Analyze circuit to determine output:

mbxs3

Joined Oct 14, 2009
170
Can someone please analyze this circuit and determine what the output voltage will be?

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Kermit2

Joined Feb 5, 2010
4,162
That seems to be a 3 volt zener, right?

What happens when you make it a 5 volt, or a 10 volt zener?

Believe it or not, those are hints at what the output will be.

SgtWookie

Joined Jul 17, 2007
22,230
The primary intent is for the circuit to be a ~5 second delay. Once C1 charges to ~1,7v, the output is turned on.

You should see somewhere around 23.1v-23.5v depending on the output load; load shouldn't draw more than about 25mA without causing problems.

BTW, that's a 4.7v 1W Zener.

SgtWookie

Joined Jul 17, 2007
22,230
Thanks, Kermit2 - I realized that in my state of exhaustion yesterday evening (lots of yard work in humid 100°F weather) that I'd transposed the 1N4372A to 1N4732A; the latter of which is in fact a 4.7v 1 Watt Zener.

That DOES change the output voltage, but I'll need to throw a model for that Zener into my simulator to make sure.

[eta]
Couldn't find a model for that particular diode in a short period of time; but the output should now be approximately 25v, give or take some, depending on the load.

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Kermit2

Joined Feb 5, 2010
4,162
We all make mistakes, and I seem to make more than my fair share most days.

mbxs3

Joined Oct 14, 2009
170
Thanks for breaking it down for me guys.

The output load is a 26vdc relay that is PCB mounted. I don't know the current requirements of the relay.

I guess that when the signal at power source #2 is high, the output of the circuit is capable of energizing the relay, when the signal at power source #2 goes low, the relay is de-energized.

SgtWookie

Joined Jul 17, 2007
22,230
What is the part number of the relay? I could see if it would simulate OK or not, but I'd need to know the resistance of the coil.

If the relay does not have a built-in diode, or if the diode is burned open, it would take out (fry) the Zener and Q6 rather quickly

mbxs3

Joined Oct 14, 2009
170
Relay part number is W260-A1A-004L1.

SgtWookie

Joined Jul 17, 2007
22,230
Leach Int'l
Interesting, those are mil-spec parts.
http://www.leachintl2.com/english/english1/vol0/properties/00028.html
The coil has 700 Ohms resistance, so ~35.7mA current.

I'm seeing about 24.5v with that much of a load on it, and it takes an extra second for the voltage to reach that point.

Note that there is no reverse-EMF suppression diode in these relays. This circuit really needs a reverse-EMF diode added, or Q6 is going to be blasted.

What's this circuit in? A vintage military aircraft? We used extremely similar relays in F-4 Phantoms.

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mbxs3

Joined Oct 14, 2009
170
Yeah, P-3's.

SgtWookie

Joined Jul 17, 2007
22,230
Cool. It's amazing that they're still flying those birds; they were old when I was on active duty.

I added some more test points so that you can get an idea of the voltages over time. If you need anything else, holler.

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mbxs3

Joined Oct 14, 2009
170
Thanks Sgt. They are some oldies. As always, I appreciate the info you provide.

SgtWookie

Joined Jul 17, 2007
22,230
You're welcome - anything to keep NAVAIR up there.