Overvoltage/Overcurrent Protection

[Ron Reeland]

Hi:
I have a Eico 666 vacuum tube tester that I built as a kit in 1963. I also have other tube testers that I have restored. Some are military types. One thing these vintage testers have in common are delicate panel meters. And nearly impossible to obtain a replacement meter for these old units. There is a real disaster that could occur in that a shorted tube under test or an error in setting the tester controls might burn out or mechanically damage the meter.

The method currently employed to protect the delicate meters is to connect two silicon diodes such as 1N4007 across the meter terminals in opposite polarities. The purpose of the diodes is to absorb any destructive over-voltage and or over-current surges. There are other diodes that are purported to conduct at lower voltages and react faster, but still short of absolute protection.

These 1N4007 diodes begin heavy conduction at about 0.7 volts, although there is some leakage present at lower voltage. However, on my Eico 666 tester, the meter has a resistance of 1000 ohms and the maximum current that the meter can handle is about 200 microamps which is just short of pegging the meter indicating needle. And the voltage applied is 0.2 volts or 200 millivolts.

Thus the diodes do not begin conducting at this low 0.2 voltage but maybe still provide some degree of protection for the meter, at least in theory. "Maybe" is the key word. A person cannot experiment with the theory as a damaged meter results in a useless tube tester.

So, I am wondering if there is some sort of device or circuit that could provide protection that can be triggered at such very low votage thresholds?

Thanks,
Ron Reeland

[wraper]

No way 0.7V*700uA = 0.5mW of power is capable of destroying the meter.

[PGPG]

No way 0.7V*700uA = 0.5mW of power is capable of destroying the meter.

Are you sure?
When I start with electronic (end of 60s) I had two 10x12cm 100uA meters. From one I have build transistor voltmeter, from second frequency and capacitance meter.
I suppose that if I drive them with 500uA the pointer would fly so fast to the end of the scale that when it hit the stop it would either break or bend, or jump over the stop and never come back.

[wraper]

No way 0.7V*700uA = 0.5mW of power is capable of destroying the meter.

Are you sure?
When I start with electronic (end of 60s) I had two 10x12cm 100uA meters. From one I have build transistor voltmeter, from second frequency and capacitance meter.
I suppose that if I drive them with 500uA the pointer would fly so fast to the end of the scale that when it hit the stop it would either break or bend, or jump over the stop and never come back.

It will hit the stop quite fast, yes. However to jump over it or bend, it would need to be of extremely bad build quality.

[PGPG]

However to jump over it or bend, it would need to be of extremely bad build quality.

As I remember stops were made by may be 1..1.5mm length elastic tube on a wire bent at a 90 degree angle. It was positioned to be exactly at pointer way. But everything was very delicate (have to work with only 100uA) and if hit hard it could probably bend enough (also pointer was enough long to not be very rigid. With these two my meters I have never checked it
But the first ohmmeter I had (not multimeter, but only ohmmeter, scale selection not by switch but by using different socket) I got free as it was damaged. The problem was the pointer out of stop (not at scale end but at scale beginning). The reason was probably not electric but I suppose someone drop it on the flor. When I opened it moving the pointer to the right side of stop was no problem based only on flexibility of everything. Because of it I suppose that jumping over stop can happen when driven 5 times to strong.
The bigger problem was that one of the two very delicate springs was bent and the individual coils are rubbing against each other. It took me a long time to fix this. I used two needles to manipulate with it. The problem was that spring shape have to be different when you press it than it will be just after.

[wraper]

It's delicate. However how much force/acceleration do you think can provide 0.5mW of power? I've never seen panel meter going bad from pointer hitting the limit too hard despite abusing some in the past. Dropping them on other hand can kill them easily.

[PGPG]

However how much force/acceleration do you think can provide 0.5mW of power? I've never seen panel meter going bad from pointer hitting the limit too hard despite abusing some in the past.

I have also didn't seen it. I'm only not sure if we can be sure it can't happen.