Post by thread:[EE]: Want advice on electronic load ( EL )

I have posted a hand drawn schematic at http://oh-god.com:5080/dir/Electronic-load/ The application is a battery tester. I want to load a 1.5 to 5V battery with a load current of zero to 40A. To the right you see I plan to drive the op amp with a 16 bit DAC. R3 shuts off the load if the DAC is disconnected. I put in R1 and R2 as insurance. I am not sure I need them at all. The EL can be very slow. A response time of 1 second is okay. Rsense is .01 ohms. Thus I figure that Vsense will vary from 0.0 to 0.4V. The FET will be using a heatsink ( and maybe a fan ) and will be rated at about 80A and 300W. I have calculated the power dissipation of the FET to be a max of 40A times 5V which is 200W. ( Some of that heat will appear across Rsense ) DAC = DAC8574 FET = FQP90N10V2 OP AMP = ??? Will this idea work or have I missed the banana boat. What would be a good op amp to maintain precision ? The op amp need not be fast. Low power is obviously not a factor. MA WFT Electronics Denver, CO 720 222 1309 " dent the UNIVERSE " All ideas, text, drawings and audio , that are originated by WFT Electronics ( and it's principals ), that are included with this signature text are to be deemed to be released to the public domain as of the date of this communication . WFT Electronics Denver, CO 720 222 1309 " dent the UNIVERSE " All ideas, text, drawings and audio , that are originated by WFT Electronics ( and it's principals ), that are included with this signature text are to be deemed to be released to the public domain as of the date of this communication . WFT Electronics Denver, CO 720 222 1309 " dent the UNIVERSE " All ideas, text, drawings and audio , that are originated by WFT Electronics ( and it's principals ), that are included with this signature text are to be deemed to be released to the public domain as of the date of this communication .

