Strange circuit - Page 1
Forum section: electronics / projects

author: james_s
author_rank: Super Contributor
timestamp: July 21, 2018, 04:51:35 pm
content: I was browsing through some Heathkit schematics and came across this one for a synthesized chime accessory for a 1970s digital clock. At a glance there's nothing too surprising about the circuit but then I looked closely at IC322 which is a CMOS hex inverter. This is where it gets weird, Vss and Vdd are both tied together and go to the input of the audio amplifier. The signals from the tone generation circuit go to the output of the inverters and the inputs are connected to the envelope generator circuits, those transistors over to the right.Now normally I would just assume the schematic is wrong, however Heathkit was well known for providing very good documentation so I'm inclined to believe it. What is going on here though? Is this some kind of clever use of a trait of the MC14049 that I'm not aware of?

author: drussell
author_rank: Super Contributor
timestamp: July 21, 2018, 05:48:48 pm
content: Quote from: james_s on July 21, 2018, 04:51:35 pmIt says right on the schematic:QuoteSo, to answer your question... Yes.

author: james_s
author_rank: Super Contributor
timestamp: July 21, 2018, 06:15:06 pm
content: Oh, I didn't even see that note. I've never seen a circuit like this before, are there other examples or did somebody at Heathkit come up with the idea? I'm still not quite sure how it works, I guess I'll have to try drawing it with the internal circuit of the IC.

author: JS
author_rank: Frequent Contributor
timestamp: July 21, 2018, 07:26:13 pm
content: You can also find circuits using opamps power pins as output, or thibgs like that. The trick is to think the IC as a circuit block counting the power pins as IO rather than a functional block that just makes what it's supposed to do when used as supposed. Even in such case the power pins should be treated as signal pins where the power comes in, but there's also precense of signal currents and then the decoupling should be used to put thode currents where they go.JS

author: james_s
author_rank: Super Contributor
timestamp: July 21, 2018, 07:49:27 pm
content: I'd be curious to see examples of more unusual circuits like this. Tricks like using digital ICs as analog elements are the sort of thing I don't think many people consider doing anymore. Everything is just a microcontroller and some glue components.

author: mikerj
author_rank: Super Contributor
timestamp: July 21, 2018, 08:04:32 pm
content: Some more details inthis PDFby the greatDon Lancaster..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 21, 2018, 10:34:21 pm
content: Some other features you may be unaware of: logic input ESD protection diodes are actually weak BJTs. Simply put, they are diodes that share a common base -- thus making an emitter, collector and common base between them. The hFE is usually low, less than 1. One consequence: when forward-basing the VSS-side diodes (i.e., pulling the input below VSS), about half the current flows to VDD, increasing power dissipation! The equivalent circuit is a grounded-base transistor, so this makes sense, but you don't think of it if it's always drawn as a diode.The current that flows into VDD, also flows through a diode structure. Normally these diodes are well enough isolated / shorted across, but if you push enough current, they will feed back into each other -- SCR action. For 74HC, this takes >100mA, and typically draws 140mA "short circuit" (needless to say, things start to get hot at 5V).Likewise, shared ESD diodes have hFE between each other, like between adjacent inputs with respect to VDD. The hFE is usually quite small, like 0.03. Here, forward biasing the diode (input above VDD) increases leakage current in the other input(s). Unlikely to be a problem in most circuits, but if you were counting on low leakage (say for a high impedance input or timer circuit), avoid biasing the ESD diodes.Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2018, 04:19:09 am
content: Quote from: james_s on July 21, 2018, 06:15:06 pmCMOS inverters can also be used as self powered synchronous bridge rectifiers. ED or EDN showed it in one of their user submitted design ideas.Quote from: JS on July 21, 2018, 07:26:13 pmI really like this one and Fairchild showed this configuration in their 1976 linear databook. Tektronix used with the 741 in the Tektronix 7A29 vertical amplifier in 1979.In the full realization, you get two high impedance inputs, one low impedance input, and two outputs. Plus the current feedback output stage multiplies the slew rate yielding a better full power bandwidth.

author: exe
author_rank: Unknown
timestamp: July 22, 2018, 09:16:48 am
content: Quote from: David Hess on July 22, 2018, 04:19:09 amI wasn't able to find it. If/when you have time, could you please share the link?

