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Over voltage protection circuit suggestions?

Hello,

a couple of us on the pinball newsgroup have been wondering about a design for an over voltage protection circuit we would like to add to our games.

Here is the problem: On some pinball games there is a pcb dedicated to providing the +5.0 DC volts for the logic boards. Apparently when this board fails, it fails and then sends anywhere between 10-12 VDC to the circuit boards. On some of these circuit boards there are ancient (read obsolete and expensive) TTL and CMOS chips, that this novice suspects would not tolerate 10-12 VDC very well.

  1. Should I be worried about sending 10-12 VDC to these dinosaur chips?
  2. If so, are there any websites or ideas you could share in regards to designing an over voltage protection circuit for this power supply pcb? I know I won't be competent with an ASCII drawing; I could certainly fax the schematic if you so desired.

Thank you in advance for any and all thoughts you may have.

Regards, Dan

Reply to
Dan Beck
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One solution is a crowbar circuit plus a fuse. A crowbar is simple ... resistors, a zener diode and an SCR. Try Google.

Reply to
Charles Schuler

Yes.

What you would want to do is put a device after the regulator that will act when the voltage has gone past a point(i.e., your over voltage). You could just use a 5v regulator if the currents are not to large.

I would imagine that using a relay would be best and simplest. If you use a

12V relay then it will not trip unless the voltage goes above 12V.

Heres a site after a quick search:

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You'll need to protect the relay from kickback but thats pretty easy. Here the power for the relay will not be a steady 12V as in the pdf but will be the supply over your pcb.(so you don't need to change the pcb out) This way when the pcb outputs 12 volts it will then have enough to drive the relay but any lower and it won't. (or if you have a constant 12VDC source you can use that if you wanted)

In other cases one might want to know what the pcb is actually doing. If its just a power supply and if the current is not to high then it might be easier to replace it using a modern linear regulating circuit and would be much more failsafe. (And then tend to have many protection mechanisms involved)

Jon

Reply to
Jon Slaughter

Hi, Dan. As Mr. Schuler suggested, a "crowbar" circuit is the standard mode of protection for this type of problem. Here's a simple one which will trigger and pop a fuse if the supply exceeds about 6.2V or so (view in fixed font or M$ Notepad):

| | + 3AG 2A + | o---o_/ \\o-o-----o---------o | | | | 1N4733A /-/ | SCR | Vz = 5.1V ^ V S2010 | | - |Power o----/| |Supply | | Vout | .-. | | 2.2K| | | | | | | | '-' | | - | | - | o----------o-----|---------o | (created by AACircuit v1.28.6 beta 04/19/05

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Here's a Wikipedia link which could help you understand the circuit:

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The idea is, if power supply voltage exceeds 6.3V (which even TTL circuits can momentarily withstand), there will be enough voltage across the 2.2K resistor to trigger the gate of the SCR. The SCR will act like a "crowbar" across the power supply, shorting it out (and bringing the output voltage down to about 1.2V or so). This would normally put a lot of stress on the SCR, but that's where your fuse comes in. It will open up, saving the SCR so it can save the day next time.

It's a heck of a lot easier to replace a $0.50 fuse than a lot of obsolete TTL.

Good luck Chris

Reply to
Chris

Thank you gentlemen, for the abundance of information! I think the crowbar circuit is the way to go.

Regards, Dan

Reply to
Dan Beck

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