# High Side VS low side

A

#### Abstract Dissonance

Jan 1, 1970
0
http://www.national.com/pf/LM/LMD18400.html

I do not understand what the High Side Drive really is doing?

It says on page 8 that high side drive can protect from being "energized"...
what is energized? The way I see the circuit is that if the load is to
high(low resistance) then there will be a lot of current and then the
circuitry can sense this and turn use some type of circuit limiting method?

If thats right then I do not get the low side drive. Looks to me, with the
circuit that is shown, there is no difference. Cause it just shows that the
load is shorted before the switch but I see the same can happen with the
high side(if it is shorted to ground before the switch in the high side
drive circuit)? Although is the current limiting is inside the circuit then
I do see a different... i.e., no way to short before the circuit limiting
circuitry.

Is this making sense?

Thanks,
Jon

J

#### John Popelish

Jan 1, 1970
0
Abstract said:
http://www.national.com/pf/LM/LMD18400.html

I do not understand what the High Side Drive really is doing?

It says on page 8 that high side drive can protect from being "energized"...
what is energized?
(snip)

In a system that is enclosed in metal (like an automobile), the most
common short circuit on any wire is a short to the grounded body. If
the load is permanently attached to the hot wire, and the power is
switched in the ground wire (low side switching), any short to ground
on the wire that gets switched off and on will just look like the
power switch is on, and the load will be energized (turned on).

If the load is permanently connected on its ground side, and the power
is switched through the hot or positive lead (high side switching),
and the switched wire gets shorted to ground, the high side switch
circuit will see the load in parallel with the short to ground, when
it tries to turn the load on, and the excess current can be sensed and
the switch turned back off.

B

#### Brian

Jan 1, 1970
0
Abstract Dissonance said:
http://www.national.com/pf/LM/LMD18400.html

I do not understand what the High Side Drive really is doing?

It says on page 8 that high side drive can protect from being
"energized"... what is energized? The way I see the circuit is that if the
load is to high(low resistance) then there will be a lot of current and
then the circuitry can sense this and turn use some type of circuit
limiting method?

If thats right then I do not get the low side drive. Looks to me, with the
circuit that is shown, there is no difference. Cause it just shows that
the load is shorted before the switch but I see the same can happen with
the high side(if it is shorted to ground before the switch in the high
side drive circuit)? Although is the current limiting is inside the
circuit then I do see a different... i.e., no way to short before the
circuit limiting circuitry.

Is this making sense?

Thanks,
Jon

On the high side drive, if the wire that is going out to the load is
shorted, that would be the same as if you put a shorting wire across the
load. Nothing would happen until the drive tries to turn the load on (in
which case the drive would see a short) and the + supply would see a short.

On the low side drive, if the wire that is going out to the load is shorted,
the short would be doing the same thing that the drive would be doing
(shorting the ground side of the load to ground). This would turn on the
load, before the drive turns it on. As far as the load is concerned, it is
being told to turn on. As far as the + supply is concerned, there is no
short (only a load being turned on).
Hope this helps
Brian

A

#### Abstract Dissonance

Jan 1, 1970
0
Ok guys, I think I got it.

Thanks,
Jon

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