Pololu Robotics & Electronics
Menu
My account Comments or questions? About Pololu Contact Ordering information Distributors

Pololu Forum

An old AC adaptor with an output rating of 6vdc malfunctioned and output nearly 10vdc, frying a Hi-Tec HS-485HB servo motor

Hello. I’m an amateur at electronics/ robotics/ software/ etc. and I could really use some help understanding how to safely power Maestro servo controllers and servo motors.

I own a Micro Maestro 6-channel servo controller, and I was powering three Hi-Tec HS-485HB servo motors with 5vdc supplied from a voltage regulator to the GND and PWR pins on the board. The motors worked well, but I wanted to experiment with an old external AC adaptor by powering one of the three servos with 6vdc instead of 5vdc (I know 6vdc is safe to use with these servos). I pulled out the GND and PWR wires from the board and hooked them up to the output voltage coming from the AC adaptor. I kept the signal wire the way it was. After trying to test it, I noticed there was some smoke in the air. The AC adaptor fried my servo! I quickly removed power to the servo. I tried reconnecting it back to the Maestro, but the servo was so far gone that it created a short in the voltage regulator powering the other servos. The bad servo now has that burnt electrical smell. I tested the AC adapter with two separate multimeters and found that it was putting out nearly 10 volts! I believe this extra voltage is what killed my servo. The AC adaptor has gone bad, and is not putting out the correct amount of voltage now.

Why did this older AC adaptor go bad? Could this happen to my other older AC adaptors if I consider using them? Do you have any advice on what the safest way to power servo motors and/or Maestro controllers is?

Thank you for your time.

-Isaiah

1 Like

Some of my cheapo unregulated DC output AC adapters only output the rated voltage if you sink the rated current. If you sink less, the voltage is higher. Maybe your adapter is the same? You can measure it with various power resistors as the load and see.

1 Like

That is typical of unregulated AC adapters. The nominal voltage is specified at a typical load.

You undoubtedly overloaded the adapter at the same time the servos were fried.

In general, servo power supplies need to be regulated, and able to supply 4.8 to 6V, at current of least (number of servos)*(servo start/stall current) Amperes. The start/stall current of the HS-485HB servo is 1.2 Amperes.

1 Like

Thank you. This is informative. I’ll include an image of the dc regulator I’m using to power my servos.

Is this a safe device to use? It’s a dc to dc regulator and it displays both voltage going in and voltage going out.

Also, because the start/stall current of the HS-485HB servo is 1.2 Amperes, does this mean that wiring up 4 of these identical relays in parallel to the Maestro, I’ll need 4.8 amperes in total to power the servos? I’m confused.

Thank you.

Hello, Isaiah.

As jlo and Jim have pointed out, the AC adapter could have been the culprit, as unregulated AC adapters typically output a higher voltage when not under a load. However, I also wanted to point out that if you want to power one of your servos from a different voltage, you should also be sure that the servo still shares a common ground with the Maestro.

It sounds like you might have disconnected the power and ground wires from the Maestro and connected a separate power supply to the servo, leaving the signal wire as the only wire connected from the Maestro to the servo, which means they do not share a common ground. In this kind of setup, you should also have a ground wire from the Maestro to the servo. It is unlikely that not having a common ground would have caused damage to the servo, but without a common ground it would still not work properly (you might get some erratic or jittery movements, if anything).

As far as the regulator you posted about, without information about it’s specifications, it is difficult to make any meaningful assessment. In general, it is good practice for your supply to be able to handle the full load of all of your devices, so ideally, your power supply and regulator would be able to handle at least 4.8A. Depending on your application and what the servos are doing, you might be able to get away with slightly less (e.g. if only 1 or 2 of the servos is moving at a time).

It sounds like you are referring to the regulator when you say relay. If that is the case, please note that we do not recommend connecting the outputs of regulators in parallel. A better solution would be to choose a more powerful regulator that can handle the full current demands of your system.

