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Author Topic: $3 dummy load  (Read 1158 times)

KM6BSQ

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$3 dummy load
« on: August 08, 2019, 02:12:15 AM »

I made $3 dummy load.
It's made of RFP 250-50 and 1/4" aluminum angle I found on a street.


In 90 sec of applying 100w RTTY it gets hot. I can still touch it, around the chip, but not for very long. Edges obviously remain cooler.
Antenna analyzer shows
impedance 49.7 ... 52.3 Ohm on 3 ... 50Mhz range
return loss > 30dB 0-50MHz

https://imgur.com/a/mMTKQTP

Did not want to move the connector and RFP 250-50 significantly apart, I was afraid to introduce unwanted reactance.

Please tell me if anything is wrong with this design.
« Last Edit: August 08, 2019, 02:30:29 AM by KM6BSQ »
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KU3X

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RE: $3 dummy load
« Reply #1 on: August 08, 2019, 04:10:11 AM »

Get a one gallon paint can, fill it with mineral oil, cut your aluminum so it fits in the paint can and drop it in. The oil will give you a much better duty cycle. You can put a barrel connector on the lid and make a short jumper up out of RG 316. You can get an empty paint can
at the paint store. You can do the same with a quart paint can if you don't need to increase the duty cycle too much.

Barry, KU3X
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K3UIM

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RE: $3 dummy load
« Reply #2 on: August 08, 2019, 06:37:13 AM »

 RFP 250-50 ??? Whatzitt?
Charlie
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Charlie. K3UIM
Where you are: I was!
Where I am: You will be!
So be nice to us old fogies!!

K0UA

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RE: $3 dummy load
« Reply #3 on: August 08, 2019, 06:49:27 AM »

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73  James K0UA

AK0B

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RE: $3 dummy load
« Reply #4 on: August 08, 2019, 06:52:19 AM »

the RFP 250 - 50 is a RF load resistor on ebay for about $2.50 with free shipping.  I agree put it in a paint can from your local paint store filled with mineral oil.  Works great.

Stan AK0B
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KM6BSQ

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RE: $3 dummy load
« Reply #5 on: August 08, 2019, 06:58:10 AM »

I’m aware of paint can design. What would it give me over this? I presume a better heat dissipation over passive air cooling.
Is 90sec not enough? How long is a typical emission test?
This is exactly why I posted it here - to understand why do I need the oil can. Thank you. 
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K0UA

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RE: $3 dummy load
« Reply #6 on: August 08, 2019, 08:34:53 AM »

I’m aware of paint can design. What would it give me over this? I presume a better heat dissipation over passive air cooling.
Is 90sec not enough? How long is a typical emission test?
This is exactly why I posted it here - to understand why do I need the oil can. Thank you. 

The 90 seconds. IS likely enough for tune up and testing. The Oil designs will let you exceed the air rating of the resistor somewhat and give you a much longer time curve. BUT downside, it is oil, and you runs some risk of spillage, Overheat it and you can build pressure in the can so most include a covered vent.  Think about a bush hog's speed reduction gearbox, it includes a vent that is covered but still allows venting to atmosphere.  It is your dummy load design, and if it meets your goals, it is "good enough".  I applaud you.
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73  James K0UA

K3UIM

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RE: $3 dummy load
« Reply #7 on: August 08, 2019, 10:17:39 AM »

Thank you, Stan. Perhaps I could use one and go full bore with my overpowering 5 watt qrp rig?? Hi.
Charlie
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Charlie. K3UIM
Where you are: I was!
Where I am: You will be!
So be nice to us old fogies!!

KM6BSQ

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RE: $3 dummy load
« Reply #8 on: August 08, 2019, 01:47:49 PM »

Is no one concern that the coax shield is connected to RF resistor legs via heatsink and thermal paste HY510? Sure it's squished together so there is some contact between the resistor pads and heatsink acting as a conductor.