Hi Michael, By the way, didn't there used to be someone by the name of "Gus" who used to post to the list with a signature involving WFT Electronics? Do you know him? Basically you are on the right track with this. The biggest problem I see is that it is unrealistic to think that you will be able to dissipate 200W from one FET in any typical type of package. FET specs can be very misleading. Usually their max dissipation number assumes that you manage to keep the case at 25C, which would pretty much require cooling with liquid nitrogen or lots of water through a copper heatsink. No real application is going to manage to carry away 200W from that package without the case getting very hot. They give junction to case thermal resistance of 0.6 deg C/Watt and typical case to sink of 0.5. Then, you need to figure at least 1 deg C/Watt for thermal spreading resistance through the heatsink material. If you manage to have a huge fan-cooled heatsink, you might manage to keep the rest of the resistance to ambient at 1 deg C/Watt. That's a total from junction to ambient of 0.6+0.5+1+1=3.1 deg C/Watt. If ambient is 25C, and the max junction temperature is 175 C, then this leaves you with a deltaT of 150C max. 150C/3.1 degC/Watt=48W. Note that this leaves no margin for calculation approximation error, accidental overload, part to part variation, or any kind of safety factor (i.e. keeping the Tj max below 175C to increase semiconductor reliability). i think that you are going to end up with multiple devices in parallel. If you end up doing that, note that you cannot just parallel FETs directly when they are operating in the linear (half-on) mode as you are using them here. That means that you would need ballast resistors and at that point it would probably be better to go with several BJTs in parallel. One way to reduce your device power dissipation is to have some power resistors in the circuit. I once built one of these battery constant-current dischargers which used two power resistors (call them R1,R2) in series with several BJTs in parallel across one of the two resistors (R2). When the BJTs were fully off, the circuit resistance was R1+R2. With the BJTs fully on, it was roughly R1. I could then vary the current between those two extremes by varying the drive to the BJT. The maximum BJT dissipation was then roughly 1/4 of what it would be if there were no power resistors (max power for the BJT in this case was at half of max current and half of max voltage). Another potential problem is op-amp instability. You have the FET in the feedback loop of the op-amp. The FET has a high gate capacitance and non-linear behavior, especially as the current gets low. You may have to add a compensation network (some Rs and Cs) in the op-amp circuit to prevent oscillation. You can tell by prototyping it, putting a scope on the op-amp output, and then driving the DAC with step functions (i.e., command a 0.5A to 10A step) and watch for ringing on the scope. As for an op-amp choice, it will be challenging to get near 16 bit accuracy. There will be Rsense inaccuracies, op-amp noise, and op-amp offset error. Analog devices makes some amazing zero-drift op-amps now (which auto-zero their own offset error). A decent general purpose 1mV offset op-amp is the LMC6482. Sean On Tue, Nov 4, 2008 at 1:32 PM, Michael Algernon <spam_OUTpicTakeThisOuTnope9.com> wrote: {Quote hidden}> > I have posted a hand drawn schematic at > http://oh-god.com:5080/dir/Electronic-load/ > > The application is a battery tester. I want to load a 1.5 to 5V > battery with a load current of zero to 40A. To the right you see I > plan to drive the op amp with a 16 bit DAC. R3 shuts off the load if > the DAC is disconnected. I put in R1 and R2 as insurance. I am not > sure I need them at all. The EL can be very slow. A response time of > 1 second is okay. Rsense is .01 ohms. Thus I figure that Vsense > will vary from 0.0 to 0.4V. > The FET will be using a heatsink ( and maybe a fan ) and will be rated > at about 80A and 300W. I have calculated the power dissipation of the > FET to be a max of 40A times 5V which is 200W. ( Some of that heat > will appear across Rsense ) > > DAC = DAC8574 > FET = FQP90N10V2 > OP AMP = ??? > > Will this idea work or have I missed the banana boat. What would be a > good op amp to maintain precision ? The op amp need not be fast. Low > power is obviously not a factor. > > MA > > > > > > WFT Electronics > Denver, CO 720 222 1309 > " dent the UNIVERSE " > > All ideas, text, drawings and audio , that are originated by WFT > Electronics ( and it's principals ), that are included with this > signature text are to be deemed to be released to the public domain as > of the date of this communication . > > > WFT Electronics > Denver, CO 720 222 1309 > " dent the UNIVERSE " > > All ideas, text, drawings and audio , that are originated by WFT > Electronics ( and it's principals ), that are included with this > signature text are to be deemed to be released to the public domain as > of the date of this communication . > > > WFT Electronics > Denver, CO 720 222 1309 > " dent the UNIVERSE " > > All ideas, text, drawings and audio , that are originated by WFT > Electronics ( and it's principals ), that are included with this > signature text are to be deemed to be released to the public domain as > of the date of this communication . > > -

> On Nov 4, 2008, at 5:42 PM, Sean Breheny wrote: > > Hi Michael, > > ( snip ) > i think that you are going to end up with multiple devices in > parallel. If you end up doing that, note that you cannot just parallel > FETs directly when they are operating in the linear (half-on) mode as > you are using them here. That means that you would need ballast > resistors and at that point it would probably be better to go with > several BJTs in parallel. > G'day Sean Why would not three FETs in parallel work in the linear mode ? Each would carry a certain amount of the current and dissipate part of the heat. As the FET heats up does it carry more or less current for a given Vgs ? According to the spec, I think the resistance goes up with temperature. Thus I would expect FETs in the linear area to current share. Am I wrong ? MA

FETs have two principal modes: linear mode and ohmic mode. In the linear mode (which occurs when Vdrain-gate <Vth), the FET acts like a constant current source/sink. The current depends mainly on Vgs and not very much on Vds. The current has a positive temperature coefficient in this mode and parallel FETs in this mode are likely not to share current equally. In the ohmic mode (which is when the gate is more than Vth above BOTH source and drain), the FET acts like a variable resistor where the resistance is a function of Vgs. In this mode, the RESISTANCE has a positive temperature coefficient and parallel FETs will share current just fine. Sean On Tue, Nov 4, 2008 at 8:56 PM, Michael Algernon <.....picKILLspam@spam@nope9.com> wrote: {Quote hidden}> > G'day Sean > > Why would not three FETs in parallel work in the linear mode ? Each > would carry a certain amount of the current and dissipate part of the > heat. As the FET heats up does it carry more or less current for a > given Vgs ? According to the spec, I think the resistance goes up > with temperature. Thus I would expect FETs in the linear area to > current share. Am I wrong ? > > MA > -

