Over voltage protection circuit suggestions?

[Dan Beck]

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

 

[Charles Schuler]

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.

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

 

[Jon Slaughter]

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?
Yes.

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.

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:
http://www.jaycar.com.au/images_uploaded/relaydrv.pdf

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

 

[Chris]

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

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 www.tech-chat.de)

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

http://en.wikipedia.org/wiki/Crowbar_(circuit)

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

 

[Dan Beck]

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

Regards,
Dan