Low Volts High Current!

The car battery you have there consists of a number of cells, which are connected in series. The voltage between the two outermost placed contacts is 12 Volt.

In between them there are other connections which carry different voltages. If you can remove the plastiv cover on the battery, or drill through it, you can connect to any number of cells you like, in series.

But I am pretty sure there are lead-acid batteries made for 7-8 Volt for sale in UK. A country filled with motorcycles of all ages and needing batteries of different voltages.

The lead-acid battery technology is well suited to delivering very high currents for a short time. It will probably be the most cost-effective in this case.

On 13 Nov 2004 16:04:23 GMT, Roger Johansson wroth:

I think you have the most satisfactory solution.

The actual voltage isn't extremely important as long as it doesn't cause the motor to explode immediately upon application. Since a computer is involved, the power, amps times voltage, is available as one of the parameters and that should be good enough even if the voltage is a little bit off from the desired 7.2 volt figure or if it sags a little during the test.

Why throw a lot of electronics at the problem when a little software can do the job?

Jim

Indeed, but you'll have to agree it's a big efficiency improvement over my 42V to 1.8V linear regulator / seat warmer.

Eh- you have approximately doubled the required AH of the battery.

The answer to this may be a hobbyized version of a multiphase buck converter- maybe summing currents at the load destination - to break the problem into manageable pieces:

In article , Winfield Hill wrote: [.....]

The TIP35 and TIP36 transistors are fairly cheap. I think I'd start like this:

Hmm, TIP35 transistors are not Darlington types, which means low beta, but also lower Vbe. If each transistor conducts 15A with a beta of 15, then the TIP36 PNP has to handle 10A, which it can do just fine. If its gain is 25, the LM317 has to handle 0.5A, which is also fine. At 500mA the LM317's dropout voltage is 1.7V, which with a Vbe of 2V for a TIP36 at 10A gives us an overall regulator dropout voltage of 3.7 volts Adding this to the 7.2V output tells us the battery must stay above 10.9 volts for regulation at 150A. If the transistor betas are a bit higher, and if the Vbe voltages are a bit lower, the battery will be able to sag more, and there may be room left for a cable drop. Now working the TIP35 pathway, say the emitter resistors drop 0.2V and Vbe = 2V, this leaves 1.5 volts for the TIP36 Vce(sat), assuming a 10.9V battery. Everything is working out, because 1.5 volts is what the TIP36 datasheet tells us to expect for Vce(sat) at 10A.

So this is a viable approach, perhaps even better than my drawing.

Here's a small NPN power transistor spec comparison table. All are big TO-218 style plastic packages, except the 2n6284 is TO-3 (rated Tj = 200C). The TIP35 and TIP36 are NOT Darlington types.

part thermal Hfe @ Ic Vbe @ Ic number C/W Pd 10A 15A 10A 15A ------ --- --- ---- ---- ---- ---- TIP35 1.0 125W 30 >15 < 2V 2V (also PNP TIP36) 2n6284 1.09 160 750 500 < 2 2.5 BDW83C 0.96 130 750 >100 < 2.5 ? TIP142 1.0 125 500 ? 2 ~ 2.5 All these transistors are inexpensive and widely available, but they're comparatively wimpy. For example, a 2n5686 (see below) is a 50A npn TO-3, rated at 300W, and it's much better suited for the task, with higher beta and lower Vbe. But it's $11.76 at DigiKey. The 60A MJ14002 is a bit less at $10.36 qty 10, stocked by DigiKey.

part thermal Hfe @ Ic Vbe @ Ic number C/W Pd 10A 15A 10A 15A ------ ----- ---- ---- ---- ---- ---- 2n5686 0.584 300W 70 50 0.85V 0.9V TO-3 non-Darlington MJ14002 0.584 300W 100 70 0.8V 0.85V " "

Using one of these transistors would open the possibility of battery voltages lower than 10.9V, although using a low-dropout LT1085, etc., instead of a LM317 (0.9V compared to 1.7V at 0.5A) would be required. Then a 10V sagging battery + cable voltage could be allowed at 150A.

I recently built an electronic load that could handle 100 A using a power hexfet from Intl. Rectifier. This one can handle 150 A with proper heatsinking:

Yes that's a good point. I learned about this by examining my 1985 Cougar with the help of Chilton and Haynes manuals. The engine compartment is filled with 20, 30, 40 and 60A distribution fuses for various things, plus single 175A "Mega fuse" to the generator and "distribution block" (called a fuseable link on some models). Mine is a removable plastic-encased fuse.

There is also a 15A and an 80A fuse bypassing the Mega fuse. In the case of these Ford models the starter motor is _not_ fused. With some effort these big replaceable fuses can be procured as automotive spare parts.

I've added a capacitor to the drawing called "Cbig" and labled some resistors.

The TIP35 and TIP36 are slow. They are extra slow when their collector voltage drops below about 2V. Along with the usual bypass an LM317 needs, you also need some added largish capacitor to make up for the fact that it will take the TIP35,36 stuff time to throttle up and down. The minimum value of this capacitor I would extimate like this:

Assume that the 150A draw starts suddenly. Assume that the voltage can droop or spike up by 0.1V.

The TIP35 and 26 have a rise and fall time of about 0.25uS (Check data sheet to be sure)

The TIP36 has to turn on and then the TIP35 so the total time is 0.5uS.

Rather than having to integrate, we just say the TIP35 and TIP36 stuff doesn't do anything for 0.5uS and get an over estimate of the minimum.

In 0.5uS, 150A will transfer a charge of 75uC.

