Power supply for breadboards and PLC

Thread Starter

zer0net

Joined Nov 13, 2011
6
Hello everyone,
I'm thinking of picking up one of the Click plc just for playing around. It needs about 24vdc 1.3A power. Instead of buying a Click power supply can I just use a simple wall plug in 110vac to 24vdc 2A adapter cut off the front end or use a DC female connector (from eBay) and use that instead? Same powersupply for breadboards

Another quick question I have is, if I supply something 2A of current but it really only needs to use 1A does that mean that the equipment will only take what it needs or am I force feeding it more? Like at home normally we use 15A breakers but we have wide range of equipment plugged in from 2000watt stereo to battery charger. I'm a little confused.

Any help will be appreciated
 

tom66

Joined May 9, 2009
2,595
A device will only draw as much current as it needs.

For example a 1.3A power supply can supply 1.3 amps without ripple increasing too far, or the voltage sagging, or the over-current protection engaging or in extreme cases smoking components (in the PSU)...!

The 2A can do up to 2 amps... You may notice a pattern...

Both are more than capable for typical breadboard projects. Trying to get 2A through the breadboard may lead to the breadboard melting, so it should be enough ;).
 

Thread Starter

zer0net

Joined Nov 13, 2011
6
Thanx for the reply,
So if i have a 24VDC 2A power supply, with 1A fuse on the L line, if the PLC tries to draw more than 1A it should protect against it correct?

you said that 2A might melt the breadboard, is there a way to step down the current without stepping down the voltage?
though i may only need 15mA for LEDs (to begin with) on breadboard. So 2A should be ok if i'm drawing less than 1A in the breadboard right?
 
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Adjuster

Joined Dec 26, 2010
2,148
Thanx for the reply,
So if i have a 24VDC 2A power supply, with 1A fuse on the L line, if the PLC tries to draw more than 1A it should protect against it correct?

you said that 2A might melt the breadboard, is there a way to step down the current without stepping down the voltage?
though i may only need 15mA for LEDs (to begin with) on breadboard. So 2A should be ok i guess.
It is a very good idea to have some means of limiting the current to a safe value, in case of a fault such as a short-circuit. This is especially advisable when using a breadboard, which is very prone to wrong connections being made or wires touching by accident.

The term "step down"is best avoided in this context though. Stepping voltages or currents up or down normally refers to transformation, which may lead to confusion. What you need is a fuse, an automatic circuit breaker, or an electronic current limiter - the latter is provided in some regulated power supplies. Models designed for laboratory use often include a user-adjustable limiter.

As long as there is no short-circuit or breakdown in the equipment, a device designed for a given voltage will only draw its required current. The fact that much more current may be available from the supply makes no difference. It is a simplification to consider appliances as simple resistances, but (depending on voltage) one could consider a mains electric cooker as several ohms resistance, and a small light bulb as a few hundred ohms.

Each draws its own current according to its resistance. To mitigate accidents, the cooker circuit may have a 30A fuse, the light 5A
(an illustration only: actual fuse ratings would depend on voltage, power ratings, and wiring codes).
 
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strantor

Joined Oct 3, 2010
5,322
Thanx for the reply,
So if i have a 24VDC 2A power supply, with 1A fuse on the L line, if the PLC tries to draw more than 1A it should protect against it correct?

you said that 2A might melt the breadboard, is there a way to step down the current without stepping down the voltage?
though i may only need 15mA for LEDs (to begin with) on breadboard. So 2A should be ok if i'm drawing less than 1A in the breadboard right?
Are you talking about putting the fuse on the AC side? because (assuming 120VAC) in order for you to draw 1A from the mains (L) you would need to pull 5A from the 24VDC. Put you fuse on the (+) side of the 24VDC.
 

Thread Starter

zer0net

Joined Nov 13, 2011
6
Are you talking about putting the fuse on the AC side? because (assuming 120VAC) in order for you to draw 1A from the mains (L) you would need to pull 5A from the 24VDC. Put you fuse on the (+) side of the 24VDC.
my mistake... I meant the DC side..
No I will not be putting it on the AC.