Would not that introduce some capacitance when HY510 acts as a dielectric? Is that what I'm seeing when SWR slowly but steadily increases with the increase of frequency?
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WB6BYU

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RE: $3 dummy load
« Reply #9 on: August 08, 2019, 07:47:29 PM »

I don't know that particular resistor package, but generally a resistor or transistor in such a package is bolted
to the heat sink, and the bolt provides the electrical connection, while the paste (a very thin layer) improves
the thermal conductivity.

Seems fine to me.  The increasing SWR may be due to the lead length, the response of your meter with
frequency, or a characteristic of the resistor.

If you need to dissipate  more heat, adding a small fan blowing on the resistor is less messy than putting
it in a can with mineral oil.

VK2TIL

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RE: $3 dummy load
« Reply #10 on: August 08, 2019, 10:58:29 PM »

It's a simple & practical, albeit ugly, way to build a dummy load.

Keeping the resistor and the connector close together is good; if they are separated they should be joined by a 50-ohm line, either co-ax or the microstrip/stripline for which this kind of package is intended.

As 'BYU said, there is adequate electrical connection to the resistor flange which, in the "TC" model, is the ground end of the internal resistor.

The return loss/SWR is very good; 30dB RL = ca. 1.07:1 SWR.

The device is specified at max. SWR of 1.30:1 over its 3 GHz range.

If the amount splattered-around is any indication, there may be far too much heat transfer compound; the thickness of compound between the mating faces should be virtually zero, say 0.001".

Too much compound gives less heat transfer than no compound at all.

But, if you are happy with the appearance and with the 90-second time limit, you have a useful item of test equipment.

Oil will increase the 90-second period but it will also increase the cooling-down time (unless measures are taken to cool the oil).

SWR may also increase but probably not significantly for most work.



« Last Edit: August 08, 2019, 11:00:35 PM by VK2TIL »
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KM1H

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RE: $3 dummy load
« Reply #11 on: August 09, 2019, 08:48:08 AM »

All those type devices (at least of any quality above floor sweepings) have a spec sheet showing the derating required from a perfect heat sink. It aint pretty.

Carl
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AE5GT

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RE: $3 dummy load
« Reply #12 on: August 09, 2019, 03:06:53 PM »

Is no one concern that the coax shield is connected to RF resistor legs via heatsink and thermal paste HY510? Sure it's squished together so there is some contact between the resistor pads and heatsink acting as a conductor.

Would not that introduce some capacitance when HY510 acts as a dielectric? Is that what I'm seeing when SWR slowly but steadily increases with the increase of frequency?

 You just connect it to the top with a ring terminal then thermal grease isn't an issue. The biggest issue is mechanical stability for the lead , they break easily . Most of the cheap ones one Ebay are pulls , but they seem to work fine.
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K8AXW

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RE: $3 dummy load
« Reply #13 on: August 11, 2019, 10:41:06 AM »

The 90 second test isn't the problem. The problem is the duty cycle.  How long do you have to wait before you get another 90 seconds test  time?

This is the reason for the paint can with oil.  It increases the duty cycle.
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W9IQ

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RE: $3 dummy load
« Reply #14 on: August 11, 2019, 12:32:13 PM »

There are other factors at play than just the oil such as the can geometry and the location of the resistor within the can.

Since oil has a high specific heat, it will generally take longer to cool down than say an effective heatsink and a fan. The role of the oil is generally to increase the effective power dissipation of the resistor thereby allowing longer "key down" times. But this will often come at the price of a longer cool off time in order to shed the heat that has accumulated in the oil.

If you look at a product like the MFJ-250 oil based dummy load, they specify that you can run 1 kW for 10 minutes but it unfortunately doesn't specify how long you need to wait before repeating.

Some commercial dummy loads utilize an oil bath but instead of a can they have a large finned heatsink in contact with the oil. This greatly helps to shed the heat that has been transferred from the resistor to the oil thereby improving the duty cycle.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.
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