Notifications
Clear all

Parallel circuit voltage


DavidL
(@davidl)
Eminent Member
Joined: 4 months ago
Posts: 28
Topic starter  

Hi,

I'm doing some tests to determine if I can power multiple components from a single battery and I came across some unexpected voltage readings. I would appreciate any help figuring out where I'm going wrong.

I have a breadboard with two components; an HC-SR04 sensor and an SG90 servo. Both components get power from the same breadboard power rail, and are grounded to the same breadboard ground rail. The breadboard gets 5v from a breadboard power supply. The multimeter probes are plugged into the same power and ground rails as the components, on the far end of the breadboard (past the components). The multimeter measures 4.98 volts if the servo is inactive. When the servo is running (actively turning), the multimeter measures between 4.75 and about 4.9 volts.

My (mis)understanding was that this setup constitutes a parallel circuit, and that the voltage should stay the same. I've seen tutorials that use a similar setup to power two lightbulbs, and the voltage at both light bulbs is the same

I've tried with two multimeters, so I think I'm misunderstanding something. Any help is appreciated.

Thanks.


Quote
robotBuilder
(@robotbuilder)
Noble Member
Joined: 3 years ago
Posts: 1480
 

@davidl

The voltage should remain the same across the components connected in parallel with a constant voltage source unless there is measurable resistance along the power rails between the connections.  A power source also has an internal resistance although this can be electronically stabilized up to some maximum current being drawn. Thus the more items added in parallel the higher the voltage drop across the power source. Is the power source a battery?

 

 


Ron liked
ReplyQuote
Ron
 Ron
(@zander)
Famed Member
Joined: 2 years ago
Posts: 3045
 

@davidl Sounds totally normal. The 2 lightbulbs you mentioned are powered by 120VAC? It sounds like your breadboard power rails are being powered by one of those breadboard power supplies. Go check the specs on it, especially the amount of current and then check the amount of current the servo draws. Under no load will always be higher. Your understanding of a parallel circuit is slightly off. You might want to study that theory a bit more.

According to ohms law, any path from power supply + to - must drop all the voltage available BUT the current will be different in proportion to the Resistance of that path, the formula therefore is I = E/R. Most likely with an increase in I the power supply will approach it's limits and as I goes up E will come down.

I hope that helps.

It is considered poor judgement to traverse a chasm in 2 leaps.


ReplyQuote
Will
 Will
(@will)
Noble Member
Joined: 1 year ago
Posts: 2017
 

@davidl

What you are seeing is what happens when an ideal power source meets the real world 🙂 When the servo is connected, it draws a lot of power (especially compared to the sensor) and the power supply can't maintain the 5V that it normally supplies and drops down accordingly.

You can imagine the same effect if you took 10 rechargeable AAA batteries and connected them in series for a (nominal) 12V supply. Now, imagine trying to use that to start your car ...

Normally, in circuits where you're expecting large draws of current (like servos), you'd add in an electrolytic capacitor across the power supply to help smooth out temporary jumps in demand. You might experiment by adding a 100uF (or whatever you have) across the + and - terminals of your breadboard supply and try your experiment again. You'll probably still run the voltage lower, but it should delay the drop.

I was kidnapped by mimes.
They did unspeakable things to me.


ReplyQuote
DavidL
(@davidl)
Eminent Member
Joined: 4 months ago
Posts: 28
Topic starter  

@robotbuilder Thanks for the response!

Yes, the power source is a battery.  I think I misunderstood parallel circuits.  I didn't realize that adding components would increase the voltage drop. 

 

I appreciate the help!

 

David


ReplyQuote
Ron
 Ron
(@zander)
Famed Member
Joined: 2 years ago
Posts: 3045
 

@davidl Adding components will increase the current drawn from the supply. All legs of a parallel circuit drop all the voltage. Just apply ohms law to each path.

It is considered poor judgement to traverse a chasm in 2 leaps.


ReplyQuote
DavidL
(@davidl)
Eminent Member
Joined: 4 months ago
Posts: 28
Topic starter  

@zander Thanks for the response. 

I'm not sure which light bulb example I saw, but I think it probably was 120 volts.  You are correct that the breadboard is getting power from a breadboard power supply.  I'll also do some more studying, and I will check the specs and find out how much current is supplied vs how much is used by the servo. 

Thanks for pointing me in the right direction!  

 


Ron liked
ReplyQuote
DavidL
(@davidl)
Eminent Member
Joined: 4 months ago
Posts: 28
Topic starter  

@will 

Hi Will, thanks for the response.  I think I understand what you're saying about the power supply not being able to maintain the full voltage when the servo is drawing power.  I like the capacitor experiment idea, and I'll be giving that a try.

 

Thanks


Ron liked
ReplyQuote
robotBuilder
(@robotbuilder)
Noble Member
Joined: 3 years ago
Posts: 1480
 

@davidl

The equation changes with capacitive and/or inductive loads. The sonar sensor and servo motors are not constant resistance loads.

Just saw posts that appeared since this one so point was taken about the servo motors.

