# Help with test system

Discussion in 'General Electronics Chat' started by rezvisquikz, Jun 26, 2016.

1. ### rezvisquikz Thread Starter New Member

Jun 9, 2016
15
0
I need to test a piece of wire by passing 1A(AC or DC), for 42 days straight non-stop.

I have chosen to pass 1A(AC) using a variac. The specification of the variac is given Vp-240v, Vs-250V(max) Is-5A(max) & 1.25kVA.

Loads available to use 3x100W & 1x60W lighbulbs, I hope this can be used as loads.

Can anyone kindly use calculation method to deduce how I can achieve 1A.

2. ### KeepItSimpleStupid Distinguished Member

Mar 4, 2014
2,833
497
Can't. The light bulb is a current dependent resistor.

Use an ammeter and leave it in the circuit.

Use a fixed resistor. Check the effect of the wire length/resistivity. e.g. R=pL/A

3. ### rezvisquikz Thread Starter New Member

Jun 9, 2016
15
0
Can I use resistor 'ARSOL HS50'? I wanted to use the resistors earlier but was scared if they were going to get very warm.

4. ### KeepItSimpleStupid Distinguished Member

Mar 4, 2014
2,833
497
1 Amp at 6V is 6 W.
Two 5W identical resistors in parallel is 10 W.

R=6/1, so 6 ohms, So two 12 ohm 5W resistors in parallel will work at 6 V IF the resistance of the "wire resistance" is much less than the resistor.

5. ### rezvisquikz Thread Starter New Member

Jun 9, 2016
15
0
Ok, I have found some ARCOL HS50 1kohm resistors. I soldered 5 of them in parallel giving 200 ohms. Each of these are 50W resistors.

What voltage should I apply using the Variac. So confused I am just worried that 1A through the resistors will run for 42 days and it can damage the resistors ?

6. ### KeepItSimpleStupid Distinguished Member

Mar 4, 2014
2,833
497
Nice numbers: V= IR or 1*200 or 200 V
P = (I^2)*R so P = 1 * 200 or 200 Watts.

Since the resistors are all the same and there are 5 of them, they should dissipate 200/5 Watts each or 40 Watts each. You have a 50 W resistor. That's about 20% more than the resistor can handle, so you should be OK.

200 W is not the best use of power, though. Let you figure out Killo-Watt-Hours for 42 Days.
If you were able to use 1 amp at 1V, that's 1W for 42 Days.

So, you have the following variables to work with:
1) Electricity costs
2) Specific values/wattages of resistors and availability
3) A maximum voltage
4) You also have a voltage step-size because of the transformerVariac.
5) 1 Amp is a given
6) Will there be any resistance temperature changes? Will you need a current regulator?

rezvisquikz likes this.
7. ### rezvisquikz Thread Starter New Member

Jun 9, 2016
15
0
@KeepItSImpleStupid thanks mate, you cleared everything in your first 2 paragraphs, I could relate to these calculations coz I am new to electronics.

8. ### Sensacell Moderator

Jun 19, 2012
1,930
591
If your goal is to pass "1 amp through a wire" you could use a 6 or 12 Volt power transformer powered from your variac.
This will make things cooler-safer-better.

Connect a 1 ohm 5 W resistor in series with the wire, connect this to the secondary of the transformer.
Connect primary of the transformer to the variac, now dial in 1 Volt across the resistor - voila! you have 1 amp.
This setup will burn 1 watt of power and you won't get electrocuted messing with the non isolated output of the variac.

9. ### KeepItSimpleStupid Distinguished Member

Mar 4, 2014
2,833
497
if you want a challenge, you could build the circuit in Figure 26; here: http://www.ti.com/lit/ds/symlink/lm117.pdf

You still needs the bypass caps and a heatsink and suitable DC supply, but it could be fun too.

10. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,369
1,476
What are you trying to test by passing 1 amp for 42 days through this piece of wire?

11. ### Techno Tronix Member

Jan 10, 2015
139
10
I think you are talking about "ARCOL HS50"? Which is design for direct heatsink mounting with thermal compound to get maximum output.

12. ### Alec_t AAC Fanatic!

Sep 17, 2013
8,656
1,975
I'm puzzled as to why you want to test a piece of wire . What are you expecting it to do or not do? And why 42 days? Is this a Hitchhiker's Guide project? .

13. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,033
1,991
Me three - why?

ak

14. ### rezvisquikz Thread Starter New Member

Jun 9, 2016
15
0
Sorry for such a late reply, it was a requirement at work, mainly a validation test requirement setup. Thanks

15. ### hp1729 Well-Known Member

Nov 23, 2015
2,306
283
1 Amp at any particular voltage?

16. ### BR-549 AAC Fanatic!

Sep 22, 2013
4,003
1,030
You are wasting your time, unless all you want is to verify the wire current. And for that all you need is a current source. It will tell you nothing about how the wire preforms in an environment, or under stress.

For that you need to consider both voltage and temperature range. And that would still just be basic.

Will the wire be moving? Is the wire close to other conductors?

There are many, many other considerations. I can't imagine a real test without specs.

17. ### hp1729 Well-Known Member

Nov 23, 2015
2,306
283
42 days = 1,000 hours. A nice round number.

18. ### ian field AAC Fanatic!

Oct 27, 2012
6,205
1,117
Bulbs very loosely approximate to a constant current - as long as you don't mind the turn on surge.

They're not very precise - but more stable than fixed resistors if the VAC tends to vary.

The simplest electronic way is to put a DC constant current circuit across the + & - terminals of a bridge rectifier, and put the AC terminals in series with the load.