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schematic

simulate this circuit – Schematic created using CircuitLab

I am working with PD Buddy Sink, a small USB-C PD negotiation and power breakout board. It takes USB-C PD input, performs the PD negotiation, and produces DC power. My problem is that the PD Buddy Sink does not block reverse current flow. I am attempting to use this device to charge an SLA battery. I have a SLA-05VDC-SL-A relay-based charge controller just after the sink. The problem is occurring when the relay closes and voltage from my battery bridges through the relay and into the sink's output, causing it to attempt to negotiate with the battery, resulting in a never-ending startup cycle and no resulting "forward" current flow.

I have spoken with the maker of this device and she recommended a diode. I've tried a Schottky 1N5822 and a Rectifier RL207, both of which are leaking reverse voltage on the order of about 15-20 millivolts. This is still too much for the PD Buddy Sink, resulting in the setup cycle.

Is there another solution to my problem to effectively "completely" eliminate reverse current flow from my battery/relay to the sink? I've considered a buck converter to drop the sink's 15-20 volt output to 14.x volts, but not sure if that would prevent the reverse flow I'm working against now.

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    \$\begingroup\$ What do you mean by "leaking reverse voltage...about 15-20 millivolts"? That doesn't make sense. \$\endgroup\$ Apr 20 '20 at 17:17
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    \$\begingroup\$ Welcome to EE.SE. A schematic is better than words. You can add one in using the CircuitLab button on the editor toolbar. Double-click a component to edit its properties. 'R' = rotate, 'H' = horizontal flip. 'V' = vertical flip. Note that when you use the CircuitLab button on the editor toolbar and "Save and Insert" on the editor an editable schematic is saved in your post. That makes it easy for us to copy and edit in our answers. You don't need a CircuitLab account, no screengrabs, no image uploads, no background grid. \$\endgroup\$
    – Transistor
    Apr 20 '20 at 17:24
  • \$\begingroup\$ @ElliotAlderson Apologies for the incorrect terminology. I mean when I use my voltage meter on my SLA battery, I get 12.7 volts. When I put a diode between the volt meter and the battery positive terminal, the diode blocks 12.68 volts, "leaking" approximately 20 millivolts in reverse. \$\endgroup\$
    – Nat West
    Apr 20 '20 at 17:37
  • \$\begingroup\$ @NatWest ok, this got me even more confused. Current can be leaked or blocked, voltage not. We'll not be smarter about what you're describing until you add that schematic. \$\endgroup\$ Apr 20 '20 at 17:58
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    \$\begingroup\$ @MarcusMüller I've added a schematic. In the diagram I've included, my voltage meter is measuring 12.68 volts. I need it to read zero. \$\endgroup\$
    – Nat West
    Apr 20 '20 at 18:10
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enter image description here

Figure 1. The RL207 datasheet explains it.

If your multimeter has a 10 MΩ input impedance then 1 μA will give V = IR = 1μ × 10M = 10 V. This is very close to what you are seeing.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. (a) The circuit redrawn in a more conventional manner with + on top. (b) Loading the circuit.

Try adding a resistance to shunt the stray current away from the multimeter and measure the voltage again. Report back!

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  • \$\begingroup\$ Now to find a resistor. My garage is not stocked for these kinds of things. I'll probably have to break open something to get one. Would I be able to find one in an AC-DC wall wart? \$\endgroup\$
    – Nat West
    Apr 20 '20 at 19:06
  • \$\begingroup\$ After applying a 100 k-ohm resistor, the voltage at the meter went to zero, exactly as you suggested @Transistor. (I'm using a Fluke 179.) But my problem still stands. Something inside the PD Buddy Sink is causing it to continually reset when hooked up to the battery. I'm reaching out to the manufacturer for assistance. \$\endgroup\$
    – Nat West
    Apr 21 '20 at 22:32
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So, with the clarifying schematic:

Your diode is in reverse operation, so it will leak very little current. In fact, let's have a look at the reverse current plot:

RL207

RL207 reverse characteristics

(rated reverse voltage = 1000V, so you're worst case in the 1% range – reading the urve at [%] = 0 is kind of unscientific, but the takeaway is: < 1µA current leakage.

1N5822

1n5822 reverse characterisitics

(same here: < 10 µA leakage at 12 V)

Assessment

Now, how much is 10 µA? Well, your USB devices won't mind these, as even if protection diodes would have to short 10 µA from 12V to ground, that'd be 120 µW of power. The technical term for that is "nothing". I'd say you've "completely" eliminated reverse current flow.

Does it discharge your batter? You've got a lead-acid battery. These have a typical self-discharge of 4% per month, meaning that just not using it at all will look like there's a resistor connected, discharging it. A month has 720 hours. If your SLA has 1 Ah of capacity, it will lose 4% of 1 Ah in 720 hours. That makes 55 µAh per hour, i.e. a self-discharge current more than 5 times as high than what is leaked.

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  • \$\begingroup\$ Hi Marcus, thanks for this. I'm not concerned about the SLA running low on power when not in use. I'm just trying to eliminate any current getting to my PD Buddy Sink, which can't handle any voltage. Here is the schematic for it: cdn.hackaday.io/files/20424873820416/pd-buddy-sink_sch_1.0.pdf \$\endgroup\$
    – Nat West
    Apr 20 '20 at 18:51
  • \$\begingroup\$ ... sigh. Voltage doesn't "get anywhere". You're still confusing voltage and current. And I specifically addressed your USB device getting reverse current in my answer! \$\endgroup\$ Apr 20 '20 at 19:03
  • \$\begingroup\$ My apologies. I'm only measuring voltage hence my consistent misuse of the terminology. I must not be understanding your assessment. I understand that we are talking about a very small amount of current. However, my situation is that whatever current is getting past the diode is too much for the PD sink. I'm going to try the resistor per Transistor's comments and will report back. I don't understand the needs of the PD Buddy Sink (schematic linked above). Might there be something in there that can further clarify my situation to provide a good solution? \$\endgroup\$
    – Nat West
    Apr 20 '20 at 19:59
  • \$\begingroup\$ "whatever current is getting past the diode is too much for the PD sink." that is not true. How would that be true? if that was true, your PD sink would be destroyed by random noise. \$\endgroup\$ Apr 20 '20 at 20:39
  • \$\begingroup\$ @NatWest Ok, let's put it the other way around: you seem to have a source for "even the leakage current of a diode can damage the device". Could you point us to that source? Because honestly, looking through that schematic, I don't find the least indication of that! And it would be pretty senseless to debunk a claim that came from thin air, just as spending an hour debunking that stones can autonomously fly – since they can't, and it's not clear why they even should. \$\endgroup\$ Apr 20 '20 at 20:48

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