Using 1N4001 as Zener Diode

This is an interesting schematic. I hadn't noticed this one before on Sam's website. Apparently the two 1N4001s are connected in series to be used as two zener diodes. Why not use a single 1N4002? Maybe to hand select for the right combination of the two to give the total breakdown voltage. I'm guessing about 150V each. Never tried it, tho.

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Watson A.Name - "Watt Sun, Dar
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I don't see any 1n4001's being used as Zener diodes at all. I see two standard 1n4001 diodes being used to provide an additional voltage drop of .6-.7v each diode.

Why not use a single 1n4002? Because it would have the same forward biased voltage drop as a single 1n4001.

David

Reply to
David

That is a really weird circuit. It looks to me that all three diodes in the string are drawn backwards... it doesn't seem to do anything sensible as shown.

John

Reply to
John Larkin

Thats a typical circuit to extract a stable clock from a bridge rect.

Cheers

Reply to
Martin Riddle

That's how it looks to me, too. :-|

Bob

Reply to
Bob Parker

"John Larkin" schreef in bericht news: snipped-for-privacy@4ax.com...

If C1A is charged, and C1B is discharged (after a flash), and the bridge is zerocrossing the AC input, well I don't know ;)

Weird indeed, but not sure if the diodes are reversed.

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Reply to
Frank Bemelman

Looks like a reverse blocking 11.4V Zener to me.

Reply to
EEng

I agree it is weird and I need to look into it more closely. I have tried to contact the owner to see if they can more carefully trace the relevant portion of the circuit.

Realize though that the output of the bridge is 120 Hz pulses and what's relevant is what happens during the rise of each pulse to determine if the SCR gets triggered (to charge the cap) or doesn't (to light the ready light). I think that if Q1 were NPN and the pot were a higher value, it would make more sense with the diodes in their current direction.

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Reply to
Sam Goldwasser

In article , snipped-for-privacy@locl.net.spam mentioned...

Looked to me like the base of the transistor was 300V or more, and the bottom of the CR7 1N4001 would have a lot lower voltage on it. But then now that I've looked at it more, I see your point. I'm now wondering what the whole purpose of the transistor is.

Anyway, it looked to me like when the unit was first turned on, the two 1N4001s would have over 300V across them, until C1A charged up. And whenever the flash using C1A was triggered, the cap would discharge and the diodes would again have over 300V across them. I guess I'll have to do some more studying of the circuit to get uncornfused.

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Watson A.Name - "Watt Sun, Dar

I don't think Samuel M. Goldwasser did a very good job redrawing it.

If you reverse the thyristor, reverse CR5, 6 and 7. Swap the location of Q1 and R1 then it makes sense.

That way Q1 and so the thyristor are on until the voltage across R3 exceeds the 12v across the zener and diode combination (roughly).

Not sure why there are two diodes. I know the temperature coefficient of a silicon diode is an inverse match for a zener of about 6v. Are two diodes a worthwhile match for a 10v zener?

Reply to
nospam

Sorry about the incorrect attribution of blame in my other post, I had a feeling the circuit must have come from a traced PCB.

Reply to
nospam

A standard diode in series with a zener diode as in CR5 and CR6 is a standard way to temperature compensate a zener. Zeners also have to be protected from carrying any forward current (shifts the zener voltage). CR7 probably compensates for the Vbe of Q1. Some designers tend to get totally anal retentive about temperature compensation.

In a circuit like this, the SCR is used to charge the storage caps from the DC source. It is supposed to allow for a fast charge with a trickle supplied by IL1 lamp. The SCR has to turn off before the strobe is triggered or the lamp will stay continuously on (and be destroyed).

In Royer type dc-dc converters used in many older flashes, the converter is current limiting and shuts down when the lamp flashes.

I agree though that something in the direction of the D5,6,7 string seems to be amiss

Oppie.

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Oppie

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No, the attribution is more or less correct. I'm the one who traced it and was bothered by the regulator from the beginning.

It would almost make sense if Q1 were an NPN but that part number is definitely PNP and I didn't pick it out of thin air.

May have to remain a mystery until someone can find one of these units to look at. The owner has since given it away unfortunately.

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Reply to
Sam Goldwasser

The PNP turns the SCR off when the voltage gets to the desired setpoint. It's fighting the gate pullup resistor.

Actually, it will work OK if the diodes are flipped over. But it's still kind of weird. The 1N4001s mainly keep from blowing out the base of the PNP, I think. This circuit looks more fiddled than designed to me.

John

Reply to
John Larkin

I am very rusty in this, but I seem to recall, that an PNP transistor operated as an NPN has a very low Von (as compared to a "normal" coupled/biased NPN transistor) when used as a saturated switch.

So if this is the case my suggestion is.

When the capacitors reach full charge the Q1 is driven on preventing the SCR form firing.

The only thing I miss is R4 to be connected to the cathode of the SCR in order to provide a ground path for the Q1 base current.

/John

Reply to
=?ISO-8859-1?Q?John_Damm_S=F8r

I think that there are more "Fiddled Than Designed" circuits in use than most of us would dare to believe. A few years ago I was repairing switchmode DC-DC power converters made by a US company I won't mention, which use a 555 at the centre of a mass of components to control the switching. As time went on, I got more and more certain that probably no-one ever knew how how they really worked. There was just no logic to it. They'd just added more and more components until they got the result they were after. Of course I could be mistaken. :) Some of the older Panasonic VCRs have circuitry which seems to be ridiculously complex for what it actually does, too.

Bob

Reply to
Bob Parker

Someone should just build the thing, and test it.

: )

Reply to
Mark Jones

I'll see your Panasonic and raise you one H.P. ;-)

Reply to
Baphomet

I tried to simulate it in Spice and the computer started to smoke :-)

Reply to
Baphomet
[snip]
[snip]

You're confusing inverted-mode (swap emitter and collector connections) with "PNP transistor operated as an NPN", which can't be done.

See "InvertedTransistors(Hunter).pdf" on the S.E.D/Schematics page of my website.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Jim Thompson

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