Botbitz 85a esc..... in a heavy?

[Max]

I've been thinking again about TZ85As in a heavy and was wondering if there was a way of hacking the esc so it acts just as a simple PWM controller (no reverse) so all the Mosfets are being used at once, you could then use a separate H-bridge using automotive relays to deal with the reversing/braking and the TZs are just altering the speed. It would be similar to a 4qd controller but replacing the MOsfets bank with a tz85a. I'm not an electronics engineer so don't know if this would work in reality or if it offered any benefit over just using a tz85a by itself but it sounded like a good idea to me!

[Shakey]

I see what you mean but the weak point in that is the relay. The TZ85's FETs are good up to 600A (Note the tracks and pins are not this good... ie. the tracks will melt before the FETs). Even if you disregarded that IIRC you'd still get ~250A continuous before the pins are in trouble with all FETs in parallel.

Also to add to this I was able to use 2 wheelchair motors at 25v with 2 TZ85's with ~100kg on 10inch wheels without significant heating even after lengthy drive periods (The motors got hotter than the TZ85). It's a seriously overspecced bit of kit for the featherweights it sees.

[overkill]

Marto custom programmed a TZ85a to act as a one way weapon controller for me, so it can be done. The problem is that you don't get all the FETs conducting at the same time; the most you can get is half of them IE all the high-side or all the low-side. It increases the current capacity but not by a huge amount.

[Max]

The problem is that you don't get all the FETs conducting at the same time; the most you can get is half of them IE all the high-side or all the low-side.

Ah that's what I feared. Could you have the same setup with the external relay h-bridge then use one of these for the pwm control: http://uk.rs-online.com/web/p/igbt-transistors/7917422/

[overkill]

In theory that will work and you wouldn't need such a large ESC to drive the outboard H-bridge. I tried that on a smaller scale and the PWM waveform out of the H-bridge looked nothing like the input - doing this probably needs an electrical engineer with experience in FET circuits. I would be very interested in any ideas for getting something like this to work!

I used the Wasp ESC from Robot power in my experiments: http://www.robotpower.com/products/wasp_info.html

[Max]

I was thinking an arduino would be the easier way to control the IGBT and relays for the h-bridge. May get one of those IGBTs just to experiment with, can anyone make enough sense of the datasheet to figure out what voltage/current it takes to turn on the IGBT/activate the gate?

[overkill]

Are you still going for a single PWM transistor and a relay H-bridge? IMHO, that's doomed to failure unless you can find an absolutely massive relay. What was the reasoning behind selecting that IGBT rather than a FET? IGBTs are generally used when you need to switch high voltages and FETs do a better job switching high current at lower voltages. That particular IGBT is not going to switch much more current than a TZ85a in a real-world situation; the large current figures are for 1 millisecond at 25 degrees.

[Max]

The relays should be alright as the pwm transistor would shut off the current whilst the relay switched then turn it back on once it has switched. This avoids sparking/arcing which is how relays normally fail, once closed they can take a lot more than their rated current.
I had noticed that a MOSFET was more suitable for low voltages than an IGBT but my issue is with connecting the mosfets, they all seem to have such tiny pins which are hard to connect a large wire to, IGBTs seem to be larger packages which are easier to connect to and you seem to only need one rather than having to connect a bunch in parallel. I am after simplicity but high current handling. Maybe the advantages of the IGBT aren't worth it though.

[overkill]

There are plenty of FETs in large packages like the one you found - if you have some spare gold bars to pay for them . I did some detailed research and concluded that unless you need to switch thousands of amps, FETs in a smaller package like the TO-247 has the best combination of current capability, heat transfer and size. I picked the IRFP4004 FET and mounted four of them to a small heatsink with copper strips to connect the source and drain pins. Its fairly compact and will comfortably switch 120A continuously in a real-world situation - more if I increased the heatsink or used a fan.

If you want to DIY a single direction FET switch,have a look at the OSMC circuit: http://www.robotpower.com/downloads/...2sch-clean.pdf. you just need to duplicate one of the low-side FET legs and then find a suitable driver. The HIP driver chip isn't necessary as you are only driving one string of FETs and you don't need the voltage pump as all the FETs are on the low side of the circuit.

[Max]

I guess this is essentially a package of mosfets: http://docs-europe.electrocomponents...6b800bbecc.pdf

Could be used with a switching frequency of up to 3kHZ but I fear that could be a bit too low. Not sure how well it would perform though and imagine when used in this sort of application as with mosfets the 100a rating would drop down a lot.

[overkill]

I have lost count of how many of those I have blown up! I used two 100A SSRs to switch a short Mag motor (no PWM) and they regularly died if the motor came close to stalling. Internally, they have two FETs in TO-220 cases. The other problem with these SSRs is impact resistance; mine had a thin a brittle plastic shell that quickly shattered. I had to cut the top of the shell off and inject potting epoxy to make them rugged enough for combat use.

On the other hand, they are super easy to control and the screw-on terminals make wiring easy too.

[marto]

I tried something similar to this had a tendency to blow up although I think it could have been due to doing pwm in two micros. It might work but I couldn't be sure. Big heatsink block helps a lot and you might just get away with running a single 85A on a reasonably geared motor.

Steve

[maddox10]

So... I gave up on the 150A version. It looked promising and I could easily pull 500A through it but the idea of pairing up two TZ85as is inherently flawed.

Pity

It all works fine until one of the ESCs browns out. And if you happen to be using very low inductance motors, like I am, this can happen even at very low PWM duty cycle. If the motors are stalled even at low throttle this can cause the input voltage to drop very low and the ESC to reset. As there is communication between the two paired ESCs to ensure they are synced and running properly this means that if they don't reset at exactly the same time or if one powers back up first they can enter a situation where there is shoot through in the ESC causing it to blow up.

Ain't the browning out of the control circuits solvable with a seperate powersource for those?

There is no way to solve this with software. It could be solved in hardware but its easier to just use a different ESC. For the middle they are 2 85As running on separate brushes.

A not uncommon trick. Over here there is a machine running astro's on XXL's in Mirror mode.

I do not know how much current is coming out of the batteries but I think it is a lot. The 8AWG wire from the batteries is too hot to touch after just a few minutes of driving. So it will be interesting to see what happens.

Lets go on with the test please

Steve