Trying to cheat ohm's law

Started by July 7, 2015
```For a circuit I described here recently, I want to be able to cause a brief, temporary, high current draw on a 12v power supply. I'd like to draw about 2A for a few ms, and at least 0.5A for perhaps 100ms.

I have a resistor + capacitor combination that gives a curve I like. A regulated 12v supply that claims to produce 5A, with a 33000uf cap in series with 3.9 ohm resistor across it, is loaded to 2+A and is still pulling 500ma, 200ms later.  +12v--3.9ohm--33000uf--Gnd

The problem is, if I understand things, the resistor would have to handle about 30W for a few milliseconds. I'd like to avoid using a component that physically large and pricey. I'm guessing I could get away with 5W because the high current is brief, but I want this circuit to work reliably for years.

Is there something smaller/cheaper I can use?
```
```On Tuesday, July 7, 2015 at 1:46:45 PM UTC-4, scott....@gmail.com wrote:

> Is there something smaller/cheaper I can use?

Never mind. I was put off by 2\$ and 3\$ resistors, but I found 3.9 ohm 10W for \$0.64.
```
```On Tue, 07 Jul 2015 10:46:41 -0700, scott.a.mayo wrote:

> For a circuit I described here recently, I want to be able to cause a
> brief, temporary, high current draw on a 12v power supply. I'd like to
> draw about 2A for a few ms, and at least 0.5A for perhaps 100ms.
>
> I have a resistor + capacitor combination that gives a curve I like. A
> regulated 12v supply that claims to produce 5A, with a 33000uf cap in
> series with 3.9 ohm resistor across it, is loaded to 2+A and is still
> pulling 500ma, 200ms later.  +12v--3.9ohm--33000uf--Gnd
>
> The problem is, if I understand things, the resistor would have to
> handle about 30W for a few milliseconds. I'd like to avoid using a
> component that physically large and pricey. I'm guessing I could get
> away with 5W because the high current is brief, but I want this circuit
> to work reliably for years.
>
> Is there something smaller/cheaper I can use?

Almost certainly yes.  Find a resistor that is specified by it's
manufacturer for transient loads; the ones that I've designed in will
have a graph in the data sheet that plots the load power against duration
(or average power, or energy, or something -- you'll have to do some math
to get it into the form you need).

I can't give you any part numbers or manufacturers off the top of my
head, but they're out there, and for that brief of a load you probably
won't need much more than something rated for your average power
consumption.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
```
```On 07/07/15 18:46, scott.a.mayo@gmail.com wrote:
> For a circuit I described here recently, I want to be able to cause a
> brief, temporary, high current draw on a 12v power supply. I'd like
> to draw about 2A for a few ms, and at least 0.5A for perhaps 100ms.
>
> I have a resistor + capacitor combination that gives a curve I like.
> A regulated 12v supply that claims to produce 5A, with a 33000uf cap
> in series with 3.9 ohm resistor across it, is loaded to 2+A and is
> still pulling 500ma, 200ms later.  +12v--3.9ohm--33000uf--Gnd
>
> The problem is, if I understand things, the resistor would have to
> handle about 30W for a few milliseconds. I'd like to avoid using a
> component that physically large and pricey. I'm guessing I could get
> away with 5W because the high current is brief, but I want this
> circuit to work reliably for years.
>
> Is there something smaller/cheaper I can use?
>

Yes - use a 7-10W resistor.

30W for a few mS is of the order of 0.1J of energy. The question is: how
much will 0.1J heat up a resistor body?

Answer - not much. If you want to get technical, look at the mass of the
body and guesstimate a specific heat capacity and work out a pulse
temperature rise.

There are extremes to this - if the pulse is very high for a very short
time, it could heat the resistor material (wire?) to melting point
before the energy dissipates throughout the body of the resistor.

Hence suggesting a 7-10W resistor as these are cheap, not very big and
very common. My gut instinct says that will be fine and in fact you
could go lower still, possibly.
```
```On Tue, 7 Jul 2015 10:46:41 -0700 (PDT), scott.a.mayo@gmail.com wrote:

>For a circuit I described here recently, I want to be able to cause a brief, temporary, high current draw on a 12v power supply. I'd like to draw about 2A for a few ms, and at least 0.5A for perhaps 100ms.
>
>I have a resistor + capacitor combination that gives a curve I like. A regulated 12v supply that claims to produce 5A, with a 33000uf cap in series with 3.9 ohm resistor across it, is loaded to 2+A and is still pulling 500ma, 200ms later.  +12v--3.9ohm--33000uf--Gnd
>
>The problem is, if I understand things, the resistor would have to handle about 30W for a few milliseconds. I'd like to avoid using a component that physically large and pricey. I'm guessing I could get away with 5W because the high current is brief, but I want this circuit to work reliably for years.
>
>Is there something smaller/cheaper I can use?

Just a power MOSFET, an OpAmp and a few resistors and a capacitor.

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```On Tuesday, July 7, 2015 at 6:34:14 PM UTC-4, Tim Watts wrote:
> On 07/07/15 18:46, scott.a.mayo@gmail.com wrote:
> > For a circuit I described here recently, I want to be able to cause a
> > brief, temporary, high current draw on a 12v power supply. I'd like
> > to draw about 2A for a few ms, and at least 0.5A for perhaps 100ms.
> >
> > I have a resistor + capacitor combination that gives a curve I like.
> > A regulated 12v supply that claims to produce 5A, with a 33000uf cap
> > in series with 3.9 ohm resistor across it, is loaded to 2+A and is
> > still pulling 500ma, 200ms later.  +12v--3.9ohm--33000uf--Gnd
> >
> > The problem is, if I understand things, the resistor would have to
> > handle about 30W for a few milliseconds. I'd like to avoid using a
> > component that physically large and pricey. I'm guessing I could get
> > away with 5W because the high current is brief, but I want this
> > circuit to work reliably for years.
> >
> > Is there something smaller/cheaper I can use?
> >
>
> Yes - use a 7-10W resistor.
>
> 30W for a few mS is of the order of 0.1J of energy. The question is: how
> much will 0.1J heat up a resistor body?

So here's a cool thing, around room temp everything has
about the same volumetric(sp) heat capacity.  ~3 J/(K*cm^3).

George H.
>
> Answer - not much. If you want to get technical, look at the mass of the
> body and guesstimate a specific heat capacity and work out a pulse
> temperature rise.
>
>
> There are extremes to this - if the pulse is very high for a very short
> time, it could heat the resistor material (wire?) to melting point
> before the energy dissipates throughout the body of the resistor.
>
> Hence suggesting a 7-10W resistor as these are cheap, not very big and
> very common. My gut instinct says that will be fine and in fact you
> could go lower still, possibly.

