RE: Interfacing a coil to the C64?

From: Gideon Zweijtzer (
Date: 2002-09-11 09:24:21

> [...]
> This is pretty simple to someone with knowledge.  So here I go:
> I would like to control solenoid with c64.  So, I use
> of IPS511 mosfet switch, taking TTL in.  So now I just need
> some good power.  For this purpose, it is employed the
> LMD18400.

As far as I can see, the LMD18400 is a quadruple high-side switch as well,
and not a power regulator. The IPS511 is indeed also a high-side switch.

Some notes on your design:

1) Where do you get your 32V from? How much current can that supply source?

2) The LMD18400 can only accept 28V as a maximum input voltage. The 32V that
you have is too high for the LMD18400. Once again, the LMD18400 is not a

3) I would place the diode that you use for kickback over the coil, not over
the coil+switch.

4) The IPS511 has a minimum input voltage on it's switch input of 4V. This
is not compatible with TTL, since TTL has a Voh of 2.4V. So you need
something between the C-64 user port and the IPS511. A HCT inverter would
do, I think?

5) Using a big capacitor for the transient current is a good idea. However,
you have to check what the equivalent series resistance (ESR) is of the
elco. Usually with elco's that big, the ESR is also quite high, which means
that the maximum current you can draw from it is quite poor.

On the other hand, I just realized, if the coil really behaves as a coil at
the moment you turn it on, the impedance should be _higher_ than the 16-ohms
that you probably measured (which is the DC-resistance). (The impedance
increases when the freqency increases (Z='omega' * L). From this point of
view, the elco doesn't add much.

6) Note that at 32V, a resistance of 16 ohms yields a current of 2A, not 500

Considering the above, I think you have either:
1) a power supply that is not capable of sourcing enough current.
2) a poor switch voltage, that results in the IPS511 not fully 'opening'.


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