# Variable R Load

Discussion in 'General Electronics Chat' started by R!f@@, Mar 1, 2014.

1. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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OK.. So I want to make a Very high power Variable test Load I can use for AC and DC.

See this

I am gonna get 10 pieces. So I can test Amps and Supplies and load transformers to find there capacity.
I can series / Parallel them to get the desired value and Power.
To get other values I intend to series connect a 100Ω 100W Wire wound pot so I can get other values besides that from series or parallel connecting the 8Ω ones.

I went for the 8Ω cause I can make 2Ω or 4Ω high power easily to test amps which is my primary concern. I need to measure amps without deafening me

And to widen the test unit I intend to measure my Tx for currents like 10A at around 70VAC. I would need 7Ω, 700W. So 8Ω,1000W would be close enuf for practical needs.

I need an advice on a rheostat value to get odd values. I will connect the rheostat to the resistors in series if needed and vary to get the desired R Load value.

What would you suggest. What I had in mind is this.
Will it do or do I need a higher power or resistance or lower Ω with higher power

I am asking cause I know there are those who might have done this.

My applications will be testing
1. Amps of around 800W or so Max.
2. Testing PSU's of 80V @ 10 amps max
3. Transformers 80V @ 10 Amps max.

2. ### studiot AAC Fanatic!

Nov 9, 2007
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I am sure you know that the current in the pot you linked to is limited to 1 amp max (100W = 1*1*100).

Multiple high power resistors can cost a small fortune.

Further their actual resistance will vary considerably with temperature and therefore current.

Since I know you are quite handy, why do you not make your own from resistance wire?

Several years ago I used the wire from an electric fire bar to make a pair of load resistors for testing amps and power supplies and transformers.

After unwinding I measured the resistance and length, thus accurately determining the length for 1Ω.

I then fixed pairs of terminals to a base in two lines, spaced at the 1Ω distance apart.

I then wound the wire back and fore in zig zag fashion between pairs of terminals to fashion two 10Ω resistors, tapped at 1Ω intervals.

The zig zag means that it is definitely non inductive. The wire is good for quite a few amps.

This simple cheap device has served me well as I can measure the current and voltage for accurate work or just use the resistance taps for 5% work to quickly and easily measure a table of values and draw a load graph for any given device under test.
I did rather abuse my resistors, the wire glowed red/white hot under some loads so the resistance definitely variedand the current/voltage measuremt method came into its own, You certainly need to take this into account with any high power wire wound resistor.

Somewhere here I think I've posted a photo before, but search isn't working today.

You can add many refinements to this, such as encapsulating the wire in cement for better heat dissipation and themal stability.

Edit : Search is working now see post#7 here

Last edited: Mar 1, 2014
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3. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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What size of nichrome can handle 10 Amps safely ?

4. ### studiot AAC Fanatic!

Nov 9, 2007
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5. ### inwo Well-Known Member

Nov 7, 2013
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Is shipping from USA a possibility to your location?

6. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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Shipping from USA or from any where is possible. I buy from ebay and Aliexpress.

The Wire should be cool as possible u know. Our normal temp is ≈30°C.

Ali gives price in KG's but I do not think I would need a KG.

If going for nichrome I believe I can go for short wires. The maximum I would need to measure is around 80V AC or DC. And current max will be 16 Amps (rarely).

If wire can handle 16 amps I would need is 5Ω to test 80V

So I think I can do with 8Ω of wire. current handling needs to be ≈16 Amps.

7. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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So would AWG #8 do for me ?

I think I need 100Ω of wire.

8. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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Crap! I get 100Ω fro AWG 8 is 2500 feet.

I think I am better off with resistors

9. ### strantor AAC Fanatic!

Oct 3, 2010
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Resistors like these could eliminate the need for the rheostat. Alternately, stacked rheostats like these could disperse the power through several, instead of just one.

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10. ### inwo Well-Known Member

Nov 7, 2013
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This is why I asked about shipping.
Don't know if it is practical.

I feel a little uneasy self promoting, but I listed these to help out.

They are priced at a fraction of cost.
But shipping may be a problem.

Probably less than the wire and porcelain would cost.

The 2 ohm/300 watt comes out to wire that handles ≈12amps (12A X 24V = 288W)

Can easily be tapped and the wire is wound on a nice form.

Have other sizes and another building I haven't looked in yet.

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11. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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OK so I changed my mind.
I will forget the VR.

I am getting 10 of these instead.

So like 62 bucks I get 10pcs 8Ω and 10pcs 1Ω

So I can make 90Ω, 1000W.

I can go from 1Ω to 90Ω and total current will be 10 Amps. for a single resistor.

How about tht ?

12. ### inwo Well-Known Member

Nov 7, 2013
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I'll add these to the list.

15ohm 50watt

\$1 each...........................

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13. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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Thanks.
Can you get the shipping to Rep of maldives.
I think they are pretty heavy.

14. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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@inwo.

can you compare the links I showed. (They are total with shipping ) to ur items.
I think from USA it would cost me more.

15. ### inwo Well-Known Member

Nov 7, 2013
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I think so too.
\$40-\$50

I see aliexpress all the time googling.

Is that a good company?
How does one pay?

I've been tempted to order from them, but never have.

Last edited: Mar 1, 2014
16. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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@ strantor.

I checked Ali but I don't wanna use those VR's cause I want to mount them. The rotary type is what I prefer but as studiot said they carry 1 amps. I need atleast 10amps.
So I found the option in post#11

17. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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Thanks though.

18. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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I have been buying stuff from Ali for quite a while. it seems they are on ebay ( the same sellers u know). Ebay ships them from china so I thought what the heck.
They are cheaper than ebay. for the same stuff.

The best part is money back guarantee. I got my money back twice. If they fail to read my message and ship any thing different ( color ) I get my money and keep the goods and if the components ( like caps ) are bad I get replacement and I can extend my money back protection to any no. of days I want.

I pay with my Visa debit. All the options are there except paypal. So no fees I pay with our USD exchange rate

They are better than ebay to me. cause I lost every laptop battery I bought from ebay and money too. .

You should try it.
You get ur money back if the item is not the same as in picture or if the seller misses to read ur message at payment (comment) and ships anything but what you asked.

I get to keep 2 items atleast cause of different colors.
I got replacement transistor for a tested bad one from a batch.

As far as I see, Aliexpress is far batter than ebay.

19. ### inwo Well-Known Member

Nov 7, 2013
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tnx

I may have confused them with Alibaba.com which have bad reviews and wants bank transfer.

With CC there is some protection.

I'm going to look for a tv part now.

ps.

Found them in favorites. Almost ordered a board from them yesterday, but didn't trust them.

20. ### R!f@@ Thread Starter AAC Fanatic!

Apr 2, 2009
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I also did a western union transfer for Asus Tablet PC.
And I got the goods from DHL. All good.

But now I pay through Visa.
You should try Aliexpress

You can order and buy. they will give you a 19day period to pay. If you do not pay order is cancelled automatically and no problem. If you don't want it just don't pay. That's all.