The voltage change on the Cbig will be:

V = Q / C

where Q is the charge C is the capacitance

We solve for C

C = Q / V = 75uC / 0.1 = 750uF

Now here's the messy bit. We have assumed tha Cbig has no resistance. All real parts have some resistance. We now need to find the data sheet on some lets say, 1000uF capacitor with a nice low Equivelent Series Resistance (ESR) to see how much drop will appear in the ESR. If that plus the droop in the 1000uF is more than the specification, you need a larger capacitor.

Finding R1:

If the circuit is lightly loaded, we want the LM317 to pass all of the current. This means that there should be less than about 0.6V on R1 when a "small current" is drawn. We will define a small current as 50mA os we can say

R1 = 0.6V / 0.05A = 12 Ohms

So we make R1 about 12 Ohms.

About R2:

R2 could in most cases be zero. It lowers the gain of the TIP36 part of things and works to reduce the risk of oscillation. I'd make this a low inductance resistor of about 0.01 Ohms or a short length of wire.

About R3:

We want about an Amp flowing in the TIP36 before the TIP35s are brought into things. R3 is something like 0.1 to 1.0 Ohms.

About R4:

This is actually 10 resistors, one resistor in the emitter leg of each TIP35. When a bipolar transistor heats up, its Vbe drop decreases. If R4 wasn't there, the hottest TIP35 would end up taking on more than its share of the load, get hotter, draw more, get hotter and etc. This could destroy one transistor.

Thermal resistance is a lot like electrical resistance. Heat sources are like current sources. All of the heat must flow out of one transistor through all of the stuff it goes through to get out to the environment. In process, one transistor will tend to warm up its brothers. To find the minimum R4 we start with assuming a current flows in one transistor and work out how much its temperature rises above the coolest of its brothers. To make the math easy, just assume 1A flows.

One you have the temperature rise, assume that the individual R4 must add at least 2.5mV of drop for every amp flowing. It is likely that this will be such a small value that using 10 wires running from the TIP35s to the load instead of one big oun will be enough to do it.

A note about layout:

You want Cbig to be as close to the load as practial. Ideally, there should be 12 wires to the (+) side of the load. The LM317, the TIP36 and all the TIP35s each have a wire. These wires should be tightly bundled together.

A note about fuses:

They make special fuses for things like car starter motors. They look like a thick cable with a wide section in it. Put one of these between the car battery and the circuit. A car battery can produce nearly infinite current for a short time.

Hmm... once I ripped 6 huge electrolytic caps from some old machine

- it might have been a photocopying machine, dunno. It had some kind of primitive fixed disk in it (and a bubble memory of the magnetic bead variety, imagine that.) Anyways the caps were at least 4" in diameter by 10" long and were rated for 16V/250,000uF each. So yep, I wired them all in parallel and charged them to 12V through a 5kW autotransformer (used as a potentiometer.)

WOW, instant spot-welder. Jumping wires, permanent magnetization of nearby metal objects, neat. Today, you could probably do even better with the 2.0 Farad "supercaps" sold to car audio enthusiasts. (Their explosion-resistance should be checked though...)

But the point is this, an idea might be to use a linear supply to charge an array of supercaps to about 8V, then (using an automotive solenoid or two) short these to your motor. This will provide a large inrush current for a short time, enough to get your motor spun up. Also switch on a big 7.2V linear supply to power the motor as the caps discharge.

Or better yet, use a PWM method and straight 12V to drive the motor, as car batteries are cheap compared to kW-rated supplies. If RPM is the goal, PWM is the best method to obtain accurate speed of a DC motor. A microcontroller could sample the shaft RPM and base the PWM duty cycle off that easily. If the design goal is measuring how much power it takes to spin the motor at that speed (how resistive the load is) then use PWM for accurate speed but measure the motor's steady-state current and calculate the power delta from that.

IGBT's make a good PWM driver stage with their built-in fast commutating diodes, high voltage and current ratings, and simplified drive characteristics. If a very low Rds is achieved, (max Vgs), (relatively) small heatsinks could be used with (relatively) little loss. (Might not make a full seat-warmer, but that motor is going to get hot no matter what.) And hey, the motor placed under your seat would give a massage along with heat!

-M

You could charge up some ultracapacitors and then place them across your load with a big switch. You can buy some 2700 FARAD 2.5V ultracapacitors at

Asa

The Supercaps have such a high ESR that you can't get much current out of them. You also can't charge them fast or discharge them fast, which can be a real pain when designing high-speed testers for products that include them.

I'd say instead that switchers are too complicated for this application in the first place and they have the very real disadvantage that it is harder to take advantage of an intermittent load - because switchers are smaller and lighter, there is less thermal inertia and they must be rated closer to the peak load - meaning we have to buy power handling capabilities that we don't need.

I *would* consider modifying the circuit to use (an) IGBT's instead of bipolars for the pass transistors; IGBT's are available in bigger packages than bipolar with screw connectors more suitable to the bus-wiring one needs for the 150A. The IGBT would have to go 'below' the motor in order to drive it.

In article , Frithiof Andreas Jensen wrote: [...]

I disagree with this. A 150A switcher is not all that hard to do and would make a lot less heat.

No, if you have to make the devices rated at X, you have to make them rated at X. This is not a capability you don't need. By definition, you need it.

I think the one IGBT would cost as much as the 11 transistors in my design and not be as good of a design. IGBTs that are rated for linear operation are less common than those rated for switching. The electronics to control the IGBT would be more complex than my proposed circuit. All round, I think this application is better with bipolars.

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What happens to your circuit after you have spun the motor up to 150A and it (your circuit) decides to reduce the current?

DNA