My new design ( thanks Sean Breheny ) uses 4 FETs. Three of them provide fixed current loads of 10A each. Each of the 3 can be switched on or off. The fourth provides a variable load of 0 to 10A. By mixing and matching , you can get a programmed load from 0 to 40A. I am looking for comments and advice. I calculate the FETs will now have to tolerate 10A max and 50W power dissipation. http://oh-god.com:5080/dir/Electronic-load/ MA WFT Electronics Denver, CO 720 222 1309 " dent the UNIVERSE " All ideas, text, drawings and audio , that are originated by WFT Electronics ( and it's principals ), that are included with this signature text are to be deemed to be released to the public domain as of the date of this communication .

FWIW we use several electronic loads from 300 Watts to 4 kW and they are all MOSFET based. Maxim had an app note on an op amp based load and I wouldn't be surprised if IR had information on paralleling MOSFETS. As an aside how many in this group used paint can full of oil to keep a dummy load cool? The common problem I see with a current sensing load it the initial turn on transient. Since the power supply output is zero the MOSFETS are driven into saturation. Depending on the response of the load and the protection in the supply this can create some interesting problems. Crowbar protection on a 4 kW supply or a 20 kW inverter for example. The better loads allow for slew rate control and constant resistance modes that help some. Tom

On Fri, Nov 7, 2008 at 10:57 AM, Tom <tomliveshereKILLspamsbcglobal.net> wrote: > The common problem I see with a current sensing load it the initial turn on > transient. Since the power supply output is zero the MOSFETS are driven into > saturation. Depending on the response of the load and the protection in the > supply this can create some interesting problems. Crowbar protection on a 4 > kW supply or a 20 kW inverter for example. The better loads allow for slew > rate control and constant resistance modes that help some. Interesting. Last time I was in a power electronics lab and the electronic load we have are only for lower power. So when we needed to test bigger converter or inverter module (up to 5KW), then we had to use a big bank of resistor with fans for cooling. I also saw people using a bank of lamps as load. Very good heater in Winter time. But in summer time, oops. I am thinking motors can be useful for higher power load as well. Xiaofan

Hi Tom, I'm sure you can find some good FET app notes, but just beware that most FET app notes which talk about paralleling mosfets are talking about doing so with the FETs used as switches (fully on or off) not in the linear region. I'm not sure I understand what you mean with the turn-on transient. Do you mean when the load is on but nothing is supplying it and then you suddenly apply the supply to the load? Sean On Thu, Nov 6, 2008 at 9:57 PM, Tom <.....tomliveshereKILLspam.....sbcglobal.net> wrote: {Quote hidden}> FWIW we use several electronic loads from 300 Watts to 4 kW and they are all > MOSFET based. Maxim had an app note on an op amp based load and I wouldn't > be surprised if IR had information on paralleling MOSFETS. As an aside how > many in this group used paint can full of oil to keep a dummy load cool? > > The common problem I see with a current sensing load it the initial turn on > transient. Since the power supply output is zero the MOSFETS are driven into > saturation. Depending on the response of the load and the protection in the > supply this can create some interesting problems. Crowbar protection on a 4 > kW supply or a 20 kW inverter for example. The better loads allow for slew > rate control and constant resistance modes that help some. > > > Tom > > -

