4511 bcd to 7 segment help needed

[tommy]

I am building a basketball scoreboard for my son. I have an old PLC which I yanked out of a machine from work but I only had enough outputs to drive 4 per number. The 4511 seemed like the answer since it only needs 4 inputs to convert to 7 segments. I have the thing built and wired but it keeps burning up chips. I assume I have something calculated incorrectly with the circuit and could use some help. I am a PLC / electrical guy with a foggy memory of my electronics schooling and could use someone to double-check my circuit. Briefly, I'm using 4 relay outputs on the plc to feed the four inputs on the chip with 10K resistor in parallel to ground on each. I'm using 12.5 vdc. The output of the chip is feeding small circuit board type relays with a resistance of 720 ohms. The relays in turn are switching power to the lights (I hacked up our Christmas tree lights because they were free - trying to keep the thing on budget, ha ha). What is happening is this: By changing the combination of the four inputs the outputs respond properly, but after only a few tries the relays start buzzing and everything freaks out. I'm assuming the chips are toasting. Do I need to review the resistor values to lower the current? According to the spec sheet on the 4511 the max output current is 25ma. Would that be total for all 7 outputs or per output? I realize this is kind of a bonehead project compared to some of the high tech things I'm reading about on this forum, but to my 10 year old son this is the big time and I hate to let him down. I'm much more at home controlling machines with PLC's (that's what I do) and am humbling myself to the electronics wizards I hope to meet to find some help...

[audioguru]

Hi Johnny,
Welcome to our forum.
The 4511 can drive up to 25mA on each output so your 720 ohm relays (12V?) are no problem with your 12.5V supply. The outputs of the 4511 are active-high, so the other end of your relay's coil should connect to ground.

It is good that you have pull-down resistors on the 4511's inputs and the input relay contact should apply only the 12.5V supply voltage to the inputs.

Your problem is probably cause by the inductive high-voltage spike produced by the relay coil when its drive is turned-off, just like old car ignition breaker points and spark coils. The voltage spike will be arrested if you connect a small diode across each output relay coil with its cathode at the output of the 4511, and its anode to ground. The voltage spike will be reduced to -0.7V which maybe is also too much for the outputs of the 4511 that normally never receive a negative voltage. Therefore I suggest adding an additional diode from each 4511 output to each output relay coil/arresting diode combination, with its anode connected to the 4511 and the cathode to the coil/diode.

[EnigmaOne]

I'm also wondering what method you're using to drive the relays.

In looking at the data sheet/app notes ( http://www.ortodoxism.ro/datasheets/fairchild/CD4511BC.pdf ), it would be my first inclination to drive the relays in the same manner as shown on page 6 for a common anode LED display, adding clamping diodes of something on the order of 1N4001 - 1N4004.

Driving the relay with a general purpose NPN transistor, like the 2N3904, ( http://www.ortodoxism.ro/datasheets/fairchild/2N3904.pdf ) would definitely save your ICs from serious harm on the output-side in most cases.

See the attached: 4511-relay.drive.png

[EnigmaOne]

After having a bit of a snooze, I find myself considering the current-sourcing capability and ripple output of the DC power supply that you might be using here.

If you enable the LT' (lamp test) input of all of the 4511s that you're using (I'm assuming 5: - 2 each for the team scores and 1 for the quarter indicator), you're talking about 35 relays (can you use hockey pucks instead of the relays?) kicked-in and holding, in addition to the current requirements of the remainder of the circuitry (4511s, PLCs, etc); which, depending on the power supply involved, might be enough to bring out some ripple/instability on the positive supply rail. (The buzzing/chattering relays are usually an indicator of a voltage output that isn't flat.)

If you've clamped the relays--and are buffer-driving them--and still have problems with toasted chips, I'd certainly look to the power supply output quality and the specific logic-input technique that you're using with the 4511s.

Oh well...I'm off to get some more coffee.