side question, but I found a high current transformer in the garbage bin after a factory was being gutted. The input terminal strip has a selection of input voltages from 220 to 265VAC. The output wires are between 3-4AWG. I input a low voltage 60Hz signal to find out the step down ratio and by calculations it will take 220VAC down to 15VAC. I am wondering if the current capability of the secondary can be approximated by looking at the wire gauge. 3-4AWG wire is rated at 70A minimum.

Why am I asking this side question? I collect parts for use in future projects and would like to be able to label this one for future use in a power supply - sort of on topic.

Thank you all for your replies. I am studying them all.


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sat 19, 2018 8:24 pm 
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http://www.antekinc.com/as-3430-300va-30v-transformer/
http://www.antekinc.com/content/AS-3430.pdf

I would use this transformer for a 35 volt 5 amp supply , the sag on a toroid is a lot less than an e-core transformer .

With a 100 ohm ballast load it should be about right .

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sat 19, 2018 9:14 pm 
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Quote:
I am wondering if the current capability of the secondary can be approximated by looking at the wire gauge. 3-4AWG wire is rated at 70A minimum.


Wire size is one factor. More important is core size/weight. Can you give core dimensions? A 200VA transformer will weigh about 5 lbs. http://www.hammondmfg.com/185.htm

Rich


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sat 19, 2018 10:36 pm 
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It is 7" long by 6" high by 4" deep including the winding bulge. It weighs about 10 lbs.


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sat 19, 2018 10:53 pm 
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You can get in the ballpark by looking at the overall size, but the size of the core is the major indicator. "core" = the lamination stack.

Example:
the Hammond 185E10, looks to be an EI-100 lamination** a bit over 1" thick. the 185G10 is an EI-125 (proportionally thicker)


**Standard lamination sizes here: http://laminationspecialties.com/produc ... roductId=1

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sun 20, 2018 8:24 am 
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Steffen wrote:
By the way, forgive a side question, but I found a high current transformer in the garbage bin after a factory was being gutted. The input terminal strip has a selection of input voltages from 220 to 265VAC. The output wires are between 3-4AWG. I input a low voltage 60Hz signal to find out the step down ratio and by calculations it will take 220VAC down to 15VAC. I am wondering if the current capability of the secondary can be approximated by looking at the wire gauge. 3-4AWG wire is rated at 70A minimum.

Why am I asking this side question? I collect parts for use in future projects and would like to be able to label this one for future use in a power supply - sort of on topic.

Thank you all for your replies. I am studying them all.

I bet this transformer is from an Arc Welding Machine which basically under a secondary short circuit condition can supply 100A and more. Unless you want to built a welder or need a boat anchor I wouldn't bother with it.

Your calculation of the secondary open circuit voltage is probably correct. To be sure, simply plug the primary to your 115V AC and measure the secondary voltage, at 230V the secondary voltage will be double of what you measured.


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sun 20, 2018 6:03 pm 
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Oh, that makes perfect sense - there is a lot of welding done in that complex of factories. Plus, that would explain better than anything the wire gauge. However, why would you say to forget about it? I would think that this would be a GREAT high current transformer for powering my 12V dynamotors for my WWII equipment, for example. (Most of my dynamotors are 28V based, but I have 12V versions too.)


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sun 20, 2018 6:12 pm 
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A welding transfomer has considerable voltage sag built into it and has very poor regulation. Once the arc is struck the voltage drops to about 1/4 of peak .

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