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

[leorcc]

Wheelchair motors typically do not draw much current so I guess that would not be a problem.

[daveimi]

I'm using wheel chair motors!

[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.