Dummy load

Doctore · Aug 17, 2013

I want to build dl like here https://www.electro-tech-online.com...emp-control-fan-load-bank-pwm-circuit.128753/
But I need help because my englisch and electronic skills are not the best.
And I start with my questions here https://www.electro-tech-online.com...emp-control-fan-load-bank-pwm-circuit.128753/
But this is not my thread, so I create new one.
First I draw schematic:

View attachment 75382

ronv suggest add 75R resistor between the output of each Op amp and the FETs.
Like this?:

View attachment 75400

Mosfets, I use irf2805pbf.
About cooling, I want to use 12pcs of 120mm pc fans and car radiator, this give me few °C less than room temperature.

Thanks for the replies.

ronv · Aug 17, 2013

This transistor may be better:
https://elcodis.com/parts/5879280/AP92T12GP-HF.html#datasheet
I found it on the second web site you posted. It has a safe operating area that includes DC.
The example is correct, except the resistor goes on the FET driver op amps not the temperature sensor.

Doctore · Aug 18, 2013

Sorry friend but I have this in my shop, but 1000+pcs and costs are 2$/pc.
Look here for mosfets: **broken link removed**
But price must be <4PLN for 10+ pcs, so If IRF2805PBF is bad I can get IRF1407PBF

ronv · Aug 18, 2013

Yes, I see that now.

The problem is that if the part does not have a spec for safe operating area that includes DC (always on) you cannot be sure it will be safe to use. I'm not sure I looked at all the "big" ones but I didn't see any others that looked good. Maybe you can look again. If you look at the safe operating area curves you should find one good for about 10 amps at 20 volts drain to source on the DC curve.

Doctore · Aug 18, 2013

"safe operating area curves" - I'm newbie in electronics and this is unreadable for me
But previosly you tell me IRF2805PBF and IRF1407PBF are good...
Other mosfets I don't find.
What is price of shipping from digikey to poland?

KeepItSimpleStupid · Aug 18, 2013

SOA is a term that encompases a lot of stuff. It might mean if something, then something is permissable otherwise it's not. Example, Aa device might be able to handle 10 A at 10 V, but only 2 at 200 V. It might withstand 100 A, if for 100 uS or less. That info is spelled out in the datasheets.

I belive, that digikey has various locations, but there may be closer options for you suh as RS Electronics and Farnell.

Check: https://www.digikey.pl/

ronv · Aug 18, 2013

Good idea KISS!

Farnell Poland has the STP80NF12 for 9 PLN which is not to bad a price.

Doc, Maybe it would be a good idea to tell us what you are going to do. Sometimes it works better than just going with what is already there. For example are you making a power supply load or maybe a battery load? Why? How many volt/amps?

()blivion · Aug 18, 2013

You should put decoupling/stabilizing capacitors from ground to VO and from ground to +VS on IC7, as close to the part as possible.

Linear regulators like to oscillate without stabilizing caps.

Doctore · Aug 19, 2013

Lol, farnel - never!
I can buy in farnell with orders for 300+PLN!
And 9PLN for one fet! Very expensive!
On digikey I can't find shipping prices.
I find more on tme...

And old equstion, I correct add 75R resistor?:
https://www.electro-tech-online.com/attachments/dummy-png.75400/

Doctore · Aug 19, 2013

misterT · Aug 19, 2013

Doctore said:
On digikey I can't find shipping prices.

Shipping is 18 EUR to finland and if you order more that 65 EUR worth, then shipping is free.. I didn't find shipping info to Poland, but I think they are quite similar (both are members of EU).

ronv · Aug 19, 2013

I think I already looked at all of those from that supplier.
Here is where the resistor goes.
I would be surprised if you find a good FET for less than 9. Lets confirm the current and voltage.

()blivion · Aug 19, 2013

Strictly speaking, one does not have to get the best FET's money can buy to make this project work. You can use pretty much any FET. However, the better each individual FET is, the fewer you will need to reach your total combined wattage. Which means less soldering, less cost, and less chances to screw something up.

When searching for a FET, the factor I would start filtering by is package type, then sort by highest wattage. A higher wattage should more or less be inline with a larger SOA, only wattage is more easy to sort by and work with. Confirm a workable SOA before purchasing of course. I would look for TO-220 FET's at 300 watts a piece or better, that was about what we did last time.

Doctore · Aug 20, 2013

Now schematic is 100% complete and good?

For mosfets, already I do not have ideas......
From digikey shipping cost are too expensive, tme is ok, but no needed mosfets, farnell not for little projects, fuq....

()blivion · Aug 20, 2013

Now schematic is 100% complete and good?

Looks mostly good, though you should still put some decoupling capacitors from pins 1-3 and pins 2-3 for IC7.

For mosfets, already I do not have ideas......

Start with a shop you know serves your area, then search by highest watts in the TO-220 package. Only pick parts over 300 watts.

Most online shops have powerful searches that let you find parts by part specs very easily. Doing it this way, with the specs above, will turn a selection of 10,000+ parts into ~100 or so, and most of what's left will work fine for this project.

From digikey shipping cost are too expensive

It must be out of their way to serve your area or something, we will have to use someone else then.

tme is ok, but no needed mosfets,

I don't even know what you are saying here, sorry.

farnell not for little projects, fuq....

Farnell/Newark certainly does sell small volumes for little projects. Maybe not in your area though? They also have perfectly acceptable MOSFET's for this project.

Doctore · Aug 20, 2013

tme is ok, but no needed mosfets,
I don't even know what you are saying here, sorry.

I mean, tme is ok but don't sold mosfets what we need.

most of what's left will work fine for this project.

So why when I showed 23 mosfets, none is not good for this project...

Farnell/Newark certainly does sell small volumes for little projects.

I mean this is not for low cost projects, I can buy in farnell with minimum 100$ - I do not want spend to much for this project.

EDIT: Now is good?

ronv · Aug 20, 2013

Soa

Here are 2 graphs of safe operating area. One is the 1407 the other the AP92T12 that I posted. Since almost all the power is disapated in the FETs the voltage across them is almost the full supply voltage (25 volts) and about 5 amps. If you look at the curves there is no DC line for the 1407, but if you used your imagination and drew one it might be 1 amp at 25 volts, while the other is good for maybe 15 amps. Big difference.

()blivion the block in Doctor's schematic is the temperature sensor not a regulator, so doesn't need the capacitors.

I'll look for some other suppliers or FETs tonight.

ronv · Aug 20, 2013

I lied, The last one on your list should work.

https://www.tme.eu/pl/details/sup57n20-33/tranzystory-z-kanalem-n-tht/vishay/sup57n20-33-e3/#

()blivion · Aug 20, 2013

Oh!!!! Temp sensor NOT regulator... i see... i see...

Doctore, the capacitors I told you to add, yeah.............. I'm guna have to ask you to just go and remove those now please .

Doctore · Aug 21, 2013

I lied, The last one on your list should work.

**broken link removed**

Doctore, the capacitors I told you to add, yeah I'm guna have to ask you to just go and remove those now please .

Ok

EDIT: Now is all good?:

And SOA, I have explained a little bit, but language barrier...

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