The AC to DC adapter that I intend to use is http://www.ebay.ca/itm/AC-100V-240V-Converter-Adapter-DC-24V-2A-Power-Supply-/220882463928?pt=LH_DefaultDomain_0&hash=item336d9eecb8#ht_1902wt_765

I will either cut off the end or use http://www.ebay.ca/itm/5-5x2-1mm-Female-DC-AC-Power-Plug-Connector-Adapter-Survey-CCTV-Camera-/280742488508?pt=LH_DefaultDomain_0&hash=item415d8e69bc#ht_2851wt_820

It seems to be the easiest/cheapest way.
 

Thread Starter

zer0net

Joined Nov 13, 2011
6
It is a very good idea to have some means of limiting the current to a safe value, in case of a fault such as a short-circuit. This is especially advisable when using a breadboard, which is very prone to wrong connections being made or wires touching by accident.

The term "step down"is best avoided in this context though. Stepping voltages or currents up or down normally refers to transformation, which may lead to confusion. What you need is a fuse, an automatic circuit breaker, or an electronic current limiter - the latter is provided in some regulated power supplies. Models designed for laboratory use often include a user-adjustable limiter.

As long as there is no short-circuit or breakdown in the equipment, a device designed for a given voltage will only draw its required current. The fact that much more current may be available from the supply makes no difference. It is a simplification to consider appliances as simple resistances, but (depending on voltage) one could consider a mains electric cooker as several ohms resistance, and a small light bulb as a few hundred ohms.

Each draws its own current according to its resistance. To mitigate accidents, the cooker circuit may have a 30A fuse, the light 5A
(an illustration only: actual fuse ratings would depend on voltage, power ratings, and wiring codes).
Thanx for the info, very helpful :)

Its good to know that I dont have to worry about regulating current aside form a fuse to protect electrical components.
 

Lundwall_Paul

Joined Oct 18, 2011
226
This brings up some good questions. You design a circuit and want to protect it. What are the general rules for selecting a proper fuse? I only guess that you want to select a fuse that is a small percentage over the real current that the circuit draws but how is that figured?
A lot to consider fast blow over slow blow, voltage ratings, in rush current....
 

bountyhunter

Joined Sep 7, 2009
2,512
Are you talking about putting the fuse on the AC side? because (assuming 120VAC) in order for you to draw 1A from the mains (L) you would need to pull 5A from the 24VDC. Put you fuse on the (+) side of the 24VDC.
No. Power factor for a single phase AC is about 1.8X. AC RMS current is close to 2X what would be calculated based on DC load power.
 

strantor

Joined Oct 3, 2010
5,322
No. Power factor for a single phase AC is about 1.8X. AC RMS current is close to 2X what would be calculated based on DC load power.
I was only considering the step-down from 120VAC to 24VAC; not the power factor. So if it were just a step down transformer (no rectifier), 5A draw on the 24VAC side would equal 1A on the primary side, correct? Then how do we apply the power factor after the rectifier for the DC side? is it 1.8 * 5A = 9A?
 

strantor

Joined Oct 3, 2010
5,322
This brings up some good questions. You design a circuit and want to protect it. What are the general rules for selecting a proper fuse? I only guess that you want to select a fuse that is a small percentage over the real current that the circuit draws but how is that figured?
A lot to consider fast blow over slow blow, voltage ratings, in rush current....
This is debated, but my observation has been that it is generally accepted that fusing is to protect the wiring, not the circuit. the circuit should be protected by other means.
 

Adjuster

Joined Dec 26, 2010
2,148
Unless you want to wire your circuit like an electric fire, with cable (and all the internal parts ?)* rated for 15A or higher, then you need a lower rated fuse somewhere. A 15A overload could fry the thinflexible cable appropriate to a small appliance.

*Edit: Somebody may trot out the old saw that the cable, not the equipment is to be protected. Nevertheless you may want to use internal wiring, switches etc. commensurate with the cable. In case of a short, you want the FUSE to blow, not for instance some random bit of PCB trace, which may present a fire hazard.
 
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