Hot chip

Tumbler · 2011-10-27 20:48

Heys

Trying to use a 74595 at 3.3V. (with a PPDB)
When i wire pin 10 to +3.3V the chip gets really hot.
First i was thinking for a shortcut, but building it 4 times from scratch.
Just blowed my 4 th chip.
What's wrong here?

My MM74HC595 seems identical with a 74595

LoopyByteloose · 2011-10-27 21:02

Recheck your wiring. You may have something on a wrong pin or completely reversed polarity. It would not hurt to use a 10K resister to pull up the reset while limiting the current. Make sure you were sold the right chip.

tonyp12 · 2011-10-27 21:06

You are sure you are using 3.3v to power it and 3.3.v max for the clock/serial pins?
If not go with 74AHCT595.

I connect pin 10 direclty to Vdd (3.3v) and it works.
How high are your OHM values on your resistors?
It's rated max 35 mA per pin.

kwinn · 2011-10-27 21:09

What Loopy says. I have used 74HC595 chips wired as in the posted diagram but with 3.3V instead of 5V with no problems.

Tumbler · 2011-10-27 21:15

I'm pretty sure i use the right connections.
Biulded it 4 times

Like i said, im using the propeller development board. I'm sure those resistors are ok.
Used a 10k resistor (pullup) to 3.3V

MY 5th chip is getting hot here.
No output anymore.

Now i have 5 DIP mini electric cookplates. Maybe Barbie can use them.

tonyp12 · 2011-10-27 21:19

Can you take a picture of it on the PPDB board with the wiring?
You are placing the dip chip along the center channel of the protoboard?, just checking to make sure.

Cluso99 · 2011-10-27 23:15

Silly question I suppose... You do have the chip the right way around... the pic of the chip is from the top, not underneath.

kwinn · 2011-10-28 08:01

There is no way that circuit should blow a 74HC595 if wired as shown and used with a good 3.3 or 5V supply. I have used it for a status led driver on numerous projects wired almost exactly as shown in your diagram. The only difference was the led resistor values, and mine were lower, so would draw more current.

I would suggest checking the voltage/signals on all the pins using pin 8 as the reference (ground), preferably with a scope so you can see the signals, or with a meter if a scope is not available. All the voltages should be positive with respect to pin 8.

JimInCA · 2011-10-28 08:10

Given that this circuit is hand wired, my suggestion is to wire up the circuit testing as you go along to see when it starts to get hot. Start by applying just Vcc and Gnd, power up the chip and see if it gets hot. If not, power it down and then add the IO signals and test again. Then go on the the LEDs enabling only one at a time. This should lead you to what signals are causing the chip to get hot. Something has to be causing the chip to draw a lot of current in order for it to get hot, it's just a matter of determining which pin to find the cause.
I hope this helps...
Jim...

erco · 2011-10-28 08:25

You've made the same mistake 5 times now. Learn pin numbers from the top AND bottom views. Before you fry chip #6, wire it up in a solderless breadboard to verify function. Per JimInCA, add wires slowly and keep checking for meltdowns.

Tumbler · 2011-10-28 20:58

Now i have some 74HC595's
Seems they are working perfect.

Removing and replaced by a MM74HC595N results in hot chip.
hallelujah

LoopyByteloose · 2011-10-29 05:53

I always hate to see somebody having to burn up several chips before they locate the problem, but the usual issue is wrong assembly. Here is seems that the MM74HC595 is not the same as a generic 74HC595 or a defective batch. It might be worthwhile to read the specific pdf for differences. Recently, I purchased some 24lc32 EEPROMs, but they sold me 24lc32A chips. The latter have an additional active pin for write protection. So even the same part in different generations can behave differently. And vendors can be rather slack at keeping up with such changes.

Hot chip

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