Dave's Dummy Load - with extra features.

[Kibi]

Hi

I have been meaning to build one of these for a while. I finally got around to it this week.
I have mostly used parts that I already had in stock. The only things I bought specifically for this project were the 10 turn pot, heatsink and meter.

The circuit is based on Dave's design on Youtube alongside his "World's simplest soft latching power switch". The soft latching switch is included mainly because I think it's cool .
One feature I really wanted to include was the ability to set the load value prior to connecting the load. It's very difficult to judge where you are with a 10 turn pot. You could just wind the pot down to 0 and then work up from there, but it's a bit cumbersome I think.
 I found an old junk PCB with a Hamlin reed relay on it. This particular relay has C/O contacts and has a 5v coil with the flyback diode built in. I consumes 25mA, but this is on momentarily, so not a problem for my battery powered design. The relay switches the meter between "set" and "read" values via a momentary push button on the front of the case.
A last minute feature I added was a pair of 2mm jacks to connect a DMM to if you want to monitor the voltage of your load at the same time. Not strictly necessary, but it offers rather a convenient place to connect standard 2mm meter leads.

I thought about including maximum power protection, but with the current limited at 2.5A there will be no problems as long as you don't exceed 50V.
The MOSFET's will blow up at 60V anyway, so you have got a whole set of other problems if you do exceed the maximum voltage. The power is well within the limits of the MOSFET pair if you stay below 50V.
I suppose a maximum voltage lockout could be employed, but I couldn't be bothered and I just wanted to get this thing done. I can't imagine myself going beyond ~25v anyway, so there is no problem for me.

The actual layout of the circuit and the use of strip board could be improved upon especially in an amplifier such as this with high currents involved, but it worked in the end. 

So, first off, design the case. It's a Hammond case that I bought for another project, but found it to be unsuitable in the end, so I used it for this project.

Here all the apertures are cut and holes drilled and threads tapped as necessary. A tungsten carbide drill bit snapped in one of the holes, so it's there to stay, (me being a dumb-ass).





Power resistor (10W) and a pair of FQP50N06L MOSFET's are bolted down and wired up. The FQP50N06L is one of my personal favourites.





The "wiring" is complete and ready to be tested.




I then performed some testing and it didn't go too well. It worked, but it oscillated like a good 'un, 750KHz to be precise! Not too surprising really, with all those long wires and a badly laid out board, but short of sitting down and designing and fabricating a board, I cured the oscillations with a 10nF across + and - of the comparator section. It then settled down and worked well.

Here is the completed project.

On the left front of the case is the set/read button. Without a load connected, you hold down the button and dial in your preferred load current.




On the rear are the load connections, 2mm sockets for meter leads and the power button.





I have included the circuit diagram that I drew if anyone is interested.

[SeanB]

Nice looking. I have broken off taps before in blind holes, so am accumulating a lot of tap extractors....... To remove the broken bits in future you need to get some spring steel strip, grind it down and use it to hook the bit loose and pull it out of the hole. With thermal cycling the piece there will eventually come loose and fall out.

[jnd]

I'm curious how well these MOSFETs work since they not designed for linear operation but for switching and this is even logic level one. Maybe the oscillation was sign of that.

[Kibi]

Nice looking. I have broken off taps before in blind holes, so am accumulating a lot of tap extractors....... To remove the broken bits in future you need to get some spring steel strip, grind it down and use it to hook the bit loose and pull it out of the hole. With thermal cycling the piece there will eventually come loose and fall out.

Ja, I wasn't too stressed about getting it out because it's only one of the holes to hold the heatsink on to the case. I was however a bit stressed about losing a tungsten carbide bit.
 One of my other projects coming up involves drilling and blind tapping 5 heatsinks with 15 holes each and I mean accurately! I need to have all my shit together and not be a doos when I start that job!

[Kibi]

I'm curious how well these MOSFETs work since they not designed for linear operation but for switching and this is even logic level one. Maybe the oscillation was sign of that.

After curing the oscillations, there was only slight ringing on the pulses. Not enough to worry about though.
You do need a logic level MOSFET for this design.

[SeanB]

Make a steel plate jig to drill the holes. Then you just clamp it to the heatsinks and drill the holes in each accurately and easily. Added bonus is you only have to be accurate once, and your copies will faithfully copy all flops you made and did not catch. Then you are only going to kick yourself once for being such a doos.

Tungsten drills are nice, I use Cobalt steel ones myself, much cheaper, but a big improvement over HSS for cutting soft metals. For stainless I use HSS, as I generally average 10 holes per bit, unless I am using a press. for those blind holes I would recommend getting some rolled taps, as you will end up with a stronger thread. Cutting soft metal with a tap makes it weaker, especially in a small diameter hole. I will not say how much grief I had getting a broken tap out of a high carbon hole, eventually I got it out, and without too much damage to the hole, so was able to drill it out and retap to fit a recoil insert. Funny enough the imperial sized kits were ultra cheap, around $10, with the metric ones being close to $50.

[Kibi]

Make a steel plate jig to drill the holes. Then you just clamp it to the heatsinks and drill the holes in each accurately and easily. Added bonus is you only have to be accurate once, and your copies will faithfully copy all flops you made and did not catch. Then you are only going to kick yourself once for being such a doos.

I like your thinking. I think I'll do that. Like you say, if I screw up the die, then I just have to make a new one, cheaper than screwing up a $50 heatsink.
Thanks for the tip. 

