Power supply ground clarification.

[Ken_Shea]

In searching the forums for help on the possibility of hooking two PS together, there is the mention of;

1) Another thing to watch out for with separate supplies is if the commons are at ground potential

2) The fact is if you are working with supplies that have isolated grounds you can do it

The terms in bold are what I need clarification on and how can I determine what my PS have ?


Thanks
Ken

[Al_The_Man]

Simply measure continuity between the black wire in the HD connectors (power off) and the case of the supply, you should have very high resistance (meg-ohms) most are isolated here and the ground connection is made by at least on ground securing screw on the mother board.
Al.

[Ken_Shea]

AL,
I am getting no reading at all, and best I can see no ground connection on the MB.

http://cgi.ebay.com/Power-One-HD48-7...QQcmdZViewItem



Ken

[Al_The_Man]

I though you meant PC power supplies as some have done, these linear supplies do NOT usually come with ground connected and it is done at the discretion of the user.
Also I assumed you meant in series, I am not sure I would want to connect this type in parallel, as they each have electronic regulation circuitry, and some have over-voltage crow-bar, This is a SCR placed across the output, and if the voltage goes up by reason of PS defect or large back EMF the SCR fires, placing a high load on the DC output and causes the PS to shut down untill powered down and up again.
Some thing to look out for in Linear supplies of this type.
Al.

[Ken_Shea]

Thanks for the information Al.

Ken

[hixfixit]

Running power supplies in parallel is not recommended at all for a situation where there current required is higher than the individual capacity of the power supplies. Some very expensive power supplies are actually designed for load sharing. Most are not.

If one power supply has a higher voltage than the other one (the difference does not need to be great), it will do all/most of the work, causing more strain on it and leading to earlier failure. Then you will have a situation where you have one power supply left trying to do work it was never intended to, which will cause overload protection to trip, power supply death or in the worst case scenario, a fire.

When you use diodes, that's normally for setting up an 'OR' type configuration. In other words, one fails, and the other one kicks in transparently, thus you have redundancy. It can also be used to protect one power supply from the other in the event of one failing in such a way that it causes current to flow into the working one, thus destroying both. It goes without saying that the diodes should be of sufficient rating to handle the current draw.

In summary, we should NOT recommend to dealers that they do this unless it's for redundancy. They should get a bigger power supply....end of story. In the event that they decide to parallel them anyway contrary to our advice, they don't blow up when you parallel them without diodes in place and are very unlikely to. The diodes however, should be in place nevertheless for some kind of protection in the event of failure of one of the power supplies.

[gar]

071015-0852 EST USA

Ken:

The power supply you referenced lists the input voltage as 100 - 240. My guess is that is an incorrect statement. More likely it should read
120 xor 240 +4%/-17%. I am guessing it should tolerate 125 V as a max.

Most commerical linear regulated supplies run very hot at maximum input voltage with maximum output load current. Basically marginal design. For good reliability you need to derate the output current rating. If enclosed it is worse.

.

[Ken_Shea]

hixfixit,
Thanks for the additional information. these have an adjustable voltage up about 6V and down about 10V so they could be balanced if that was the only issues I guess.

Gar,
You are absolutely correct on both counts, the voltage and heat. A decal on the PS indicates a either or voltage and also indicates that a 15cfm air-flow is required if used at full load.


I have decide to build a single PS, was hoping to avoid this as the more I read the more confused I get on what specific components are needed.

Thanks
Ken

[gar]

071015-0739 EST USA

Ken:

If all you want is a power supply to supply motors, either stepper or servo, then generally there is no particular need for a regulated supply.

What is the power supply for and what voltage, current, and regulation do you need?

.

[Ken_Shea]

Gar,

The voltage and current information I have is:
1) Servos are 36VDC
2) Maxium continuous current is 3amp
3) Power delivered to load is 86 watts
4) Was wanting to build a PS that will handle 3 of these servos at maximum load with some reserve.


Ken

[gar]

071015-0811 EST USA

Ken:

In all likelyhood you will not need a continuous steady state 9 A. But it may be useful to design for 12 A as your full continuous duty load. This will provide lots of margin.

The bigger question is what maximum voltage your servo drivers can tolerate and what minimum voltage can you work with.

Almost any power supply can be approximated as a constant voltage source, Vs, with a series internal resistance of Rs. Then your output voltage will be
Vo = Vs - (Il * Rs) where the load current is Il.

If you assume a 10% internal impedance, then your no load to full load output voltage will change by 10%.

.

[Ken_Shea]

gar,
12amp sounds good.

The drivers are Rutex 2010 rated at 100V and 20 amp.

"what minimum voltage can you work with"
Not sure what you need to know here?


Ken