Blocking Diodes

[dereckbc]

This is a common application for critical equipment like plant control systems that require reliable power or for UPS logic power supplies.

Bob I am familiar with that application as I work in the telecom industry and everything has dual DC input. Difference is the equipment is built for dual inputs, and the steering diodes are built into the equipment and not something you have to add.

When I heard the term Blocking Diode got me thinking solar PV

 

[hurk27]

If you have load-share capable power supplies, and then use steering diodes on the outputs, then (in general, there may be exceptions, but I've never seen one) the power supplies can no longer load-share. For load share capable PSUs to load share, their outputs must be directly connected together.


This is because of the properties of a diode. A diode will only conduct when it is "forward biassed", which means that the anode is more positive than the cathode, and that voltage differential exceeds (silicon) 0.7v.

Which ever diode has the highest voltage source will reverse bias all the other diodes, so only one conducts.

This mechanism ensures that if you have two sources of power and a diode steering mechanism, the source with the higher voltage wins. Its a very neat and reliable answer to high availability power, fire and burglar alarm panels use steering diodes for just this purpose; if the mains is on, the power comes from the mains supply. If not, then the next source of power is the standby battery.

Using biasing/balancing resistor on the output at the rating of the supply will allow all power supply's to contribute equally, if the power supply has adjustable voltage regulation, and you set it to over come any voltage drop across the resistor, it will maintain system voltage, and limit each power supply to its own rating, other wise like you said one will runaway with all the load.

This is a common design in any power amp where you have ganged parallel output transistors, the biasing resistors prevents one transistor from taking all the load.

But this probably is not the intended design of these power supply's, as Dereck said, they are for redundancy, encase one fails so the others can keep going.

 

[gar]

100929-0721 EST

WastefulMiser:

You need to study the characteristics of semiconductor diodes. There is not a constant forward voltage drop. There are two major factors for a given part number. These are forward current and the junction temperature.

A quick illustration on a small sample of 1N4148s. Two diodes from same batch. Ambient temperature about 70 F. As the current thru the diode is changed its junction temperature changes. Thus, one waits a short time after changing the current to measure the voltage.
1N4148 is a small signal diode.

Current . Diode 1 Diode 2
0001 MA 0.614 V 0.615 V
10.4 MA 0.728 V 0.727 V
106. MA 1.024 V 1.014 V
233. MA 1.069 V 1.053 V

An MR752 is a larger diode, higher current rating
235 MA 0.72 V then added some heat to one lead 0.63 V.

Really the voltage drop is related to current density per unit area, the junction temperature, the material, and processing. Total current is current density times chip area.

You need a characteristic curve for a particular diode to determine current vs voltage, or at least one point and general information of the curve shape.

Some sites to look at:
http://en.wikipedia.org/wiki/Diode_modelling
http://www.physics.csbsju.edu/trace/CC.html


Your next problem is load sharing between the power supplies. You need to know what the peak current load on the supplies will be. Can one supply safely and reliably support this peak load. If so, then you really don't care about how the load is distributed. Otherwise there needs to be a load balancing method that includes the diodes.

How well regulated does the voltage need to be after the diodes? This will greatly affect how you do load sharing.

.

 

[WastefulMiser]

Thank you everyone for your replies.

gar, I appreciate your detail analysis. At one time I knew my electronics well enough to pass tests but I have unfortunately slept since. Last month I actually flew home to get my old text books (and to see my family), but my mother gave (threw!!) away all my books.

More or less it is my fault as I told her: "If I wanted to keep something I would have it with me -- you can do whatever you want with what's left."

gar: "How well regulated does the voltage need to be after the diodes?"

Sorry for the simple response, but: whatever it takes to change the PLC inputs from a 1 to a 0 and a 0 to a 1 and to activate the coils for the IRP's. What's the magic number? I really do not know. This really helps me in understanding all the variables involved in such designs.

 

[hurk27]

Sorry for the simple response, but: whatever it takes to change the PLC inputs from a 1 to a 0 and a 0 to a 1 and to activate the coils for the IRP's. What's the magic number? I really do not know. This really helps me in understanding all the variables involved in such designs.

Depending upon the PLC manufacture design, and the operating voltage, and whether it's I/O is a pull up or pull down?

take a 24 volt I/O pull up"

a "1" would be above 22 volts and a "0" would be below a 2 volts for a change state.

 

[hurk27]

Thank you everyone for your replies.

gar, I appreciate your detail analysis. At one time I knew my electronics well enough to pass tests but I have unfortunately slept since. Last month I actually flew home to get my old text books (and to see my family), but my mother gave (threw!!) away all my books.

More or less it is my fault as I told her: "If I wanted to keep something I would have it with me -- you can do whatever you want with what's left."

Boy I can attest to the long time of being out of the class room, although I have been keeping up with the INTERNET and trying not to get to far behind with the newer technology, but its a 24/7 job just trying to do that, its been 30 years since I sat in a class at University of Florida or Mid Florida Tech, and stuff you don't use in everyday work goes by the wayside so fast that having reference material is a must, and like you I had lost all my old books and other things, but my lost was because of a disgruntled ex-wife who took them out of spite, over the years I have tried to replace some of the books, but there were some you can't get unless you are enrolled in the school, so trust me it was an irreplaceable loss.

I have allot of respect and much thanks for members like Gar who have allowed me to re-learn many things I have long forgot, and even learned things I never learned in school, who have been able to keep up more in certain areas where I have not been able to because of not being job related info you would use and keep in memory.

 

[gar]

101001-0822 EST

WastefulMiser:

I suspect that most PLC loads would pull-in (relay or solenoid type device) at something below 75% of nominal voltage. But you need to test or get specifications on the devices. You also have to consider the voltage drop in the PLC output card, and wiring voltage drops.

On PLC inputs I would probably agree with hurk on the low end of designing for maybe 2 V as a maximum value for a logic 0 on a 24 V system. I think the high end threshold will be considerably lower than 22 V. Here you need to test the thresholding points of the inputs of your PLC inputs. Compare your test results with the PLC input specifications.

With some quantitative information on your specific system you can make some decisions.

Also looks at the maximum voltages allowed on the various devices. I believe you will find a lot of latitude on on voltage. If the high voltage limits will allow, then I would suggest you adjust the power supply voltages to get 24 V after the diodes at full system load. Then under light load this voltage might rise somewhat less than 1 V. Under no load maybe 1.5 V.

Your bigger problem maybe power sharing between the supplies if maximum load is 1000 W.

.