```
```On Tue, 7 Jul 2015 18:17:32 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Tuesday, July 7, 2015 at 6:34:14 PM UTC-4, Tim Watts wrote:
>> On 07/07/15 18:46, scott.a.mayo@gmail.com wrote:
>> > For a circuit I described here recently, I want to be able to cause a
>> > brief, temporary, high current draw on a 12v power supply. I'd like
>> > to draw about 2A for a few ms, and at least 0.5A for perhaps 100ms.
>> >
>> > I have a resistor + capacitor combination that gives a curve I like.
>> > A regulated 12v supply that claims to produce 5A, with a 33000uf cap
>> > in series with 3.9 ohm resistor across it, is loaded to 2+A and is
>> > still pulling 500ma, 200ms later.  +12v--3.9ohm--33000uf--Gnd
>> >
>> > The problem is, if I understand things, the resistor would have to
>> > handle about 30W for a few milliseconds. I'd like to avoid using a
>> > component that physically large and pricey. I'm guessing I could get
>> > away with 5W because the high current is brief, but I want this
>> > circuit to work reliably for years.
>> >
>> > Is there something smaller/cheaper I can use?
>> >
>>
>> Yes - use a 7-10W resistor.
>>
>> 30W for a few mS is of the order of 0.1J of energy. The question is: how
>> much will 0.1J heat up a resistor body?
>
>So here's a cool thing, around room temp everything has
>about the same volumetric(sp) heat capacity.  ~3 J/(K*cm^3).

Cool, but a 1206 resistor may not have good transient thermal coupling
from the thin resistive element into the alumina substrate.

30 w for a few ms will damage a conventional thickfilm or thinfilm
1206 resistor; trust me on that, we've proven it recently.
Surface-mount wirewounds are the way to go for big power peaks.

--

John Larkin         Highland Technology, Inc
picosecond timing   laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

