Power Consumption Question

IVB

Senior Member
If I have a 3A power supply on 24x7, does that mean there's a constant draw of 3A from the electric company? Or does that mean that I can draw up to 3A, but it'll take only as much energy as I need?

I just got that 300lb SmartHome maglock, need to get it 315mA. This is something the wife actually loves, and if it works out in practice, she wants 2 more. RadioShack sells a 3A p/s, I only need 1A, but their price is pretty cheap and I can finish up this project tomorrow.
 
IVB said:
If I have a 3A power supply on 24x7, does that mean there's a constant draw of 3A from the electric company? Or does that mean that I can draw up to 3A, but it'll take only as much energy as I need?
Power supplies are normally specified by the power that can be produced on the output of the supply. Normally it will specify a current capability at a certain voltage. The amount of power the supply draws from the primary source (120volts) is dependant on several factors.....mainly on secondary current draw, but also efficiency, etc.

Let's take a typical example...Let's say you have a 3A 12VDC supply; the supply is 50% efficient and the load is 1 amp. In this case, the primary will draw 0.2 amps. This is calculated by dividing the voltage ratio to get the primary current, then dividing by the efficiency.
 
IVB,

What are you using that 300 lb maglock for? I hope it's not an outside door.

300 lbs won't be enough. You need a 600 lb lock.
 
Let's take a typical example...Let's say you have a 3A 12VDC supply; the supply is 50% efficient and the load is 1 amp. In this case, the primary will draw 0.2 amps. This is calculated by dividing the voltage ratio to get the primary current, then dividing by the efficiency.
Thanks very much for spelling that out. In your example, would it be 0.2amps, or 2amps? I'm drawing 1amp and am 50% efficient, so I need twice as much as I produce.

IVB,

What are you using that 300 lb maglock for? I hope it's not an outside door.

300 lbs won't be enough. You need a 600 lb lock.

Oh, good to know before I bought more. This one is ok as it's technically an outside door, but it's the outer door of our mudroom which is a flimsy thing with tons of glass.

So...do you sell any 600lb locks? I looked, but didn't see any on your site.
 
So that 300 pound mag lock is working out OK? I have a need to put one on an outside gate (if someone would apply more than 300 lb to it the gate would give). How much did you get it for? I'm wondering if I could somehow make it water tight by mounting it in some plastic "project" boxes (anyone have any ideas on this ??). Which one did you wind up using?

Thanks,

BSR
 
IVB said:
Let's take a typical example...Let's say you have a 3A 12VDC supply; the supply is 50% efficient and the load is 1 amp. In this case, the primary will draw 0.2 amps. This is calculated by dividing the voltage ratio to get the primary current, then dividing by the efficiency.
Thanks very much for spelling that out. In your example, would it be 0.2amps, or 2amps? I'm drawing 1amp and am 50% efficient, so I need twice as much as I produce.
Your original post asked how many amps on the 120 Volt primary would the load be pulling. In the example, you are forgetting to divide by the primary/secondary voltage ratio. If the secondary is supplying 1 amp at 12 Volts, then the primary will be drawing 0.1 amp at 120 volts. Then take efficiency into account. Here is the equation:

(secondary amps) X (secondary volts) = (primary amps) x (primary volts) x efficiency

So primary amps = (sec volts) x (sec amps) / [(primary volts) x efficiency]
 
BraveSirRobbin said:
So that 300 pound mag lock is working out OK? I have a need to put one on an outside gate (if someone would apply more than 300 lb to it the gate would give). How much did you get it for? I'm wondering if I could somehow make it water tight by mounting it in some plastic "project" boxes (anyone have any ideas on this ??). Which one did you wind up using?

Thanks,

BSR
SmartHome had a 15% off sale, plus free shipping, plus this was $138. hence it came out to about $118 shipped.

Can't tell you how it's working out yet - I set it up, but found out that I don't have a 12VDC power source with 315mA available at the location I want to run the wiring to.

I'm hoping to get to RadioShack today to get this, so I can at least turn it on to see how it works out. I don't think you have much in the way of watertight to worry about, as the wire seems pretty embedded in the lock on the one side. On the other side, you could just put it in a watertight single-gang box [way too big], with some silicon gel or the like. That's what I used on my outdoor speakers to insulate the terminals.

to sandpiper, thanks for that explanation. I'm going to have to read and absorb that, as cocoontech/HA is the first time i've had to really understand all this electrical stuff. Your explanation makes total sense, makes me feel better about how much this would cost from a power perspective.
 
BSR,

Seco-larm has a unit 941SA300 (300#) that sells for $86.

i assume you know there are units for gates that are weather-proofed, but cost much more.

bob
 
bobr said:
BSR,

Seco-larm has a unit 941SA300 (300#) that sells for $86.

i assume you know there are units for gates that are weather-proofed, but cost much more.

bob
Hi Bob;

Thanks for the reply. I do remember looking up something weather proof and found out that indeed they are very expensive. That's why I was thinking about making an inexpensive (indoor) one rain tight.

Thanks for the tip on the Seco-larm. I'll try looking it up tonight or tomorrow.

Regards,

BSR
 
Couple of things.

1) Mag locks also come in outdoor models BSR.

2) We carry a full line of them and have them in stock. I'll make sure that they get on the website right away. Sorry about that. I thought they were already there. 300# - $86, 600# 114, 600# Weather Proof - $145. We also carry the full line of brackets that might be needed with them.
 
AutomatedOutlet said:
Couple of things.

1) Mag locks also come in outdoor models BSR.

2) We carry a full line of them and have them in stock. I'll make sure that they get on the website right away. Sorry about that. I thought they were already there. 300# - $86, 600# 114, 600# Weather Proof - $145. We also carry the full line of brackets that might be needed with them.
Well, glad I overpaid for mine to that epitome of customer service, SmartHome.

You'll have email in about 3 mins...
 
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