Brandon

Yes, I meant to say servos when I said relays.

Yes, I was not sharing a common ground between the Maestro and the servo.

Thank you Brandon.

I just found out that the regulator in the image I sent has a maximum output current of 3 amps. So I’ll need something that can output more amperage than this. For my setup, I want to program a sequence that controls the movement of an animatronic prop for Halloween. I know that the number of servos I’ll need to get all the different movements I want will be seven or eight in total. All of these will be moving at roughly the same time in sync with each other. I’ll be using 4 Hi-Tec HS-485HB servos. The other 3 or 4 servos will have to be rated at a higher output torque because of higher work loads.

Do you have any recommendations on a specific voltage regulator I should use to be able to handle the high current demand?

I really appreciate the help.
Thank you.

Isn’t connecting a voltage source to the GND and PWR pins of the Maestro considered wiring the servos in parallel? Am I incorrect about that?

Before you choose a regulator, you need to decide on the power requirements, and that means deciding on the final servo complement.

As I mentioned before, the safest approach for a servo power supply is to make sure that it can supply more current than the total of the servo start/stall currents.

Why? Because when servos are overloaded or misbehaving, the voltage of an inadequate power supply will drop, which triggers all the servos to start moving.

The result is a wildly twitching, totally out of control gizmo, which might be fun to watch for a few moments.

Yes, connecting the servos to the Maestro power rail will power them in parallel; I was referring to supplying power to that rail with multiple regulators in parallel.

As Jim mentioned, you should determine the maximum current draw of your final system, and use that information to find an appropriate voltage regulator (as well as AC adapter or battery). If you can tell us the combined maximum current draw of all of your servos as well as details about your power supply, I might be able to help select an appropriate regulator.

By the way, you mentioned needing 7 or 8 servos in total for your final application; since the Micro Maestro only has 6 servo channels, you might consider upgrading to a 12-channel Mini Maestro. Also, if your current demand is high enough, it could be useful to split the Mini Maestro’s servo power rail into banks and use separate regulators to power each bank.

Brandon

Thank you Brandon. All this info is really helpful.

I’ll get back to you on the total current draw when I get the math sorted out.

For splitting the maestro power rail into 3 separate power banks for 7 or 8 servos, would the servos be powered externally, with an additional GND wire going back to the board from the GND wires on the servos?

Thanks,

-Isaiah

The servo power rail on the Mini Maestros can be split into separate banks of 6 servos by cutting the trace on the underside of the board. For the 12-channel Mini Maestro, this means you can split the power rail into 2 banks, and power each of them from a separate power source. The cuttable trace is indicated by the dashed line in this picture:

If you want 3 separate banks, you might consider the 18-channel Mini Maestro:

Powering the servos separately from the Maestro board like you described is also an option.

Brandon

1 Like

Thank you Brandon.

So, I think I’ve made the decision to stick with just 3 servos; 3 Hitec HS-485HBs, and each one requires 1.2 amps of start/stall current. So I’ll need at least 3.6 amps. These servos will be running at the same time when working.

So, would the suggested AC adapter/transformer I’d need to buy be a 6vdc 5amp max power supply? And then, should I feed this through a voltage regulator that can output 5 or more amps of current?

Thank you.

-Isaiah

A 6V, 5A supply should be fine for 3 of those servos. A lot of modern wall-wart style power adapters have built-in switching regulators, so you probably shouldn’t need an external regulator unless your AC adapter is unregulated.

Brandon

Do you have a specific product you’d recommend?

Thank you.

Thrift shops are a great source of cheap, regulated, high quality AC adapters, as many are discarded by people every day. The voltage and current characteristics are always printed on the enclosure.

Of course you can always buy suitable ones on line.

Unfortunately, I do not have any specific recommendations for a 6V power adapter (the closest we carry is a 5V, 5A one). However, as Jim mentioned, they are widely available, so you can probably find one locally, or search around online.

Brandon