SparkFun Forums

[paynterf]

I'm trying to use the VCC output of a Sparkfun PowerCell to drive the 5V DC (168 ohm) coil of a DPDT relay. The relay is supposed to be energized whenever the PowerCell has 5V applied, either via the USB connector or via the 5V input. However, at least with the unit I have here, it doesn't work.

If I drive the relay with a 5V lab power supply, it works fine and exhibit the appropriate current flow for a 168-ohm coil resistance (approx 30mA).

If I leave one wire to the relay coil disconnected so I can first make the USB connection and then apply VCC to the relay, it works fine (I can hear the relay click and measure the change in contact connections).

if I connect both wires (VCC and GND) to the relay coil, and then plug in the USB connector, the relay does not engage (no sound, no change in contact connections)

What gives here - is there some startup problem with the PowerCell with inductive loads? Is there any workaround to this issue?

TIA,

Frank

[Mee_n_Mac]

The intelligent charger IC is probably tripping on it's under voltage lockout. From the PDF ...

There is also an under voltage lock out (UVLO) set at 2.6V with a couple of resistors, along with a jumper on the back of the board to disable the UVLO. The UVLO prevents your battery voltage from dropping too low and damaging the battery. Only advanced users should disable the UVLO.

I'm unsure why you think it should be a relay driver.

[paynterf]

The intelligent charger IC is probably tripping on it's under voltage lockout. From the PDF ...

There is also an under voltage lock out (UVLO) set at 2.6V with a couple of resistors, along with a jumper on the back of the board to disable the UVLO. The UVLO prevents your battery voltage from dropping too low and damaging the battery. Only advanced users should disable the UVLO.

I'm unsure why you think it should be a relay driver.

Hmm, interesting - but the relay coil is connected to the output of the boost regulator, so I don't see how the UVLO comes into play.

In any case, I tried disabling the UVLO via the provided shorting pads, but the only difference it made in the behavior was that with it disabled (shorted), VCC won't run the relay coil in *either* configuration (USB first, then coil connections made or USB with coil already connected to VCC). When I re-enable the UVLO by un-shorting the pads, it reverts to the original behavior (will properly energize the coil when the coil connection is made after USB power is applied, but not if the coil connections are made and then USB power is applied).

In the 'bad' configurations, the VCC output is around 1V DC when connected to the relay coil, so maybe the output has switched to some sort of high impedance state?

Frank

[paynterf]

Just for grins, I tried the same experiment with a second PowerCell, and got exactly the same behavior

Frank

[UhClem]

It appears that the inductive load is causing trouble with the soft start. Try it with a resistor equal to the DC resistance of the coil to verify. If it works with the resistor then it is the inductance.

If so then you are going to have to add a circuit to delay applying power to the coil.

[paynterf]

David

Good idea about checking with a resistive load. Unfortunately, it appears that the problem isn't load reactance - rather it is load current. With various resistive loads from about 1KOhm to 10KOhm, I was able to figure out that the problem starts occurring right around 1mA load current. If the load draws less than 1mA the VCC output will immediately rise all the way up to 5V. Above that amount, and the output will limit to approx 1mA.

In all cases, momentarily interrupting the load will cause the PowerCell VCC output to rise all the way to 5V, at which point it has no trouble sourcing at least 30mA.

This seems a bit strange for an output that is provided explicitly to power unknown peripherals, but both the units I have exhibit the same behavior. Unless I am badly misinterpreting the datasheet for the TPS61200, this is not correct behavior. From https://www.sparkfun.com/datasheets/Pro ... s61200.pdf, I see "Startup into Full Load at 0.5 V Input Voltage", which I take to mean I could have a 600mA load (assuming the USB power source could handle it) and the device would still start up correctly as soon as the input voltage rises above 0.5V. Am I missing something?

Frank

[UhClem]

I just realized that it appears that you are using this without a battery attached. This means that the charge controller might be the problem.

Have you tried using a battery?

[paynterf]

David,

The batteries don't get connected to the PowerCell until *after* the relay gets thrown. The whole point of this exercise is to manage a situation that requires two Li-Ion batteries in series to run the motor controller & Arduino Uno, but the batteries can only be charged in parallel (or independently, which is what I plan to do, using two PowerCells). So, the solution was to add a DPDT relay that switches the battery stack between series and parallel, and the natural switching signal for this is the VCC output of one of my two PowerCells.

Frank