 

 

 

This post was modified 2 weeks ago 2 times by robotBuilder

ReplyQuote
DaveE
(@davee)
Honorable Member
Joined: 1 year ago
Posts: 565
 

Hi @davidl,

   You have received some good advice ... I thought it might be helpful to further clarify a couple of things .. sorry if this reads more like a sermon (Yes, I have 'history' on subjecting people to sermons 🙄 🙄 ) , and apologies for any bits that are 'too simple':

  • Capacitors of say 100uF are often used as 'charge reservoirs' (or decoupling capacitors) to help maintain the power voltage level when a higher current peak demand occurs. These are often needed for ellectronic circuits (analogue and digital) to help ensure the active devices (ICs, transistors, etc.) have 'smooth' power source.
    • However, for a capacitor of modest capacity (say 100 uF), and low impedance loads like servo motors, the capacitor will only have enough charge to support the voltage for a tiny fraction of second.
    •  
    • This means you are unlikely to see any difference of voltmeter readings due to adding a 100uF capacitor, as your voltmeter is likely to take a large fraction of second to give even a single reading, and typically a second to give a 'constant' reading.
    •  
    • However, a capacitor may be able to momentarily reduce voltage fluctuations, and hence prevent electronic devices such as microcontrollers being 'confused'. Thus, in some cases an extra capacitor can fix a problem, even though you may not have instrumentation to observe the capacitor's 'smoothing' effect directly.
    •  
    • To see the effect of the capacitor, you need alternative instrumentation, typically an oscilloscope which can display voltage changes on a microsecond to picosecond time scale (depending on budget!)
    •  
  • When voltages are lower than you expect, it is wise to do some investigation as to 'Why?'.
  •  
    • Start by measuring the voltage at source, e.g. at the output pins of battery or power supply. Continue to monitor the voltage as you change the load. If the voltage changes significantly with the load, then maybe your power source has insufficient current capability.
    •  
    • Of course, batteries tend to show a voltage drop when current is drawn, especially when they approaching end-of-life, and a drop of say 0.1V on a 1.5V cell would often be 'normal' behaviour.
    •  
    • Mains powered supplies tend to be better regulated, but even with voltage regulators, small voltage changes are still to be expected as 'normal'. 
    •  
    • After checking out the power at source,  'walk your meter probes' around the circuit, checking physical points which are 'directly' wired to the power source pins, looking for voltage drops in addition to those seen close to the source. These can arise for a number of reasons, including resistance of (thin) wires, poor contacts and so on.
    •  
    • Small currents of say a few milliAmps to power a low power microcontroller are unlikely to exhibit significant voltage drops of this type, but say 0.1 Ohm resistance of a wire to a motor drawing 1 Amp, will produce a voltage drop of 0.1 Volt.
    •  
    • Remember that voltage drops due to wires being too thin, dirty contacts, etc., can apply to both the voltage rail (e.g. +5V) and the 0V return path.

Good luck with your project and best wishes, Dave


ReplyQuote
DavidL
(@davidl)
Eminent Member
Joined: 4 months ago
Posts: 28
Topic starter  

@robotbuilder Thanks for the response.  I will do some research on that.  It looks like it's not as simple as I thought.

 

 


ReplyQuote
DavidL
(@davidl)
Eminent Member
Joined: 4 months ago
Posts: 28
Topic starter  

@davee Sermons are welcome, and at this stage, "too simple" is "just right" for me.

Thanks for the pointers about the capacitor.  I haven't gotten around to adding them yet (hopefully this weekend), so it's good to know what to expect and what not to expect.  If I understand correctly, adding a capacitor might help smooth the power, but the multimeter isn't likely to show the impact.

I will also do the voltage tests you suggested.  Most of my tests were at the beginning of the power rails and the end of the power rails, not much in between.

Thanks!


ReplyQuote
DaveE
(@davee)
Honorable Member
Joined: 1 year ago
Posts: 565
 

Hi @davidl,

  Looks like you have the right idea.

A trick I sometimes do is to measure the voltage along the "length" of a given power rail whilst it is supplying current ... on a 'good' day the voltage will be very small, maybe too small for your meter to show, if it's not very sensitive. .... on a bad day it may be 0.1V or more.

Of course, what is 'acceptable' depends upon the application, so don't take 0.1V as an 'absolute' measure of goodness.

The advantage of this trick is you can set the meter to its most sensitive range (or it will do it automatically if it autoranges),. The disadvantage is that it is easy to get confused, especially if there is a wiring mistake or similar.

So I would always start with measuring the 'full' voltage at all points from the source to the load, and then (optionally) use the second method to 'home in' on the exact point or section that is being awkward.

Good luck with your search. Dave


ReplyQuote
Ron
 Ron
(@zander)
Famed Member
Joined: 2 years ago
Posts: 3045
 

@davidl You should try to determine the class of meter you have, is it a 1%, 5% or 20% meter. As you might guess, a 20% meter is inexpensive, a 1% is expensive. As hobbyists 5% would be ok, 1% if budget allows, the 20% is good for continuity checking and household 120VAC use but not for the kinds of in circuit tests you have been discussing. A 20% error on a 5V circuit is 1V.

It is considered poor judgement to traverse a chasm in 2 leaps.


ReplyQuote