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How to turn on 4 servo motors at once ?

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bianchi77

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Friends,

I want to turn on 4 servo motors but I can't, its MG996, I'm using ULN2003 as a driver,
Power supply 5V 1A, if I turned on 2 servo first then connect the other 2 after it's on, it's working fine, but if I connect all the servo (4) at once,
it doesn't work, the servo only shaking,

Is it because I have less power to turn on ? any ideas to solve this issue ?

Thanks
 

Hi,

It does sound like a power overload, have you measured the current of just 1 motor at turn ?

Are you running the micro on the same +5v power rail as the servos? The load of 4 motors might drop the +5v rail too low causing the pic to momentarily drop out.
 

Stall Torque and starting current are proportional. If the demand current drops the voltage too much from the effective load resistance or ESR, the motor won't move. Are these the inexpensive power hungry Chinese servos? Analog or digital?
Maybe get the current specs for the stall torque and resize power source.

The key point to remember with digital servos is to install the largest capacity battery that space/weight will allow.
Always install a battery monitor to check the operational capacity and, wherever possible, top up the charge before every flight, just to be sure.
 

I don't exactly understand your way to operate the servos. Normally, a servo would be continuously powered and controlled by PWM input.
 

SunnySkyguy said:
Stall Torque and starting current are proportional. If the demand current drops the voltage too much from the effective load resistance or ESR, the motor won't move. Are these the inexpensive power hungry Chinese servos? Analog or digital?
Maybe get the current specs for the stall torque and resize power source.

The key point to remember with digital servos is to install the largest capacity battery that space/weight will allow.
Always install a battery monitor to check the operational capacity and, wherever possible, top up the charge before every flight, just to be sure.
Click to expand...

Yes it's chinese servo and I suspect, I loss power on the first time turn on, because when I plug out 2 of them, it works and I reconnect the other two...
I'm not using battery, I'm using transformer, 1A 5V, I'm thinking about starting current is dropping out the voltage as the cause of losing power

Do you know any ideas to solve it ?

---------- Post added at 00:05 ---------- Previous post was at 00:03 ----------
wp100 said:
Hi,

It does sound like a power overload, have you measured the current of just 1 motor at turn ?

Are you running the micro on the same +5v power rail as the servos? The load of 4 motors might drop the +5v rail too low causing the pic to momentarily drop out.
Click to expand...

Yes I put the micro on the same power rail +5v as the servo, do you mean I need to separate the servo with it's own power ? It's 5V and 1A transformer, simple rectifier DC.
 

bianchi77 said:
Yes I put the micro on the same power rail +5v as the servo, do you mean I need to separate the servo with it's own power ? It's 5V and 1A transformer, simple rectifier DC.
Click to expand...


Hi,

You have not mentioned any current measurements.
Have you measured the voltage on the +5v rail when 4 servos turn on ?

You have to consider three values, power on surge, normal running and standby current.

Untill you know what they are how do you know that a 1A regulator will do or where the problem really is?

A small 100ma +5v regulator is probably all you need for the Pic ( assuming its not driving a load of leds etc).
If used with a large electro cap it will help protect the pic from the power rail surges from the servos.
 

wp100 said:
Hi,

You have not mentioned any current measurements.
Have you measured the voltage on the +5v rail when 4 servos turn on ?

You have to consider three values, power on surge, normal running and standby current.

Untill you know what they are how do you know that a 1A regulator will do or where the problem really is?

A small 100ma +5v regulator is probably all you need for the Pic ( assuming its not driving a load of leds etc).
If used with a large electro cap it will help protect the pic from the power rail surges from the servos.
Click to expand...

for 4 servos, the first time current = 1.8A, and for 2 servos 1.4A, how can I solve this ?
I try to separate power rail, not working ...?
any ideas ?

thanks
 

bianchi77 said:
for 4 servos, the first time current = 1.8A, and for 2 servos 1.4A, how can I solve this ?
I try to separate power rail, not working ...?
any ideas ?

thanks
Click to expand...

