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Zener in series to LED...

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Externet

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Would a LED turn on only at voltages above Vz + Vf + Vdrop if hooked in series ? (Vdrop for the series current-limiting resistor)
-Like to indicate valid supply, not undervoltage, above the operative minimum, or battery not in need or recharging, or signal peaked-

Have not seen that in circuits...
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

It will work to some degree, but the brightness will vary depending on the level of the voltage.
The LED starts to turn on at a lower voltage than at full brightness. First light may only be 20ua for example.
You could however use an LM431.
 

spec

Well-Known Member
Most Helpful Member
Hi,

It will work to some degree, but the brightness will vary depending on the level of the voltage.
The LED starts to turn on at a lower voltage than at full brightness. First light may only be 20ua for example.
You could however use an LM431.
Further to what you say, this approach has the advantage of precision and flexibility. It would be very accurate and the no-go voltage could be set to any voltage within certain limits. By the way, for low voltage monitoring there is the TLVH431 which has a 1.25V reference, but its maximum voltage is limited to 18V compared to 36V maximum for the LM431.

spec
 
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KeepItSimpleStupid

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Would a LED turn on only at voltages above Vz + Vf + Vdrop if hooked in series ? (Vdrop for the series current-limiting resistor)

Have not seen that in circuits...
I used it in a circuit I designed for an audio amp. I kinda wanted all of the rails (4 of them) to be within 2/3 of 50V, but the 2/3 didn't have to be anywhere near exact,
The design did exactly what it was supposed to do, but the design could have been better. If any rail fails to be >2/3 Vcc my line current limiting resistor will fry. During start-up audio is removed by an optoisolator and the speakers are disconnected. That resistor and 40,000 uF of capacitance determines the turn-on delay.
If a rail fuse pops, that fusible resistor pops too.

No method was employed to remove power if (so many seconds passed by on start-up) AND (immediately after a Power Good signal) AND any supply was <2/3 of the final value was implemented along with "some light".

The home made AMP didn't actually have a power switch either.

I accidentally reversed the PNP and NPN output transistors. The only damage was that resistor that's supposed to pop.

Now, there was ONE ISSUE: The opto's would break until a ZNR was placed across each capacitor. There was a 12 V logic supply that came up immediately. I had a relay that shorted the RESISTOR, but no means to disconnect the AC line power totally. The amps "switch" was external and was a triac built into a "fancy timer".
 

dr pepper

Well-Known Member
Most Helpful Member
As you reach the zener voltage + the led voltage the led wil turn on gradually, the zener will have some affect on this too.
 
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