Simple reverse-polarity-protection circuit has no voltage drop

10/22/2011 3:54 AM EDT

you could have a millisecond of reversed power until the relay operates if this is directly connected to your circuit.

10/24/2011 3:36 PM EDT

Figures 3

10/24/2011 3:38 PM EDT

The basic idea is sound but relays take a few msec to energize and open the NC contacts. This short reverse-voltage pulse is enough to damage some circuits, although most wouldn't be affected.

To avoid applying *any* reverse voltage, wire the circuit with the opposite sense so that the relay's NO contacts pass the supply current and the relay only energizes with correct polarity. This way, reverse voltage will never be applied to the supplied circuit.

10/24/2011 3:46 PM EDT

Are Fig. 3 and Fig. 4 reversed? Shouldn't the NC terminal be connected to the source (passing source to output) when the polarity is correct?

10/24/2011 3:59 PM EDT

Diagrams have been swapped. Figure 4 cannot have any power flowing through the red LED as drawn. If you swap the diagrams with the lables the process is correct and you get no power to the load until the input power has been verified to be of the correct polarity by the relay control circuit.

10/24/2011 4:11 PM EDT

TTo clarify by swapping labels, I mean Relay NO (Normally Open) should be NC (Normally Closed) contact and vice versa. Connections would then be correct if Figure explanations for figure 3 and figure 4 are swapped.

10/24/2011 4:16 PM EDT

By swap labels, I mean the relay normally opened and normally closed (NO and NC) respectively are swapped. And then the Figure 3 and Figure 4 descriptions are listed under the wrong diagram, they need to swap as well. Then the control circuit correctly applies power to the load after input power bias has been verified.

10/24/2011 4:22 PM EDT

Hey! half my comment was deleted! I should have gotten credit for finding the error first, but instead I look like a dolt ;) LOL

I'm so mad (not really.... it's funny actually)

10/24/2011 4:51 PM EDT

Nice idea, but.... you've reversed the figures.

10/24/2011 5:25 PM EDT

The problem of this circuit is that the load sees the reverse polarity during the response time of the electromechanical relay. This, as we all know, most electronics will be fried by then. I would rather waste little power to the relay and require the relay to be ON only when the correct polarity power is connected and then connect that power to the load.

10/24/2011 5:50 PM EDT

If you think that a Red Led is a sufficent indicator then it may be suitable for battery powered applications, but I dont think so :( Prefer to use mosfets.

Delibalta

10/25/2011 3:31 AM EDT

Protects the load from connecting the source reverse polarity, but doesn't provide protection if the load is connected cross-eyed.

10/25/2011 8:23 AM EDT

In the 'old days' a battery symbal was such that the long bar was designated (+) positive and the short bar was (-) negative. Has that chnaged at some point? If not, Fig. 3 is forward biasing D(1) and Fig. 4 is reverse biasing D(1). The relay positioned shown would be valid only if B(1) was turned around.

10/25/2011 9:08 AM EDT

I use a similar approach for mobile amateur radio equipment that could be incorrectly connected to field supplied power sources. But I agree with the previous comment that the master relay should only be pulled in and provide power to the loads after the source is properly protected.
73
KD4MRS

10/25/2011 2:50 PM EDT

The issues with this circuit are:
1- The short life of the Relay.
2- The "bounce" issue
There are other ways to solve that issues without voltage drop or creating the relay damage & spikes.
FYI:
Electronics is an art, hope your teacher told you, always ask your self what about the MTBF [Google it] and what about causing an unwanted dangers signals! its means: Good Thinking==> Top Results, as a beginner design its OK -Good Luck.
dov.rossitzan@argoncrop.com
www.argoncorp.com

10/25/2011 11:13 PM EDT

@Klaus Hauptmann
it seems editors have mistakenly interchanged the discriptions for fig 3 and fig 4

10/26/2011 4:40 AM EDT

Agree relay should be operated when supply polarity correct.
Relays do not need as much current to stay operated (hold) as they do to operate. Therefore to save power a [large electrolytic cap in parallel with a resistor to set the hold current] in series with the relay coil would save power.
Relay reliability can be greatly improved by connecting a snubber resistor (about 50 ohms) in series with a cap (100nF for example) across its contacts to quench sparks.

10/26/2011 1:07 PM EDT

While a novel approach there are several shortcomings, some of which have been mentioned:

1. Relay activation time: Can take several milliseconds to fully switch, by then the circuit could be damaged.

2. Contact bounce as mentioned.

3. Reverse surge through contacts could shorten life.

4. Size and cost of relay.

I recently used an LTC4411 Ideal Diode for a reverse polarity circuit. Good for 2.6A at 5.5V, and super low leakage. Otherwise find the lowest Vf Schottky diode that you can.

