# IC specs, rated current/voltage

Thread Starter

#### quadhed

Joined Jan 13, 2016
48
Hello. Does anyone know if directly (without current limiting resistors) connecting two 12V rated chips would destroy them? It seems to me if they are rated at the same voltage they would draw the rated current as well (one into another). Correct?

#### dl324

Joined Mar 30, 2015
16,943
connecting two 12V rated chips would destroy them?
What chips? Connecting them to what and how? A schematic or block diagram would be helpful.

#### Papabravo

Joined Feb 24, 2006
21,227
Integrated Circuit (IC) chips are not normally connected to a power source with current limiting resistors. One possible exception is the LED or an LED string. Since they are current devices when forward biased, they need that current limiting resistor to limit the maximum current they will draw from an arbitrary voltage source.

Thread Starter

#### quadhed

Joined Jan 13, 2016
48
What chips? Connecting them to what and how? A schematic or block diagram would be helpful.
Say just a 555 to a 4017 decade counter with both rated at 12 V.

#### dl324

Joined Mar 30, 2015
16,943
Say just a 555 to a 4017 decade counter with both rated at 12 V.
Both devices can safely be connected to 12V without current limiting resistors because both are rated for supply voltages higher than that. Who made you think current limiting resistors were required?

#### Papabravo

Joined Feb 24, 2006
21,227
The little boy who wasn't there. He's been a PITA since I was a kid and didn't want to admit to making a mistake.

Thread Starter

#### quadhed

Joined Jan 13, 2016
48
Both devices can safely be connected to 12V without current limiting resistors because both are rated for supply voltages higher than that. Who made you think current limiting resistors were required?
The spec sheet has max output current and input for the ics. I'm assuming these are the currents at the rated voltage.

#### dl324

Joined Mar 30, 2015
16,943
The spec sheet has max output current and input for the ics. I'm assuming these are the currents at the rated voltage.
It would be helpful if you started being more specific.

#### Papabravo

Joined Feb 24, 2006
21,227
It would be helpful if you started being more specific.
What happens if the "rated voltage" is 3-18 volts as it would be for a 4000 series CMOS part like the 4017?
Also the 555 is both a bipolar part and a CMOS part. Which of those alternatives are you considering?

Thread Starter

#### quadhed

Joined Jan 13, 2016
48
What happens if the "rated voltage" is 3-18 volts as it would be for a 4000 series CMOS part like the 4017?
Also the 555 is both a bipolar part and a CMOS part. Which of those alternatives are you considering?
The TTL 555.

Thread Starter

#### quadhed

Joined Jan 13, 2016
48
It would be helpful if you started being more specific.
If you have a 555 ttl version operating at 18 volts Max it is my belief that, at that voltage,the max current would be output at pin 3.
Is this what the 200 mA max on the datasheet at pin 3 means?

#### Papabravo

Joined Feb 24, 2006
21,227
If you have a 555 ttl version operating at 18 volts Max it is my belief that, at that voltage,the max current would be output at pin 3.
Is this what the 200 mA max on the datasheet at pin 3 means?
TTL is a bipolar technology. If you read the datasheet carefully you will note that +18 Volts on Vcc will very likely let the magic smoke out of the part.

Last edited:

#### dl324

Joined Mar 30, 2015
16,943
If you have a 555 ttl version operating at 18 volts Max it is my belief that, at that voltage,the max current would be output at pin 3.
Is this what the 200 mA max on the datasheet at pin 3 means?
As I said, a 555 timer would be okay operating at 12V. At 18V, it would be at it's absolute maximum, and no designer worth beans would operate a device at it's absolute maximum parameter.

Even the manufacturer, National Semiconductor in this case, only characterizes operation at 5-15V.

If a device survived operating at it's absolute maximum, which it isn't guaranteed to do, any spike on the supply voltage that caused it to exceed the abs max by even a small amount could be enough to cause the device to act like an SCR.

They characterized the device sinking or sourcing 200mA, so that should be safe:

Thread Starter

#### quadhed

Joined Jan 13, 2016
48
As I said, a 555 timer would be okay operating at 12V. At 18V, it would be at it's absolute maximum, and no designer worth beans would operate a device at it's absolute maximum parameter.

View attachment 250811
View attachment 250812

Even the manufacturer, National Semiconductor in this case, only characterizes operation at 5-15V.

If a device survived operating at it's absolute maximum, which it isn't guaranteed to do, any spike on the supply voltage that caused it to exceed the abs max by even a small amount could be enough to cause the device to act like an SCR.

They characterized the device sinking or sourcing 200mA, so that should be safe:
View attachment 250813
I'm really trying to understand the datasheet. What do all the multiple ISOURCEs and ISINKs mean?

#### Papabravo

Joined Feb 24, 2006
21,227
I'm really trying to understand the datasheet. What do all the multiple ISOURCEs and ISINKs mean?
In the TTL world the transistors are not symmetrical. This means that they can SINK way more current in the low state then they can SOURCE in the HIGH state. IN CMOS the output stages are symmetrical and they can SOURCE as much current as they SINK. It allows the TTLL designer to avoid a situation where the driving requirements for a device are not exceeded by connection of too many inputs to an output.

In the original 7400 series TTL parts it was common to have an output that was able to SINK 16 mA while maintaining the output at a valid logic '0'. A TTL input would source 1.6 mA into a low output and was referred to a 1 Unit Load. So the output capability was capable of driving 10 Unit Loads. I don't remember off the top of my head anymore how much current a TTL output could source while maintaining a valid logic '1' but it was a good deal less than it could sink.

#### dl324

Joined Mar 30, 2015
16,943
What do all the multiple ISOURCEs and ISINKs mean?
They're illustrating the typical increase in the collector-emitter voltage drop for various load currents and supply voltages.

For current sink, when they give worst case numbers, you should use them when designing circuits. Typical is how most devices will behave. For Vcc=5V, they had a line for 8mA, but didn't give any voltage drop information.

For current source they gave less data. In this case, the worst case numbers are the minimums.

Note that the maximum sink/source currents tested are equal. That isn't always the case. They also don't list any qualifications for those currents, so you can assume that they're safe continuous currents.

#### BobaMosfet

Joined Jul 1, 2009
2,113
Hello. Does anyone know if directly (without current limiting resistors) connecting two 12V rated chips would destroy them? It seems to me if they are rated at the same voltage they would draw the rated current as well (one into another). Correct?
Read the appropriate datasheet for the appropriate chip. And here:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

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