Dumb question, but I need to know

Thread Starter

birckcmi

Joined Jan 1, 2018
91
Hello,I am (still) building a power supply for a plastic bender. I managed to buy an old HP6433BC Lab PS that will do the job, but I would also like to put together a smaller and much lighter PS for the device alone. I've assembled all the components to deliver 24DC volts at Max 14.6 A through a 9-60volt adjustable regulator Max 20 A, OR a 7-80 VDC regulator Max 30 A. My question is: does the 20-30 Amp final DC output mean I will need a heavy duty power cord for the enclosed switching AC-DC PS that connects to the 120AC mains? 12 ga.? 14 ga.? 16 ga.?
Plus, if anyone sees any major safety problems with the series of components I'm using, please give a shout.
 

Papabravo

Joined Feb 24, 2006
12,926
The output power you require is 80V * 30A = 2.4 kW. I would say that at those power levels you might need the largest diameter wire you can afford. This is really nothing to mess around with. You do realize that the required input power is going to be greater than the output power. It could be several tens of percentage points higher.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
That's pretty clear, Papabravo. So the figures that need to be multiplied to arrive at wattage are the highest possible volts and highest possible Amps to be output at the delivery end?
 

wayneh

Joined Sep 9, 2010
16,254
That's pretty clear, Papabravo. So the figures that need to be multiplied to arrive at wattage are the highest possible volts and highest possible Amps to be output at the delivery end?
That's true if those two things can actually happen at the same time. You need to know the actual worst-case scenario, and that includes allowing for Murphy's Law. A kid or a pet or you yourself will eventually turn any dials to their maximum settings. You have to design for safety against any possible user choices.

If I read your post correctly, you'll have max 24VDC at up to 15A? That's 360W. Allowing for some inefficiencies, that's maybe 500W drawn on your input power cord. That's not a problem. A 1500W hair dryer is not unusual and will have a normal power cord.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
I am sure you are right. In fact, now that I have taken another look at it, the online Nichrome wire calculator tells me I will need 20 watts at 4 amps and 5 volts to reach 400˚F (@2 ft. of22 AWG wire, yielding 1.34 ohms). So even 360 watts is overkill. I'll re-design the circuit. Thanks for setting me straight. I'm glad I asked.
 

MisterBill2

Joined Jan 23, 2018
5,209
Hello,I am (still) building a power supply for a plastic bender. I managed to buy an old HP6433BC Lab PS that will do the job, but I would also like to put together a smaller and much lighter PS for the device alone. I've assembled all the components to deliver 24DC volts at Max 14.6 A through a 9-60volt adjustable regulator Max 20 A, OR a 7-80 VDC regulator Max 30 A. My question is: does the 20-30 Amp final DC output mean I will need a heavy duty power cord for the enclosed switching AC-DC PS that connects to the 120AC mains? 12 ga.? 14 ga.? 16 ga.?
Plus, if anyone sees any major safety problems with the series of components I'm using, please give a shout.
Short answer about does the LINE CORD need to be heavy duty. NO. The reason is that normally power in = power out, neglecting losses. so if the power out is at 24 volts and the power in is at 120 volts, then, neglecting losses, the current in would be one fifth as much, since the voltage is about 5 times greater, Power in watts = volts x amps, (no losses considered). So the power cord needs to be good for at least 6 amps. An adequate safetymarging says use a power cord rated for ten amps. So 16 gage wire should be adequate.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
That's a reasonable question. I think it's partly that I don't like working with a circuit jacked straight into the wall socket, either in the form of a variac (ugh!) or even a more isolated regulation circuit. AC @ 60Hz is 5X more likely to kill me than DC. The other part is that it's worth my while to learn more about AC-to-DC conversion(and plain AC), Ohm's Law, and what it will take to get the job done with 12 or 24 volts rather than 120. So I ask questions on this forum, get good answers, read up on what I've learned (See list below), and hope I can come up with a design that's a lot safer than what I've seen. Most of those designs, particularly the ones powered by a variac, would probably have clumsy me deep-fried within minutes.
References:
Building Power Supplies
David Lines Radio Shack/Archer
Understanding Electricity and Electronics G. Randy Slone TAB/McGraw-Hill
Complete Idiot’s Guide to Electronics101 Westcott & Westcott Alpha Books
Electronics for Dummies Shamieh & McComb Wiley
Encyclopedia of Electronic Circuits Graf McGraw-Hill
 

shortbus

Joined Sep 30, 2009
7,461
Not wanting to start a fight, but... What is the difference in using a DC power supply that plugs in the wall, and an AC transformer? The DC power supply has a transformer in it too.

AC @ 60Hz is 5X more likely to kill me than DC.
Not when the voltage out is the same. And the AC to DC thing was way back when Edison killed an elephant.:)
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
As I said, I have a lot to learn, and I'm learning more by crossing from AC to DC, and doing the job with DC. It makes the design more complicated, so more interesting. I have the time to learn as much as I can. If you look at the YouTube demos of How I Built a Plastic Bender, It's a wonder those guys are still alive, assuming that they are. DC or AC, the setups are creepy. I'm trying to improve on all of it.
 
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Sensacell

Joined Jun 19, 2012
2,565
I am sure you are right. In fact, now that I have taken another look at it, the online Nichrome wire calculator tells me I will need 20 watts at 4 amps and 5 volts to reach 400˚F (@2 ft. of22 AWG wire, thermal yielding 1.34 ohms). So even 360 watts is overkill. I'll re-design the circuit. Thanks for setting me straight. I'm glad I asked.
That calculation assumes you are measuring the temp of a bare wire- with little thermal load.

You will need far more power to actually do any real "work"

It all comes down to simple economics - you need controllable power at the lowest cost.

Ill bet a big fat 300 w dc switch mode supply and a simple PWM circuit will be cheaper than a variac and a transformer any day of the week- commodity products are cheap, exotic stuff is expensive.

Both setups are equally safe if wired correctly.
 

Thread Starter

birckcmi

Joined Jan 1, 2018
91
OK, that's worth investigating. I have on hand a 24DCV enclosed switching power supply yielding 14.6 Amps, and a selection of DC controllers: 5-36V@15Amps, 9-60V @ 20Amps, and 7-80V PWM@ 30Amps. I'm leaning toward the 5-36V @ 15Amp controller because it would provide a range of wattages without much likelihood of overheating anything, maxing out at 540W. According to the online Nichrome Calculator, Getting 2 feet of 20ga. wire (.65ohms/ft) up to 800˚F requires 52 watts ,e.g.,6.3 Amps@ 8.3V. The Makrolon Polycarbonate user guide recommends, for bending sheets (regardless of thickness), 350˚F max.
I think I have what I need? Some of this stuff I've had for a while, and some I bought for this purpose. I also went for a used Hewlett-Packard/Harrison adjustable bench power supply for future projects. Since that weighs 35 pounds, I'd like to build a more maneuverable power source that will fit onto the wire heater. I don't mind spending the money it takes to do it right-which is why I've kept posting on this forum. Thank you for your comment.
 
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