Xiaofan Chen wrote: > On Fri, Nov 7, 2008 at 10:57 AM, Tom <EraseMEtomlivesherespam_OUTTakeThisOuTsbcglobal.net> wrote: >> The common problem I see with a current sensing load it the initial turn on >> transient. Since the power supply output is zero the MOSFETS are driven into >> saturation. Depending on the response of the load and the protection in the >> supply this can create some interesting problems. Crowbar protection on a 4 >> kW supply or a 20 kW inverter for example. The better loads allow for slew >> rate control and constant resistance modes that help some. > > Interesting. Last time I was in a power electronics lab and the electronic > load we have are only for lower power. So when we needed to test > bigger converter or inverter module (up to 5KW), then we had to use a > big bank of resistor with fans for cooling. I also saw people using > a bank of lamps as load. Very good heater in Winter time. But > in summer time, oops. > > I am thinking motors can be useful for higher power load as well. > > Xiaofan I used an old amplifier heatsink and a pile of panel mount 50W 1 ohm resistors that I got from ebay as my test load for a 14v power supply. I've also used a 1 ohm 250 watt resistor in a bucket of water to test high current low voltage sources. - Martin

I recently needed to test some power supplies in the few dozen watt range... Somewhere I came across an appnote for a constant current "load" basically using an opamp and a FET... One of the opamp inputs was tied to a multiturn pot wired as a voltage divider... the other one was tied to the top of a low-side current sense resistor. The output of the opamp was tied to the input of the FET. So.. the opamp would increse or decrease it's output (turning the FET on or off more or less), until the voltage at the top of the current sense resistor matched the voltage out of the voltage divider... want more current, turn up the voltage from the voltage divider. This worked pretty good, although I was getting some "current ringing", and I really didn't want to mess with it... So I went and bought one of these: http://www.circuitspecialists.com/search.itml?icQuery=electronic+load Perhaps a similar approach (the opamp/fet combination) would work, although using the output from the ADC to set the opamp input voltage. -forrest

{Quote hidden}> > On Nov 6, 2008, at 7:57 PM, Tom wrote: > > FWIW we use several electronic loads from 300 Watts to 4 kW and they > are all > MOSFET based. Maxim had an app note on an op amp based load and I > wouldn't > be surprised if IR had information on paralleling MOSFETS. As an > aside how > many in this group used paint can full of oil to keep a dummy load > cool? > > The common problem I see with a current sensing load it the initial > turn on > transient. Since the power supply output is zero the MOSFETS are > driven into > saturation. Depending on the response of the load and the protection > in the > supply this can create some interesting problems. Crowbar protection > on a 4 > kW supply or a 20 kW inverter for example. The better loads allow > for slew > rate control and constant resistance modes that help some. > > > Tom Do you suppose a FET could be operated in a paint can full of oil to cool it ? The max load voltage would be 5V and the gate voltage might be 10-12 volts. MA

In message <C13C4F75-5FFD-4A3A-8E92-DA05BBF2700Bspam_OUTnope9.com>, Michael Algernon <@spam@picKILLspamnope9.com> writes > >I have posted a hand drawn schematic at >http://oh-god.com:5080/dir/Electronic-load/ > >The application is a battery tester. I want to load a 1.5 to 5V >battery with a load current of zero to 40A. What type of battery? Is there a specific reason to need to load test it or can you test using conductance instead? -- Clint Sharp

> > On Nov 7, 2008, at 1:26 PM, Clint Sharp wrote: > > In message <KILLspamC13C4F75-5FFD-4A3A-8E92-DA05BBF2700BKILLspamnope9.com>, Michael > Algernon <RemoveMEpicTakeThisOuTnope9.com> writes >> >> I have posted a hand drawn schematic at >> http://oh-god.com:5080/dir/Electronic-load/ >> >> The application is a battery tester. I want to load a 1.5 to 5V >> battery with a load current of zero to 40A. > What type of battery? Is there a specific reason to need to load > test it > or can you test using conductance instead? > -- > Clint Sharp > -- Thanks Clint My customer wants to program a variable current load on his batteries. I don't know why ...... MA WFT Electronics Denver, CO 720 222 1309 " dent the UNIVERSE " All ideas, text, drawings and audio , that are originated by WFT Electronics ( and it's principals ), that are included with this signature text are to be deemed to be released to the public domain as of the date of this communication .

