# 100 Amp DC Load 24 volt in

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#### bodgeroger

##### New Member
Hi Guys, new to this electronics stuff, but i need to build a variable dc load i have built a couple but all have ended in loss of magic smoke.
i am thinking perhaps an infinite number of 2n3055 transistors is not the way to go but i just cannot think how to do this with MOS FETS or IGBITS.

Anybody got any ideas i could try??

tks bodgeroger

#### tcmtech

##### Banned
You could go with a Massive IGBT block! They can be found on eBay for a reasonable price. This one here would set you back around $50 -$75 on eBay. And it Has a 4100 watt dissipation capacity!

Just make sure your heat sink is big enough! Several square feet with big honkin fins! Or water cooled works very well for compact applications.

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#### Willbe

##### New Member
Iron coat hanger wire has 6x the resistance of copper and can stand this current if you submerse it in water.
No shock danger at 24v.

#### MikeMl

##### Well-Known Member
How long does it have to dissipate 2.4KW?

Water cooled resistors can dissipate KW, especially if you turn the water to steam.

#### bodgeroger

##### New Member

Water cooling is out, nice idea but implementing a portable load would be really difficult, now a huge chunk of aluminium with "stonking fins" is good and with the addition of a fan too, well this is the direction to go.

time for dissipation is variable, i have had power supplies being burnt in at 100 amps over the weekend ready for installation Monday morning.

As to the igbit block this is very good and i do have access to some of these but i do not know how to drive them especially with a varying load, I.E. turn the volume control and current goes up or down!!

#### ronsimpson

##### Well-Known Member
I used car headlights for a load. Two bulbs for 24 volts. I have a bank of bulbs with switches to switch each pair. No heatsink but you can get a sun tan.

#### bodgeroger

##### New Member
Yes i have already done that, trouble is that the office lookis like a lighthouse or somthing out of x files!!!!!

#### tcmtech

##### Banned
Using IGBT's for a variable load is easy you just use a simple pot with one end at the collector and the other at the emitter. The variable goes to the gate.
For unattended running I would highly recommend a thermal switch that would disconnect the collector connection of the pot. If it over heats it will shut it down.

#### MikeMl

##### Well-Known Member
Here is a picture of something that I built years ago. It is a computer-controlled capacity tester for 35Ah lead-acid aircraft batteries. The two big coils on top are "locomotive braking resistors" that I bought from a surplus dealer. They are rated at 1200W each.

I have two of the braking resistors left over from that project. One is 0.069Ω and the other is 0.150Ω., each rated at 1200W in free air. I=24/(0.069+0.150)= 109.6A.

If you can use these, contact me via PM.

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#### bodgeroger

##### New Member
Hi MikeMI,

Now that is what i had in mind, but i was planing to use the transistors to provide the load, but scanning through a few IGBIT datasheets it looked like they turned on with the meerist sniff of gate voltage, but as if what you say is true then a simple pot would work and point noted about over heating and flames

anyone want to post a circuit for me??

what about thermal stability as the IGBIT gets hotter wont it "runaway"

Roger

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