Current or voltage problem

RE: Current or voltage problem

benjaminpeere — Wed, 09 Jan 2019 11:09:28 GMT

Thanks for reply, I already know this, this is actually what I am doing, and this is not the problem, the problem is that I can't connect to the Bluetooth chip when it is powered via the LDO if I use the 2 batteries in series, of course the motor is not powered via the LDO it would have no sense to do this. And I think haakonsh is right, and my LDO has thermal shutdown and it shutdown because I provide him too much power, I looked for a new LDO that should be able to do the job

RE: Current or voltage problem

awneil — Wed, 09 Jan 2019 10:57:09 GMT

[quote userid="13562" url="~/f/nordic-q-a/42234/current-or-voltage-problem/164483"]reduce the current consumption from 1.4A[/quote]

As shown in my other post, the way to do that is to not supply the motor through the regulator!

It is highly unlikely that a motor requires a regulated supply.

[quote userid="74715" url="~/f/nordic-q-a/42234/current-or-voltage-problem"] I put the batteries in series to have more power[/quote]

But you totally defeat that by putting it through a series regulator!

RE: Current or voltage problem

awneil — Wed, 09 Jan 2019 10:43:28 GMT

[quote userid="74715" url="~/f/nordic-q-a/42234/current-or-voltage-problem/164519"]Do you think it will work if I use a voltage divider[/quote]

No.

Remember Ohm's law: the voltage dropped by the resistor is going to vary with the current through it. Because your stepper motor draws a large current, that will be a large voltage!

This is all basic electronics - nothing specific to Nordic.

You need to understand voltage, current, resistance, and power.

The thing to do in cases like this is to separate your low-voltage, low-power supply (for the microcontroller) from your high-power supply (for the motor) - something like this:

                                        +-----------+
                                        |           |
 high voltage supply ------+----------->+ regulator +-----------+-------------
                           |            |           |           |
                           |            +-----------+           |
                           |                                    |
                    +------+------+                      +------+------+
                    |    high     |       controls       |     low     |
                    |    power    +<---------------------+    power    |
                    |   load(s)   |                      |   load(s)   |
                    +------+------+                      +------+------+
                           |                                    |
                           |                                    |
                           |                                    |
GND -----------------------+------------------------------------+--------------

RE: Current or voltage problem

haakonsh — Wed, 09 Jan 2019 10:14:06 GMT

I think you need to scope the supply and find out what's really going on before you start fixing problems that might not exist.

RE: Current or voltage problem

benjaminpeere — Tue, 08 Jan 2019 12:43:13 GMT

Do you think it will work if I use a voltage divider bridge to reduce the input voltage?

RE: Current or voltage problem

haakonsh — Tue, 08 Jan 2019 10:10:07 GMT

"But I Don't understand because the LDO is supposed to support up to 25V Input voltage, why would 8.4V would be a problem?"

The LDO is not a good solution when you need to supply a large amount of current and step down a large amount of voltage because the LDO removes an amount of power equal to (Vin - Vout) * I from the supply. The power removed is converted to heat, and this is a fundamental limitation of any LDO.

"How am I suppose to reduce the current drawn from the LDO if this is the problem?"

If you cannot reduce the current consumption from 1.4A to less than:

I = Pd / (Vin-Vout) = 1200mW / (8.4V - 3.3V) = ~235mA,

Then you need to reduce the input voltage to the LDO to:

Vin = Vout + (Pd / I) = 3.3V + (1200mW / 1.4A) = 4.157V.
These calculations are only valid given that you're drawing 1.4A from the LDO as you previously stated.

If you cannot lower the current drawn from or the input voltage to the LDO then you need to either use a hefty heatsink, or use a Switch Mode Power Supply, commonly known as a Buck/Boost Regulator.

"In output i have a stable 3.3V (I measured it with my multimeter)"
You need an oscilloscope. The supply can be very noisy and the multimeter will probably not capture it.

RE: Current or voltage problem

benjaminpeere — Tue, 08 Jan 2019 09:37:41 GMT

But I Don't understand because the LDO is supposed to support up to 25V Input voltage, why would 8.4V would be a problem? also it is absolutely not hot when I touch it.

How am I suppose to reduce the current drawn from the LDO if this is the problem?

I am sure that it is not due to the stepper motor, because the stepper motor is not on the circuit at this moment.

RE: Current or voltage problem

haakonsh — Tue, 08 Jan 2019 09:26:15 GMT

According to the BA33DD0T datasheet the thermal dissipation of the LDO is ~1200mW. You need to dissipate (8.4V - 3.3V) * 1.4A = 7.14W. If the LDO has a thermal shutdown then that might reset the nRF51 module continuously. If that's the cause then you need to either add the required heatsink or reduce the current drawn from the LDO.

Another probable cause is supply noise from the stepper motor, if that's the case you need to filter the nRF51 module's supply.

Either way you need to scope your power supply and see what's going on.