View Full Version : Which power supply to run servos directly off PDB?

02-02-2007, 11:17 PM
Hi guys,

The power supply for the PDB is either 9 VDC and 300 mA or 7.4 VDC and 1000 mA. However, I am holding a standard servo in my hands. Voltage drop from Vdd to Vss is 5 VDC on the PDB. Current is less than 1000 mA or 300 mA, depending on the power supply purchased. However, the servo draws 300 mA at rest and 4200 mA when locked. So, do I need to purchase a 9000 mA or more power supply? In other words, if I want to run standard servos off my PDB, do I need to buy a 7.4 VDC and 9 amps plus power supply?


02-02-2007, 11:26 PM
I suggest using the 7.5v @ 1amp power supply when powering servos.· Here is a link for more information:· http://www.parallax.com/detail.asp?product_id=750-00009. (http://www.parallax.com/detail.asp?product_id=750-00009)· The 7.5v power supply will be sufficient for this project.



Dave Andreae

Parallax Tech Support·

Mike Green
02-02-2007, 11:34 PM
You won't be able to run the servo when stalled off the PDB's 5V supply since the 5V regulator will shut down much above 1000ma. You should really have a separate supply if you're going to use a large servo or several smaller ones. Servos are not really designed for higher voltages (like 7.4). Although they will work, the brushes inside will wear out a little quicker with the higher voltages. Best would be a 6V supply with enough current capacity. If you intend intermittent use, a 6-7.2V battery pack with a float charger would be ideal. It could supply several amperes for a few minutes at a time while the float charger could supply the "resting current" plus a small charging current.

If you want to run the servos off the 7.5V supply, you could put one or two diodes in series and each will drop about 0.7V. The diodes need to be rated for the current.

02-03-2007, 12:57 AM
So, let me get this straight, you can run standard servos off the PDB with no load for testing purposes only if you have a 7.5 VDC 1000 mA power supply. However, in order to pull a servo with load, I'll need to use my Parallax Servo Controller.

Mike Green
02-03-2007, 01:06 AM
You can run the servos off the PDB, but you have to supply enough power to the servo motor. The (+) terminal of the servo should be connected to a source of 5-6V at enough current to supply the drain of the servo motor with whatever mechanical load you want to use. The ground (-) should be connected to a common ground (with the PDB). The control line can be driven by the PDB or a PSC or anything else that can supply the pulses. The PSC happens to provide separate terminals for the servo motor power source (as well as a microcontroller that can independently drive the control lines). The PDB doesn't have a way to connect the servo power terminal to a supply separate from the regulated +5V unlike say the BOE, but you could set up a little jumper board or even use the breadboard to let you do this.

02-03-2007, 01:46 AM
Mike Green,

Is this O.K. then?
P1 is the I/O pin going to the BS2p40 on the PDB. S/+/- is the connection to the servo. 6VDC is a power supply or battery separate from the power system on the PDB. In other words, I don't have to use the PSC nor the 2 servo jacks on the PDB. I can just rig my own connections per the above schematic?

That's a mad tight idea! Ur the best!


Post Edited (latigerlilly) : 2/2/2007 6:52:36 PM GMT

Mike Green
02-03-2007, 01:54 AM
Yes, that's what I was suggesting. I have a BOE-BOT with 3 servos. All 3 run directly off the 6V battery. The BOE can run off the same battery because the regulator is very low dropout and works just fine from 6V. I think the PDB uses a different regulator that requires a little higher voltage (like 7.5V) to regulate properly.

02-03-2007, 05:01 AM
Yes, the above will work, as long as you make sure the battery '-' is connected to the BOE Vss line.

That's called a "common ground", and is needed so the Servo can properly read the "p1" control signal.

And even though the battery '-' is connected to VSS, the battery '+' must NOT be connected to Vdd.

02-03-2007, 06:53 AM

Is this what you mean?

Mike Green
02-03-2007, 07:13 AM
Yes. Sorry. I assumed you knew. You always have to connect the grounds together. The servo and the Stamp use the ground as a reference point for the control signal. It won't work if the grounds are not connected together.