10/26/2011 10:25 PM EDT

RElay life would not be a consideration since in the correct connection the relay is not operated, thus there would be no error. Likewise, relay power is not an issue because the relay is only engaged when the connection is reversed. The one potential for serious damage is in the time between the power being connected incorrectly and the time the relay contacts open. This damage is prevented if there is an open power connecting switch in series, following this protective circuit. In that case the results would be as claimed. If a power control relay is utilized to switch the protected device's power, than a simple diode in the control relays coils circuit could offer complete protection. Of course, then there is the power relay coil current as wasted power.

10/28/2011 3:35 PM EDT

I dont know how many times this circuit has been "re-heated" since the "ideas" columns in the 1970s, but it seems to crop up at regular intervals. It is hardly original.

10/28/2011 8:11 PM EDT

It would be safer to have the relay open by default, as damage may occur before the contacts disengage. Old, old idea, but there are entire generations of EEs who need the common sense...

11/1/2011 6:39 PM EDT

1.There some mistakes with battery electrodes images on Fig.3 and Fig.4.
2.Fig.3 the battery’s electrode positive terminal should be shown as a long and the negative terminal should be shown as a short on schematics. It means the positive battery terminal attached to ground. So the relay should be ON and contact S1 should be connected to NO and LED is ON.
3.Fig.4 the negative battery terminal is attached to ground, relay is OFF.
4.The relay contact resistance is not equal to zero and is dependent of relay design and could be in range from 5-10mOhms for high power relays and up to 0.25 Ohm or higher for reed relays, so it could be comparable with MOSFET Ron resistance.
5.Use a relay for switching power for a load with a capacitor in parallel will be significantly worse than use MOSFET.

12/20/2011 1:56 PM EST

One other option that would be simple and less expensive would be to use a polymer PTC PolySwitch device in series, and then place the diode in the parallel leg. If you accidentally plug in the battery backwards, the diode would be forward biased, and you would have a high short circuit current that would quickly trip the PolySwitch device. Since the diode is in the parallel leg, it would add no voltage drop. Depending on the PolySwitch device used, it would add very little resistance to the circuit. The additional cost of these 2 components will be much less than a relay circuit, and the PolySwitch device would help provide resettable overcurrent protection with no warranty returns or service calls.

1/4/2012 12:30 AM EST

This is an interesting idea, but it does have at least one flaw.

The design assumes that the power connected backwards will supply enough voltage and current to power the relay coil. Suppose the device is battery powered and the replacement battery, plugged in backwards, is used and doesn't have enough power to operate the coil. (Many people will 'try' a used battery to limp along until a new replacement can be acquired.)

It is quite possible that the power supply may not be able to activate the relay, yet still has enough power to damage the circuit it is intended to protect.

9/7/2012 1:42 PM EDT

People still make relays??? ;)

9/7/2012 5:43 PM EDT

An alternative solution is MOSFET with an ideal diode controller. When the FET is on, there is very little drop across it.

2/18/2013 3:16 PM EST

It is not reliable enough, due to there is a time between applying wrong polarity to disconnect relay, it can be several milliseconds. In addition, relay has small MTBF in contrast with solid-state device.
Any way it may be helpful for a FM radio!

2/22/2013 2:35 PM EST

Nice try.. but not really all that useful.
As stated in earlier comments:
- too slow to protect many electronics circuits.
- complexity impacting reliability
- P channel MOSFET would likely provide nearly the same benefits without the drawbacks.
Alternatively, if a fuse (self healing?) is already a requirement in the circuit or the power source is current limited (higher impedance battery?), a diode across the input (not in the normal current path) will protect the circuitry down stream without causing a voltage drop.

ALL solutions for protecting against reverse polarity have some losses. Even if it is only the drop across a few micro ohms or minor leakage current.

NO voltage drop? Zero! ? really?
If I have told you once , I have told you a million times, stop exaggerating! ;)

2/23/2013 1:56 PM EST

It is possible to have a fuse feed the protected circuit and use a reversed biased diode connected to the gate of an SCR. If reverse polarity is applied, the diode fires the SCR which crowbars the fuse, blowing it open to protect the downstream circuit from reverse polarity. A couple of caveats: the circuit will see the voltage drop of the SCR for a short time until the fuse clears, and this crowbar protection scheme requires a fuse replacement. I have used this circuit in applications where the caveats were not critical. Protection was speedy and both technically and cost effective.

Best regards,

Myron Boyajian