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12v 4 - 500W PSUs: ATX vs single voltage unit

npomeroy

Dec 9, 2012
19
Joined
Dec 9, 2012
Messages
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I am designing a setup that requires 24 separately controlled 130W heaters. The "heaters" are constructed from silicone-insulated resistance cable and I'd prefer to keep it low voltage as I'll be building it myself. So far I have controlled each heater via a FET switched from a Picaxe chip on a 1-second PWM duty cycle. So I need about 3000W of 12v supply (or possibly 18 or 24v). So, say ten 400W supplies may be obtained.

The cheapest local choice seems to be computer ATX power supplies like this 500W one
http://www.ascent.co.nz/productspecification.aspx?ItemID=415767 for about US$50

One would have thought "plain" switch mode PS would be cheaper but in our market even a 150W unit from Jaycar is US$84 and higher power ones if available are even dearer.
http://www.ascent.co.nz/productspecification.aspx?ItemID=415767

Aliexpress from China shows much cheaper switch mode single voltage units like this 400W one for US$40 shipped
http://www.aliexpress.com/item/S-40...nic-LED-Transformer-AC-DC-CNC/1971159120.html

So what is my question...?
1. For my purpose are there pros and cons of bottom-end ATX supplies vs "plain" switch mode power supplies?
2. For either power source is a problem created by the power demand flicking on and off all the time? (As 3 - 4 heaters would be controlled from each PS I'd try to stagger the PWM pulses to be out of phase).

I understand that using the non-ATX units gives me choices of 18 or 24v which may be advantageous in choosing resistance ratings and reducing currents (I have heard of linking ATX supplies to give 24v but am not keen on that).

regards
Nelson
PS (excuse the pun) I'd be happy to use simple wire-wound transformers and rectify etc but they are even dearer for that power, and would be heavy to import from China.
 

Kiwi

Jan 28, 2013
471
Joined
Jan 28, 2013
Messages
471
Hi Nelson,

Just a few questions etc off the top of my head;
1- Why dc? Could you modify your circuit to run the heaters on ac straight off a transformer without the need for rectifiers and filtering etc?

2- Is this in a domestic or industrial location? I was wondering how you are going to get 3KW of mains supply. Normal NZ wall socket is 230V10A(~2.3KW). Not sure I would trust continuously running one at this load.

3- I believe that the negative of ATX power supplies is difficult to isolate from the grounded case. You would also have no warranty on the power supply once modified.

4- My understanding is that anything connected to mains in NZ should have a Certificate of Compliance(CoC). I doubt if power supplies from Aliexpress would have one. Might have insurance issues.

5- Do you know a friendly local electrician that you could ask for advice?

Cheers,
Brendan
 

npomeroy

Dec 9, 2012
19
Joined
Dec 9, 2012
Messages
19
Hi Nelson,

Just a few questions etc off the top of my head;
1- Why dc? Could you modify your circuit to run the heaters on ac straight off a transformer without the need for rectifiers and filtering etc?

Yes AC is OK but plain AC transformers are even more expensive.

2- Is this in a domestic or industrial location? I was wondering how you are going to get 3KW of mains supply. Normal NZ wall socket is 230V10A(~2.3KW). Not sure I would trust continuously running one at this load.

Domestic. I'm assuming I'll use 2 or 3 separate circuits.

3- I believe that the negative of ATX power supplies is difficult to isolate from the grounded case. You would also have no warranty on the power supply once modified.

I don't see why the ground needs to be isolated - smaller versions have worked fine without modification.

4- My understanding is that anything connected to mains in NZ should have a Certificate of Compliance(CoC). I doubt if power supplies from Aliexpress would have one. Might have insurance issues.

Yes, a thought - then locally sourced ATX may be best.

5- Do you know a friendly local electrician that you could ask for advice?

Maybe.

Cheers,
Brendan

Also, in terms of overall load, after the temperatures have stabilised each heater is only ON for 50% or less of its duty cycle, so the main precaution is to not switch everything on at the same time. I could even have a timed set of relays that brought each PS on in sequence after a power cut.
 
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