Michael Algernon wrote: > My customer wants to program a variable current load on his > batteries. I don't know why ...... > Is this a one-off? If so, he/you might want to seriously consider one of the electronic loads I mentioned in a previous posting to the list. I also found a couple of circuits which were similar to the one I was talking about... The one which is best described is: http://electronicdesign.com/Articles/ArticleID/4056/4056.html The schematic is at http://electronicdesign.com/Files/29/4056/Figure_01.gif What I was suggesting is to hook your ADC up to the + terminal instead of using the pots as the voltage divider. The general idea can be used for constant voltage as well..... As long as you can produce a voltage on the - terminal of the opamp which corresponds to the actual current or voltage on the output, and put the desired matching voltage on the + terminal, the load will "seek" to the current or voltage, since the opamp wants it's inputs to match.... If you increase the voltage on the + terminal, it will increase the output voltage until the voltage coming off of the current sense resistors matches. -forrest

> > On Nov 8, 2008, at 9:01 PM, Forrest W Christian wrote: > > Michael Algernon wrote: >> My customer wants to program a variable current load on his >> batteries. I don't know why ...... >> > Is this a one-off? If so, he/you might want to seriously consider > one > of the electronic loads I mentioned in a previous posting to the list. He wants 20 of them. {Quote hidden}> I also found a couple of circuits which were similar to the one I was > talking about... The one which is best described is: > > http://electronicdesign.com/Articles/ArticleID/4056/4056.html > > The schematic is at http://electronicdesign.com/Files/29/4056/Figure_01.gif > > What I was suggesting is to hook your ADC up to the + terminal instead > of using the pots as the voltage divider. The general idea can be > used > for constant voltage as well..... As long as you can produce a > voltage > on the - terminal of the opamp which corresponds to the actual current > or voltage on the output, and put the desired matching voltage on > the + > terminal, the load will "seek" to the current or voltage, since the > opamp wants it's inputs to match.... If you increase the voltage on > the > + terminal, it will increase the output voltage until the voltage > coming > off of the current sense resistors matches. > > -forrest > > > WFT Electronics Denver, CO 720 222 1309 " dent the UNIVERSE " All ideas, text, drawings and audio , that are originated by WFT Electronics ( and it's principals ), that are included with this signature text are to be deemed to be released to the public domain as of the date of this communication .

Sounds like you picked up that project from guru --- On Sat, 11/8/08, Michael Algernon <spamBeGonepicspamBeGonenope9.com> wrote: {Quote hidden}> From: Michael Algernon <TakeThisOuTpicEraseMEspam_OUTnope9.com> > Subject: Re: [EE]: Want advice on electronic load ( EL ) > To: "Microcontroller discussion list - Public." <RemoveMEpiclistTakeThisOuTMIT.EDU> > Date: Saturday, November 8, 2008, 12:56 AM > > > > On Nov 7, 2008, at 1:26 PM, Clint Sharp wrote: > > > > In message > <C13C4F75-5FFD-4A3A-8E92-DA05BBF2700BEraseME.....nope9.com>,> > Michael > > Algernon <EraseMEpicnope9.com> writes > >> > >> I have posted a hand drawn schematic at > >> http://oh-god.com:5080/dir/Electronic-load/ > >> > >> The application is a battery tester. I want to > load a 1.5 to 5V > >> battery with a load current of zero to 40A. > > What type of battery? Is there a specific reason to > need to load > > test it > > or can you test using conductance instead? > > -- > > Clint Sharp > > -- > > Thanks Clint > My customer wants to program a variable current load on his > > batteries. I don't know why ...... > MA > > > WFT Electronics > Denver, CO 720 222 1309 > " dent the UNIVERSE " > > All ideas, text, drawings and audio , that are originated > by WFT > Electronics ( and it's principals ), that are included > with this > signature text are to be deemed to be released to the > public domain as > of the date of this communication . > > --

