APM Power Modules

Preamble

This blog leads on from the blogs Flight Controllers, see the section on APM/Additional Required Extras, (in particular Safetly Use CLONE APM Power Modules), as well as Low Voltage Warning for CC3D. There are varying reports that claim that these devices can pass the full battery voltage to the flight controller and fry it, but there are also others that say that this is scaremongering – so who is telling the truth? Also there are cheaper clones versions that lack certain components. This article explores both of these points.

Types of board

There are a couple of types of this board, principally the discontinued 3DR, see APM Power Module with XT60 Connectors Kit, and its direct clones (with or without the regulator and capacitor), and there are a couple of other Power Module derivatives (v1 and v5):

APM power module with regulator
APM power module with regulator

from Apm 2.6 2.5 2.52 Machtmodul mit 5.3v DC bec Bewertungen and Apm 2.6 2.5 2.52 Machtmodul mit 5.3v DC bec

3DR power module front
3DR power module front

and

3DR power module rear
3DR power module rear
RTFlyer Power Module
RTFlyer Power Module
Power Module v1
Power Module v1

rear:

Power Module v1 rear
Power Module v1 rear

Also, from APM2.5.2 APM2.6 Flight Control Board Pixhawk Power Module V1.0 Output BEC 3A Deans T Plug

Zoomed in:

And from APM Power Module V1.0 Vision 30V/90A with BEC for RC Model

High voltage power module V5
High voltage power module V5

and

From APM Power Module 3A (V5.0) U-BEC UBEC Ardupilot 2.5 2.6 Pixhawk XT-60, front:

APM power module 3A v50 UBEC (Front)
APM power module 3A v50 UBEC (Front)

Rear:

APM power module 3A v50 UBEC
APM power module 3A v50 UBEC
Geeetech XZN 3DR Power Module
Geeetech XZN 3DR Power Module

Schematic

hardware_designs / 3DR-APM25-Power-Module-V10

classic-bec10-mr-2

Zoomed

classic-bec10-mr-2_crop

Missing components

The cheaper 3DR clones are missing two components:

Regulator

There is a regulator board, for the (3+2) 5 pins, that attaches to the rear of the board.

From APM2.6/2.5/2.52 power module/current module with 5.3VDC BEC With XT60:

APM Power module showing regulator
APM Power module showing regulator

Here is a close up view:

3DR power module rear
3DR power module rear

It should be possible to be able to get these regulators seperately

Capacitor

There is a tantalum SMD capacitor, as can be seen here (from High Voltage APM Power Module with 3A UBEC), note that the positive terminal is at the top, marked by the brown line:

APM 3DR power module pinout showing SMD capacitor
APM 3DR power module pinout showing SMD capacitor

The SMD capacitor is a 300 μF, 25 V (E rating = 25V), (see Online calculator – 3 digit capacitor, Reference on KEMET SMD Tantalum Capacitors, The Footprint Files – Electrolytic Capacitors and Installing Capacitor Kits) and has marked on it:

307

25E

319

kemet_smd_tantalum_markings

and

SMD capacitor markings - Voltage rating
SMD capacitor markings – Voltage rating

 

Arguments against worrying about the clones

I think you are unnecessarily scaring people. Yes, the switching regulator is absent and that’s why they are cheap. In fact I like them this way. I want to measure v/i but don’t need the reg. The capacitor is missing because it is only needed when the regulator is fitted. If you want the regulator then it easy to purchase and fit yourself. I have done this to three of mine and the caps are easy to get too. It is not possible for this circuit to apply full battery voltage to your APM if the regulator is missing. There is just no physical connection other than through a resistor and the normal failure mode of resistors is open circuit. In fact, a failed regulator is much more likely to short the battery to the APM as with a switching regulator there IS a direct connection to the battery through the switching FET and inductor. A shorted FET WILL smoke the APM and this IS a common FET failure mode. There is also no need to change the cable wiring if the regulator is not fitted. The 5V wire just goes nowhere which effects nothing.

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