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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 17:39:19 -0400
List-post: <>

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 

"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 

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 reverse 
>> 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 voltage 
>>>> 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 look
>>>>> 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 black
>>>>> 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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