[Bored@Work]

I am a little bit worried about the power switch at the back. Imagine you use the load for a while and it gets hot. Then you want to turn it off and you have to reach behind the hot dummy load to reach the switch.

[Kibi]

I am a little bit worried about the power switch at the back. Imagine you use the load for a while and it gets hot. Then you want to turn it off and you have to reach behind the hot dummy load to reach the switch.

Mmm, that's a good point. The only reason it went on the back was because the front panel is fairly crowded already.
Also, I didn't ever think about switching the load off in that way. I always thought about turning the load it's self off if anything went wrong, just as you might if you were using a bog standard resistor as a load.
However, due to the design of the power switch circuit, another switch can be added elsewhere without modifying the circuit.
I do like your idea though, it might be worth investigating.

[dcel]

Strange, I can not see your posted pics.
I'm very interested to see your spin on Dave's Dummy load.

Chris

[EEVblog]

I like the final physical look, nice!

Dave.

[nctnico]

What you need to look out for is the safe operation area. Even at 50V this can be a very serious problem! Last year I wanted to build a dummy load with two 600V 100A transistors (these http://www.ineltron.de/english/mitsubishi-data/transistor/QM100DY-H.pdf) in parallel. A closer look at the DC safe operation area showed that the transistors I had would be close to destruction when handling 50V at 2A! So thats a no-go. To satisfy my requirements (400V / 20A / 300W) I would need to parallel several TO220 devices rated for large DC currents and get the current sharing right. Devices rated for large DC currents are hard to find. You'll end up with transistors intended for audio amplifiers. Those are not cheap at all.

Another problem with your circuit is that you'll need to match the MOSFETs very closely. When used in their linear region MOSFETs have a negative temperature coefficient just like bipolar transistors. This means that when the temperature rises one MOSFET will do all the work if the match is bad (and it usually is). Closely look at figure 2 in the datasheet of the MOSFETs you used ( http://www.fairchildsemi.com/ds/FQ/FQP50N06L.pdf). And if you take a look at the Safe Operation Area graph (figure 9) then you can see that this MOSFET can handle no more than approx. 250mA at 50V DC. That should be 2.5A at 50V. So its OK. Thanks Kibi for your remark below.

In the end I choose to buy a couple of DC loads from Array (model 3710).

[Kibi]

I like the final physical look, nice!

Dave.

Thanks Dave. I wish I could say the same about the inside. I'll design a board some time when I stop doing 16 hour days

Another problem with your circuit is that you'll need to match the MOSFETs very closely. When used in their linear region MOSFETs have a negative temperature coefficient just like bipolar transistors. This means that when the temperature rises one MOSFET will do all the work if the match is bad (and it usually is). Closely look at figure 2 in the datasheet of the MOSFETs you used ( http://www.fairchildsemi.com/ds/FQ/FQP50N06L.pdf). And if you take a look at the Safe Operation Area graph (figure 9) then you can see that this MOSFET can handle no more than approx. 250mA at 50V DC.

True, I think some current sharing resistors would help with balancing the load between the two devices.

Looking at the SOA graph (Fig. 9) of the Fairchild datasheet, I read the maximum DC current curve as 2.5A @ 50V, not 250mA.

[giorgos]

I made this circuit and I realized that i can set the load current up to 1A maximum. How can I use it for higher currents, for example 5A?

[IanJ]

What you need to look out for is the safe operation area. Even at 50V this can be a very serious problem! Last year I wanted to build a dummy load with two 600V 100A transistors (these http://www.ineltron.de/english/mitsubishi-data/transistor/QM100DY-H.pdf) in parallel. A closer look at the DC safe operation area showed that the transistors I had would be close to destruction when handling 50V at 2A! So thats a no-go. To satisfy my requirements (400V / 20A / 300W) I would need to parallel several TO220 devices rated for large DC currents and get the current sharing right. Devices rated for large DC currents are hard to find. You'll end up with transistors intended for audio amplifiers. Those are not cheap at all.

Another problem with your circuit is that you'll need to match the MOSFETs very closely. When used in their linear region MOSFETs have a negative temperature coefficient just like bipolar transistors. This means that when the temperature rises one MOSFET will do all the work if the match is bad (and it usually is). Closely look at figure 2 in the datasheet of the MOSFETs you used ( http://www.fairchildsemi.com/ds/FQ/FQP50N06L.pdf). And if you take a look at the Safe Operation Area graph (figure 9) then you can see that this MOSFET can handle no more than approx. 250mA at 50V DC. That should be 2.5A at 50V. So its OK. Thanks Kibi for your remark below.

Yup, one reason I chose to use a couple of IXTK46N50L mosfets in my load.

The IXTK46N50L (500v, 46A) mosfet overcomes the problem of running in the mosfets linear mode by extending the transistors’ FBSOA. I.E. ETI (Electro-Thermal-Instability) as a result of positive feedback within the mosfet. Per the datasheet, the IXTK is ideally used in programmable loads, battery chargers & current regulators etc.

Expensive though!

Datasheet article here - http://www.datasheetarchive.com/indexer.php?file=DSASW00214358.pdf&dir=Datasheets-SW11&keywords=IXTK46N50L&database=user-highscore#

My IXTK46N50L's are driven using an LM8272 op-amp which is designed to drive large capacitive loads. I've been up to 25v @ 10 amps........haven't tried any higher as I'd need better cooling.

Ian.

[Smokey]

For a good source of high SOA rated parts, check out the DIYAudio guys and see what they used for their classAB amps.