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Re: [Amps] Series PC Power Supplies

Subject: Re: [Amps] Series PC Power Supplies
From: "Will Matney" <>
Date: Thu, 01 Jun 2006 18:39:46 -0400
List-post: <>
I want to correct something I said;

"The same diode arrangement can be used here, but balancing resistors can be 
used with them"

This would not be the same diode arrangement. In series, the diodes are across 
the + and - terminals. In paralell, the diodes are wired in paralell. A low 
value resistor can be placed in series with each diode in paralell also.



*********** REPLY SEPARATOR  ***********

On 6/1/06 at 5:39 PM Will Matney wrote:

>By balancing, I don't mean balancing the current or voltage, but balancing
>the system so that it does not fail catastrophically because of the
>failure of one supply. 
>"It's preferable to use identical power supplies but is not a requisite in
>a series cascade supply. If dissimilar power supplies are connected in
>series, the maximum available current will be that of the supply with the
>lowest current rating. The supply with the lowest isolation voltage should
>be placed closest to ground. The diodes conduct the reverse current that
>could result if that particular power supply was not turned on with the
>other supplies in the cascade. The diodes protect the filter capacitors
>and the series pass transistors. The AC line power should be applied to
>each simultaneously". These quoted sentances can be found in the book
>"Regulated Power Supplies" by Irving Gottlieb.
>However, in series connected supplies, connecting different regulator
>circuits in series can still vary by what would seem a small amount from
>one another. Each regulator set in each supply can lead or lag the others
>when they respond to the same current change in time. This is because they
>are each individually controlled by their own regulator circuits. This
>lead and lag in responce times can cause current spikes in the individual
>supplies. They do make some supplies which their regulation circuits can
>be tied together to try and keep the responce times the same, but still it
>does not work as good as it should. In paralelled supplies, this can be
>worse to where one supply could be carrying a good bit of the load out of
>the set. The same diode arrangement can be used here, but balancing
>resistors can be used with them. Most supplies with multiple pass
>transistors have individual balancing resistors around 0.1 to 0.5 ohms
>each so one transistor will not be drawing more curren
> t than the rest, or not be as dramatic. However, when paralelling
>supplies this way, dont count on having any close regulation.
>*********** REPLY SEPARATOR  ***********
>On 6/1/06 at 4:49 PM Jim Tonne wrote:
>>Good explanation of why and how! Thanks.
>>However there seems nothing to "balance"!
>>- Jim WB6BLD
>>----- Original Message ----- 
>>From: "Peter" <>
>>To: "Jim Tonne" <>
>>Cc: <>; <>
>>Sent: Thursday, June 01, 2006 4:36 PM
>>Subject: Re: [Amps] Series PC Power Supplies
>>> PSU manufacturers recommend adding these diodes when placing units in 
>>> series. Major reason is that should one unit shut down and not be 
>>> generating power the output rectifier and filter network becomes
>>> driven by the remaining unit(s)
>>> Peter G3YYN
>>> Jim Tonne wrote:
>>>> Will:
>>>> If the supplies are in series wouldn't each be passing
>>>> the same number of amps?  What is there to "balance"?
>>>> What do you mean by "placing rectifiers across..." each
>>>> supply?
>>>> - Jim WB6BLD
>>>> ----- Original Message ----- 
>>>> From: "Will Matney" <>
>>>> To: <>
>>>> Sent: Thursday, June 01, 2006 3:07 PM
>>>> Subject: Re: [Amps] Series PC Power Supplies
>>>>> Another recommended practice is to place rectifiers across the + and
>>>>> of each supply to try and balance the load. If the load is not
>>>>> across each, one supply can be overburdened if just slightly off from 
>>>>> the others. The rectifiers need to be rated at the same current 
>>>>> capability of the total supply in a series connection. The low side 
>>>>> supplies ground will be the supply ground and the ground for the last 
>>>>> rectifier in the string. This is shown in several power supply books 
>>>>> that mentions seriesing and paralelling supplies for increased
>>>>> or current. Generally though, this practice of using several supplies
>>>>> not really recommended because of the balancing problems.
>>>>> Best,
>>>>> Will
>>>>> *********** REPLY SEPARATOR  ***********
>>>>> On 6/1/06 at 11:57 AM Joe Isabella wrote:
>>>>>> Two things...
>>>>>> 1. You can get a brand new 25A switching supply for $80-90 if you
>>>>>> around.  I bought my Samlex 25A for $80.
>>>>>> 2. How did you series the supplies you have?  The "first" one will 
>>>>>> provide
>>>>>> your "ground" connection.  Take the red wires from that and connect 
>>>>>> them
>>>>>> to the black wires of the "second" supply.  Now take the red wires
>>>>>> that one, and connect them to the black wires of the "third" supply, 
>>>>>> which
>>>>>> will provide the positive (15 VDC from the black wires of the "first"
>>>>>> supply to the red wires of the "third" supply, in this case) to your 
>>>>>> rig.
>>>>>> Make sure you tied the orange wires to each supply's red wires so
>>>>>> all
>>>>>> see "power good" line properly.  Make sure you have isolated the
>>>>>> leads from chassis ground on the second & third supplies as as the 
>>>>>> article
>>>>>> says since they need to be floating (otherwise, the 5V from the first
>>>>>> second supplies will be going straight to ground).  If you didn't 
>>>>>> isolate
>>>>>> them, you'll only see the 5V from the "third" supply...
>>>>>> Joe, N3JI
>>>>>> ----- Original Message ----
>>>>>> From: aborg <>
>>>>>> To: Amps Amps <>
>>>>>> Sent: Thursday, June 1, 2006 9:09:38 AM
>>>>>> Subject: [Amps] Series PC Power Supplies
>>>>>> Hello Amp'ers,
>>>>>> Quick technical question. I'm embarrassed that I
>>>>>> can't figure this out, but not quite a super tech yet.
>>>>>> Here it is. I am attempting to series 3 pc pwr
>>>>>> supplies. I can't seemed figure why I'm only seeing 5+
>>>>>> volts. The link to the article is below. Here is an
>>>>>> excerpt from the article. "Our second means of
>>>>>> utilizing this low cost power is to connect the +5 VDC
>>>>>> outputs of three separate supplies together in series.
>>>>>> This will provide you with +15 VDC at 25 amps on up to
>>>>>> the maximum rating of the supplies you have acquired."
>>>>>> Can someone be kind enough to relieve from my
>>>>>> embarassment ?
>>>>>> Thanks
>>>>>> Mike-KK4MS
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