Heh heh are you the guy bidding against me ? MA On Nov 11, 2008, at 6:49 AM, alan smith wrote: Sounds like you picked up that project from guru --- On Sat, 11/8/08, Michael Algernon <RemoveMEpicEraseMEEraseMEnope9.com> wrote: {Quote hidden}> From: Michael Algernon <RemoveMEpicspam_OUTKILLspamnope9.com> > Subject: Re: [EE]: Want advice on electronic load ( EL ) > To: "Microcontroller discussion list - Public." <RemoveMEpiclistTakeThisOuTspamMIT.EDU> > Date: Saturday, November 8, 2008, 12:56 AM >> >> On Nov 7, 2008, at 1:26 PM, Clint Sharp wrote: >> >> In message > <EraseMEC13C4F75-5FFD-4A3A-8E92-DA05BBF2700BspamspamBeGonenope9.com>,> > Michael >> Algernon <RemoveMEpicKILLspamnope9.com> writes >>> >>> I have posted a hand drawn schematic at >>> http://oh-god.com:5080/dir/Electronic-load/ >>> >>> The application is a battery tester. I want to > load a 1.5 to 5V >>> battery with a load current of zero to 40A. >> What type of battery? Is there a specific reason to > need to load >> test it >> or can you test using conductance instead? >> -- >> Clint Sharp >> -- > > Thanks Clint > My customer wants to program a variable current load on his > > batteries. I don't know why ...... > MA > > > WFT Electronics > Denver, CO 720 222 1309 > " dent the UNIVERSE " > > All ideas, text, drawings and audio , that are originated > by WFT > Electronics ( and it's principals ), that are included > with this > signature text are to be deemed to be released to the > public domain as > of the date of this communication . > > --

no....I looked at it, talked to them...it appeared to be a can of worms, that I preferred not to get involved with. When the spec's are not clear, I avoid those projects. --- On Tue, 11/11/08, Michael Algernon <picSTOPspamspam_OUTnope9.com> wrote: {Quote hidden}> From: Michael Algernon <spamBeGonepicSTOPspamEraseMEnope9.com> > Subject: Re: [EE]: Want advice on electronic load ( EL ) > To: "Microcontroller discussion list - Public." <KILLspampiclistspamBeGoneMIT.EDU> > Date: Tuesday, November 11, 2008, 1:14 PM > Heh heh are you the guy bidding against me ? > MA > > On Nov 11, 2008, at 6:49 AM, alan smith wrote: > > Sounds like you picked up that project from guru > > > --- On Sat, 11/8/08, Michael Algernon <EraseMEpicEraseMEnope9.com> > wrote: > > > From: Michael Algernon <@spam@pic@spam@spam_OUTnope9.com> > > Subject: Re: [EE]: Want advice on electronic load ( EL > ) > > To: "Microcontroller discussion list - > Public." <spamBeGonepiclistKILLspamMIT.EDU> > > Date: Saturday, November 8, 2008, 12:56 AM > >> > >> On Nov 7, 2008, at 1:26 PM, Clint Sharp wrote: > >> > >> In message > > > <.....C13C4F75-5FFD-4A3A-8E92-DA05BBF2700Bspam_OUTnope9.com>,> > > Michael > >> Algernon <TakeThisOuTpic.....TakeThisOuTnope9.com> writes > >>> > >>> I have posted a hand drawn schematic at > >>> http://oh-god.com:5080/dir/Electronic-load/ > >>> > >>> The application is a battery tester. I want > to > > load a 1.5 to 5V > >>> battery with a load current of zero to 40A. > >> What type of battery? Is there a specific reason > to > > need to load > >> test it > >> or can you test using conductance instead? > >> -- > >> Clint Sharp > >> -- > > > > Thanks Clint > > My customer wants to program a variable current load > on his > > > > batteries. I don't know why ...... > > MA > > > > > > WFT Electronics > > Denver, CO 720 222 1309 > > " dent the UNIVERSE " > > > > All ideas, text, drawings and audio , that are > originated > > by WFT > > Electronics ( and it's principals ), that are > included > > with this > > signature text are to be deemed to be released to the > > public domain as > > of the date of this communication . > > > > --