A question about current

[Dana Raymond]

If you are trying to connect your HDD Controller to some custom electronics,
then, yes, 20mA is going to be a problem. Thats a pretty high current, so
you will need to add a buffer of some sort. However, a 74HC04 will never
present that kind of input load, so there's some confusion here. My question
to you is: Why do you think the load is 20mA?

To answer the 3.3V to 5V question can you provide the specific logic
families (or part numbers) of the driver and the load? The answer is complex
and is based on the kinds of families involved.

Ioh is specifified as a negative number because current is flowing OUT of
the pin, not in.

Hope this helps.
Dana Frank Raymond

 

[Costas Vlachos]

Hi everybody,

I have a specification for a hard disk drive (HDD) controller and it has
the following text:


OUTPUT SIGNALS

HIGH Voh = 4.6V ~ Vcc (Ioh = -2mA)
LOW Vol = 0.6V (Iol = 6mA)


My questions is, if the output signal from the HDD controller is attached
to an input, which requires 20mA, could any damage be done the HDD
controller? Am I right in assuming that an input would require a certain
amount of current? As an example, lets say I connect the output signal
from the HDD controller to an inverter (74HC04), I am assuming that the
inverter will try and pull a certain amount of current from the HDD
controller. Also, how come in the above text the Ioh parameter is minus
2mA, why is it minus and not positive?

The minus in the current above simply indicates that the current when the
output is LOW will be of opposite direction to the current when the output
is HIGH. Another way of saying it would be that the HDD controller's output
can "source" 2mA and can "sink" 6mA. Usually source currents are designated
positive and sink currents negative, it seems that it's the other way around
in your case, but you get the idea.

Regarding the amount of current the inverter's input will draw, don't worry,
it will be nowhere near 20mA! Logic inputs usually have high impedance,
which means that they only require very little current to be driven (I think
it's less than 2mA for TTL, only uA for CMOS). You can find this info in the
data sheet of the IC/device you're using. For example, the input of your
high-speed CMOS inverter will draw a maximum of 1uA = 0.001mA. Don't worry
about it, nothing will get damaged.

My second question is similar to the first. If I have a 3.3V output and I
connect it to a 5V input, could any damage be done to the 3.3V system? I
guess the answer is related to the first question.

Again, nothing will get damaged, but it may not work. You have to make sure
that the 5V input's HIGH and LOW voltage thresholds are within the 3.3V
output's HIGH and LOW voltage levels. For TTL, the input thresholds are at
least 2.0V for HIGH and at most 0.8V for LOW. For your HC inverter they
depend on Vcc. Again, they can be found in the data sheet.

Hope this helps.

cheers,
Costas
_________________________________________________
Costas Vlachos Email: [email protected]
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[Edward]

Hi everybody,

I have a specification for a hard disk drive (HDD) controller and it has the
following text:


OUTPUT SIGNALS

HIGH Voh = 4.6V ~ Vcc (Ioh = -2mA)
LOW Vol = 0.6V (Iol = 6mA)


My questions is, if the output signal from the HDD controller is attached to
an input, which requires 20mA, could any damage be done the HDD controller?
Am I right in assuming that an input would require a certain amount of
current? As an example, lets say I connect the output signal from the HDD
controller to an inverter (74HC04), I am assuming that the inverter will try
and pull a certain amount of current from the HDD controller. Also, how
come in the above text the Ioh parameter is minus 2mA, why is it minus and
not positive?

My second question is similar to the first. If I have a 3.3V output and I
connect it to a 5V input, could any damage be done to the 3.3V system? I
guess the answer is related to the first question.

As you can probally tell from the above, I don't really know what im talking
about so if you want me to expand on the above in anyway, just say.

Thanks in advace,

 

[Fred Abse]

Hi everybody,

I have a specification for a hard disk drive (HDD) controller and it has
the following text:


OUTPUT SIGNALS

HIGH Voh = 4.6V ~ Vcc (Ioh = -2mA) LOW Vol = 0.6V
(Iol = 6mA)


My questions is, if the output signal from the HDD controller is
attached to an input, which requires 20mA, could any damage be done the
HDD controller?

Probably not, but it probably won't work because of voltage drop.

Am I right in assuming that an input would require a certain amount of
current?
Yes

As an example, lets say I connect the output signal from the HDD
controller to an inverter (74HC04), I am assuming that the inverter will
try and pull a certain amount of current from the HDD controller.

Yes, but certainly well under 2mA. Microamps tops.

Also, how come in the above text the Ioh parameter is minus 2mA, why is
it minus and not positive?

Because it is OUT OF the terminal. Convention is that positive currents
flow INTO a node.

What it means is that the device will sink 6mA and source 2mA

My second question is similar to the first. If I have a 3.3V output and
I connect it to a 5V input, could any damage be done to the 3.3V system?
I guess the answer is related to the first question.

An input should be high impedance so it shouldn't draw enough current to do
any harm, but the Voh of the 3.3V device might not be high enough to
reliably cause the 5V device to change state. YMMV.

As you can probally tell from the above, I don't really know what im
talking about so if you want me to expand on the above in anyway, just
say.

What on earth that draws 20mA do you want to connect to an HDC?

 

[Fred Abse]

Usually source currents are
designated positive and sink currents negative,

A quick look at a Texas TTL databook shows the opposite.

"Traditional" node-admittance network analysis uses the same convention,
ie. to a node is positive, from a node is negative.