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2018, 11:22:22 am
content: Quote from: exe on July 22, 2018, 09:16:48 amThis was pre-internet so unless they posted it, there is no link.

author: mark03
author_rank: Frequent Contributor
timestamp: July 22, 2018, 08:50:11 pm
content: Quote from: David Hess on July 22, 2018, 11:22:22 amI'm very curious about this too, so if anyone knows more...

author: David Hess
author_rank: Super Contributor
timestamp: July 22, 2018, 10:05:36 pm
content: It duplicatedthis circuitwith a single IC. As discussed at the link, it is only really suitable for a square wave source and of course relatively limited power.DC comes out the power pins and the inputs and outputs are driven by AC. A single 14 pin hex inverter IC was used with two pairs of three inverters tied in parallel for lower channel resistance and then the pairs connected in antiparallel. At least that is how I remember it.This is just an extreme version of powering a CMOS circuit which is missing ground or power connections through the protection diodes at the inputs and outputs.

author: chris_leyson
author_rank: Super Contributor
timestamp: July 22, 2018, 10:18:48 pm
content: I remember working on some CMOS logic that one day magically burst into life all by itself with no power applied, it was getting its power from the 10MW radar about a mile down the road.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 22, 2018, 10:48:23 pm
content: Quote from: David Hess on July 22, 2018, 10:05:36 pmI also remember a variant of that circuit (mostly just the addition of resistors and zener diodes) that claims to be a low voltage grid tie inverter, although I'd imagine it doesn't work very well if it even works at all.

author: Cyberdragon
author_rank: Super Contributor
timestamp: July 23, 2018, 08:33:32 pm
content: Quote from: chris_leyson on July 22, 2018, 10:18:48 pmSSSHHHHH! You'll attract the wireless power wankers!

author: exe
author_rank: Unknown
timestamp: July 23, 2018, 09:05:54 pm
content: Quote from: Cyberdragon on July 23, 2018, 08:33:32 pmBTW, is wireless power a thing? I wanted to do RF energy harvesting to supply a blinking LED, but some preliminary calculations showed it wouldn't work for me. Mainly, I didn't find a good broad-band rectifier with good-enough efficiency. (Some random research papers I found in the Internet agree with my observations).

author: james_s
author_rank: Super Contributor
timestamp: July 23, 2018, 09:08:54 pm
content: Wireless power transmission absolutely works, provided the amount of power needed is tiny so that terrible efficiency isn't a problem. Crystal radios have been around for over a century and they are wirelessly powered. If you stand out under a high tension transmission line with a fluorescent tube at night it will glow, that's wireless power transmission.It's not going to replace wires for powering useful stuff, but it can be done for novel purposes.

author: exe
author_rank: Unknown
timestamp: July 23, 2018, 09:12:04 pm
content: Quote from: james_s on July 23, 2018, 09:08:54 pmYeah, I understand. I built a radio like that in my childhood. Directed power transmissions also works reasonably with two directed antennas (or in close proximity, example: inductive chargers).But what about a typical household away from 10MW radars and an overhead power grid?

author: james_s
author_rank: Super Contributor
timestamp: July 23, 2018, 09:18:59 pm
content: Well if the power isn't there then you won't be able to harvest it. If you want to light up an LED you could probably pick up enough RF from broadcast radio/TV transmitters with a suitable antenna, but you won't be charging your mobile phone or powering your laptop.

author: duak
author_rank: Super Contributor
timestamp: July 23, 2018, 10:57:59 pm
content: I remember seeing a linear DC power supply kit that connected an output pin of an LM324 quad opamp directly to Vcc. Turns out it used that opamp to act as a shunt regulator to keep the chips Vcc below its absolute maximum rating of 32 VDC. With a 25 VAC transformer the unloaded raw DC was typically 38 - 40 V. It struck me as being a "Sword of Damocles" circuit with the ability to destroy itself. I suppose if the opamp was sentient it would likely be sweating.I also remember working with a fellow who was debugging a TTL circuit that had a verified 6+ V on one of its signals. This was patently impossible because the only power supply was 5 V and there were no inductors. Turns out one of the Vcc bypass caps was accidently installed between the anomalous signal and another output and it formed a charge pump.