reverse polarity protection - Page 1
Forum section: electronics / beginners

author: FrankBuss
author_rank: Unknown
timestamp: August 14, 2019, 10:37:45 am
content: I'm creating a PCB for a client for a guitar amp and fuzz, see attached circuit diagram. It should be possible to power it from a 9 V battery and from an external power supply. Note that it uses -9 V and 0 V because of the PNP transistors.Hereis a full example of a very similar fuzz.For reverse voltage protection I added some diodes, but how can I protect it, if the power supply is gavanically coupled to the GND of the audio input stage through the output connector? The usual power supply would have the same GND as the audio device. When plugged in the wrong way, the -9 V of the device would be GND, and the GND of the device would be +9 V. This would then short circuit to the GND of the audio device.For protecting the device and for decoupling the battery and external power supply input, I added 2 diodes. For gavanically coupled GNDs, I added the fuse, but the client doesn't like this, because the usual musician might have problems replacing it. Is there a better way to protect this path without a fuse?

author: capt bullshot
author_rank: Super Contributor
timestamp: August 14, 2019, 10:52:32 am
content: If cost doesn't matter too much, I'd consider a normally open dual pole relay (breaking -9V and GND), closing only when all conditions are met. I'm too lazy ATM to think about it, maybe one could build a similar circuit using MOSFETs instead the relay, don't know if it's worth, since a suitable relay could cost < 1EUR.

author: FrankBuss
author_rank: Unknown
timestamp: August 14, 2019, 11:07:37 am
content: Sounds a bit complicated. I think I will use just use a 100 ohm / 1 W resistor in place of the fuse. This would filter some of the noise of an external power supply as well in combination with C2.

author: Kjelt
author_rank: Super Contributor
timestamp: August 14, 2019, 02:35:37 pm
content: Maybe too simple but why not use a full Graetz bridge and only connect the output to the Vin and GND ?That way there is no chance the - and + will be reversed, only problem is the double diode drop.Also there are these thermal fuses that come back after cooling down, forgot their name, pretty expensive though IIRC.

author: Kasper
author_rank: Frequent Contributor
timestamp: August 14, 2019, 03:04:20 pm
content: Ti app note slva139 might be useful for this.www.ti.com› lit › slva139PDFReverse Current/Battery Protection Circuits - Texas Instruments

author: FrankBuss
author_rank: Unknown
timestamp: August 14, 2019, 03:17:56 pm
content: Thanks, the bridge rectifier and using a FET are both good solutions for reverse polarity protection, but both don't help with the problem that the power supply would be shorted, if the GND of the power supply is the same GND as the GND of the devices connected to J1 or J2. But I think a resistor would work.Thanks for theresettable fuse idea, might work, too. They are not too expensive.

author: Peabody
author_rank: Super Contributor
timestamp: August 14, 2019, 03:43:29 pm
content: Am I the only one who doesn't understand why we have -9V and 0V, and why we apparently don't have a common ground?

author: FrankBuss
author_rank: Unknown
timestamp: August 14, 2019, 04:09:25 pm
content: Quote from: Peabody on August 14, 2019, 03:43:29 pmSee the full example I linked in the first posting:https://fuzzcentral.ssguitar.com/3knob.phpWith a battery it is no problem, the battery plus is the circuit GND and the battery minus is the circuit minus. A problem could happen, if someone uses an external power supply, which is not galvanically decoupled to the input/output devices and has the same GND as these devices, because the plus of the power supply would be the GND of the circuit, which then would be shorted to the GND of the power supply, through the GND of the devices connected to input/output. It shouldn't happen, manual would say to use isolated power supplies, but sometimes people don't follow the instructionsI plan now to use a big 100 ohm / 1 W resistor, which is not perfect, but would protect it.

author: mariush
author_rank: Super Contributor
timestamp: August 14, 2019, 05:23:32 pm
content: Any particular reason you're not simply using a voltage doubler / inverter IC?These only require a couple of ceramic (or electrolytic or whatever) capacitors to invert the voltage and they're available in through hole versions.You can also parallel some of these to get lower output impedance, more current etc ... ex see page 9 in the datasheet for TC7660 (see below, I've attached the datasheet).20..25mA :https://www.digikey.com/short/p59hcc40mA : TC7662 :link45mA : ICL7660 :link(note some 20mA max versions exist, link above filters and shows only 45mA versions)80mA : TC962 :https://www.digikey.com/products/en?keywords=tc962100mA : LT1054 :https://www.digikey.com/products/en?keywords=lt1054So you could one of these in inverter configuration and use a cheaper 9v or 7.5v DC adapter and you simply invert the voltage to -9v -ish...I filtered the ones in the link above to only show the ones which can handle 12v or more, because a lot of these simple charge pump regulators can only handle up to 6v or so.Then you could use a simple barrel jack connector which disconnects the battery when the dc adapter is inserted (maybe have a bulk capacitor so that circuit doesn't reset at that disconnect moment) or you could use some kind of OR / power management chip to switch between battery and DC in ... ex ICL7673 or LTC4412 or whatever)