Hi,

If you measure currents like that, you can be assured the actual peak is a lot higher.

Even with some large caps on the +5v 1A regulator it surely be dropping the voltage.

The solution, much bigger PSU and a regulator of 3 or 5A rating at 5V.

I'm not into servo /modelling but have to ask what load you are trying to move with those motors, can you get smaller ones to do the same fuction with less power ?
 

wp100 said:
Hi,

If you measure currents like that, you can be assured the actual peak is a lot higher.

Even with some large caps on the +5v 1A regulator it surely be dropping the voltage.

The solution, much bigger PSU and a regulator of 3 or 5A rating at 5V.

I'm not into servo /modelling but have to ask what load you are trying to move with those motors, can you get smaller ones to do the same fuction with less power ?
Click to expand...
I think it's a small digital servo
MG996, I can't find the power needed, but i'm sure it's small since the stall up to 11kg for 6v

Any ideas to solve it ?
 

Your power source is underrated for current.
Consider starting current for each servo worst and and sum then add 20% for margin.
Then consider surge time constant and increase surge capacitor values.
Ic = C dv/dt
so if low ESR cap is selected to supply 5V with only 10% drop in 100mS for example.
Say you wanted to supply 1amp from transformer and 1 amp from Cap.
you need C= Ic dt/dv = 1A * 100 ms / 0.5V = 0.2 Farad supercap...$$ so better to simply increase 5V supply rating to 2A or 3A
If you have an old PC PSU,, consider that. or extend the Disk molex cable out to your circuit +5, Gnd .. and put a PTC protection rated for your needs . They only cost 25 cents.
 

SunnySkyguy said:
Your power source is underrated for current.
Consider starting current for each servo worst and and sum then add 20% for margin.
Then consider surge time constant and increase surge capacitor values.
Ic = C dv/dt
so if low ESR cap is selected to supply 5V with only 10% drop in 100mS for example.
Say you wanted to supply 1amp from transformer and 1 amp from Cap.
you need C= Ic dt/dv = 1A * 100 ms / 0.5V = 0.2 Farad supercap...$$ so better to simply increase 5V supply rating to 2A or 3A
If you have an old PC PSU,, consider that. or extend the Disk molex cable out to your circuit +5, Gnd .. and put a PTC protection rated for your needs . They only cost 25 cents.
Click to expand...
what's regulator chip for 5V 2A any ideas ?

thanks
 

bianchi77 said:
what's regulator chip for 5V 2A any ideas ?

thanks
Click to expand...

Hi,

There are not that many 2A plus fixed regulators to choose from at a reasonable price.

The best one I can see is the LT1084 5A device.

If thats not available, then an adjustable regulator at 2A seems the best option.
Look for L200, L350T or LD1085P.

In case you are wondering, you can Not use two 1A regulators together on the same line.

You could however use two 1A regulators if you use two ULN driver chips, each one powering just 2 servos, and power for each ULN from its own 7805.
As you have shown the servos can take around 1A each it would be wise to parallel at least two of the driver segments as they are only rated at 500ma each.
 
wp100 said:
Hi,

There are not that many 2A plus fixed regulators to choose from at a reasonable price.

The best one I can see is the LT1084 5A device.

If thats not available, then an adjustable regulator at 2A seems the best option.
Look for L200, L350T or LD1085P.

In case you are wondering, you can Not use two 1A regulators together on the same line.

You could however use two 1A regulators if you use two ULN driver chips, each one powering just 2 servos, and power for each ULN from its own 7805.
As you have shown the servos can take around 1A each it would be wise to parallel at least two of the driver segments as they are only rated at 500ma each.
Click to expand...

Thanks for the info mate.

---------- Post added at 23:36 ---------- Previous post was at 23:31 ----------

abit expensive for LT1084 5A, USD10 for a piece....
 

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