Reverse battery or DC polarity protection - Revision history

Mkrdwiki: /* How to implement a reverse-polarity protection properly */

2012-12-28T23:21:17Z

How to implement a reverse-polarity protection properly

← Older revision		Revision as of 19:21, 28 December 2012
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	1. Use a fuse! Don't substitute a PCB trace for a fuse!		1. Use a fuse! Don't substitute a PCB trace for a fuse!

	2. Use a PTC resettable fuse in place of a single-use conventional type.		2. Use a PTC resettable fuse in place of a single-use conventional type.

	3. Use a forward-biased diode instead of a reverse-biased one. Engineers do not like this method because the forward-biased diode reduces the power supply voltage by VF, and dissipates VF*IF loss power. A Schottky diode will have a voltage drop of about 0.3V, however.		3. Use a forward-biased diode instead of a reverse-biased one. Engineers do not like this method because the forward-biased diode reduces the power supply voltage by VF, and dissipates VF*IF loss power. A Schottky diode will have a voltage drop of about 0.3V, however.

	4. For full operation with any polarity input, a rectifier bridge can be used. However, the voltage and power loss will be double that of a single forward-biased diode.		4. For full operation with any polarity input, a rectifier bridge can be used. However, the voltage and power loss will be double that of a single forward-biased diode.

	5. Use a FET sensing switch. Since most supplies already have a high-side P-channel FET switch for power On/Off function, this low-resistance switch can be used for polarity protection. Only the sensing circuit has to be reverse-polarity tolerant; as soon as it detects the incorrect polarity it is to disable the rest of the circuit by turning the MOSFET OFF.		5. Use a FET sensing switch. Since most supplies already have a high-side P-channel FET switch for power On/Off function, this low-resistance switch can be used for polarity protection. Only the sensing circuit has to be reverse-polarity tolerant; as soon as it detects the incorrect polarity it is to disable the rest of the circuit by turning the MOSFET OFF.

Mkrdwiki at 23:20, 28 December 2012

2012-12-28T23:20:37Z

← Older revision		Revision as of 19:20, 28 December 2012
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	[[File:CB Radio blown trace3.jpg\|CB Radio blown trace]]		[[File:CB Radio blown trace3.jpg\|CB Radio blown trace]]



			I fixed the radio by placing a jumper where the trace lifted completely off the board. I tinned another trace (since overheated copper loses its electrical properties). And I have bypassed the trace-connector-trace arrangement with a second wire jumper (white wire).

			Note that I should have wired in a fuse, since I now have a thicker wire going to the diode. With this arrangement, the diode is likely to overheat before the new wire does.

			Repair effort:

			[[File:CB Radio repaired trace.jpg\|CB Radio repaired trace]]

			[[File:CB Radio repaired trace2.jpg\|CB Radio repaired trace]]


			==How to implement a reverse-polarity protection properly==

			1. Use a fuse! Don't substitute a PCB trace for a fuse!
			2. Use a PTC resettable fuse in place of a single-use conventional type.
			3. Use a forward-biased diode instead of a reverse-biased one. Engineers do not like this method because the forward-biased diode reduces the power supply voltage by VF, and dissipates VF*IF loss power. A Schottky diode will have a voltage drop of about 0.3V, however.
			4. For full operation with any polarity input, a rectifier bridge can be used. However, the voltage and power loss will be double that of a single forward-biased diode.
			5. Use a FET sensing switch. Since most supplies already have a high-side P-channel FET switch for power On/Off function, this low-resistance switch can be used for polarity protection. Only the sensing circuit has to be reverse-polarity tolerant; as soon as it detects the incorrect polarity it is to disable the rest of the circuit by turning the MOSFET OFF.

Mkrdwiki at 23:12, 28 December 2012

2012-12-28T23:12:13Z

← Older revision		Revision as of 19:12, 28 December 2012
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	There was no fuse; additionally, the excessive current flowed across a whole board, up a connector, and back fully across a second board before finding the reverse diode! This is the mayhem that resulted:		There was no fuse; additionally, the excessive current flowed across a whole board, up a connector, and back fully across a second board before finding the reverse diode! This is the mayhem that resulted:

			[[File:CB Radio blown trace.jpg\|CB Radio blown trace]]

			[[File:CB Radio blown trace2.jpg\|CB Radio blown trace]]

			[[File:CB Radio blown trace3.jpg\|CB Radio blown trace]]

Mkrdwiki at 22:55, 28 December 2012

2012-12-28T22:55:31Z

← Older revision		Revision as of 18:55, 28 December 2012
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	[[File:CB radio blown trace.png\|Reverse polarity protection]]		[[File:CB radio blown trace.png\|Reverse polarity protection]]


			There was no fuse; additionally, the excessive current flowed across a whole board, up a connector, and back fully across a second board before finding the reverse diode! This is the mayhem that resulted:

Mkrdwiki at 22:54, 28 December 2012

2012-12-28T22:54:17Z

← Older revision		Revision as of 18:54, 28 December 2012
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	[[File:Reverse-polarity-protection(1).png\|Reverse polarity protection]]		[[File:Reverse-polarity-protection(1).png\|Reverse polarity protection]]

			This method is picked because it is cheapest and least lossy. I will discuss more appropriate methods later.

			Diode D1 does not do anything, and does not waste any power on all of those occasions that the power is applied correctly. However, if the polarity is reversed:

			[[File:Reverse-polarity-protection.png\|Reverse polarity protection]]

			Then diode D1 conducts, and blows out the fuse.

			That would keep the device protected. However, I came across a device recently which used a ''PCB trace'' for the fuse!


			I forgot the convention that the white stripe on a power cord connects to the center conductor of a DC "barrel" style connector.

			When I opened up my Cobra handheld CB radio, I have discovered that this is what they used for their protection:

			[[File:CB radio blown trace.png\|Reverse polarity protection]]

Mkrdwiki at 22:49, 28 December 2012

2012-12-28T22:49:35Z

← Older revision		Revision as of 18:49, 28 December 2012
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	[[File:Reverse-polarity-protection(1).png~~\|thumb~~\|Reverse polarity protection]]		[[File:Reverse-polarity-protection(1).png\|Reverse polarity protection]]

Mkrdwiki at 22:49, 28 December 2012

2012-12-28T22:49:19Z

← Older revision		Revision as of 18:49, 28 December 2012
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	Engineers often pick this as their reverse battery or DC supply protection feature:		Engineers often pick this as their reverse battery or DC supply protection feature:


			[[File:Reverse-polarity-protection(1).png\|thumb\|Reverse polarity protection]]

Mkrdwiki at 19:55, 10 December 2012

2012-12-10T19:55:16Z

← Older revision		Revision as of 15:55, 10 December 2012
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	,,		Engineers often pick this as their reverse battery or DC supply protection feature:

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2012-12-10T19:27:49Z

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