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Modem power supply problems

Discussions pertaining to the electrical issues (e.g. circuitry, power, etc.) of Pumpkin's CubeSat kit.

Modem power supply problems

Postby aek » Sun Jul 12, 2009 12:05 am

See http://www.cubesatkit.com/docs/datasheet/DS_CSK_FM430_710-00252-C.pdf .

R23 sets the current limit for the MHX to 820mA by default. You can increase that to 1.2A by changing R23 -- see Maxim MAX890 datasheet for setting the resistor.

It is likely that even at 1.2A limit, an 'x20 will trip the current limit. At that point, you'll need to reduce the x20's output power, or follow my suggestion to provide more current via a direct connection to +5V_SW.

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Re: Modem power supply problems

Postby Albert » Sun Jul 12, 2009 11:45 am

I'm working with an FM430 Rev C and an MHX 920A transceiver. I've been trying to test the modems (one in the FM430 and the other connected to a PC) but it doesn't work.

I have seen that the +5V_SW signal falls under 3V every time the transciever receives data to transmit via TXD pin. After this, +5V_SW power is again right and modem restarts. So, it's impossible to transmit anything.

I have connected two modems to two test boards to a PC and I know that both them work and I can even transmit in both ways between the modems.

I suspect that there could be some problem with the current limit in the +5V_SW but I'm not sure ... Any idea? Thanks!

[This message has been edited by Albert (edited July 12, 2009).]

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Re: Modem power supply problems

Postby aek » Sun Jul 12, 2009 11:59 am

There are several issues here:

1) The Rev and earlier FM430s have a relatively low maximum current on +5V_SW (now called MHX_PWR in Rev D), set by the use of a MAX890 -- 1.2A. The resitor used in Rev C actually sets the current limit to be a bit lower, because at the time the MHX2400 was the only MHX that was available, and it required ca. 800mA on transmit. The x20 series (MHX920, MHX2420) require substantially more current from the +5V_SW supply -- around 2-3A (!) peak, as reported by x20 users.

So, for Rev C and earlier, if you are using an x20 MHX unit, you need to provide external circuitry on another module that feeds +5V into +5V_SW at currents in the 2-3A range. Or, you can reduce the transmit power of the x20 unit, but I do not know how low you have to go before the current drawn drops below 1.2A.

2) The Clyde Space EPS (if you are using it) cannot feed the currents required by the x20 series ... its auto-reset circuitry will trip (I am told) when trying to feed an x20-series transmitter. This means that you will needs another approach to delivering the +5V that the MHX requires.

3) Pumpkin's Rev D Motherboard uses a Micrel part (2425?) with a >3A limit (though it is set to around 2A at Pumpkin -- see MB datasheet). Therefore it might be able to support the x20 MHXs directly, but you still need an EPS that doesn't object to the high currents drawn from the +5V_SYS bus.

So, you are basically fighting the fact that the x20 series MHX units were released after the FM430 Rev C was designed. And the x20 series draw peak currents much higher than the stated values in the Microhard datasheets. Microhard is aware of this problem -- you may want to contact them directly.

P.S. Unfortunately the MAX890 and Micrel parts have different pinouts and are not interchangeable.

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[This message has been edited by aek (edited July 12, 2009).]

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Re: Modem power supply problems

Postby a901395 » Fri Feb 26, 2010 2:48 am

Dear,
I tried to connect two MHX2400 in their respective development board and work correctly. Now when I connect one of them in the CubeSat Kit Development Board, I have problem with power supply. The CubeSat Kit Development board can be powered via USB or electrically.If none of the ways I feed the transceiver.

What can I do?
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Re: Modem power supply problems

Postby a901395 » Mon Mar 01, 2010 7:26 am

Dear,
I have two transceivers microhard MHX-2400. When I connect in their respective development boards are working properly. It connects to 12 V, because if I connect to 5V will not work. The Cubesat works with 5 V, then to put one of the transceivers in the development board of Cubesat and another on MHX-2400 development board, I fail to connect between them. I think the problem is that the development board of Cubesat can not feed the transceiver. It's correct? What can I do?
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