# Steering Diode

Discussion in 'Electronic Basics' started by Sir Charles W. Shults III, Feb 18, 2004.

1. ### Sir Charles W. Shults IIIGuest

The explanation on the page you supplied was quite clear and concise.
If diode D2 is not forward biased, it cannot conduct. If 15 volt power is
available, D2 is reverse biased and power from the battery cannot pass
through it. Only when then 15 volts fails can D2 conduct battery power.

Cheers!

Chip Shults

2. ### John PopelishGuest

electricked wrote:
(snip)
Yes.

3. ### John PopelishGuest

Yes.

At the plant where I work, the power house has a series of 135
(nominal) DC buses that all the critical relay logic runs off, to make
sure things work through a power blip. There is a big transformer
supply (with cap bank storage) for each bus that is set by transformer
taps to produce about 140 volts. There is a single backup transformer
operated supply (with cap bank) that serves as secondary source for
all these first line supplies. It is set to about 138 volts. Then
there is a common battery bank and charger system that runs at about
136 volts at no load as the third source for all systems. The main
and backup supplies are diode connected same as your example. There
is a current measurement on each supply, so we can tell if any
backups are carrying load. A while back, we got current on both the
main and backup supplies for several banks, and we found defective
rectifiers on the first line supplies that increased their ripple
voltage enough that the second line supply was supplying current
during the dips.

4. ### electrickedGuest

Hello all,

I have a question about the diagram and explanation given on the following
page: http://ourworld.compuserve.com/homepages/g_knott/elect212.htm

It says that while the power supply lasts the equipment will be powered by
it and if it fails, the battery will start conducting and give power to the

Now, I understand how this works, but what troubles me is that if both are
in great operating order, why doesn't the battery conduct and the power
supply wait until the battery is dead? Is it because the power supply
provides a higher voltage? Please explain.

Thank you!

--Viktor

5. ### electrickedGuest

That's exactly what the page said. But why? Why can't the battery conduct to
start with and not the power supply? Is it because the battery's voltage is
lower than the power supply's? I don't get it.

--Viktor

6. ### electrickedGuest

That's exactly what the page said. But why? Why can't the battery conduct
to
What if the power supply was 10V and the battery was 12V? Would the battery
supply the power until it's finished and then the power supply take over?
See what I mean?

Thanks for speedy response!

--Viktor

7. ### electrickedGuest

Thanks John! Makes perfect sense now.

8. ### R. Steve WalzGuest

-----------------
Yes, the PS voltage is higher, and that presents 14.4V on one side
of the steering diode, and 12V on the other, reverse-biasing it,
and reverse-biased it remains OFF.

The battery might not even need the diode, that is, it might as well
be recharged while the PS is on, if the battery's requirements were
simple, or if it had a switch to turn off the battery positive when
it was "full", as long as we don't exceed its charging maximums.

But if 15V is left across a nominally 12V battery, it will indeed
overcharge it and damage or overheat it.

-Steve