```
```scott....@gmail.com wrote:

>
>
> Never mind. I was put off by 2\$ and 3\$ resistors,
> but I found 3.9 ohm 10W for \$0.64.

** 10 watt wire-wound resistors are tough components. Most are rated to accept  10 times rated power for 5 seconds and can usually stand up to 100 times for a few milliseconds.

So in your app there is no problem at all.

Ones like the 6.8ohm example below are often used in line with the AC supply to large transformers to reduce inrush surge currents at switch on -  where peak values might be up to 40amps during first half cycles. Normally the resistor is bypassed by a pair of relay contacts after a short time.

http://casamodularsystems.com/images/Restistors/6R8-J-10W-WW-AX-PW10-IRH_640p.jpg

....  Phil

```
```On 08/07/15 04:41, John Larkin wrote:

> Cool, but a 1206 resistor may not have good transient thermal coupling
> from the thin resistive element into the alumina substrate.
>
> 30 w for a few ms will damage a conventional thickfilm or thinfilm
> 1206 resistor; trust me on that, we've proven it recently.
> Surface-mount wirewounds are the way to go for big power peaks.

I don't doubt it - those things are pretty small.

That's why I suggest a 7-10W to the OP :)

```
```On Tuesday, July 7, 2015 at 11:41:39 PM UTC-4, John Larkin wrote:
> On Tue, 7 Jul 2015 18:17:32 -0700 (PDT), George Herold
> <gherold@teachspin.com> wrote:
>
> >On Tuesday, July 7, 2015 at 6:34:14 PM UTC-4, Tim Watts wrote:
> >> On 07/07/15 18:46, scott.a.mayo@gmail.com wrote:
> >> > For a circuit I described here recently, I want to be able to cause a
> >> > brief, temporary, high current draw on a 12v power supply. I'd like
> >> > to draw about 2A for a few ms, and at least 0.5A for perhaps 100ms.
> >> >
> >> > I have a resistor + capacitor combination that gives a curve I like.
> >> > A regulated 12v supply that claims to produce 5A, with a 33000uf cap
> >> > in series with 3.9 ohm resistor across it, is loaded to 2+A and is
> >> > still pulling 500ma, 200ms later.  +12v--3.9ohm--33000uf--Gnd
> >> >
> >> > The problem is, if I understand things, the resistor would have to
> >> > handle about 30W for a few milliseconds. I'd like to avoid using a
> >> > component that physically large and pricey. I'm guessing I could get
> >> > away with 5W because the high current is brief, but I want this
> >> > circuit to work reliably for years.
> >> >
> >> > Is there something smaller/cheaper I can use?
> >> >
> >>
> >> Yes - use a 7-10W resistor.
> >>
> >> 30W for a few mS is of the order of 0.1J of energy. The question is: how
> >> much will 0.1J heat up a resistor body?
> >
> >So here's a cool thing, around room temp everything has
> >about the same volumetric(sp) heat capacity.  ~3 J/(K*cm^3).
>
> Cool, but a 1206 resistor may not have good transient thermal coupling
> from the thin resistive element into the alumina substrate.
>
> 30 w for a few ms will damage a conventional thickfilm or thinfilm
> 1206 resistor; trust me on that, we've proven it recently.
> Surface-mount wirewounds are the way to go for big power peaks.

Well not many cm^3 in a 1206... (one here is ~0.02 inches tall so
~2.5E-3 cm^3, ~7E-3 J/K, 0.1 J is about a 14 degree rise...
Assuming I punched in all the numbers correctly. )

OK poor coupling... so 30 W for ~3 ms blows up a film 1206?

George H.

OK so
>
>
> --
>
> John Larkin         Highland Technology, Inc
> picosecond timing   laser drivers and controllers
>
> jlarkin att highlandtechnology dott com
> http://www.highlandtechnology.com

```